JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B11202, doi: /2010jb007433, 2010

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010jb007433, 2010 Magma plumbing system of the 2000 eruption of Miyakejima volcano, Japan, deduced from volatile and major component contents of olivine hosted melt inclusions Genji Saito, 1 Yuichi Morishita, 1 and Hiroshi Shinohara 1 Received 29 January 2010; revised 30 June 2010; accepted 26 July 2010; published 11 November [1] Chemical analyses of 56 olivine hosted melt inclusions and their host olivines from the explosive eruption of Miyakejima volcano, Japan, on 18 August 2000 were carried out by electron probe micro analyzer and secondary ion mass spectrometry in order to investigate the magma plumbing system of the eruption. The bimodal olivine core composition, relatively small rims, and diffusion profiles of the Mg rich olivines indicate the mixing of evolved magma and less evolved magma in 2 months before the eruption. The major element composition of the Mg poor olivine hosted melt inclusions (Mg poor OLMIs) is similar to that of the groundmass. The Mg rich olivine hosted melt inclusions (Mg rich OLMIs) have a SiO 2 and K 2 O poor but Al 2 O 3 and CaO rich composition, compared to the whole rock composition of the products. Most of the Mg rich OLMIs have slightly higher H 2 O ( wt %) and S ( wt %) contents and a lower Cl ( wt %) content than the Mg poor OLMIs, whereas they have similar CO 2 ( wt %) content. These results suggest that CO 2 rich gas was added to the evolved magma in a shallow magma chamber, followed by decompression degassing in a pressure range of MPa and the rapid ascent of the less evolved magma from a deep magma chamber up to a depth of about 2 km. The ratios of the H 2 O and S contents of the Mgrich OLMIs are similar to that of the volcanic gas emitted from the summit after the 2000 eruption, which suggests shallow degassing of the less evolved magma in the conduit since the eruption. Citation: Saito, G., Y. Morishita, and H. Shinohara (2010), Magma plumbing system of the 2000 eruption of Miyakejima volcano, Japan, deduced from volatile and major component contents of olivine hosted melt inclusions, J. Geophys. Res., 115,, doi: /2010jb Introduction [2] The volatile content of magma is one of the most important controlling factors for magma ascent and volcanic eruption [e.g., Blake, 1984], as well as for the evolution of magma [e.g., Coats, 1962; Sisson and Grove, 1993]. Because the volatile content of magma changes in processes such as crystallization, magma mixing, and pressure decrease, information on the volatile content of magma helps us to construct quantitative models of these magma processes. Melt inclusion analysis is a powerful method for estimating the volatile content of pre eruptive magma [e.g., Anderson, 1973; Johnson et al., 1994; Lowenstern, 2003]. Because melt inclusions can preserve the volatile content of the melt at the time of its entrapment, we can evaluate magma processes by comparing the major element composition of the melt inclusions and host phenocrysts with the volatile content of the inclusions. With regard to the investigation of 1 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan. Copyright 2010 by the American Geophysical Union /10/2010JB basaltic volcanoes, the volatile analysis of melt inclusions trapped in olivines is an effective tool because olivines commonly crystallize during the ascent and evolution of basaltic magma. [3] Several previous studies on melt inclusions trapped in olivines revealed the existence of a large variation in the volatile content of basaltic magma for various tectonic settings and volcanoes [e.g., Anderson and Brown, 1993; Roggensack, 2001; Metrich et al., 2004; Spilliaert et al., 2006; Johnson et al., 2008]. Olivine hosted melt inclusion analyses of the 1959 eruption of Kilauea volcano, which is considered to be representative of ocean island basalt volcanoes, indicate H 2 O poor magma (<1 wt % [Anderson and Brown, 1993], whereas those of subduction related volcanoes indicate a relatively H 2 O rich composition of up to 6 wt % [Sisson and Layne, 1993]. In addition, large variations in the volatile content of the basaltic melts from a single eruption have also been observed in some volcanoes; these variations were considered to have been caused by the degassing and evolution of the magma during its ascent. For example, Roggensack [2001] showed that olivine hosted melt inclusions from the 1974 eruption of Fuego volcano had large variations in their H 2 O(3 5wt%)andCO 2 (up to wt %) 1of29

2 contents and concluded that these were caused by the mixing of less evolved basaltic magma ascending from a deeper part with evolved magma in a shallow magma chamber. For Etna volcano, Metrich et al. [2004] and Spilliaert et al. [2006] reported that large variations in the H 2 O and CO 2 contents of olivine hosted melt inclusions were observed for products of the eruptions. They concluded that these volatile variations resulted from the degassing of gasrich, less evolved magma by a pressure decrease and the subsequent addition of gas to the degassed basaltic magma at a depth of 4 6 km. [4] In this study, we conducted volatile analyses of the melt inclusions in olivines from a bomb and lapilli from the summit eruption of Miyakejima volcano, Japan, on 18 August 2000, in order to investigate the magma degassing and evolution processes of this eruption. The Miyakejima 2000 eruption offers an excellent case for the investigation of the process of magma ascent based on melt inclusion studies because many geological, geophysical, and geochemical observations were carried out during the eruption sequence [e.g., Nakada et al., 2005]. Saito et al. [2005] analyzed the volatile content of the melt inclusions in the summit eruption products, but they mainly analyzed plagioclase hosted melt inclusions; as a result, few analyses of olivine hosted inclusions have been performed. Saito et al. [2005] observed two types of olivines in the bombs and lapilli: dominant Mg poor core olivines and rare Mg rich core olivines. Because the Mg rich olivines could crystallize in a magma that is less evolved compared to the magma in which the Mg poor olivines crystallized, volatile analyses of the melt inclusions trapped in these olivines can provide information on the degassing and evolution of the magma during its ascent. Using the combined geological, petrological, and geochemical observations from the 2000 eruption, we investigated the magma plumbing system of the eruption. 2. Outline of the 2000 Eruption of Miyakejima Volcano [5] Miyakejima is an active volcanic island located 200 km south of Tokyo on the Izu Mariana arc system. The history of the development of Miyakejima volcano during the last 10,000 years is divided into four stages: 10 7 ka, ka, 2.5 ka to the early 15th century, and from the 1469 eruption to the present [e.g., Tsukui et al., 2002, 2005]. The volcanic activity in 2000 began with an earthquake swarm at a depth of 3 km beneath the summit of Miyakejima volcano on 26 June [Sakai et al., 2001]. After the earthquake swarm migrated toward the west, there was a minor submarine eruption off the western coast of Miyakejima on 27 June. An extensive earthquake swarm and crustal deformation then occurred in July and August [Japan Meteorological Agency, 2000; Kaneko et al., 2005; Nakada et al., 2005], indicating the intrusion of a dyke at a depth of 2 13 km, up to 20 km northwest of Miyakejima [Nishimura et al., 2001; Sakai et al., 2001; Toda et al., 2002]. The intruded magma is thought to have been tapped from a magma chamber beneath Miyakejima [Geshi et al., 2002]. Subsidence of the summit area began with minor phreatic eruptions on 8 July and continued through mid August. Several phreatic and phreatomagmatic explosions occurred during this subsidence. The eruptive activity culminated with the largest summit explosion on 18 August [e.g., Nakada et al., 2005]. Major eruptions ceased at the end of August, but then intense and continuous degassing from the summit began [Kazahaya et al., 2004]. A high rate of gas emission corresponding to more than 120 kg s 1 of SO 2 continued from the end of August 2000 onward and peaked at 630 kg s 1 in December 2000 [Kazahaya et al., 2004]. The emission rate gradually decreased to about 20 kg s 1 at the end of 2009 [reports by Japan Meteorological Agency, 2010; jma/menu/volcanomenu.html]. The total amount of SO 2 emitted from the end of August 2000 to March 2009 was estimated to be 24 megaton (Mt) (reports by K. Kazahaya k/monthly.htm). [6] During the 2000 eruption, two magmas of different composition erupted successively [Amma Miyasaka et al., 2005; Kaneko et al., 2005]. The magma that erupted from the submarine craters on 27 June was aphyric basaltic andesite (<7 vol % phenocrysts). The whole rock and mineral composition is similar to that of the magma from the eruption of 1983, which suggests that the magma of the submarine eruption was a remnant of the 1983 eruption. The magma that erupted as volcanic bombs from the summit on 18 August was plagioclase phyric basalt (20 vol % phenocrysts). Petrological observation of the essential ejecta on 18 August indicated that new basaltic magma had ascended from a deeper chamber [Amma Miyasaka et al., 2005: Kaneko et al., 2005]. Amma Miyasaka et al. [2005], who carried out petrological studies on volcanic rocks resulting from eruptions over the past 500 years, concluded that shallow andesitic and deeper basaltic magmas successively erupted from different craters. Many geophysical [Sakai et al., 2001; Furuya et al., 2003; Nakada et al., 2005; Uhira et al., 2005] and petrological [Amma Miyasaka and Nakagawa, 2003; Amma Miyasaka et al., 2005] studies have suggested that the depths of the andesitic and basaltic magma chambers are 3 5 km and about 10 km below sea level, respectively. Kaneko et al. [2005] suggested that the basaltic magma was injected into the shallow magma chamber and mixed with the basaltic andesite magma before the submarine eruption: a suggestion that is based on the wide range of Cl contents in the melt inclusions in the plagioclase of the 27 June submarine spatter. [7] Saito et al. [2005] analyzed the volatile content of the melt inclusions in the plagioclase and olivine from a bomb and lapilli from the summit eruption on 18 August 2000, in order to investigate the magma ascent and eruption processes of the basaltic magma. Their results indicate that the melt in the basaltic magma has wt % H 2 O, <0.011 wt % CO 2, wt % S, and wt % Cl. The variation in the volatile content suggests degassing of the magma during its ascent up to a depth of about 1 km. They estimated the initial bulk volatile content of the basaltic magma to be wt % H 2 O, wt % CO 2, wt % S, and wt % Cl, on the basis of the volatile content of plagioclase hosted melt inclusions and the chemical composition of the volcanic gas emitted from the summit. They also suggested that the basaltic magma ascended from the deeper chamber due to a decrease in magma density caused by volatile exsolution with a pressure decrease. The highly vesiculated magma, which had bubbles of at least 30 vol %, 2of29

3 Table 1. Major Element Analyses (in Weight Percent) of Eruptives From the Eruption on 18 August 2000 of Miyakejima Volcano and a GSJ Reference Sample (JB1a) a Sample Bomb Lapilli b Bomb 819B1 c JB1a Recom d This Study e SiO (0.36, 0.69) TiO (0.01, 0.87) Al 2 O (0.09, 0.61) FeO* (0.02, 0.31) MnO (0.00, 0.00) MgO (0.06, 0.81) CaO (0.02, 0.22) Na 2 O (0.09, 3.12) K 2 O (0.01, 0.38) P 2 O (0.00, 1.71) Total (0.56, 0.56) a FeO*, total iron as FeO. b A powder sample of a single lapillus. c A representative analysis of the essential product from the eruption on 18 August 2000 of Miyakejima volcano [Saito et al., 2005]. d The recommended values are after Imai et al. [1995]. e Averages of analyses of 5 glass disks made from JB1a. Values in parenthesis are standard deviations and coefficients of variation (standard deviation/average 100). contacted groundwater at sea level, causing the large phreatomagmatic eruption on 18 August. 3. Samples and Analytical Procedures 3.1. Samples [8] The samples used for melt inclusions in this study were an essential bomb (13 cm in size; in Table 1) and lapilli (1 4 cm in size; and ) from the eruption on 18 August 2000, which were collected on the side of the volcano 2 4 km from the summit. The bomb and lapilli contained 15 vol % plagioclase phenocrysts, a few vol % of olivine phenocrysts, and a few vol % of clinopyroxene phenocrysts [Saito et al., 2005]. Saito et al. [2005] described two types of olivine in the bomb and lapilli. One type has a large and homogeneous core of Fo with a size of less than 0.6 mm; this type is common and has a euhedral shape (Fo is the forsterite % in the olivines; that is, Mg/(Mg + Fe) 100% in mol). The other type has a normal zoning profile with a more Mg rich core (Fo ) and a larger phenocryst size (0.4 1 mm); this type is less common. Most of the inclusions in the olivines from the 2000 eruption are less than 0.1 mm in size (section 4.3). This makes it difficult to prepare double thin sections of the inclusions for Fourier transform infrared spectrometry (FTIR), which is the conventional method for analyzing the H 2 O and CO 2 in melt inclusions. Therefore, in order to accumulate data on the H 2 O and CO 2 contents of the inclusions, for this study, we used secondary ion mass spectrometry (SIMS) because SIMS measurements do not require difficult sample preparations X Ray Fluorescence Analyses [9] The major element compositions of the bomb and lapilli were determined by a wavelength dispersive X ray fluorescence analysis (XRF) (Axios, PANalytical Co. Ltd. in Geological Society of Japan (GSJ), National Institute of Advanced Industrial Science and Technology [AIST]) (Table 1). Glass disks were prepared for the XRF analyses using 1:10 mixtures with lithium tetraborate, following the procedure of Togashi [1989]. Repeated analyses of rock standard JB1a, prepared by the GSJ [Imai et al., 1995], indicated small coefficients of variation (= standard deviation/ average 100) and good agreement with the recommended values (Table 1) Electron Probe Micro Analyzer Analyses [10] Olivine phenocrysts were picked from the crushed samples and mounted in epoxy resin. Synthetic and natural glass samples for SIMS calibration were also put in the same mount sample. This mount sample was ground with sand paper to expose the melt inclusions and then polished with diamond powder (1 mm). Sample observations by backscattered electron images and chemical analyses of the melt inclusions, their host olivines, and the natural and synthetic glass samples for SIMS calibration were carried out using an electron probe micro analyzer (EPMA) (JEOL JXA 8900 in GSJ, AIST). The EPMA analytical conditions for the analysis of the olivines were the same as those reported by Saito et al. [2002]. We measured the compositional profiles from core to rim across the zoned olivines by EPMA. The major element compositions and the S and Cl contents of the melt inclusions were analyzed by EPMA with an accelerating voltage of 15 kev, a beam current of 12 na, and a defocused beam with a 5 mm diameter. A more detailed description of the analytical conditions and the results of the repeated analyses of the reference glasses can be found in Appendix A Glass Samples for SIMS Calibration [11] To ensure precise SIMS measurement of the H 2 O and CO 2 contents of the melt inclusions, we prepared glass samples for SIMS calibration. For the calibration, we used six natural basaltic glass samples and seven synthetic basaltic glass samples (Table 2). The H 2 O and CO 2 contents of the glass samples were determined by FTIR (Nicolet Magna IR 550 FTIR spectrometer in GSJ, AIST). Doubly polished thin sections of the glass samples were prepared according to the procedure of Lowenstern and Mahood 3of29

