MIZUHO AMMA-MIYASAKA 1 * AND MITSUHIRO NAKAGAWA 2

Transcription

1 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 PAGES 2113± DOI: /petrology/egg072 Evolution of Deeper Basaltic and Shallower Andesitic Magmas during the AD Eruptions of Miyake-Jima Volcano, Izu--- Mariana Arc: Inferences from Temporal Variations of Mineral Compositions in Crystal-Clots MIZUHO AMMA-MIYASAKA 1 * AND MITSUHIRO NAKAGAWA 2 1 HOKKAIDO BRANCH, GEOLOGICAL SURVEY OF JAPAN, AIST, N8W2, KITA-KU, SAPPORO , JAPAN 2 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, GRADUATE SCHOOL OF SCIENCE, HOKKAIDO UNIVERSITY, SAPPORO , JAPAN RECEIVED SEPTEMBER 2, 2002; ACCEPTED MAY 29, 2003 Miyake-jima volcano has erupted at least 13 times during the period To understand the historic magmatic processes, we focus on the mineral assemblage and chemical compositions of crystal-clots in single samples from each of the eruptions. Most of the historic lavas consist of nearly aphyric to weakly porphyritic basalt to andesite, but there also exist megacryst-bearing rocks. The megacrysts are considered to be xenocrysts from a deep-seated plutonic body. Many samples of each eruption contain two types of clots beside megacrysts, termed here B-type and A-type. The B-type clots are composed of olivine, clinopyroxene and plagioclase, whereas the A-type clots additionally contain magnetite and orthopyroxene. Compositional relationships between these mafic minerals suggest that the minerals in the same type of clots are in equilibrium. Comparing the chemical compositions of the minerals in the two types of clots in each sample, they are derived from distinct magmas: the B-type clots from basaltic magma and the A-type clots from andesitic magma. During the historic activity, the magma plumbing system appears to have included two magma storage systems: a deep-seated basaltic and a shallower andesitic one. In many cases, basaltic magma has injected into shallower andesitic magma to form mixed magma; however, andesitic magma has sometimes erupted alone without extensive injections of basaltic magma. Temporal variations of mineral compositions in the clots and estimated whole-rock compositions of the end-member magmas suggest that the basaltic magma has differentiated gradually since 1469, and that its magmatic temperature has fallen from 1220 to 1180 C. Conversely, the andesitic magma has changed in a complex fashion to become more mafic (the magmatic temperature rose from 1050 to 1100 C). As a result of this study, it is estimated that the basaltic magma after the 1983 eruption was the least mafic, and the andesitic magma the most mafic, of the historic eruptions. KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing INTRODUCTION The present state of the magma plumbing system beneath an active volcano provides important clues about the nature of the volcanic activity and the potential for future eruptions. Temporal changes in the magma plumbing system can be evaluated through detailed petrological studies of stratigraphically wellconstrained eruption sequences. Many studies have focused on the magma plumbing system and eruption processes based on the eruptive sequence of a single eruption (e.g. Reagan et al., 1987; Wolfe et al., 1987; *Corresponding author. Telephone: Fax: m.miyasaka@aist.go.jp Journal of Petrology 44(12) # Oxford University Press 2003; all rights reserved