4 Table 2a. Chemical Composition (in Weight Percent) of Volcanic Glass Samples for SIMS Calibration a Sample JDF 40 b cv (%) JDF2 10 b cv (%) MTB 20 b cv (%) CYM 10 b cv (%) GLP cv 10 b (%) KLER 10 b cv (%) SiO (0.57) (0.49) (0.46) (0.36) (0.20) (0.66) 1.3 TiO (0.05) (0.03) (0.03) (0.02) (0.02) (0.04) 1 Al 2 O (0.15) (0.10) (0.15) (0.10) (0.11) (0.11) 0.8 FeO* (0.27) (0.10) (0.13) (0.12) (0.08) (0.17) 1.5 MnO 0.22 (0.02) (0.02) (0.03) (0.02) (0.03) (0.03) 17 MgO 6.98 (0.10) (0.05) (0.09) (0.09) (0.07) (0.07) 1 CaO (0.08) (0.06) (0.09) (0.06) (0.08) (0.05) 0.5 Na 2 O 2.79 (0.16) (0.15) (0.13) (0.15) (0.13) (0.13) 4.7 K 2 O 0.18 (0.01) (0.01) (0.01) (0.01) (0.01) (0.02) 3 P 2 O (0.02) 16 nd 0.05 (0.01) (0.02) 101 nd 0.06 (0.03) 51 H 2 O 0.35 (0.003, 12) (0.01, 10) (0.06, 15) (0.03, 13) (0.02, 11) (0.02, 8) 4 CO (0.003, 12) (0.003, 10) (0.002, 15) (0.006, 13) (0.002, 11) (0.001, 8) 36 S (0.006) (0.007) (0.005) (0.004) (0.005) (0.006) 5 Cl (0.004) (0.003) (0.003) (0.003) (0.004) (0.003) 19 Total a FeO*, total iron as FeO. Major elements, S, and Cl concentrations of the glasses were analyzed by EPMA. Water and CO 2 concentrations of the glasses were measured by FTIR. Values in parenthesis represent the standard deviation of EPMA. For H 2 O and CO 2, the standard deviation (or range) and number of FTIR analyses are shown. cv (%), coefficient of variation (= standard deviation/average 100). JDF and JDF2, Juan de Fuca Ridge basaltic glass; MTB, Mariana Trough basaltic glass; CYM, Cayman Trough basaltic glass; GLP, Galapagos Ridge basaltic glass. KLER, Kilauea East Ridge basaltic glass. b Number of EPMA analyses. [1991]. We used the total OH peak at 3550 cm 1, or the OH peak at 4500 cm 1, along with molecular H 2 O peaks at 5200 cm 1 and/or 1630 cm 1 for the H 2 O analysis, depending on the H 2 O abundance. We used carbonate peaks at 1515 and 1435 cm 1 for the CO 2 analysis. The absorption coefficients were calculated on the basis of the major element compositions [Dixon et al., 1995]. The H 2 O and CO 2 contents were calculated using Beer s law. The detailed procedures, conditions, and errors for the FTIR analyses were described by Saito et al. [2001]. We carried out 8 15 analyses for each natural basaltic glass sample in different positions (Table 2a). The H 2 O and CO 2 contents of the six natural glass samples were in the range of wt % and wt %, respectively (Table 2a). [12] The synthetic basaltic glass samples were made in high pressure experiments, which were performed using an internally heated pressure vessel with a rapid quenching device installed at GSJ, AIST. The starting material was powder from the groundmass part of the bomb from the eruption of Miyakejima volcano on 18 August The sample powder ( mg) was enclosed in cylindrical Pt capsules (20 30 mm in length and 3 mm in diameter) with a mol L 1 Na 2 CO 3 solution or distilled H 2 O(0 6 mg) that were then welded. The capsules were kept for 3 9 hat 1200 C 1300 C and 195 MPa and then quenched. The experimental products were glassy and brown to black in color. We cut these experimental products into segments and took the central portion for EPMA, FTIR, and SIMS analyses. The products were composed almost entirely of glass with/without very rare crystals that were 0.1 mm in size (Figure B1). All of the products had tiny bubbles that were smaller than 0.1 mm. Backscattered electron images of the products suggested that the fraction of bubbles in each product was less than 0.1 vol %. To evaluate the chemical homogeneity of the glass products, we measured the contents of major elements, S, and Cl in the cross sections of the glass products (Figure B1) using EPMA, and the H 2 O and CO 2 contents using FTIR. Our analyses indicated that all of the glass products had chemical homogeneities suitable for use as reference glasses in the SIMS calibration. A more detailed description of the chemical analyses of the products is provided in Appendix B. The H 2 O and CO 2 contents of the seven synthetic basaltic glass samples ranged from 1.1 to 3.8 wt % for H 2 O and from to wt % for CO 2 (Table 2b). In addition, degassed basaltic glass samples were prepared by fusion of the volcanic glass samples (JDF and MTB) at 1300 C under vacuum. It is expected that the H 2 O and CO 2 contents of the degassed basaltic glass samples are 0 wt %. Therefore, the basaltic glass samples containing H 2 O(0 3.8 wt %) and CO 2 ( wt %) were prepared for the SIMS calibration in this study SIMS Analyses [13] Before the SIMS analyses, the mounted samples were polished with diamond powder (1 mm) and Al 2 O 3 powder (1 mm). All the mounted samples were dried for several days at 50 C in an oven that was exhausted by a vacuum pump, before being coated with gold for the SIMS analyses. [14] We used a Cameca ims 1270 SIMS (installed at GSJ, AIST) to measure the H 2 O and CO 2 contents of the melt inclusions. Cs + ions were used as the primary beam and negatively charged secondary ions of 1 H, 12 C, and 30 Si were collected [Hauri et al., 2002]. The defocused Cs + primary beam was restricted to 25 mm in diameter by a circular aperture in order to obtain a homogeneous primary beam of about 1 na. Negative ions of 1 H, 12 C, and 30 Si were detected using a total impact energy of 20 kv (a primary accelerating voltage of 10 kv and a secondary extraction voltage of 10 kv). A 1 1 mm square field aperture was introduced into the secondary ion optics, limiting the analyzed area of the sample surface to a central square measuring mm, in order to avoid the crater edge effect. A normal incidence electron gun was used for charge compensation on the sample. We adjusted the electron gun conditions based on Kita et al. [2004]. These adjustments were carried out on an aluminum mesh plate for each measurement day. Energy filtering was applied to prevent the H + ions from adsorbing H 2 O on the sample and the inner wall of the instrument. The energy slit 4of29

5 Table 2b. Chemical Composition (in Weight Percent) of Synthetic Basaltic Glass Samples for SIMS Calibration a Range of cv (%) Reference Glass c Min Max eg060802ht3 cv 29 b (%) eg060802ht1 3 cv 19 b (%) eg060802lt4 cv 21 b (%) eg060802lt2 cv 15 b (%) eg cv 18 b (%) eg cv 15 b (%) Sample eg cv Number b 16 b (%) SiO (0.44) (0.64) (0.76) (0.46) (0.60) (0.37) (0.29) JDF TiO (0.05) (0.03) (0.05) (0.05) (0.04) (0.04) (0.04) JDF Al2O (0.12) (0.20) (0.27) (0.11) (0.12) (0.10) (0.09) JDF FeO* (0.21) (0.18) (0.29) (0.22) (0.51) (0.18) (0.20) JDF MnO 0.21 (0.03) (0.02) (0.03) (0.03) (0.03) (0.03) (0.03) JDF MgO 4.23 (0.04) (0.08) (0.09) (0.05) (0.06) (0.05) (0.05) JDF CaO 9.99 (0.07) (0.08) (0.06) (0.06) (0.13) (0.09) (0.08) NBS621 Na 2 O 2.65 (0.13) (0.15) (0.13) (0.13) (0.20) (0.11) (0.12) JDF K 2 O 0.47 (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) JDF P2O (0.02) (0.02) (0.02) (0.02) (0.01) (0.02) (0.02) MTB H2O 3.3 (0.4, 8) (0.1, 11) (0.1, 13) (0.2, 2) (0.1, 2) (0.02, 5) (0.02, 5) Saito2001 CO (0.003, 11) (0.003, 12) (0.003, 13) (0.004, 6) (0.003, 8) (0.003, 6) (0.002, 5) Saito2001 S (0.002) (0.004) (0.003) (0.013) (0.010) (0.004) (0.003) NBS610 Cl (0.003) (0.004) (0.004) (0.004) (0.004) (0.004) (0.004) MTB Total a FeO*, total iron as FeO. Major elements, S and Cl concentrations of the glasses were analyzed by EPMA. Water and CO 2 concentrations of the glasses were measured by FTIR. Values in parenthesis represent the standard deviation of EPMA. For H2O and CO2, the standard deviation (or range) and number of FTIR analyses are shown. cv (%), coefficient of variation (= standard deviation/average 100). The synthetic glasses were made by high pressure experiments. The starting material is powder of the groundmass part of the bomb of the 18 August 2000 eruption of Miyakejima volcano (see text). b Number of EPMA analyses. c Representative values of cv (%) of repeated analyses of reference glasses (Table A1) are shown for comparison (see text). Saito2001 means data from Saito et al. [2001, Table 2]. was set at +20 ev for the lower edge with an energy band width of 20 ev. The secondary ions of 1 H (< counts per second, cps), 12 C (< cps), and 30 Si ( cps) were detected by an electron multiplier. A single analysis consisted of 20 cycles of measurement of the 1 H (1 s), 12 C (10 s), and 30 Si (1 s) masses by a switching magnet. We made SIMS calibration lines for H 2 O and CO 2 on each measurement day using the reference glasses. A more detailed description of the SIMS analyses is provided in Appendix C. Repeated analyses of the same melt inclusions (a2 ph1 i1 and a2 ph3 i1) on different days resulted in measurements of 1.9 ± 0.2 wt % H 2 O and ± wt % CO 2 on inclusion a2 ph1 i1 (n = 2) and 3.0 ± 0.1 wt % H 2 O and ± wt % CO 2 on inclusion a2 ph3 i1 (n = 2). These results indicate that the analytical error is not more than ±0.2 wt % for H 2 O and ± wt % for CO Results 4.1. Whole Rock Composition [15] The bomb ( ) and lapillus ( ) have almost identical whole rock compositions (Table 1). In addition, their compositions are identical to the whole rock composition of bomb 819B1, which was used as a sample of the summit eruption on 18 August 2000, by Saito et al. [2005], which indicates that these products originated from the same magma Host Olivine Composition and Zoning [16] The olivines analyzed in this study have core compositions ranging from Fo 70 to Fo 85 (Figure 1). By combining our observation with the results of Saito et al. [2005] (section 3.1), we found that the distribution of the core composition is bimodal, with peaks at Fo and Fo (Figure 1a). In this study, we refer to olivines with a core composition of Fo as Mg poor olivines and olivines with a core composition of Fo as Mg rich olivines. Figure 2 shows backscattered electron images of these two types of olivines. Mg poor olivines have a homogeneous core of Fo and a narrow rim (<0.1 mm) of Fo This type is common; its size is smaller than 1 mm and it has a euhedral shape. The other type, Mg rich olivines, has a homogeneous core of Fo and a normally zoned rim of mm. The rim composition of the Mg rich olivines is Fo This type rarely occurs and has a larger phenocryst size (0.7 2 mm). [17] The bimodal chemistry of the olivine cores and the lack of an intermediate core composition indicate that the Mg rich olivines and Mg poor olivines originated from two different magmas. The rim composition of the Mg rich olivines is similar to that of the Mg poor olivines (Figure 1b), which indicates that the Mg rich olivines reacted with an evolved melt, crystallizing the Mg poor olivines before the eruption. [18] We measured the compositional profiles from core to rim of 10 Mg rich olivines using EPMA. Two types of profiles were observed (Figure 3). One type has a homogeneous core and normal zoning at its rim (Figure 3a); 6 of the 10 olivines analyzed showed this profile. The other type has two steps of normal zoning around its rim and a homogeneous core (Figure 3b); 4 of the 10 olivines analyzed showed this profile. The second step of normal zoning 5of29

6 Figure 1. Chemical composition of (a) olivine phenocryst cores of a bomb ( ) and lapilli ( and ) from the eruption on 18 August 2000, as well as the bomb and lapilli analyzed by Saito et al. [2005]; (b) olivine phenocryst cores and rims in a bomb ( ) and lapilli ( and ) from the eruption on 18 August 2000, and the bomb and lapilli analyzed by Saito et al. [2005]. Fo is the % of forsterite in the olivines (= Mg/(Mg + Fe) 100% in mol). in the outer part was probably formed by crystal growth in an evolved magma after mixing. [19] We calculated the residence time of the Mg rich olivines following the method of Johnson et al. [2008], based on the assumption that the normal zoning in the rim of the former type was produced by a magma mixing event. We used a diffusion coefficient parallel to the a and b axes coefficient, D = (m 2 s 1 ), for the Fe Mg exchange, as calculated from the equation for diffusion parallel to the c axis of Jurewicz and Watson [1988]: and D Fe Mg ¼ expð 29708=TÞ m 2 s 1 D a ¼ D b ¼ 6D c ; Figure 2. Backscattered electron images of olivine hosted melt inclusions. (a) Melt inclusions in Mgpoor olivine (a3 ph1 i1, i2, i3). (b) A melt inclusion in Mg poor olivine (a3 ph5 i1). (c) A melt inclusion in Mg rich olivine (a2 ph3 i1). This inclusion contains a small Ca rich pyroxene that is indicated by a black arrow. (d) Melt inclusions in Mg rich olivine (a2 ph5 i1, i2, i3). The inclusion a2 ph5 i1 contains a Ca rich pyroxene near the wall of the host olivine that is indicated by a black arrow. The white lines with arrows in the photomicrographs correspond to the EPMA line scans for the zoning profile. 6of29

7 Figure 3. Zoning profiles and residence time calculations of the Mg rich olivines: (a) a2 ph3 and (b) a2 ph5. Each line analysis of the olivines by EPMA was carried out along the white arrows shown in Figure 2. The x axes are the distances from the rim (mm). The y axes (Fo) show the % of forsterite in the olivines (= Mg/(Mg + Fe) 100% in mol). The calculated diffusion profiles are also shown (dotted lines for 5 days, solid lines for 10 days, and broken lines for 20 days). Both zoning profiles fit the diffusion profiles of the 10 day model, which indicates that the residence time of the Mg rich olivines in the evolved magma was 10 days. where T is the temperature of the magma. We used 1100 C as the temperature of the magma, based upon the petrological estimate by Amma Miyasaka et al. [2005]. Model calculations of the diffusion profiles at residence times of 5, 10, and 20 days for olivine a2 ph5 are shown in Figure 3a. We estimated a residence time of 10 days for olivine a2 ph5 (the bold solid line in Figure 3a). We obtained a residence time of days for the six olivines that have a homogeneous core and normal zoning at its rim. We also estimated a residence time of days for the four olivines that have two steps of normal zoning around the rim and a homogeneous core (Figure 3b), under the assumption that the normal zoning in the inner part of these olivines was produced by a magma mixing event. These results suggest that the Mg rich olivines resided in a more evolved magma for days before the eruption Description of Melt Inclusions [20] We carried out chemical analyses of 26 melt inclusions in the Mg poor olivines (Mg poor OLMIs) and 30 inclusions in the Mg rich olivines (Mg rich OLMIs). Table 3 presents descriptions and the chemical compositions of the 40 melt inclusions whose H 2 O and CO 2 contents were measured. The melt inclusions in both types are distributed in the core of the olivines and not along secondary fractures, which suggests a primary origin. Most of the inclusions are glassy and several contain a mineral and/or a bubble in the glass. Back scattered electron images of the inclusions revealed that a 1 2 mm overgrowth of host phenocrysts commonly occurred along the wall of the inclusions. [21] Most of the Mg poor OLMIs are round to elliptical in shape, with a size of mm. About two thirds of the inclusions are bubble free, whereas the others contain a bubble with a diameter that is from 10% to 30% of the inclusion diameter (Table 3a). The approximate bubble toinclusion ratio of the volume was estimated from the diameter of the bubble and the size of the host inclusion. The volume of the bubble in the inclusion was calculated from the bubble diameter by assuming a sphere shape for the bubble, and the volume of the inclusion was calculated from the length of the major and minor axes of the inclusion by assuming an ellipsoid shape for the inclusion. The estimated ratio is below 3 vol % (Table 3a). Small bubbles of less than a few vol % were probably formed by shrinkage of the melt during cooling [Anderson et al., 1989]. Using the density calculation of a melt with the groundmass composition from the 2000 eruption, Saito et al. [2005] estimated that a 1.5 vol % shrinkage of a melt with a H 2 O content of 1.4 wt % could occur with a decrease in temperature from 1100 C to 1000 C. Therefore, the small bubbles in the Mg poor OLMIs are probably bubbles caused by shrinkage. [22] The host olivine composition of the Mg rich OLMIs ranges from Fo 78 to Fo 85 (Table 3b). Most of the inclusions are round to elliptical in shape, with a size of mm. Several inclusions exhibit an irregular or quadrilateral shape, with a size of mm. About one third of the inclusions are bubble free, whereas the others contain bubbles of various diameters that are up to 50% of the inclusion diameter. We calculated the approximate volume ratio in the same way as we did for the Mg poor OLMIs, which resulted in a range of 1 to 23 vol % (Table 3b). Optical observations did not indicate any leakage features such as fractures or melt channels. If there had been leakage from the inclusion, the volatile content of the glass in the inclusion would have decreased as the volume ratio increased. However, the bubble to inclusion ratio of the volume seems to have no correlation with the volatile (H 2 O, CO 2, and S) content of the glass in the inclusions (Table 3b). It is thus likely that the large bubbles in inclusions are trapped bubbles, although the absence of a correlation between the estimated bubble toinclusion volume ratio and the volatile content of the glass could also have been caused by random cutting of the 7of29

8 inclusion during the sample preparation. On the other hand, the small bubble in an inclusion a 2 ph8 i4 was probably formed by melt shrinkage. [23] About one third of the Mg rich OLMIs (11 inclusions) contain Ca rich pyroxene as a coexisting phase in the glass. The size of the pyroxene varies from 10 to 100 mm (Table 3b). We calculated the approximate volume ratios of pyroxene to glass in the inclusions, which were estimated from the sizes of the pyroxenes, bubbles, and host inclusions, assuming an ellipsoid shape for the pyroxenes and inclusions, and a sphere shape for the bubbles. Our calculations indicated a range of vol % for the approximate volume ratios. The pyroxenes in the glass have a chemical composition of Wo En Fs and a high Al 2 O 3 content of wt % (Table 4). These chemical compositions of the Ca rich pyroxenes in Mg rich OLMIs (Table 4) are similar to those of the Ca rich pyroxenes (Wo En 40 Fs 10 ) that were experimentally produced from low MgO (<6 wt %), high alumina ( 19 wt %) basalts at a pressure of MPa, in a H 2 O saturated condition [Sisson and Grove, 1993, Figure 1]. This suggests that the Ca rich pyroxenes in the Mg rich OLMIs could be crystallized in a H 2 O saturated melt, since the Mg rich OLMIs have a major element composition (MgO < 7 wt % and Al 2 O 3 =18 20 wt %; Section 4.5) that is similar to that of the low MgO high alumina basalts investigated by Sisson and Grove [1993] Correction for Postentrapment Crystallization of Host Olivine [24] Backscattered electron images of the melt inclusions show that the width of the overgrowth is 1 2 mm. The chemical composition of the inclusions were corrected for postentrapment crystallization by adding a host olivine component in order to obtain an olivine melt equilibrium, assuming K D =0.30[Roedder and Emslie, 1970; Niihori, 2007]. The mole ratio of Fe(II) to total Fe in the melt was assumed to be 0.8 on the basis of the results of wet chemical analyses of fresh lavas from the 1983 eruption [Fujii et al., 1984]. The estimated overgrowth is 2 5 wt % for the Mgpoor OLMIs (Table 3a). We also corrected the chemical composition of the melt inclusions in the Mg poor olivines reported by Saito et al. [2005] to account for the postentrapment crystallization, finding that the estimated overgrowth is 2 5 wt %. These overgrowth corrections for Mg poor OLMIs in this study and the inclusions in the study by Saito et al. [2005] provided similar chemical compositions (Figure 4). In addition, the chemical composition of these inclusions after correction for overgrowth is similar to that of the groundmass in a bomb from the 2000 eruption. The estimated overgrowth is 5 8 wt % for the Mg rich OLMIs (Table 3b). The degree of overgrowth for the Mg rich OLMIs is a little higher than that for the Mg poor OLMIs Major Element Composition of Melt Inclusions [25] Figure 5 shows the major element composition of the melt inclusions in olivines after correction for the postentrapment crystallization of the host olivine, together with the groundmass composition of a bomb from the summit eruption in 2000, and the whole rock composition of the products of eruptions from 10 ka to The major element compositions of the Mg poor OLMIs that were obtained in both this study and by Saito et al. [2005] are similar to that of the groundmass. This result takes into account the standard deviation in the chemical analyses of the groundmass. Saito et al. [2005] reported that plagioclase hosted melt inclusions from the summit eruption in 2000 also have compositions similar to that of the groundmass. Our results indicate that the entrapment of the Mg poor OLMIs occurred just before the eruption, along with the plagioclase hosted melt inclusions. In addition, the major element compositions of the Mg poor OLMIs are similar to the whole rock compositions of the submarine eruption on 27 June 2000 and of the 1983 eruption (Figure 5). [26] The similarity between the major element compositions of the Mg poor OLMIs and the groundmass in the bomb indicates that fractional crystallization of a melt having the whole rock composition of the bomb formed the melt represented by the Mg poor OLMIs. We calculated the change in the chemical composition of a melt having the whole rock composition of the 2000 summit eruption during the crystallization of plagioclase, olivine, and clinopyroxene. We assumed that 20 vol % plagioclase (An 89 ), 1 vol % olivine (Fo 70 ), and 1 vol % clinopyroxene (Wo 39 En 45 Fs 16 ) crystallized (equal to 22 wt % crystallization) and that the densities of plagioclase, olivine, and clinopyroxene are , , and kg m 3, respectively. These assumptions were based on the mineral modes of the ejecta of the summit eruption (17 22 vol % plagioclase, vol % olivine, and vol % clinopyroxne) observed by Amma Miyasaka et al. [2005] and the chemical compositions of the minerals reported by Saito et al. [2005]. The gray arrows in Figure 5 show the changes in the melt composition. The calculated melt composition is similar to that of the Mg poor OLMIs, which indicates that the melt represented by the Mg poor OLMIs was derived by fractional crystallization of a melt having the whole rock composition of the bomb and lapilli of the summit eruption. [27] On the other hand, the Mg rich OLMIs have compositions that are SiO 2 poor (47 53 wt %), FeO poor (8 11 wt %), K 2 O poor ( wt %), Al 2 O 3 rich (16 21 wt %), and MgO rich (5 7 wt %), as compared to the Mg poor OLMIs (Table 3 and Figure 5). In addition, most of them have chemical compositions that are less evolved than the bomb and lapilli of the 2000 summit eruption. This suggests that the magma of the bomb and lapilli was derived by fractional crystallization of a melt represented by the Mgrich OLMIs. We calculated the amount of the minerals crystallized in the above fractional crystallization process, on the assumption that a melt with the composition of a Mgrich OLMI (a2 ph10 i2) crystallized plagioclase, olivine, and clinopyroxene. The chemical compositions and densities of the minerals used for this calculation are as follows: An 93 for plagioclase ( kg m 3 ), which is the highest An content that was observed by Saito et al. [2005]; Fo 84 for the olivine ( kg m 3 ) observed in this study; and clinopyroxene of Wo 55 En 27 Fs 18 ( kg m 3 ), which is the chemical composition of a coexisting pyroxene in a Mg rich OLMI a2 ph5 i1 (Table 4). A mass balance calculation indicates that the magma represented by the bomb and lapilli can be derived from a 37 vol % fractional crystallization (25 vol% of plagioclase, 6 vol% of olivine, and 6 vol% of clinopyroxene; equal to 40 wt % crystallization) 8of29