2 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Garcia et al., 1989, 1992, 1996, 2000; Wolf & Eichelberger, 1997; Marianelli et al., 1999; Nakagawa et al., 1999, 2002; Streck et al., 2002). A series of papers on the Pu'u O'o eruption of Kilauea (Garcia et al., 1989, 1992, 1996, 2000) has revealed changing magmatic processes over a period of 17 years; Reagan et al. (1987) and Streck et al. (2002) discussed the role of basalt replenishment in the long-lived (about 30 years) eruption of Arenal volcano, Costa Rica. Clearly, to study the evolution of magma plumbing systems, it is important to investigate as long a period of activity as possible [e.g. Wright & Fiske (1971) for Kilauea, Hawaii; Borgia et al. (1988) for Arenal, Costa Rica; Fichaut et al. (1989) for Mt. Pelee, Martinique; Belkin et al. (1993), Villemant et al. (1993) and Tedesco et al. (1998) for Vesuvius, Italy; D'Antonio et al. (1999) for Campi Flegrei caldera, Italy; Nakano & Yamamoto (1991) for Izu---Oshima, Japan]. In most of these studies, however, the evolution of the magma plumbing systems was discussed mainly in terms of the temporal change in whole-rock compositions. Although detailed analysis of phenocryst minerals is useful to understand magmatic processes (e.g. Nakamura, 1995; Umino & Horio, 1998; Nakagawa et al., 1999, 2002; Streck et al., 2002), a systematic mineralogical study of the eruptive activity of an active volcano for a considerable period (for hundreds to thousands years), based on the evolution of both whole-rock and mineral chemistry, has not been carried out, except for that by Borgia et al. (1988). Miyake-jima volcano has erupted at least 13 times since AD During the period , magma effused mainly from flank fissures. The most recent eruption, which began in June 2000 (Nakada et al., 2001), however, is distinctive compared with other historic eruptions. Earthquake swarms occurred beginning the night of June 26, and a submarine eruption took place on the morning of June 27. A summit eruption (ash plume) followed on July 8, and the summit area suddenly subsided. The collapse continued `silently' until mid-august, resulting in the formation of a new caldera with a volume of about 06 km 3. The volcano is now (April, 2003) discharging a large quantity of volcanic gases (SO 2, etc.) and inhabitants of Miyake-jima have been evacuated since September Several petrological studies have been carried out on recent historic eruptions, except the 2000 eruption, of Miyake-jima volcano (e.g. Iwasaki et al., 1982; Fujii et al., 1984; Soya et al., 1984; Sato et al., 1996; Amma-Miyasaka & Nakagawa, 1998, 2002). Sato et al. (1996) dealt with most of the historic lavas since They showed that whole-rock compositions became more silicic from 1643 to 1874, and then more mafic from 1940 until They proposed that mafic magma was recently injected into the magma plumbing system. Amma-Miyasaka & Nakagawa (2002) focused on the historic 1940 and 1962 eruptions and established that deeper basaltic magma and shallower andesitic magma existed beneath the volcano during this period. It is not clear, however, how long the magma plumbing system has existed and how it has evolved through time. In this paper, we describe the evolution of the wholerock and mineral chemistry of the historic lavas from 1469 to We focus on the mineral assemblage and chemical compositions of crystal-clots in single samples to clarify whether minerals in each sample are in equilibrium or not. We demonstrate the existence of two magma storage systems since 1469, one basaltic and one andesitic. We carefully investigate the evolution of both magma types for the past 500 years and the interaction between these magmas during each eruption episode. This provides important new insights into the evolution of the historic magma plumbing system, and allows us to estimate the possible state of the system just before the 2000 eruption. GEOLOGY AND ERUPTIONS OF MIYAKE-JIMA VOLCANO Miyake-jima volcano is a composite volcano with two nested calderas (Fig. 1), and is composed of tholeiitic basalt and andesite. Based on the eruption style and petrological characteristics of lavas and scoria, Tsukui et al. (2001) divided the volcanic activity of the last years into four stages: , , 2500 years BP to AD 1154, and since AD During the first stage, the main cone was constructed, and the outer caldera filled. Erupted materials are porphyritic basalts. The second stage began after a 3000 year repose period, consisting of andesitic lavas and scoria erupted from lateral and central vents. The third stage began with a large-scale eruption that formed the inner caldera, which has subsequently been filled with nearly aphyric to weakly porphyritic basalts. The latest stage, since1469, began after a 300 year repose period. The eruptions were characterized by the effusion of lavas and scoria mainly from NE---SWtrending flank fissures (Fig. 1). Eruptions occurred every years before 1811, and have become more frequent (every years) since then (Table 1). Most of the eruptions continued for a short duration (typically a day to a month) whereas the 1763 eruption was prolonged, lasting for 6 years. The volume of magma erupted is estimated to be km 3 for each eruption. Although these values may be underestimated, especially for the older eruptives, there seems to be little correlation between the eruptive mass and the duration of each eruption. 2114

3 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 1. Location of Miyake-jima volcano and distribution of lavas and pyroclastic deposits erupted during the period These eruptions were characterized by the effusion of lavas and scoria mainly from NE---SW-trending flank fissures. The 1643 lava flows are composed of three lobes: main-n, main-s and central. Modified from Isshiki (1984). Table 1: Summary of eruptive history of Miyake-jima volcano from 1469 to 1983 Age (date AD) Vent location Duration Volume* (km 3 DRE) Rocks Total phenocryst (vol. %) SiO 2 (wt %) 1983 SW flank 15 h nearly aphyric---weakly porphyritic NE flank 30 h nearly aphyric summit 25 days nearly aphyric---weakly porphyritic NE flank 23 h nearly aphyric megacryst bearing N flank days 0.01 nearly aphyric megacryst bearing W flank 10 days nearly aphyric---weakly porphyritic ? NE flank 7 days megacryst bearing SW flank 6 years nearly aphyric SW flank 14 days nearly aphyric SW flank 21 days nearly aphyric SE flank?? nearly aphyric SE flank?? nearly aphyric W flank?? nearly aphyric *Quoted from Tsukui & Suzuki (1998) except for the 1811 lavas. 2115