9 Table 3a. Chemical Composition (in Weight Percent) of Melt Inclusions in the Mg Poor Olivines of a Bomb and Lapilli From the Eruption on 18 August 2000, Whose H2O and CO2 Contents Were Measured a Inclusion b a2 ph1 i1 a2 ph1 i3 a2 ph2 i2 a2 ph11 i1 a2 ph13 i1 a2 ph14 i1 a2 ph15 i1 a2 ph16 i1 a3 ph1 i1 a3 ph1 i2 a3 ph1 i3 Phenocryst chemistry (Fo) c c73, r70 c73, r70 c73, r71 c73, r69 c73, r70 c72, r70 c73, r70 c73, r70 c72, r70 c72, r70 c72, r70 Host chemistry (Fo) d Inclusion shape r, 140 and size e 140 mm r, 56 56mm r, 56 42mm t, 71 47mm r, 49 42mm e, 45 34mm r, 71 62mm e, 40 34mm e, mm r, mm Phase f g g g, b (11 mm) g g g g, b (17 mm) g g g g Bubble (vol %) g 1 2 Pyroxene to glass (vol %) h Overgrowth (wt %) i r, 70 60mm SiO TiO Al 2 O FeO* MnO MgO CaO Na2O K 2 O P 2 O H 2 O (SIMS) j 1.9 ± 0.2 (n =2) CO2 (SIMS) j ± (n =2) S Cl FeO*/MgO S(+6)/total S k n.a. n.a. n.a. Sat. press. (MPa) by VC l Sat. press. (MPa) by PPL m a The chemical composition was corrected for host mineral overgrowth. The major element composition was recalculated on a volatile (H 2O, CO2, S and Cl) free basis. FeO*, total FeO. b First numbers represent each sample as follows: a2, bomb ; a3, lapilli ; and a4, lapilli Second and third numbers of each sample are to distinguish each phenocryst and each inclusion. c Chemical composition of cores and rims of host phenocrysts. Numbers with c represent core compositions and numbers with r represent rim compositions. Fo, forsterite content (mol %) in the olivine. d Chemical composition of host phenocrysts near the melt inclusions. Fo, forsterite content (mol %) in the olivine. e r, round shape; e, elliptical shape; t, tear drop shape; i, irregular shape; q, quadrilateral shape. f Phases in the melt inclusions: g, glass; b, bubble; ox, oxide; px, Ca rich pyroxenes. Numbers in parentheses with b represent size of bubbles. Numbers in parentheses with px and ox represent size of Ca rich pyroxenes and oxide, respectively. g Volume percent of bubble in inclusion. See text. h Approximate volume percent of pyroxene to glass in inclusion. See text. i Extent of overgrowth in inclusion. See text. j Water and CO2 contents of the inclusions were determined using SIMS. See text. k Mol fraction of S (+6) in total S in the melt inclusions calculated from the S Ka radiation wavelength, assuming that all S in the inclusions is composed of S(+6) and S( 2). n.a., not analyzed. l Saturation pressure calculated from H 2O and CO2 contents of the melt inclusions using VolatileCalc program [Newman and Lowenstern, 2002]. The SiO2 content for the pressure calculations is 49 wt %. See text. m Saturation pressure calculated from H 2O and CO2 contents of the melt inclusions using the solubility model of Papale et al. [2006]. An online version of the Papale s solubility model ( was used for these calculations. The mol ratio of Fe(II) to total Fe in the melt was assumed to be 0.8. See text. 9of29

10 Table 3a. (continued) Inclusion b a3 ph2 i1 a3 ph3 i1 a3 ph5 i1 a3 ph6 i1 a3 ph7 i1 a3 ph7 i2 a3 ph7 i3 a3 ph9 i1 a3 ph10 i1 a3 ph11 i1 a4 ph5 i1 Phenocryst chemistry (Fo) c c72, r71 c72, r71 c72, r70 c73, r71 c73, r70 c73, r70 c73, r70 c73, r71 c73, r72 c73, r71 c73, r73 Host chemistry (Fo) d Inclusion shape r, 70 and size e 70mm e, mm e, mm r, 90 80mm r, 50 50mm e, 94 38mm e, mm e, 79 51mm r, 99 87mm e, mm r, 46 44mm Phase f g, b (10 mm) g g g g g g, b (38 mm) g, b (6 mm) 2 g, b (9 mm) g g, b (5 mm), ox (15 10 mm) Bubble (vol %) g Pyroxene to glass (vol %) h Overgrowth (wt %) i SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O P 2 O H2O (SIMS) j CO2 (SIMS) j S Cl FeO*/MgO S(+6)/total S k n.a. n.a. n.a Sat. press. (MPa) by VC l Sat. press. (MPa) by PPL m 10 of 29

11 Table 3b. Chemical Composition (in Weight Percent) of Melt Inclusions in the Mg Rich Olivines of a Bomb and Lapilli From the Eruption on 18 August 2000, Whose H 2 O and CO 2 Contents Were Measured a Inclusion b a2 ph3 i1 a2 ph4 i1 a2 ph4 i2 a2 ph5 i1 a2 ph5 i4 A2 ph6 i2 a2 ph6 i3 a2 ph8 i2 a2 ph8 i4 Phenocryst chemistry (Fo) c c85, r71 c85, r71 c85, r71 c84, r70 c84, r70 c84, r73 c84, r73 c84, r72 c84, r72 Host chemistry (Fo) d Inclusion shape and size e r, mm r, mm i, mm e, mm r, mm e, mm e, mm e, mm r, mm Phase f g, px (20 15 mm) g, b (80 mm), px (60 13 mm) g, b (34 mm) g, b (51 mm), px ( mm, mm) g, px (48 24 mm) g, b (78 mm) g, b (28 mm) g, b (30 mm), px (50 40 mm) Bubble (vol %) g Pyroxene to glass (vol %) h Overgrowth (wt %) i g, b (17 mm), px (45 23 mm) SiO TiO Al2O FeO* MnO MgO CaO Na 2 O K 2 O P2O H2O (SIMS) j 2.8 ± 0.1 (n = 2) CO2 (SIMS) j ± (n = 2) S Cl FeO*/MgO S(+6)/total S k Sat. press. (MPa) by VC l Sat. Press. (MPa) by PPL m a The chemical composition was corrected for host mineral overgrowth. The major element composition was recalculated on a volatile (H 2O, CO2, S and Cl) free basis. FeO*, total FeO. b First numbers represent each sample as follows: a2, bomb ; a3, lapilli ; and a4 lapilli Second and third numbers of each sample are to distinguish each phenocryst and each inclusion. c Chemical composition of cores and rims of host phenocrysts. Numbers with c represent core compositions and numbers with r represent rim compositions. Fo, forsterite content (mol %) in the olivine. d Chemical composition of host phenocrysts near the melt inclusions. Fo, forsterite content (mol %) in the olivine. e r, round shape; e, elliptical shape; t, tear drop shape; i, irregular shape; q, quadrilateral shape. f Phases in the melt inclusions: g, glass; b, bubble, ox, oxide; px, Ca rich pyroxenes. Numbers in parentheses with b represent size of bubbles. Numbers in parentheses with px and ox represent size of Ca rich pyroxenes and oxide, respectively. g Volume percent of bubble in inclusion. See text. h Approximate volume percent of pyroxene to glass in inclusion. See text. i Extent of overgrowth in inclusion. See text. j Water and CO2 contents of the inclusions were determined using SIMS. See text. k Mol fraction of S (+6) in total S in the melt inclusions calculated from the S Ka radiation wavelength, assuming that all S in the inclusions is composed of S(+6) and S( 2). n.a., not analyzed. l Saturation pressure calculated from H2 O and CO 2 contents of the melt inclusions using VolatileCalc program [Newman and Lowenstern, 2002]. The SiO 2 content for the pressure calculations is 49 wt %. See text. m Saturation pressure calculated from H 2O and CO2 contents of the melt inclusions using the solubility model of Papale et al. [2006]. An online version of the Papale s solubility model ( was used for these calculations. The mol ratio of Fe(II) to total Fe in the melt was assumed to be 0.8. See text. 11 of 29

12 Table 3b. (continued) Inclusion b a2 ph8 i5 a2 ph10 i1 a2 ph10 i2 a2 ph10 i3 a2 ph10 i4 a2 ph12 i1 a2 ph17 i1 a2 ph19 i2 a4 ph4 i1 Phenocryst chemistry (Fo) c c84, r72 c85, r70 c85, r70 c85, r70 c85, r70 c82, r70 c84, r73 c84, r73 c84, r71 Host chemistry (Fo) d Inclusion shape and size e q, mm q, mm e, mm i, mm e, mm t, mm e, mm r, mm e, mm Phase f g g g, b (71 mm) g, b (35 mm) g g, px (14 7 mm) g, b (28 mm), px (19 10 mm) g g, b (12 mm) Bubble (vol %) g Pyroxene to glass (vol %) h Overgrowth (wt %) i SiO TiO Al2O FeO* MnO MgO CaO Na 2 O K2O P2O H2O (SIMS) j CO2 (SIMS) j S Cl FeO*/MgO S(+6)/total S k n.a. Sat. press. (MPa) by VC l Sat. Press. (MPa) by PPL m 12 of 29

13 Table 4. Chemical Composition (in Weight Percent) of Representative Ca Rich Pyroxenes in Melt Inclusions in Mg Rich Olivines a Inclusion, a2 ph4 i1, a2 ph5 i1, a2 ph5 i1, Pyroxene Size mm mm mm Chem. Comp. Wo53En28Fs19 Wo55En27Fs18 Wo48En38Fs14 SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O P 2 O S Cl a FeO*, total FeO. melt to times the original content. At present, we cannot determine which process actually occurred in the crystallization of the pyroxene, i.e., whether crystallization took place before or after entrapment of the inclusions. In order to prevent a large error in the determination of the volatile content due to postentrapment crystallization, in the following discussion, we do not include data on the five Mg rich OLMIs containing vol % pyroxene. [29] The Mg rich OLMIs have a chemical composition similar to that of volcanic rocks from the eruptions of Miyakejima volcano in 10 7 ka (Figure 5), which have the lowest SiO 2 and K 2 O composition of all eruption products from the last 10,000 years. Such less evolved magma of the melt represented by the Mg rich OLMI (a2 ph10 i2; black arrows in Figure 5). [28] The approximate volume ratios of pyroxene to glass in the Mg rich OLMIs that contain Ca rich pyroxenes were estimated to be vol % (section 4.3) (Table 3b). The inclusions containing coexisting pyroxenes (represented as gray squares in Figure 5) have compositions similar to those of the inclusions without pyroxenes. The pyroxenes could have crystallized either before or after the entrapment of the inclusions. If they crystallized after the entrapment, the composition of the melt in the inclusions changed via postentrapment crystallization, resulting in a change in the volatile content of the melt. We estimated the change in the melt composition of the inclusions caused by postentrapment crystallization from the approximate volume ratios of pyroxene to inclusion and from the chemical compositions of glass and pyroxene in the inclusions; for the estimation, we assumed that the densities of pyroxene and glass were and kg m 3, respectively. The change in the composition of the six Mg rich OLMIs containing Ca rich vol % pyroxene was very small compared to the analytical errors in the chemical analyses because of a small degree of postentrapment crystallization of the pyroxene ( wt % in the melt). These results indicate that even if crystallization of the pyroxenes occurred in these inclusions following entrapment, there was little change in the volatile content of the melt on account of crystallization. On the other hand, the above calculation for the five Mg rich OLMIs containing vol % pyroxene (Table 3b) indicates that the change in the melt composition caused by postentrapment crystallization of the pyroxene ( wt % in the melt) is beyond the analytical errors. For example, the calculation for Mg rich OLMIs (a2 ph8 i2) containing the maximum approximate volume ratios of pyroxene to inclusion (31 vol %) indicates that the chemical composition of the inclusion corrected for the postentrapment crystallization of pyroxene (SiO 2 = 47.52, Al 2 O 3 = 18.46, MgO = 7.34, CaO = 15.56, and K 2 O = 0.14 wt %) is different from the measured composition of the inclusion (SiO 2 =50.30,Al 2 O 3 = 20.22, MgO = 6.07, CaO = 11.84, and K 2 O = 0.22 wt %) (Table 3b). The crystallization of wt % pyroxene in the melt after inclusion entrapment increases the volatile content of the Figure 4. MgO versus SiO 2 contents of the Mg rich OLMIs and Mg poor OLMIs in the bomb and lapilli of the eruption on 18 August The Mg poor OLMIs obtained by Saito et al. [2005] are also shown. The open squares and diamonds indicate the chemical composition of the melts of Mg rich and Mg poor OLMIs as corrected for host mineral overgrowth (see text). The small open diamonds also indicate the chemical composition of the melt of the inclusions in the Mg poor olivines obtained by Saito et al. [2005], as corrected for host mineral overgrowth. The open circle and the error bars with the circle indicate the groundmass composition of the bomb and the standard deviation of the groundmass analyses obtained by Saito et al. [2005]. The black arrow indicates the change in the chemical composition of a Mgpoor OLMI (a3 ph7 i1) resulting from correction for a postentrapment crystallization of 4.0 wt %. The gray arrow indicates the change in the chemical composition of a Mgrich OLMI (a2 ph3 i1) resulting from the correction for a postentrapment crystallization of 5.6 wt %. All of the data were recalculated on a volatile (H 2 O, CO 2, S, and Cl) free basis. 13 of 29