4 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 ANALYTICAL METHODS For each of the eruptions, we collected samples to represent all source vents as well as the complete eruptive sequence. Mineral core compositions were determined by a single analysis from the centre of the minerals for representative samples that reflect the whole-rock variations of each eruption. The JEOL 733 and 8800 electron probe microanalysers at Hokkaido University were used for the mineral analyses. Operating conditions were 15 kv accelerating voltage and 20 na beam current with a minimum spot size of 1 mm. Each element was counted for 30 s on the peak and 20 s on the background. Corrections were made according to the ZAF method. Whole-rock compositions were determined by X-ray fluorescence (XRF), using a Philips PW-1404 system with a Rh tube at Hokkaido University. Glass beads were used for major element analysis, and pressed pellets for trace element analysis. Major element compositions were determined for 230 samples, and trace element compositions for 198 samples from the eruptive products. Whole-rock compositions of all the samples for which we have analysed mineral compositions are listed in Table 2 and representative mineral compositions are given in the Appendix. The complete wholerock and mineral composition dataset is included in Electronic Appendices 1---5, which may be downloaded from the Journal of Petrology web site at petrology.oupjournals.org/. PETROGRAPHY AND MINERAL CHEMISTRY Although most of the lavas are nearly aphyric to weakly porphyritic, with less than 9 vol. % of phenocrysts, there also exist rocks characterized by the presence of anorthite (up to 3 cm in length) and olivine megacrysts (Table 1). Phenocryst minerals are plagioclase, olivine, clinopyroxene, orthopyroxene and magnetite. The groundmass is composed of plagioclase, clinopyroxene, magnetite and brown glass, and ranges in texture from intersertal to hyalo-ophitic. Megacryst-free rocks (nearly aphyric to weakly porphyritic rocks) All of the dated lavas except for the 1811 lavas include nearly aphyric rocks (Table 1), and their modal volumes of phenocrysts are less than 3 vol. %. Some of the 1835 and 1940 summit and 1983 lavas are weakly porphyritic, containing vol. % phenocrysts. We generally call these `megacryst-free rocks'. Mafic phenocrysts in the megacryst-free rocks usually consist of clinopyroxene, orthopyroxene and olivine (Figs 2 and 3). The 1835 lavas, however, do not contain olivine phenocrysts, whereas the 1643 and 1712 lavas lack orthopyroxene. In addition, in some of the 1940 and 1962 lavas both olivine and orthopyroxene phenocrysts are absent (Amma-Miyasaka & Nakagawa, 1998). Plagioclase is the dominant phenocryst phase and its maximum size is 46 mm in length. The An content of the plagioclase phenocrysts is in the range mol % (Fig. 2). Histograms for the 1835, 1962 and 1983 samples show unimodal distributions with a peak of An ˆ , whereas the other samples show bimodal distributions. Although the peaks of the An contents are variable in each eruption, most of them are in the range of An ˆ and An ˆ Clinopyroxene less than 24 mm in length is the dominant mafic phenocryst. Most of the phenocrysts are augite, although pigeonite sometimes occurs in samples of the 1712 and 1940 lavas (Fig. 3). The Mgnumber of most of the clinopyroxene phenocrysts is in the range ; however, one phenocryst in the 1962 lava is more mafic (Mg-number 480). There is little difference in the compositions of the phenocrysts from samples of different eruption ages, although compositional variations in the 1835 and 1962 lavas seem to be smaller compared with other eruptives (Fig. 3). Olivine phenocrysts are usually smaller than 20 mm in length. The Fo content of the olivine phenocrysts is in the range mol % (Fig. 3). The variations in the phenocryst composition in the samples older than the 1763 eruption are greater than those in the samples younger than the 1874 eruption. Orthopyroxene and magnetite phenocrysts are rare and smaller than 16 mm and 04 mm in length, respectively. The Mg-number of orthopyroxene phenocrysts is in the range (Fig. 3). Although most of them are in the range Mg-number ˆ , some of the orthopyroxene phenocrysts in the 1962 and 1983 lavas have Mg-number 560. Megacryst-bearing rocks Megacryst-bearing rocks occur only in the 1811, 1874 and 1940 flank eruption sequences (Table 1), and are orthopyroxene-bearing clinopyroxene---olivine basalt to andesite (Figs 2 and 3). The total volume of phenocrysts varies from 1 to 27 vol. %, and gradually increases from the 1811 (511 vol. %), to 1874 (516 vol. %) and to 1940 (527 vol. %) eruptions. Petrographic characteristics and mineral compositions of the megacrysts in the 1940 samples were described by Amma-Miyasaka & Nakagawa (2002). Although the mineral assemblage and chemical compositions of the megacrysts in the 1874 samples are similar to those of the 1940 eruption, clinopyroxene megacrysts are also found in the 1811 megacryst-bearing rocks in addition 2116

5 Table 2: Representative whole-rock compositions during the period Age: Sample no.: * wt % SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total FeO*/MgO ppm Sc V Cr Ni Cu Rb Zn Sr Y Zr Ba Pb Ga AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA

6 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Table 2: continued Age: ? 1811? Sample no.: 1818* 1828* * 11207* 31910* * 1808* wt % SiO TiO Al O Fe O MnO MgO CaO Na2O K2O P2O Total FeO*/MgO ppm Sc V Cr Ni Cu Rb Zn Sr Y Zr Ba Pb Ga

7 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Age: Sample no.: wt % SiO TiO Al2O Fe2O MnO MgO CaO Na O K2O P2O Total FeO*/MgO ppm Sc V Cr Ni Cu Rb Zn Sr Y Zr Ba Pb Ga *Megacryst-bearing rocks. 2119

8 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Fig. 2. Histograms of An contents of plagioclase phenocryst cores in megacryst-free and megacryst-bearing rocks erupted during the period The megacryst-bearing rocks have more Ca-rich plagioclase compared with the megacryst-free rocks. The 1811 lavas consist only of the megacryst-bearing rocks. to olivine and plagioclase. The chemical compositions of these megacrysts are more Mg rich or Ca rich compared with other phenocrysts in the megacryst-free rocks. Plagioclase up to 30 mm in length forms the dominant phenocryst phase. The compositions of plagioclase phenocrysts are in the range An ˆ , nearly the same as those in the megacryst-free rocks (Fig. 2). Plagioclase megacrysts (longer than 5 mm) in all the rocks have An 490. Although bimodal distributions of plagioclase with An 490 and An ˆ are recognized in all the samples, phenocrysts with An 590 are rare in the 1811 samples. Olivine is the dominant mafic phenocryst. The compositions in the 1811 and 1940 samples range from Fo ˆ 70 to Fo ˆ 86 (Fig. 3). The 1874 lavas, however, also include Fo-poor phenocrysts (Fo 570). Olivine megacrysts (longer than 2 mm) have similar compositions (Fo 480) among the three eruptions. 2120