14 Figure 5. (a) Al 2 O 3 versus SiO 2 contents, (b) MgO versus SiO 2 contents, (c) CaO versus SiO 2 contents, and (d) K 2 O versus SiO 2 contents of Mg rich OLMIs and Mg poor OLMIs in the bomb and lapilli from the eruption on 18 August Gray squares indicate inclusions having small Ca rich pyroxene ( vol %) in the melt (see text). Gray squares with crosses indicate inclusions having large Ca rich pyroxene ( vol %) in the melt (see text). The data on the Mg poor OLMIs obtained by Saito et al. [2005] are also shown. The open circles and error bars with the circles show the groundmass composition of the bomb and the standard deviation of the groundmass analyses [Saito et al., 2005]. The whole rock compositions of the products from the summit eruption on 18 August 2000, obtained by this study and Saito et al. [2005], and those of the spatters from the submarine eruption on 27 June 2000 [Amma Miyasaka et al., 2005; Kaneko et al., 2005] are also shown. The whole rock compositions of the products from previous eruptions during the last years are also shown: eruptions from 10 ka to 7 ka [Soya et al., 1984; Tsukui et al., 2002; S. Nakano, personal communication, 2001], eruptions from 4 ka to 1962 shown as areas surrounded by broken lines and eruptions in 1983 shown as areas surrounded by solid lines. The black arrows indicate changes in the chemical composition of the melt in the Mg rich OLMI (a2 ph10 i2) by the crystallization of 25 vol % plagioclase (An 93 ), 6 vol % olivine (Fo 84 ), and 6 vol % Ca rich pyroxene (Wo 55 En 27 Fs 18 ). The gray arrows show changes in the chemical composition of the bomb ( ) by the crystallization of 20 vol % plagioclase (An 89 ), 1 vol % olivine (Fo 70 ), and 1 vol % clinopyroxene (Wo 39 En 45 Fs 16 ). All of the data were recalculated on a volatile (H 2 O, CO 2,S,and Cl) free basis. 14 of 29

15 Figure 6. (a) Water and CO 2 contents of Mg rich OLMIs with K 2 O content of less than 0.24 wt %, Mgrich OLMIs with K 2 O content of more than 0.24 wt %, and Mg poor OLMIs in the bombs and lapilli of the eruption on 18 August The normal gray squares and gray squares marked with x show Mgrich OLMIs with small Ca rich pyroxenes ( vol %) in the melt. Mg poor OLMIs whose CO 2 was not detected in the FTIR analysis [Saito et al., 2005] are also shown. The arrows with small diamonds indicate that the plotted values are maximum estimates of the CO 2 content. Error bars show the analytical error for H 2 O (±0.2 wt %) and CO 2 (± wt %) as estimated from the range of repeated analyses of H 2 O and CO 2 in olivine hosted melt inclusions (see text). A straight line labeled CO 2 /H 2 O of volcanic gas shows the mass ratio of CO 2 and H 2 O for the volcanic gas (0.051) discharged from the summit of Miyakejima since the end of August This mass ratio was calculated based on the gas composition (H 2 O:CO 2 :S:Cl = 94.9:2.0:2.9:0.2, in molar ratio [Shinohara et al., 2003]). Also shown are the solubility of H 2 O and CO 2 in a basaltic melt at MPa and 1100 C (black broken lines) and the molar ratio of CO 2 to H 2 O+CO 2 in gas phase coexisting with the melt (gray broken lines), which were calculated using the VolatileCalc program developed by Newman and Lowenstern [2002]. In these calculations, an SiO 2 content of 49 wt % for the melt is used. A change in the H 2 O and CO 2 contents of a basaltic melt having initial H 2 O and CO 2 contents of 1.9 wt % and 0.1 wt % by degassing with a pressure decrease (closed system), which was obtained by Saito et al. [2005], is also shown by a gray arrow. (b) The H 2 O and CO 2 contents of the Mg rich OLMIs are plotted with the solubility of H 2 O and CO 2 in a basaltic melt having the chemical composition of a Mg rich OLMIs (a2 ph10 i2) at MPa and 1100 C (black broken lines) and a molar ratio of CO 2 to H 2 O+CO 2 in gas phase coexisting with the melt (gray broken lines), which were calculated using the solubility model developed by Papale et al. [2006]. (c) The H 2 O and CO 2 contents of the Mg poor OLMIs are plotted with the solubility of H 2 O and CO 2 in a basaltic melt having the chemical composition of a Mg poor OLMIs (a2 ph1 i1) at MPa and 1100 C (black broken lines) and a molar ratio of CO 2 to H 2 O+CO 2 in gas phase coexisting with the melt (gray broken lines), which were calculated using the solubility model developed by Papale et al. [2006]. The symbols, the straight lines labeled CO 2 /H 2 O of volcanic gas, and the error bars in Figures 6b and 6c are the same as in Figure 6a. 15 of 29

16 Figure 7. The volatile content of the Mg rich and Mg poor OLMIs in the bombs and lapilli from the eruption on 18 August The symbols are the same as in Figure 6. (a) Water and S contents of the melt inclusions. (b) Sulfur and CO 2 contents of the melt inclusions. (c) Water and Cl contents of the melt inclusions. (d) Chlorine and S contents of the melt inclusions. The arrows with small diamonds indicate that the plotted values are the maximum estimates of CO 2 content obtained from FTIR analyses [Saito et al., 2005]. The analytical errors for H 2 O (±0.2 wt %) and CO 2 (± wt %) were estimated from repeated analyses of the H 2 O and CO 2 in the olivine hosted melt inclusions (see text). The analytical errors for S (±0.007 wt %) and Cl (±0.004 wt %) were estimated from the standard deviation of the repeated analyses of the S and Cl in the reference glasses (see Appendix A). The gray straight lines show the mass ratios of S to H 2 O (0.055), CO 2 to S (0.94), Cl to H 2 O (0.005), and S to Cl (10) for the volcanic gas discharged from the summit of Miyakejima since the end of August The mass ratios were calculated based on the gas composition (H 2 O:CO 2 :S:Cl = 94.9:2.0:2.9:0.2, in molar ratio [Shinohara et al., 2003]). The arrows show the changes in the S and Cl contents of the melt of inclusion a2 ph10 i2 by a degassing process in which all of the H 2 O in the melt is exsolved from the melt and the chemical composition of the emitted gas is equal to that of the volcanic gas. 16 of 29

17 Figure 8. (a) K 2 O versus H 2 O contents, (b) K 2 O versus CO 2 contents, (c) K 2 O versus S contents, and (d) K 2 O versus Cl contents of the Mg rich and Mg poor OLMIs in the bombs and lapilli of the eruption on 18 August The symbols are the same as in Figure 6. The small arrows with small diamonds in Figure 8b indicate that the plotted values are the maximum estimates of the CO 2 content obtained by FTIR analyses [Saito et al., 2005]. The error bars show the analytical errors for H 2 O, CO 2, S, and Cl, which are the same as those in Figures 6 and 7. The EPMA analytical error for K 2 O was estimated to be ±0.02 wt % (see Appendix A), which is less than the symbol size. The large black arrows indicate the changes in the H 2 O, CO 2, S, and K 2 O contents of melts by magma degassing with a pressure decrease. The dark gray arrows show changes in the Cl and K 2 O contents of melts by fractional crystallization without exsolution of Cl from the melt to the gas phase. The light gray arrows indicate changes in the volatile and K 2 O contents due to the proposed mixing of evolved and less evolved magmas. See text for details. erupted not only in 10 7 ka but also occasionally from 2.5 ka until the early 15th century. Tsukui et al. [2001] pointed out an abrupt increase in Mg# [¼ : Mg/(Mg + Fe) 100, in mol] of the magmas from the eruptions of 2.5, 1.3, and 0.5 ka with a slight decrease in time and presumed that undifferentiated magma was supplied intermittently to the magma plumbing system underneath Miyakejima volcano. Amma Miyasaka et al. [2005] also suggested that the magma storage system consists of independent batches of andesitic and basaltic magmas. Considering these petrological models for the magma plumbing system, the existence of Mg rich OLMIs in the 2000 eruption products indicates that less evolved magma in a deeper chamber ascended and erupted in Volatile Content of Melt Inclusions [30] We measured the H 2 O and CO 2 contents of 22 Mgpoor OLMIs and 18 Mg rich OLMIs, along with the S and Cl contents of 26 Mg poor OLMIs and 30 Mg rich OLMIs without homogenization. The analytical values of the volatile components described below were corrected to account for host olivine overgrowth. A large variation in the volatile content of the olivine hosted melt inclusions was observed, a variation that correlates with the melt inclusion composition and its host olivine chemistry. The H 2 O content of the Mg poor OLMIs ranges from 0.7 to 2.5 wt %, and the CO 2 contents range from to wt % (Table 3a and Figure 6a). The H 2 O and CO 2 contents of the two inclusions 17 of 29

18 in Mg poor olivines (1.4 wt % H 2 O and <0.009 wt % CO 2 ) analyzed by Saito et al. [2005] agree with the results of the present study. The H 2 O and CO 2 contents of the Mg poor OLMIs seem to coincide with the change in the H 2 O and CO 2 contents of a basaltic melt with an initial 1.9 wt % H 2 O and 0.1 wt % CO 2 by degassing with a pressure decrease, which was proposed on the basis of the volatile content of plagioclase hosted melt inclusions and the volcanic gas composition reported by Saito et al. [2005, Figure 6a]. The Mg poor OLMIs have an H 2 O content that is roughly similar to that of the plagioclase hosted melt inclusions ( wt % [Saito et al., 2005]), but half of them have a higher CO 2 content than the plagioclase hosted melt inclusions (<0.011 wt % [Saito et al., 2005]). The S contents of the Mg poor OLMIs obtained in this study and in the study by Saito et al. [2005] both display a large variation, from to 0.17 wt % (Figure 7). This variation is similar to that of the S content of the plagioclase hosted melt inclusions ( wt % [Saito et al., 2005]). The molar ratios of sulfate to total sulfur in the 16 Mg poor OLMIs range from 0 to 1, although most of them (13 inclusions) range from 0.2 to 0.6 (Table 3a). This variation is also similar to that of the sulfate to total sulfur molar ratio of plagioclasehosted melt inclusions ( [Yasuda et al., 2001]). The ratios of the S and H 2 O contents of the Mg poor OLMIs are similar to that of the volcanic gas that has been emitted from the summit since the end of August 2000 (Figure 7a), which suggests that the gas emission is being caused by the complete removal of S and H 2 O from the melt represented by the Mg poor OLMIs. The Cl contents of the Mg poor OLMIs obtained in this study and in the study by Saito et al. [2005] show a large variation, from to 0.12 wt % (Figure 7d). The Mg poor OLMIs have a slightly higher Cl content than the plagioclase hosted melt inclusions ( wt % [Saito et al., 2005]). The Cl content of the Mg poor OLMIs increases linearly with the K 2 O content, whereas the H 2 O, CO 2, and S contents of the Mg poor OLMIs show no correlation with the K 2 O content (Figure 8). Because the increase in the K 2 O content was probably caused by crystallization (Figure 5), the variation in the Cl content is also probably due to crystallization of the magma. [31] The H 2 O content of the Mg rich OLMIs ranges from 1.9 to 3.5 wt %, and the CO 2 content ranges from to wt % (Table 3b and Figure 6a). The CO 2 content of the Mg rich OLMIs is similar to that of the Mg poor OLMIs, but the H 2 O content is higher. The high H 2 O content (up to 3.5 wt %) of the Mg rich OLMIs with a low MgO (<7 wt %) and high Al 2 O 3 (>19 wt %) composition is compatible with the results of the petrological experiments of Sisson and Grove [1993], which showed that low MgO, high alumina basalts (<6 wt % MgO and 19 wt % Al 2 O 3 ) can be derived from primary magma with a high H 2 O content (>4 wt %). The S content of the Mg rich OLMIs shows a large variation from to wt % (Figure 7). The sulfate to total sulfur molar ratios in the Mg rich OLMIs range from 0.3 to 0.7 (Table 3b). The ratios of the S and H 2 O contents of the Mg rich OLMIs are roughly similar to that of the volcanic gas that has been emitted from the summit since the end of August 2000 (Figure 7a). The Cl content of the Mg rich OLMIs ranges from to wt %, which is considerably lower than that of the Mg poor OLMIs (Figures 7d and 8d). [32] The Mg rich OLMIs can be divided into two groups: one with lower K 2 O content (<0.24 wt %; low K 2 OMgrich OLMIs) and one with higher K 2 O content (>0.24 wt %; high K 2 OMg rich OLMIs). These two groups have different features in relation to their volatile content as follows. The H 2 O, CO 2, and S contents of the low K 2 OMg rich OLMIs vary largely with the almost constant K 2 O content (Figures 8a 8c), whereas the Cl content of the low K 2 O Mg rich OLMIs seems to have a linear correlation with the K 2 O content (Figure 8d). The distribution of the low K 2 O Mg rich OLMIs in an S Cl plot (Figure 7d) is along with the estimated change in the S and Cl contents of the melt represented by the inclusion a2 ph10 i2 by degassing. This change was calculated assuming that the degassing process removes all the H 2 O from the melt, giving the gas the chemical composition of the volcanic gas that has been emitted from the summit since the end of August 2000 (Figure 7d). The high K 2 OMg rich OLMIs have values for H 2 O, S, and Cl contents that are intermediate between those of the low K 2 OMg rich OLMIs and the Mg poor OLMIs (Figures 7 and 8). In addition, the distribution of the Cl and K 2 O contents of the high K 2 OMg rich OLMIs displays a trend that is different from that of the low K 2 OMg rich OLMIs, which indicates that the increase in the Cl and K 2 O contents of the high K 2 OMg rich OLMIs was not caused by crystallization of the magma, but rather, by the mixing of a melt represented by the low K 2 OMg rich OLMIs and a melt represented by the Mg poor OLMIs (Figure 8d). This deduction is consistent with the distribution of the high K 2 O Mg rich OLMIs in the H 2 O K 2 O, CO 2 K 2 O, and S K 2 O plots (Figures 8a 8c). The host olivine of the high K 2 OMg rich OLMI a2 ph12 i1 has a slightly Fo poor core composition (Fo 82 ) compared to that of the low K 2 OMg rich OLMIs (Table 3b). This might be caused by crystallization of the Mg rich olivine in a magma in which a small amount of evolved magma initially mixed with less evolved magma. 5. Discussion 5.1. Estimation of Changes in the H 2 O Content of Melt Inclusions by Diffusion Through Their Host Olivines [33] The H 2 O content of melt inclusions in olivines can potentially be modified by the diffusion of hydrogen [e.g., Sobolev and Danyushevsky, 1994; Danyushevsky et al., 2002; Massare et al., 2002; Hauri, 2002] and/or H 2 O [Portnyagin et al., 2008] following entrapment in the host olivines. We evaluated this possibility on the basis of our melt inclusion data. The magnitude of water loss by hydrogen diffusion is controlled by the oxidation of Fe in the melt [Danyushevsky et al., 2002]: 2H 2 OðmeltÞþ6FeOðmeltÞ ¼ 2H 2 þ 2Fe 3 O 4 or at temperatures higher than magnetite stability, H 2 Oðmelt Þþ2FeOðmeltÞ ¼ H 2 þ Fe 2 O 3 ðmeltþ: The maximum amount of H 2 O (wt %) that can undergo dissociation can be calculated as H 2 Oðmelt; wt % Þ ¼ 0:125 FeOðmelt; wt % Þ: 18 of 29