9 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 3. Core compositions of olivine and pyroxene phenocrysts in megacryst-free and megacryst-bearing rocks erupted during the period Continuous lines with numerals are isotherms of Lindsley (1983). All data are plotted in terms of quadrilateral pyroxene components diopside---enstatite---hedenburgite---ferrosilite. Clinopyroxene less than 40 mm in length is also an abundant mafic phenocryst. All of the phenocrysts are augite (Fig. 3), and their Mg-numbers are in the range The compositional range in the rocks is nearly the same as that for the megacrystfree rocks. The 1811 megacryst-bearing rocks show a compositionally bimodal distribution of the clinopyroxene phenocrysts (Fig. 3). The phenocrysts with high Mg-number often coexist with plagioclase and olivine megacrysts. Orthopyroxene and magnetite phenocrysts are rare, and smaller than 12 mm and 03 mm in length, respectively. The compositional range of orthopyroxene phenocrysts is narrow (Mg-number ˆ ) compared with the megacryst-free rocks (Fig. 3). WHOLE-ROCK GEOCHEMISTRY All of the rocks during the period are classified as tholeiites in a SiO 2 ---FeO*/MgO diagram (Fig. 4). Most of the lavas consist of medium-k basalt to andesite, whereas some of the megacryst-bearing rocks plot in the low-k field. Although lavas from the 1469 to the 1983 eruptions show variations in SiO 2 ( wt %) and K 2 O ( wt %), there are no systematic changes in their whole-rock composition with time (Fig. 5). Comparing the differentiated megacryst-free rocks in each eruption, SiO 2 and K 2 O contents seem to increase from 1469 to 1712, then decrease to 1940 and increase again to On the other hand, the chemical compositions of the differentiated megacryst-bearing rocks in each eruption are nearly constant, whereas the entire compositional ranges of SiO 2 and K 2 O have gradually become wider from 1811 to The compositional variations of both megacryst-free and megacrystbearing rocks are shown in representative Harker diagrams in Fig. 6. The SiO 2 contents of the megacryst-free rocks range from 506 to 564 wt % (Figs 4---6). Among these rocks, at least three distinct trends (I, II and III) can be recognized, especially in SiO 2 ---TiO 2, SiO 2 ---FeO*/ 2121

10 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Fig. 4. (a) SiO 2 ---FeO*/MgO and (b) SiO 2 ---K 2 O diagrams for megacryst-free and megacryst-bearing rocks erupted during the period Dividing line in the SiO 2 ---FeO*/MgO diagram is from Miyashiro (1974) and that in the SiO 2 ---K 2 O diagram from Gill (1981). All analyses are normalized to 100 wt % volatile-free with total iron (FeO*) calculated as FeO. MgO and SiO 2 ---Cr diagrams, and these seem to correspond to eruption age (Fig. 7). Trend I consists mainly of the 1469, 1535 and 1595 lavas. Two samples (e.g. No in Table 2) from one of the lava lobes of the 1643 eruption (main-n, Fig. 1) are also classified as part of this trend. The SiO 2 content of the Trend I rocks ranges from 518 to 549 wt %. This trend is the most mafic (poor in SiO 2, TiO 2, FeO*/MgO and rich in Cr) of the three trends. Trend II consists mainly of lavas from the 1643 main-s lava lobe (Fig. 1; No in Table 2) and 1712 lavas. Compared with the rocks of Trend I, those of Trend II are in the range of SiO wt %, and are characterized by lower SiO 2 and Cr, and by higher TiO 2 and FeO*/MgO. Trend III is composed mainly of the lavas, and also contains the 1643 lavas from the central lava lobe (Fig. 1; No in Table 2). The rocks of this trend have SiO 2 ˆ wt %, and are the most differentiated (lowest Cr, and highest SiO 2, TiO 2 and FeO*/MgO). There also exist some samples with lower Fig. 5. Temporal variations of whole-rock SiO 2 and K 2 O contents for megacryst-free and megacryst-bearing rocks erupted during the period FeO*/MgO and higher Cr (e.g. No of 1983 and No of 1835 in Table 2), compared with other lavas. The SiO 2 content of the megacryst-bearing rocks ranges from 514 to 555 wt % (Figs 4---6). With an increase of SiO 2, the Al 2 O 3, Cr and Ni contents decrease, whereas the TiO 2 contents and the FeO*/ MgO ratio increase. The compositional variation of the megacryst-bearing rocks in each eruption forms distinct trends in most of the variation diagrams (Fig. 6). Although the 1811 lavas consist of SiO 2 ˆ wt % andesites, the 1874 and 1940 lavas have a wider compositional range of SiO 2 ˆ wt % and SiO 2 ˆ wt %, respectively. The 1811 lavas are rich in TiO 2 and have higher FeO*/MgO, whereas the 1874 lavas are poor in TiO 2 and FeO*/MgO among the megacryst-bearing rocks. Comparing the whole-rock compositions of the megacryst-bearing rocks with the megacryst-free samples, the megacryst-bearing rocks are rich in Al 2 O 3 and Ni, and poor in TiO 2, FeO*/MgO and V. Furthermore, the felsic ends of the trends of variations of the megacryst-bearing rocks intersect with those of the megacryst-free samples at around SiO 2 ˆ wt %. 2122

11 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 6. Selected SiO 2 variation diagrams for major elements (TiO 2, Al 2 O 3 ), FeO*/MgO and trace elements (V, Cr and Ni) for megacryst-free and megacryst-bearing rocks erupted during the period MINERAL ASSEMBLAGE AND CHEMICAL COMPOSITIONS OF CRYSTAL-CLOTS It is widely accepted that two types of magmas have erupted from the tholeiitic volcanoes of the Izu---Mariana arc: `plagioclase-controlled' (plagioclaseaccumulated) and `differentiated' magmas (Nakano & Yamamoto, 1991; Nakano et al., 1991; Tsukui & Hoshino, 2002). This is consistent with the evidence that there exist no disequilibrium phenocryst assemblages, such as olivine and quartz, or olivine and hornblende, in all of the rocks formed during the eruptions of Miyake-jima volcano. Most of these rocks are, however, nearly aphyric and have less than 3 vol. % phenocrysts. This suggests that plagioclase accumulation is a minor source of compositional variation in these rocks. In addition, SiO 2 and K 2 O do not systematically change during the period (Fig. 5). If differentiation had proceeded in a closed magma storage system, these values should have increased with time. These observations suggest that simple differentiation does not play an important role during the period. On the other hand, phenocryst minerals show wide and polymodal compositional distributions (Figs 2 and 3), suggesting that all of the phenocrysts could not crystallize simultaneously from a single magma. To investigate whether these rocks are equilibrium 2123

12 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 crystallization products or not, we examined the mineral assemblage and chemical compositions of crystal-clots found in each sample, as described by Amma-Miyasaka& Nakagawa(2002), because it can be considered that minerals in the same crystal-clot have crystallized simultaneously from the same magma. Types of crystal-clots On the basis of the mineral assemblage and the chemical compositions of the minerals within the crystal-clots in the rocks, crystal-clots can be divided into four types (Figs 8 and 9): megacryst-type (M-type); basaltic-type (B-type); andesitic-type (A-type); basaltic andesitic-type (AB-type). The M-type crystal-clots are recognized in megacryst-bearing rocks, and mostly consist of Ca-rich plagioclase and Mg-rich olivine with occasional Mg-rich clinopyroxene (e.g. in the 1811 lavas). They are the least differentiated of the four types of clots: An ˆ , Fo ˆ and Mgnumber ˆ The B-type crystal-clots consist of plagioclase, clinopyroxene and olivine, characterized by the absence of orthopyroxene and magnetite. Mineral core compositions are An ˆ , Mgnumber ˆ and Fo ˆ , respectively, more evolved than those of the M-type clots. In contrast, the A-type crystal-clots are characterized by the presence of magnetite (Usp ˆ ) and orthopyroxene (Mgnumber ˆ ), as well as plagioclase, clinopyroxene and olivine. Core compositions of plagioclase, clinopyroxene and olivine in these clots are An ˆ , Mg-number ˆ and Fo ˆ , slightly more evolved compared with those in the B- type clots. In addition to these clots, a fourth type of crystal-clot (AB-type)exists in the 1535 and 1595 lavas. The crystal-clots contain Mg-rich orthopyroxene (Mg-number ˆ ; Fig 3 and 9) without magnetite. The core compositions of coexisting plagioclase and clinopyroxene are An ˆ and Mg-number ˆ , respectively. The mineral assemblage and chemical compositions are intermediate between the A-type and B-type clots. Fig. 7. Variation of TiO 2, FeO*/MgO, and Cr vs SiO 2 for megacryst-free rocks ( symbol in Fig. 6) erupted during the period The megacryst-free rocks can be divided into three trends. The 1643 lavas flowed down separately into three lobes: main N, main S and central lobes, which respectively correspond to Trend I, II and III. Compositional relationship of minerals within and among the clots The compositional relationship between olivine and pyroxenes can be investigated on the basis of the Fe---Mg distribution in these minerals. If these minerals coexist in equilibrium, the Mg-number of olivine should be nearly the same as or slightly lower than that of clinopyroxene and orthopyroxene (Obata et al., 1974; Brey & Kohler, 1990). We have determined the average compositions and compositional ranges of 2124