19 Assuming that the molar ratio of Fe(II) to total Fe in a melt in the inclusions is 0.8 (as in section 4.4), we calculated the maximum amount of H 2 O that can undergo dissociation. The calculation indicated 1.3 wt % H 2 O for a Mg poor OLMI (a2 ph11 i1) that has the highest FeO content among the Mg poor OLMIs and 0.94 wt % H 2 O for a Mg rich OLMI (a2 ph12 i1) that has the highest FeO content among the Mg rich OLMIs. These results indicate that the H 2 O content of melt inclusions might be underestimated up to about 1 wt % due to hydrogen diffusion following inclusion entrapments. We concluded, however, that such water loss from the inclusions is not significant for the following two reasons. (1) The melt should be considerably oxidized by the above reactions if there is water loss from the melt, but the oxidation states of S in the melts of the inclusions (Table 3) were not correlated with the H 2 O content. (2) Magnetite dust, which is a clear indication of Fe oxidation in a melt [Danyushevsky et al., 2002], was not observed in the melt inclusions analyzed in this study. [34] On the other hand, the reduction of Fe 2 O 3 (melt) to FeO in the above reaction can account for any water gain by hydrogen diffusion [Danyushevsky et al., 2002]. The maximum amount of water gain (wt %) can be calculated as H 2 Oðmelt; wt % Þ ¼ 0:113 Fe 2 O 3 ðmelt; wt % Þ: We calculated the maximum water gain to be 0.32 wt % for the Mg poor OLMI (a2 ph11 i1) and 0.23 wt % for the Mg rich OLMI (a2 ph12 i1). Because these amounts are similar to the analytical error of SIMS (±0.2 wt %), the water gain by hydrogen diffusion was not significant. [35] On the basis of their experimental studies, Portnyagin et al. [2008] suggested that melt inclusions in olivines can rapidly exchange H 2 O with the matrix melt via molecular H 2 O transport along dislocations in the olivine. Their experiments indicated that nearly dry melt inclusions can gain up to 2.5 wt % H 2 O if they are placed in a H 2 O bearing melt at 200 MPa and 1140 C for 2 days. Although their results are important, we would suggest that molecular H 2 O transport did not occur in our samples. Our suggestion is based on the following observations. (1) The chemical similarity between the Mg poor OLMIs in this study and the plagioclase hosted inclusions [Saito et al., 2005] indicates that both inclusions trapped the same melt (section 4.5). The H 2 O content of the Mg poor OLMIs ( wt %) is also similar to that of plagioclase hosted melt inclusions ( wt %), which indicates that there was no change in the H 2 O content of the Mg poor OLMIs after inclusion entrapment. (2) If H 2 O exchange between the low K 2 OMg rich OLMIs and the evolved melt represented by the Mg poor OLMIs occurs after the mixing of the less evolved and evolved magmas, the H 2 O content of the low K 2 OMg rich OLMIs should be equal to that of the Mg poor OLMIs because the residence time suggested by the diffusion profiles of the host olivines (10 40 days) is longer than the re equilibration time observed by Portnyagin et al. [2008]. The low K 2 OMg rich OLMIs, however, have a somewhat higher H 2 O content than the Mg poor OLMIs (Figure 6a). (3) A large emission of volcanic gas began just after the 2000 summit eruption, which suggests that the magma related to the eruption remained at a shallow depth and effectively released the volcanic gas [Shinohara et al., 2003; Kazahaya et al., 2004]. A similarity between the ratio of S and H 2 O in the volcanic gas and the ratio of S and H 2 O contents in the Mg rich OLMIs (Figure 7a) strongly suggests that there was a complete release of S and H 2 O from the magma at low pressure. If this is the case, the measured H 2 O content of the inclusions should be identical to that of the melt in the magma. In addition, as described in section 5.5, the initial CO 2 content of the deep, less evolved magma (0.15 wt %), which was estimated from the H 2 O content of the low K 2 OMg rich OLMI and the CO 2 /H 2 O mass ratio of the volcanic gas, is almost the same as the value (0.16 wt % CO 2 ) that was estimated from its S content and the CO 2 /S mass ratio of the volcanic gas. These observations indicate that the H 2 O content of the inclusions was not significantly affected by either water loss or water gain after inclusion entrapment Gas Saturation Pressure of Magma [36] The gas saturation pressure of magma can be calculated from the H 2 O and CO 2 contents of the melt inclusions and the solubility of the H 2 O CO 2 fluid in the melt. Two solubility models of the H 2 O CO 2 fluid in a basaltic melt have been developed: the VolatileCalc program [Newman and Lowenstern, 2002] and a solubility model proposed by Papale et al. [2006]. The calculation package of the VolatileCalc program has been widely used because it gives saturation pressures, open and closed system degassing paths, and isobaric and isoplethic solubility curves for a basaltic melt on the basis of only the SiO 2 content of the melt as the input. This program is capable of dealing with the melt composition from nephelinite to tholeiite, but it might overestimate the gas saturation pressure of a basaltic melt having a calcic composition, like that of the Mg rich OLMIs, because the calcium content of a melt has a positive effect on the solubility of CO 2 in basalts [Moore, 2008]. Papale s solubility model, which has been developed recently, is the only available model that accounts for compositional variation in H 2 O CO 2 solubility from basalts to rhyolites. In order to correctly estimate the gas saturation pressures using this solubility model, the ferric to ferrous iron ratio in a melt is needed because the solubility of CO 2 substantially depends on the redox condition of the melt [Papale et al., 2006]. Because we did not measure the ferric to ferrous iron ratio in the melt inclusions, a constant molar ratio of Fe(II) to total Fe of 0.8 (as in section 4.4) was used for these calculations. In the following discussion, we use both of these solubility models to estimate the gas saturation pressures of our melt inclusions. [37] Using the VolatileCalc program, the H 2 O and CO 2 contents of the 22 Mg poor OLMIs indicate a pressure of MPa (Table 3a and Figure 6a). Papale s model gave almost the same pressure range (28 84 MPa) (Table 3a and Figure 6c). The gas saturation pressure of the seven Mgpoor OLMIs that contain bubbles is the same as that of the inclusions without bubbles. The gas saturation pressure estimated from the Mg poor OLMIs by the VolatileCalc program is similar to or slightly higher than that from plagioclase hosted melt inclusions (23 60 MPa by the VolatileCalc program [Saito et al., 2005]). On the other hand, the H 2 O and CO 2 contents of the 12 low K 2 OMgrich OLMIs indicates a higher pressure range of MPa when the VolatileCalc program is used (Table 3b and Figure 6a). Papale s model provided a pressure range of 19 of 29

20 45 98 MPa (Table 3b and Figure 6b), which is lower than that obtained using the VolatileCalc program. This difference was caused by the compositional dependence of the solubility of CO 2 and H 2 O, as described by Moore [2008] (Figures 6a and 6c). Calculation of the p coefficients for 14 low K 2 OMg rich OLMIs, which was defined as a compositional parameter for the VolatileCalc program [Dixon, 1997, equation (7)], provided a value of 0.10 (average) ± 0.15 (standard deviation), which suggests that the melt composition of the Mg rich OLMIs is either at or beyond the compositional limit of the VolatileCalc program. In the following discussion, we therefore used the gas saturation pressures of the Mg rich OLMIs obtained using Papale s model. The H 2 O and CO 2 contents of two high K 2 OMg rich OLMIs (a2 ph12 i1 and a2 ph6 i3) indicate a gas saturation pressure of 36 and 54 MPa (Table 3b and Figure 6b). The pressure is somewhat lower than that of the other low K 2 O Mg rich OLMIs and is within the gas saturation pressure range of the Mg poor OLMIs Magma Evolution in Shallow and Deep Magma Chambers [38] Many geophysical [e.g., Sakai et al., 2001; Furuya et al., 2003; Nakada et al., 2005; Uhira et al., 2005] and petrological [Amma Miyasaka and Nakagawa, 2003; Amma Miyasaka et al., 2005; Kaneko et al., 2005] studies suggest that two magma chambers have existed at least for the past 500 years: a shallow magma chamber at a depth of about 3 5 km and a deep magma chamber at a depth of about 10 km. In subsequent paragraphs, we discuss the magma evolution and degassing processes of Miyakejima volcano using the working hypothesis that these two magma chambers of different depths existed before the 2000 eruption. [39] On the basis of the chemical composition of melt inclusions, their host olivine chemistry, and the whole rock composition of the 2000 eruption products, we can consider three component magmas, A, B, and C, as follows. The A magma is basaltic andesite magma in the shallow magma chamber, which erupted in 1983 and June The B magma is the parent magma of the A magma, and the A magma is the product after 22 wt % fractional crystallization of the B magma. The C magma represents a deeper sourced basaltic magma from where the B magma was produced via 40 wt % fractional crystallization. The features of these three magmas and their interrelationships are described next. [40] The major element composition of the A magma is similar to that of the Mg poor OLMIs and the groundmass of the bomb from the 2000 summit eruption (Figure 5). The bimodal chemistry of the olivine cores of the 2000 summit eruption products indicates the existence of a more evolved magma, the B magma, and a less evolved magma, the C magma. Because Mg rich olivines are rarely found in the products of the summit eruption in comparison with Mgpoor olivines [Saito et al., 2005], the mixing ratio of C magma to B magma is thought to be very small. Therefore, the B magma probably has a whole rock composition that is almost identical to that of the 2000 summit eruption products. The C magma is the less evolved basaltic magma that was recorded as low K 2 OMg rich OLMIs in the products of the 2000 summit eruption. This magma has a chemical composition that is similar to that of the magmas that erupted in 10 7 ka (Figure 5). [41] Our mass balance calculations in Section 4.5 suggest that the A magma was produced by 22 vol % (22 wt %) fractional crystallization of the B magma, and that the C magma produced the B magma through 37 vol % (40 wt %) fractional crystallization. Considering that the subsidence of a large amount of crystals is required in order for C magma to become B magma, it is likely that the fractional crystallization process of the C magma occurred in the deep magma chamber at a depth of about 10 km. We speculate that the B magma produced by the fractional crystallization in the deep magma chamber has been supplied to the shallow magma chamber, either intermittently or continuously, since the 15th century. [42] The products of the 1983 eruption and the 2000 submarine eruption were phenocryst poor, with 2 8 vol % phenocrysts for the 1983 eruption [Fujii et al., 1984; Soya et al., 1984] and 3 7 vol % phenocrysts for the 2000 submarine eruption [Amma Miyasaka et al., 2005]. These observations are consistent with the above model, which shows that fractional crystallization of the B magma produced the phenocryst free A magma. Because the A magma has existed in the shallow magma chamber since at least 1983 [Amma Miyasaka et al., 2005; Kaneko et al., 2005], it is likely that the B magma, which is its parent magma, has also existed below the A magma in the shallow magma chamber. Kaneko et al. [2005] pointed out that the magma of the 2000 submarine eruption ( A magma) included the magma of the 2000 summit eruption ( B magma); their deduction was based on the wide range of Cl contents in the melt inclusions in the plagioclases of the submarine products. Their deduction is also consistent with our model, which proposes that the shallow magma chamber was stratified into a lower B magma and an upper A magma before the 2000 submarine eruption. The above model is based on the premise that only one shallow magma chamber is present; an alternative possibility is that more than one shallow magma chamber could exist and these could be closely associated. Further discussion in this regard would require more detailed petrological and geophysical studies of the magma chamber conditions Degassing Process of the 2000 Summit Eruption Magmas [43] The Mg poor OLMIs have a wide range of CO 2 contents, from to wt %, and a relatively small range of H 2 O contents, from 0.7 to 2.5 wt % (Figure 6c). This behavior could be the result of magma degassing with a decrease in pressure because the solubility of CO 2 in a silicate melt is lower than that of H 2 O. The large variations in the H 2 O, CO 2, and S contents of the Mg poor OLMIs, with their almost constant K 2 O content (Figures 8a 8c), also suggest magma degassing with a decrease in pressure. The gas saturation pressure of the Mg poor OLMIs is MPa (Section 5.2), which corresponds to a 1 3 km depth under a lithostatic pressure gradient. As the Mg poor olivines were crystallized in the B magma, the degassing and Mg poor olivine crystallization of the B magma probably occurred at a depth of 1 3 km. This estimate suggests that the degassing and crystallization of the B magma occurred in the 20 of 29