13 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA equilibrium from the same magma, and that different types of crystal-clots have crystallized from distinct magma compositions. The Fo content of olivine phenocrysts is strongly dependent on the FeO/MgO of the magma and K D(olivine/liquid) is constant over a wide range of P, T, f O2 and H 2 O (Roeder & Emslie, 1970). Based on the Fo content of olivine in each type of clot, M-type clots are formed in the most mafic magma, and the A-type clots in the most differentiated magma. The presence or absence of magnetite and orthopyroxene in these clots suggest that the magma producing the M-type and B- type clots is basaltic, and that producing the AB-type and A-type clots is probably andesitic (e.g. Gill, 1981). Fig. 8. Photomicrographs illustrating the various types of crystalclots (M, B, A and AB) during the period The M-type photograph is through crossed nicols and the other photographs are in plane-polarized light. olivine and pyroxenes in each type of clot of each eruption age (Fig. 10). The Mg-number of clinopyroxene is slightly higher compared with that of olivine in each type of clot. Moreover, the Mg-number of orthopyroxene is nearly the same as that of olivine in the A-type clots. This suggests that the olivine and pyroxene in each type of clot have crystallized in DISCUSSION Origin of M-type clots and megacrysts Amma-Miyasaka & Nakagawa (2002) investigated the origin of M-type clots and megacrysts in the 1940 lava. We defined the M-type clots and megacrysts by their size [plagioclase of L 403 mm (where L is the radius of the equivalent circle) and olivine of L 402 mm] and the least differentiated composition of minerals (plagioclase of An ˆ , olivine of Fo ˆ and clinopyroxene of Mg-number ˆ ). Examining these crystals in detail, we pointed out the following petrographical features: (1) spherical olivine in plagioclase; (2) wide, homogeneous cores of plagioclase; (3) kink-banding of olivine megacrysts. These features cannot be explained by normal crystallization processes. The spherical olivine and wide, homogeneous cores of plagioclase can be formed by long-term diffusion, and the kink bands can be formed under conditions that can transmit strain. Such a condition could be achieved within a plutonic body. This is supported by the evidence that the mineral assemblage, chemical compositions and crystal size distributions of both the megacrysts and M-type clots are identical to those of plutonic xenoliths (smaller than 5 cm in diameter, consisting only of An ˆ anorthite and Fo ˆ olivine crystals) found in some of the 1940 lavas. M-type crystal-clots have also been recognized in the megacryst-bearing rocks of the 1811 and 1874 eruptions. The clots in the rocks of the 1874 eruption consist only of plagioclase and olivine, as in the case of the 1940 megacryst-bearing lavas. On the other hand, the clots in the 1811 lavas also contain clinopyroxene. There is no significant difference in the petrographic characteristics and chemical composition of plagioclase and olivine megacrysts among the three lavas of known age. Thus, we consider that the megacrysts in the 1811 and 1874 lavas have the same origin as those in the 2125

14 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Fig. 9. Mineral assemblage and mineral core compositions (plagioclase, clinopyroxene and olivine) in four types of crystal-clots (M, B, A and AB) in the rocks erupted during the period (see text). from N---NE flank fissures. Therefore, it may be suggested that these lavas have captured both the megacrysts and M-type clots as xenocrysts from the same body located beneath the N---NE sector of the volcano. Fig. 10. Mg-number of (a) clinopyroxene core and (b) orthopyroxene core vs Mg-number of olivine core in the A-, B- and M-type clots in the rocks erupted during the period The AB-type clots lack olivine phenocrysts. Dashed line in (a) is after Obata et al. (1974) lavas. Although the whole-rock chemistry of the megacryst-bearing rocks of each eruption seems to define distinct trends, this may reflect the difference in the ratio of component minerals in the plutonic rocks. All of the megacryst-bearing rocks erupted Evidence for magma mixing Both A-type and B-type crystal-clots coexist in each sample of the lavas, except for 1835, 1962 and Histograms of minerals occurring in A-, B- and AB-type crystal-clots in each eruption are shown in Fig. 11. Plagioclase in these lavas shows compositionally bimodal distribution. The peaks of the An contents are variable among lavas of different eruption ages. Plagioclase phenocrysts in the B-type clots are, however, always more Ca rich than those in the A-type clots in each eruption. Histograms of olivine and clinopyroxene do not clearly show compositionally bimodal distributions; however, these minerals are always more Mg rich in the B-type clots. This coexistence of two types of crystal-clots with distinct chemical compositions is considered to be a disequilibrium feature. The relationship between the crystal-clots and their host rocks can be investigated on the basis of Fe---Mg partitioning (Fig. 12). If minerals crystallize in equilibrium with the host magma, Fe---Mg distribution between the mineral-cores and the magma must be constrained by plausible partition coefficients, shown by the lines in Fig. 12. In the case of the rocks with both A-type and B-type clots, most of the mafic minerals in the clots do not plot near equilibrium. Most olivine and clinopyroxene in the B-type clots are more Mg rich than possible equilibrium, whereas those in the A- type clots are more Fe rich. Orthopyroxene, contained only in the A-type clots, is also more Fe rich than that 2126

15 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 11. Histograms of core compositions of plagioclase, clinopyroxene, orthopyroxene and olivine in A-, B- and AB-type crystal-clots in the rocks erupted during the period The M-type clots are considered to be xenocrysts and are excluded. It should be noted that the rocks containing only B-type clots have never erupted during the period required to be in equilibrium with the whole-rock FeO*/MgO ratio (Fig. 12). This suggests that the mafic minerals in the B-type clots should have crystallized from a more mafic (lower FeO*/MgO) magma than the host magma, and those of the A-type clots from a more differentiated (higher FeO*/MgO) magma. Our data suggest that most of the lavas contain two types of phenocryst that originated from distinct magmas, and indicate that these lavas could be produced by magma mixing of two end-member magmas, a differentiated A-type (andesitic) and a mafic B-type (basaltic) magma. The 1535 and 1595 lavas that have another type of crystal-clot (AB-type, Fig. 11), in addition to the A-type and B-type, might be the results of mixing between three magmas. In summary, at least two distinct magmas have existed beneath the Miyake-jima volcano since 1469, and have usually mixed during eruption. Rocks without obvious evidence for magma mixing The 1983, 1962 and 1835 lavas contain only the A-type crystal-clots (Fig. 11). Although histograms of phenocryst compositions in these lavas do not show clear bimodal distributions, the compositional range of the plagioclase phenocrysts is relatively wide (An ˆ ). The compositional variation of clinopyroxene in the 1983 lavas is also relatively extensive, compared with that in the 1962 and 1835 lavas. The compositional relationships between minerals and whole rocks also show that most of the mafic minerals in these rocks are in equilibrium with their host magma; however, Fe-rich clinopyroxene in the 1983 sample (No in Table 2) and olivine in the 1962 sample (No in Table 2) might not be in equilibrium (Fig. 12). The lack of B-type clots indicates that these lavas may have been produced without mixing with the mafic B-type magma. Although it is also possible that nearly aphyric basaltic magma mixed with the A-type magma, the proportion of the basaltic magma would be minor, because the mineral compositions are nearly in equilibrium with the host magma (Fig. 12). Considering the wide compositional variations of the phenocryst minerals (plagioclase and clinopyroxene) especially in the 1983 lavas, however, we could not exclude the possibility of mixing between distinct andesitic magmas. In conclusion, we suggest that lavas without the B-type clots are not mixing products between the A-type and B-type magmas, and that only pure A-type magma (or magmas) has erupted during the 1835, 1962 and 1983 eruptions. Furthermore, it is noteworthy that rocks containing only the B-type clots have never erupted during the period

16 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 Fig. 12. Fe---Mg partitioning between whole rock and cores of A-type and B-type olivine and pyroxenes from megacryst-free rocks. The megacryst-bearing rocks are excluded, because the FeO*/MgO of these rocks may be affected by the presence of xenocryst minerals. Lines represent partition coefficients of K D [(X FeO /X MgO ) mineral /(X FeO /X MgO ) liquid ] of Beattie (1993) for olivine and orthopyroxene, and Baker & Eggler (1987) for clinopyroxene. FeO contents of whole rocks are calculated by assuming Fe 2 /(Fe 2 Fe 3 ) ˆ 09. We have also tried Fe 2 / (Fe 2 Fe 3 ) ˆ 081 by wet chemical analysis (Isshiki, 1960; Iwasaki et al., 1982), but 09 better explains the equilibration relationships in non-mixing rocks. End-member magmas and their relationships Estimation of whole-rock compositions of end-member magmas Using the Fe---Mg mineral---melt distribution coefficients for olivine/liquid and orthopyroxene/liquid (Beattie, 1993), we can estimate the FeO*/MgO ratios of both the A-type and B-type magmas (Fig. 13). Except for the 1835, 1962 and 1983 lavas, the FeO*/ MgO ratios of the whole rocks are intermediate between those of the A-type and B-type magmas in each eruption. This is consistent with our conclusion that these rocks are mixing products between these magmas. Whole-rock FeO*/MgO ratios of the 1962 and most of the 1835 and 1983 lavas are nearly the same as the estimated FeO*/MgO ratios of the A-type magmas, whereas the whole-rock ratios of the 1535, 2128

17 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 13. Estimated FeO*/MgO ratios of A-type and B-type magmas erupted during the period We used K D(olivine/liquid) ˆ 0303, K D(orthopyroxene/liquid) ˆ 0284 of Beattie (1993) and the representative core compositions of mafic minerals in each type of clot. Calculated FeO*/MgO values of the A-type liquid are plotted at the SiO 2 -rich end and those of the B-type liquid at the SiO 2 -poor end. 1595, 1712 and most of the 1643 lavas are similar to those of the B-type magmas. FeO*/MgO ratios might reflect the mixing ratio between the A-type and B-type magmas. Relationship of the end-member magmas Plots of incompatible-element ratios (K 2 O/Ba, Y/Ba and Y/Zr) in the megacryst-free rocks define linear trends that pass through the origin with R 2 values of (Fig. 14). This strongly suggests that both the A-type and B-type magmas may have been derived from a single primary magma. This conclusion is consistent with Sr isotopic data. Notsu et al. (1983) and Notsu & Aramaki (1984) measured 87 Sr/ 86 Sr ratios of the rocks from Miyake-jima volcano, and confirmed that the isotopic ratio is concentrated in a narrow range ( ), suggesting that the source region of the magmas beneath Miyake-jima volcano is isotopically uniform. Depths of the end-member magmas The depths of the magma storage systems have been estimated based on geophysical observations carried out since the 1940 eruption. Takahashi & Hirano (1941) estimated that the size of the chamber was 27 km in radius and 100 m thick for the 1940 eruption, assuming the depth of the magma to be 3 km. Hypocentres of long-period earthquakes (Ueki et al., 1984) and geomagnetic change (Nakagawa et al., 1984) associated with the 1983 eruption suggested that magma was located at a depth of km. In addition to these shallower `sources', the inflation and deflation accompanying the 1983 eruption suggested a magma reservoir about km SW of the summit crater, at a depth estimated to be about 8 km (Tada & Nakamura, 1988). Sasai et al. (2001) also suggested that the pressure source during lies beneath the southern flank of the volcano at a depth of 8 km below sea level. Considering that the depths of short-period earthquakes are km below sea level (Minakami et al., 1963; Miyazaki & Sawada, 1984; Ueki et al., 1984), the magma storage systems appear to have existed at least at two depths (c km and 8 km). On the other hand, geobarometry did not successfully reveal the crystallization depth of the A-type and B-type magmas of the volcano. We used the pseudoternary normative diagram (cpx---ol---sio 2 ) of Walker et al. (1979) and Baker & Eggler (1983) for lavas with only the A-type clots and those with both the A-type and B-type clots, and the clinopyroxene geobarometer (Nimis, 1995) for both types of clots. These calculations show, however, no difference between the A-type and 2129