21 shallow magma chamber or in a conduit on the shallow magma chamber. The exsolution of S from the melt to the gas phase during the degassing process could have caused the large decrease in the S content of the Mg poor OLMIs. On the other hand, because sufficient exsolution of Cl from the melt to the gas phase did not occur during the degassing process, the Cl content of the melt could have increased with the crystallization of the magma (Figure 8d). [44] The low K 2 OMg rich OLMIs have a wide range of CO 2 contents, from to wt %, and a relatively small range of H 2 O contents, from 2.2 to 3.5 wt % (Figure 6b). This characteristic can be explained by magma degassing with a decrease in pressure, just as in the case of the variations in the H 2 O and CO 2 contents of the Mg poor OLMIs. The H 2 O, CO 2, and S contents of the low K 2 OMg rich OLMIs vary greatly with an almost constant K 2 O content (Figures 8a 8c). These results indicate degassing with a decrease in pressure of the C magma. The gas saturation pressure is calculated to be MPa (section 5.2); this range of pressure values corresponds to a 2 4 km depth under a lithostatic pressure gradient. This estimate suggests that in the 2000 eruption the C magma ascended from the deep magma chamber, and the degassing and crystallization of the C magma occurred at the same depth or at a slightly lesser depth than that of the shallow magma chamber. [45] The high K 2 OMg rich OLMIs have volatile contents and major element compositions that are similar to those of the Mg poor OLMIs (Figures 6, 7, and 8), which suggests that entrapment of these inclusions occurred in a mixture of the C magma and B magma. In comparison with the low K 2 OMg rich OLMIs, the high K 2 OMg rich olivines show lower gas saturation pressure for the two inclusions (36 and 54 MPa; section 5.2), which suggests that the mixing occurred when the less evolved magma ascended from a deeper part and encountered the evolved magma at a lesser depth. Because the gas pressure of these two melt inclusions corresponds to a depth of km under a lithostatic pressure gradient, the mixing is considered to have occurred at a depth of about 2 km Initial Volatile Content of Deep Less Evolved Magma [46] Saito et al. [2005] considered that the magma of the 2000 summit eruption was the source of the volcanic gas that has been emitted from the summit since the end of August They calculated the initial CO 2 content of the deep basaltic magma from the H 2 O and S contents of plagioclasehosted melt inclusions (1.9 wt % H 2 O and 0.11 wt % S) and the volcanic gas composition. The chemical similarity between the plagioclase hosted melt inclusions and the Mg poor OLMIs (sections 4.5 and 4.6) indicates that the plagioclase hosted melt inclusions were derived from the B magma. Therefore, their estimation was based on the assumption that the B magma is the source of the volcanic gas. It has been clarified in this study, however, that the C magma also contributed to the 2000 summit eruption. The volatile analyses of the low K 2 OMg rich OLMIs indicate that the C magma has higher H 2 O and S contents than the B magma. In addition, the S/H 2 O ratios of the Mg rich OLMIs are similar to that of the volcanic gas (Figure 7a). For these reasons, we propose that the C magma is the other source of the volcanic gas emitted from the summit. [47] We calculated the initial CO 2 content of the lessevolved melt from the H 2 O and S contents of a low K 2 O Mg rich OLMI (a2 ph10 i2; 3 wt % H 2 O and wt % S) in the same way as the study by Saito et al. [2005], assuming that all of the H 2 O and S are exsolved from the melt. The initial CO 2 content of 0.15 wt % was estimated from the H 2 O content and the CO 2 /H 2 O mass ratio of the volcanic gas (0.051 [Shinohara et al., 2003]). Almost the same value (0.16 wt % CO 2 ) was also estimated from the S content and the CO 2 /S mass ratio of the volcanic gas (0.95 [Shinohara et al., 2003]). These calculations indicate that if the melt represented by the Mg rich OLMI (a2 ph10 i2) is the source of the volcanic gas emitted from the summit, the initial CO 2 content of the C magma should be about 0.15 wt %. We therefore anticipate that the C magma has an initial volatile content of 3 wt % H 2 O, 0.15 wt % CO 2, and 0.17 wt % S Volatile Evolution in the Deep and Shallow Magma Chambers [48] As discussed in section 5.3, we anticipate that the B magma originated from the fractional crystallization (40 wt %) of the C magma in a deep magma chamber located at a depth of about 10 km and that it has been continuously or intermittently supplied to a shallow magma chamber since the 15th century. In addition, the previous section estimated that the C magma has an initial volatile content of 3 wt % H 2 O and 0.15 wt % CO 2. On the assumption that C magma with this original volatile content evolved into B magma by the process of fractional crystallization, we can estimate the volatile evolution of the melt during the magma evolution in these magma chambers and during its ascent. Papale s solubility model provided the gas saturation pressure (170 MPa) for a melt having the initial H 2 O and CO 2 contents and major element composition corresponding to those of the low K 2 OMg rich OLMI a2 ph10 i2. This pressure is lower than the lithostatic pressure (250 MPa) corresponding to the depth of the deep magma chamber (10 km). Therefore, the C magma began to crystallize in a gas unsaturated condition in the deep magma chamber. The B magma, which was derived by 40 wt % crystallization of the C magma, had a bulk volatile content of 5 wt % H 2 O and 0.25 wt % CO 2. We calculated the partitioning of H 2 O and CO 2 between the melt and gas in the B magma using Papale s solubility model, assuming that the major element composition of the B magma was the same as that of the bomb (Table 1). This calculation indicated that the B magma in the deep magma chamber consists of a melt with 4.9 wt % H 2 O, wt % CO 2, and an exsolved gas phase of 0.25 wt % of the magma. The B magma with this volatile content degassed with the decrease in pressure during its ascent from the deep magma chamber to the shallow magma chamber (a depth of 3 5 km). This closedsystem degassing up to a pressure of 100 MPa (approximately equivalent to a depth of 4 km) decreased the H 2 O and CO 2 contents of the melt from 4.9 wt % H 2 O and wt % CO 2 to 3.9 wt % H 2 O and wt % CO 2 ( 1 in Figure 9). The calculated CO 2 content of the melt is lower than the analyzed values of the Mg poor OLMIs from the 2000 summit eruption, and the calculated H 2 O content is higher than that of the Mg poor OLMIs. The variation in the H 2 O and CO 2 contents of the Mg poor OLMIs indicates degassing with the decrease in the pressure of the B magma at a 21 of 29

22 Figure 9. Volatile evolution processes of magmas beneath Miyakejima volcano. Water and CO 2 contents of Mg rich OLMIs with K 2 O content of less than 0.24 wt % are shown as an area surrounded byasolidlines,thoseof Mg rich OLMIs with K 2 O content of more than 0.24 wt % are shown as an area surrounded by a broken line and those of Mg poor OLMIs as an area surrounded by a dotted line. The solubility of the H 2 OandCO 2 in a basaltic melt at MPa and 1100 C (black broken lines) and the molar ratio of CO 2 to H 2 O+CO 2 in gas phase coexisting with the melt (gray broken lines) are also shown. The solubility and molar ratios were calculated using Papale s solubility model [Papale et al., 2006] and the whole rock composition of the bomb (Table 1) as the melt composition. In a deep magma chamber at depth of about 10 km (250 MPa), 40 wt % fractional crystallization of C magma having initial volatile content of 3 wt % H 2 O and 0.15 wt % CO 2 made B magma having bulk volatile content of 5 wt % H 2 O and 0.25 wt % CO 2. The B magma in the deep magma chamber consists of a melt with 4.9 wt % H 2 O and wt % CO 2 and an exsolved gas phase of 0.25 wt % of the magma. This B magma ascended from the deep magma chamber to a shallow magma chamber (3 5 km depth), causing degassing of the B magma with the pressure decrease. The decompression degassing from 250 to 100 MPa decreased the volatile content of the melt in the B magma to 3.9 wt % H 2 O and wt % CO 2 ( 1 ). Before the 2000 eruption, the addition of CO 2 rich gas to the B magma occurred in the shallow magma chamber (depth of 4 km) and caused a change in the volatile content of the melt to 2.8 wt % H 2 O and wt % CO 2 along an isobaric line of 100 MPa ( 2 ). In addition, C magma ascended from the deep magma chamber and was injected into the B magma. During the ascent of the C magma, the exsolution of H 2 O, CO 2, and S from the melt to gas phase occurred with the pressure decrease ( 3 ). The mixture of the B and C magmas ascended to a depth shallower than 3 km, accompanied by degassing of the B magma with the pressure decrease ( 4 ). depth of 1 3 km (section 5.4). Therefore, it would have been necessary for a process involving an increase in the CO 2 content and decrease in the H 2 O content to take place before the 2000 summit eruption. [49] A possible volatile evolution process that account for such a change in the CO 2 and H 2 O contents of the melt is the addition of CO 2 rich gas to gas saturated magma [Metrich et al., 2004; Spilliaert et al., 2006; Metrich and Wallace, 2008]. The addition of CO 2 rich gas to a gassaturated magma at a constant pressure causes re equilibration of melt and gas phases in the magma and changes the H 2 O and CO 2 contents of the melt along the gas saturation curve depending on the amount and composition of the CO 2 rich gas. If an increase in the CO 2 content from a melt with 3.9 wt % H 2 O and wt % CO 2 to a melt with 2.8 wt % H 2 O and wt % CO 2 occurred by this process, the change in the H 2 O CO 2 composition would have occurred along the gas saturation curve at about 100 MPa (No. 2 in Figure 9). On the basis of the H 2 O and CO 2 solubilities and the mass balance between the gas and melt phases, the amount of CO 2 supplied to the melt can be calculated to be 1.7 wt % of the melt itself. This is a minimum estimate that is based on the assumption that a pure CO 2 gas was added to a bubble free magma. As it is possible that the B magma lost bubbles due to gas melt separation during its residence in the shallow magma chamber, we would consider that the B magma was bubble free before the addition of CO 2. The C magma in the deeper part could be considered as a source of the CO 2 gas that was added to the magma, because it has a high CO 2 content (0.15 wt %). Because the gas saturation pressure of the C magma with an initial content of 3 wt % H 2 O and 0.15 wt % CO 2 is 170 MPa, the C magma can have an exsolved gas phase at a pressure of less than 170 MPa. On the assumption that the C magma is degassed at a pressure of 150 MPa (approximately equivalent to a depth of 6 km) and the exsolved gas (H 2 O+CO 2 )is supplied to the shallow magma chamber, we calculated the amount of the exsolved gas required for the increase in the CO 2 content of the melt on the basis of the H 2 O and CO 2 solubilities and the mass balance between the gas and melt phases. The Papale s solubility model indicated that the C magma has an exsolved gas phase of wt % at 150 MPa and the composition of the gas is 50 mol% H 2 O and 50 mol %CO 2. The amount of the CO 2 rich gas supplied to the melt can be calculated to be 7.3 wt % of the melt itself. As the C magma has the gas phase of wt % at 150 MPa, the amount of the C magma required to supply the CO 2 rich gas at 7.3 wt % is 170 times as much as the B magma. This is a preliminary estimate, and more detailed petrological studies are required for a quantitative understanding of the process by which CO 2 is supplied from a deeper part. In any case, our estimation suggests a larger amount of C magma than B magma degassed at a deeper part and imparted CO 2 rich gas to the B magma before the 2000 eruption. The degassing of this CO 2 rich B magma with a decrease in pressure, which was accompanied by crystallization of the Mg poor olivines, occurred at a depth of 1 3km during the magma ascent from the shallow magma chamber ( 4 in Figure 9; section 5.4). [50] The volatile contents of low K 2 OMg rich OLMIs suggest that before or during the 2000 eruption, C magma also ascended from a deeper magma chamber. Using Papale s solubility model, we calculated the change in the H 2 O and CO 2 contents of a melt in the C magma due to closedsystem decompression degassing, assuming that the major element composition of the C magma is the same as that 22 of 29