18 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 DECEMBER 2003 (A-type) and a deeper basaltic (B-type), have existed during the period The relative position of these magma storage systems may be consistent with the fact that the A-type magma (or magmas) has erupted alone during the 1835, 1962 and 1983 eruptions, whereas rocks containing only the B-type clots have never erupted during the period The absolute pressure of both magmas should be less than 3 kbar; the shallower andesitic magma storage systems might be located at a depth of km, and the deeper basaltic magma storage systems might be at 8 km. Magma storage systems and their evolution Our petrological analysis of the lavas reveals that the magma plumbing system during this period has involved two distinct magma storage systems, filled with the andesitic (A-type) and basaltic (B-type) magmas. In many of the eruptions, the resultant lavas are the mixing products between these two magmas, although shallower andesitic magmas have sometimes erupted alone. Because we can identify phenocryst minerals that are derived from each endmember magma during mixing events, the evolution of both end-member magmas can be studied by using these phenocrysts. We can demonstrate the evolution of the chemical composition of these phenocrysts and of the deduced magmatic temperature during the past 500 years. Fig. 14. Incompatible-element variation diagrams for the megacryst-free rocks erupted during the period R 2 values are for all points. B-type magmas. Whole-rock compositions are projected around 1 atm for both lavas on the normative diagram, and equilibration pressures calculated based on clinopyroxene compositions are kbar for both the A-type and B-type clots. The B-type magma appears to have entrained plutonic xenoliths before mixing with the A-type magma during the 1940 eruption (Amma-Miyasaka & Nakagawa, 2002). Thus, we suggest that basaltic magma should exist at a deeper level beneath the volcano than andesitic magma. We propose that two distinct magma storage systems, a shallower andesitic Evolution of the end-member magmas and their magmatic temperature The core composition of olivine in the B-type crystalclots has gradually become Fe rich with time (Fig. 15). The average composition has decreased systematically from Fo ˆ 75 in the 1469 lavas to Fo ˆ 69 in the 1940 lavas. Although the compositional range of clinopyroxene in the B-type magma is wide, the average pyroxene composition in each eruption is nearly the same in the range of Mg-number ˆ throughout the period According to Brey & Kohler (1990), the ratio of the Mg-number of clinopyroxene to the Fo content of olivine increased from 1 to more than 1, with falling equilibrium temperature. Therefore, a temporal change in both Mg-number of clinopyroxene and Fo content of olivine in the B-type clots would indicate that the temperature of the basaltic magma has fallen systematically with time. The temperature of the basaltic magma calculated by Loucks (1996) actually fell from 1220 C in the 1469 eruption to 1180 C in Using the temporal variation of phenocryst minerals in the B-type clots, the evolution of the whole-rock chemistry of the basaltic magma can also be evaluated. The FeO*/MgO 2130

19 AMMA-MIYASAKA AND NAKAGAWA MAGMA EVOLUTION, MIYAKE-JIMA Fig. 15. Temporal variations of core compositions of B-type clinopyroxene and olivine and magmatic temperature for the rocks erupted during the period Numbers in parentheses represent the eruption intervals of B-type magma. The B-type magma erupted every years during the period Large filled triangles are average core compositions of the minerals. ratios of the basaltic end-member magmas during were the lowest (Fig. 13). Since then, the ratio has increased, accompanied by falling magmatic temperature. In contrast, the core composition of clinopyroxene and orthopyroxene in the A-type clots has not changed systematically as it has for the B-type clots (Fig. 16). The average core composition of clinopyroxene is Mgnumber ˆ 67 in the 1469 lavas, which is the most Fe rich during the past 500 years. Although the composition has fluctuated since then, it has gradually become more Mg rich with time to Mg-number ˆ 72 in the 1983 lavas. Except for the 1763 eruption, the average Mg-number of orthopyroxene in the 1469 A-type clots reflects the most differentiated magma during the period , and has gradually become Mg rich up to Although orthopyroxene in the A-type clots in the 1763 lava is the most Fe rich among the samples, it is rare in Using the average compositions of both clinopyroxene and orthopyroxene in the A-type clots, magmatic temperatures were calculated according to the Wells (1977) geothermometer for each eruption. The temperature appears to have risen in a complex fashion from 1070 C in the 1469 eruption to 1100 C in 1983 (Fig. 16). As in the case of the basaltic magma, temporal changes in the whole-rock chemistry of the andesitic magma are also evident: the FeO*/MgO ratio has decreased from 44 in the 1469 eruption to 33 in 1983 (Fig. 13). Thus, the two magmas, which have formed endmember components for magma mixing during the past 500 years, have evolved following separate paths (Fig. 17a). The basaltic magma has become more differentiated with time while cooling, whereas the andesitic magma has become more mafic while heating. A temporal fall in the magmatic temperature of the basaltic magma could be explained by fractional crystallization of a primary basaltic magma, and it suggests that the basaltic end-member magma has been continuously in existence at least for 500 years. The complicated and reverse evolution of the andesitic magma during the past 500 years cannot be explained by a closed system as in the case of the basaltic magma. In each eruption, the deeper basaltic magma was injected into the shallower andesitic magma and erupted a mixed magma. It suggests that the shallower 2131