23 Figure 10. Schematic diagram of magma plumbing system for Miyakejima volcano. Two magma chambers have existed from at least the 15th century until the 2000 eruption: a shallow magma chamber at a depth of about 3 5 km and a deep magma chamber at a depth of about 10 km. The deep magma chamber was composed of less evolved C magma. The C magma formed B magma by fractional crystallization in the deep magma chamber (I). The B magma ascended and supplied the shallow magma chamber intermittently or continuously (II). The B magma in the shallow magma chamber formed A magma by fractional crystallization. Before the 2000 eruption, the shallow magma chamber consisted of upper A magma and lower B magma. The A magma caused the submarine eruption on 27 June Magma intrusion from Miyakejima occurred after the submarine eruption, followed by the subsidence of the summit area with minor phreatic phreatomagmatic eruptions in July and August CO 2 rich gas ascended from a deeper part and was injected into the B magma in the shallow magma chamber (III). This preceded the ascent of the C magma, which is characterized by Mg rich OLMIs, from a depth of 10 km to a depth of about 3 km (IV). A small amount of C magma was injected into the CO 2 rich B magma at a depth of about 2 km, forming a mixed magma. This magma ascended and erupted from the summit on 18 August 2000 (V). Since the end of August 2000, C magma ascending from the deeper chamber to a shallow depth through a conduit has been emitting volcanic gas from the summit. of the low K 2 OMg rich OLMI a2 ph10 i2. This calculation indicated that closed system degassing of the C magma decreased the initial H 2 O and CO 2 contents from 3.0 wt % H 2 O and 0.15 wt % CO 2 to 2.8 wt % H 2 O and wt % CO 2 at a pressure of 75 MPa and to 2.5 wt % H 2 O and wt % CO 2 at a pressure of 50 MPa (approximately equivalent to a depth of 2 km; 3 in Figure 9). The crystallization of the Mg rich olivines of the C magma occurred at a depth of 2 4 km during its ascent, resulting in the entrapment of low K 2 OMg rich OLMIs. The injection of C magma into B magma probably occurred at a depth of about 2 km because the two high K 2 OMg rich OLMIs that were trapped in the olivines after mixing have gas saturation pressures of 36 and 54 MPa (section 5.4) Magma Plumbing System of the 2000 Eruption [51] The magma plumbing system for the 2000 eruption is proposed as follows on the basis of the volatile contents and the major element composition of the olivine hosted melt inclusions obtained in this study. Two magma chambers existed beneath Miyakejima volcano before the 2000 eruption, and they have been in existence at least since the 15th century. These chambers are a shallow magma chamber at a depth of about 3 5 km and a deep magma chamber at a depth of about 10 km (Figure 10). The deep magma chamber was formed by C magma, which ascended from a deeper part. In the deep chamber, fractional crystallization of the C magma occurred, which produced gas saturated B magma. This B magma ascended from the deep magma chamber and accumulated in the shallow magma chamber. In addition, the B magma produced A magma by fractional crystallization in the shallow magma chamber. The shallow magma chamber then became stratified, with upper A magma and lower B magma. This magma plumbing system may have been in operation since the 15th century. Before the 2000 eruption, CO 2 rich gas, which was emitted from the C magma in the deeper part, ascended to the shallow magma chamber and was injected into the B magma. Therefore, just before the 2000 eruption, the shallow magma chamber consisted of both lower CO 2 rich B magma and upper A magma (Figure 10). [52] On 27 June 2000, the A magma ascended from the shallow magma chamber, causing a submarine eruption. The intrusion of magma ( 1 km 3 ) from Miyakejima to 20 km NW at a depth of 2 13 km also occurred simultaneously with the submarine eruption [Sakai et al., 2001; Nishimura et al., 2001; Toda et al., 2002]. In July, the subsidence of the summit area began, a process that was accompanied by several phreatic phreatomagmatic eruptions. The subsidence continued intermittently until mid August, finally creating a caldera with a diameter of 1.6 km and a depth of 450 m [Nakada et al., 2005]. Several reports interpreted the caldera 23 of 29

24 formation as being the result of a magma chamber depression caused by magma flow from the shallow magma chamber (Figure 10) [Uto et al., 2001; Geshi et al., 2002]. The cylindrical pit shape of the caldera indicates the subsidence of piston shaped cylindrical blocks into the shallow magma chamber [Geshi et al., 2002]. The subsidence of igneous rock ( kg m 3 ) and/or cumulate ( kg m 3 ) having greater densities than the magma into the shallow magma chamber could have caused the ascent of A magma from the chamber by stoping [Saito et al., 2005]. This ascending magma would then have come into contact with a groundwater reservoir, resulting in the eruptions in July. [53] In August 2000, the B magma began degassing and crystallizing during its ascent from a depth of 3 5 km (Figure 10). The C magma also ascended from a deeper magma chamber before or during the 2000 eruption. The ascent of the B magma and the C magma might be caused by stoping as in the case of the A magma. The injection of C magma into B magma probably occurred at a depth of about 2 km. The compositional profiles from the core to the rim of the Mg rich olivines in the 2000 summit eruption products (Figure 3) suggest short residence times of less than 40 days for the Mg rich olivines in the more evolved magma before the summit eruption. Therefore, the injection of C magma into the B magma occurred within 40 days of the summit eruption. This mixed magma would then have come into contact with a groundwater reservoir, resulting in the eruption on 18 August. [54] Intense and continuous degassing activity from the caldera floor began after the major eruptions ceased at the end of August. The similarity between the H 2 O to S ratio of the volcanic gas and the ratio of H 2 O content to S content in the Mg rich OLMIs suggests the possibility that C magma from the deeper part was the source of the volcanic gas. Kazahaya et al. [2004] suggested that the intense and continuous gas emission could have been caused by degassing due to magma convection in a conduit. We estimated the amount of C magma required to supply the SO 2 emitted from August 2000 to March 2009 (24 Mt), assuming complete degassing of the magma with an initial S content of 0.20 wt %. The amount of degassed magma was calculated to be kg, which corresponds to a volume of about 2.2 km 3. This suggests the existence of a magma chamber composed of C magma with a volume of about 2 km 3. The deep magma chamber is thought to be located at a depth of about 10 km; consequently, it is the C magma ascending from the deeper chamber to a shallow depth through a conduit that would have been emitting volcanic gas from the summit since the end of August Concluding Remarks [55] Chemical analyses of olivine hosted melt inclusions and their host olivines from the explosive eruption of Miyakejima volcano, Japan, on 18 August 2000, were carried out in order to investigate the degassing and evolution of the magma during its ascent. By combining our results with geological, petrological, and geochemical observations, the magma plumbing system of the 2000 eruption could be modeled. [56] As shown in this study, using a dataset of the volatile (H 2 O, CO 2, S and Cl) and major element contents of melt inclusions, we can estimate important parameters for modeling of the magma plumbing system, such as the depths of crystallization of magma and magma mixing, the chemical composition of the magma components of magma mixing, and the initial volatile content and volume of the deep magma chamber. This information provides a quantitative understanding of the magma ascent and of the degassing and evolution processes of magma in its passage from a deeper part to its eruption from the summit. [57] Our magma ascent model of the 2000 eruption at Miyakejima volcano suggests that the input of a large amount of CO 2 rich gas from a deeper part to the shallow magma chamber occurred just before the eruption. As proposed by Metrich and Wallace [2008], such gas flushing related to deeply derived magmatic CO 2 might be a common process in basaltic volcanoes. Further studies should be conducted so that we can understand the gas flushing process quantitatively and evaluate the role of gas flushing in basaltic eruptions. Appendix A: Analytical Condition and Errors of EPMA [58] The major element composition and the S and Cl contents of the melt inclusions and glass samples were analyzed by EPMA (JEOL JXA 8900 in GSJ). The analytical conditions were as follows: an accelerating voltage of 15 kev, a beam current of 12 na, and a defocused beam with a diameter of 5 mm. We first measured the Na in the glass using a counting time of 4 s for the peak and subsequently measured the Si and Al for 20 s; Ti, Fe, Mn, Mg, Ca, K, and P for 40 s; and S and Cl for 50 s. We performed a search for the S Ka radiation wavelength for the S analysis and determined the proportions of sulfide and sulfate in the melt inclusions [Wallace and Carmichael, 1994]. The absolute abundances of the major elements were obtained after ZAF correction of the X ray intensities. The standards were quartz for Si, rutile for Ti, corundum for Al, fayalite for Fe, manganese oxide for Mn, periclase for Mg, wollastonite for Ca, albite for Na, adularia for K, sphalerite for S, and pyrosmalite for Cl. [59] Repeated analyses of the five reference glasses were carried out in order to estimate the analytical error of our method (Table A1). The results indicate a high degree of precision for all of the major elements. The coefficients of variation (= standard deviation/average 100, cv in Table A1) for the elements were less than 5%, except for Na 2 O in the JDF glass (cv = 5.6%) and small amounts of TiO 2 (the recommended value < 0.2 wt %), FeO (<0.06 wt %), MnO (<0.19 wt %), K 2 O (<0.16 wt %), and P 2 O 5 (<0.21 wt %). The coefficient of variation for K 2 O ( wt %) was 3 5%, indicating a precision of 0.02 wt %. The coefficient of variation for S was 7 13% for a low S content (<0.1 wt %) and 4 5% for a high S content (>0.1 wt %), indicating a precision of wt % for a low S content and wt % for a high S content. The coefficient of variation for Cl was 10 11% for a Cl content of wt %. However, it increased to 16 26% for a low Cl content (<0.02 wt %). Thus, the precision of the Cl analysis was wt % for a Cl abundance of more than 0.03 wt %. 24 of 29

25 Table A1. Repeated EPMA Analyses of the Major Elements, S and Cl Contents (in Weight Percent) in Reference Glasses a Analysis JDF MTB NBS621 NBS620 NBS610 This Study cv (%) Ref b This Study cv (%) Ref c This Study cv (%) Recom d This Study cv (%) Recom d This Study cv (%) Recom d SiO (0.57) (0.46) (0.47) (0.03) (0.32) (0.1) (0.26) TiO (0.05) (0.03) (0.01) (0.003) 0.03 (0.01) (0.05) 0.07 (0.02) 31 Al 2 O (0.15) (0.15) (0.05) (0.04) 1.81 (0.04) (0.05) 1.90 (0.02) 1 2 FeO* (0.27) (0.13) (0.02) (0.003) 0.04 (0.03) (0.001) 0.06 (0.02) 38 MnO 0.22 (0.02) (0.03) (0.02) (0.02) (0.03) 63 MgO 6.98 (0.10) (0.09) (0.02) (0.03) 4.29 (0.09) (0.05) 0.08 (0.01) 17 CaO (0.08) (0.09) (0.16) (0.05) 7.76 (0.10) (0.05) (0.06) Na2O 2.79 (0.16) (0.13) (0.34) (0.05) (0.42) (0.05) (0.44) K 2 O 0.18 (0.01) (0.01) (0.05) (0.03) 0.43 (0.02) (0.05) 0.07 (0.01) 10 P 2 O (0.02) (0.01) nd nd (nd) nd S (0.006) e (0.005) e (0.005) e (0.006) e (0.007) e Cl (0.004) f (0.003) f (0.005) f (0.003) f (0.004) f Total (0.71) (0.42) (0.55) (0.57) (0.51) FeO*, total iron as FeO; cv (%), coefficient of variation (=standard deviation/average 100). a Analyses: JDF, average of 40 analyses of Juan de Fuca Ridge basaltic glass; MTB, average of 20 analyses of Mariana Trough basaltic glass; NBS621, average of 25 analyses of National Bureau of Standards (NSB) Standard Reference Material 621, soda lime glass; NBS620, average of 15 analyses of NBS Standard Reference Material 620, soda lime glass; NBS610, average of 10 analyses of NBS Standard Reference Material 610, soda lime glass. Values in parentheses in these results represent the standard deviation of EPMA analyses. b Major element analyses by XRF (H. Sakai, personal communication, 1990). c Average of 43 glasses of MTB type basalt with Mg# > 65 from the axial ridge of the Mariana Trough back arc basin between N and N by EPMA [Hawkins et al., 1990]. d Recommended values by National Bureau of Standards except for S and Cl. Values in parentheses are uncertainty estimated by NBS. e S analyses of JDF, NBS621, NBS620, NBS610 [Kohno, 1992], and MTB glass [Kusakabe et al., 1990] by the strong phosphoric acid decomposition technique [Ueda and Sakai, 1983]. Uncertainty in the measurements was ±5% [Ueda and Sakai, 1983]. f Cl analyses of JDF, NBS621, NBS620, NBS610 [Kohno, 1992], and MTB glass [G. Saito, unpublished data, 1990] by colometry method [Tomonari, 1962]. Uncertainty in the Cl analyses was estimated as ±0.001wt % based on repeated analyses of the standard samples [G. Saito, unpublished data, 1990]. 25 of 29

26 [60] The repeated analyses indicated that the average values of the major elements, S and Cl, were in reasonable agreement with the recommended values, to within three standard deviations, except for SiO 2 and TiO 2 in NBS620, MgO in MTB glass and NBS620, CaO in NBS621 and 620, K 2 O and Cl in MTB glass, and P 2 O 5. These discrepancies between the average values and recommended values for SiO 2, TiO 2, MgO, CaO, K 2 O, and Cl were not Figure B1 26 of 29