Depths of Partial Crystallization of H 2 O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7^118S)

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1 JOURNAL OF PETROLOGY VOLUME 49 NUMBER1 PAGES 25^ doi: /petrology/egm068 Depths of Partial Crystallization of H 2 O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7^118S) RENAT ALMEEV 1 *,FRANC OIS HOLTZ 1,JU «RGEN KOEPKE 1, KARSTEN HAASE 2 AND COLIN DEVEY 3 1 INSTITUTE OF MINERALOGY, LEIBNIZ UNIVERSITY OF HANNOVER, CALLINSTRASSE 3, 30167, GERMANY 2 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF AARHUS, C. F. MÒLLERS ALLE 110, DK-8000 AARHUS C, DENMARK 3 LEIBNIZ INSTITUTE FOR MARINE SCIENCES (IFM-GEOMAR), WISCHHOFSTRASSE. 1^3, D KIEL, GERMANY RECEIVED MAY 3, 2006; ACCEPTED OCTOBER 10, 2007 ADVANCE ACCESS PUBLICATION NOVEMBER 22, 2007 Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7^118S). The results indicate that midocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100^900 MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100^300 MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200^900 MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 04^1wt% H 2 O in the parental melts. These estimates are in agreement with direct (Fourier transform IR) measurements of H 2 O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 004^009 and 030^055 wt% H 2 O for depleted and enriched MORBs, respectively. Our results are in general agreement (within 200 MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation INTRODUCTION The chemical compositions of basaltic glasses recovered from mid-oceanic ridges commonly show characteristics that are believed to be the result of various processes occurring at the stage of (1) primary magma generation and (2) subsequent modification of the parental magmas in the course of fractional crystallization, magma mixing and wall-rock assimilation en route to the ocean floor. Mantle source heterogeneity or/and different extents of partial melting, and in some cases the influence of deep mantle plumes, are generally advocated to explain differences in trace element abundances and ratios as well as in isotopic compositions of MORB glasses. Geochemical data, in particular large variations in trace element ratios for most MORB suites, clearly demonstrate that the trace element abundances in basalts (including K 2 O) are principally controlled by processes occurring in the source region (composition of the source, melt fraction), although crystal fractionation also plays a role. In contrast, major element variations in a majority of MORB, as a firstorder approximation, can be directly related to the continuous evolution of the parental basaltic melts along Ol þ Pl *Corresponding author. Telephone: þ Fax: þ r.almeev@mineralogie.uni-hannover.de ß The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@ oxfordjournals.org

2 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 and Ol þ Pl þ Cpx cotectics (Mineral abbreviations: Ololivine, Pl-plagioclase, Cpx-clinopyroxene). This observation is supported by a number of experimental studies on phase equilibria in MORB-like systems in which liquid lines of descent fairly well reproduced natural petrochemical trends. Another important observation of the experimental studies was to highlight a pronounced dependence of the clinopyroxene saturation temperature on pressure (Bender et al.,1978): at higher pressure Cpx crystallizes earlier (at higher temperature), resulting in a compositional trend showing a decreasing CaO concentration and CaO/ Al 2 O 3 ratio in residual liquids with decreasing MgO concentration. This property of MORB systems allows estimation of the pressure (depth) at which the basaltic magmas last equilibrated with the Ol þ Pl þ Cpx mineral assemblage before eruption. Several semi-empirical techniques use this property to evaluate partial crystallization pressures (e.g. Ariskin et al., 1992; Grove et al., 1992; Danyushevsky et al., 1996; Yang et al., 1996; Herzberg, 2004; Villiger et al., 2007). These methods are based mostly on the results of melting experiments on MORB compositions. The pressure can be determined within a precision estimated to be 100^200 MPa. However, all these methods, as has been pointed out by Michael & Cornell (1998), have two main limitations: the models require that (1) the system is saturated with respect to Cpx (in addition to Ol þ Pl) and (2) the system is assumed to be anhydrous. The first of these limitations can be accounted for by making pressure estimates only for lavas containing less than 8 wt% MgO, as this is the value at which the CaO concentrations of most MORB compositions start to decrease with decreasing MgO content. The second limitation (anhydrous crystallization) is related to the database used for the calibration of the models, which were constructed from phase equilibria experiments conducted at nominally dry conditions. Two sources of error arise: (1) high-pressure experiments performed in a piston cylinder apparatus are never absolutely water-free, although their water activities are generally not determined (Hirschmann et al., 1998; Holtz et al., 2001; Kagi et al., 2005); (2) even the small amounts of water in typical MORBs [less than 06wt%H 2 O; see, for example, compilations of Michael (1995) and Danyushevsky (2001)] will have discernible effects on mineral cotectics and petrochemical trends (e.g. Michael & Chase, 1987; Danyushevsky, 2001; Asimow et al., 2004) and so need to be taken into account when simulating magma crystallization processes. In this paper we present a method to constrain the pressure at which crystallization occurred in hydrous MORB. The effects of pressure and of small amounts of H 2 Oon liquid lines of descent were simulated using the COMAGMAT (version 3.57) program (Ariskin et al., 1993; Ariskin & Barmina, 2004) (hereinafter referred to as COMAGMAT). The method was applied to estimate pre-eruptive conditions for basaltic magmas from four segments (A1^A4) of the Mid-Atlantic Ridge (MAR) between 7 and 118S (Moeller, 2002). Two approaches, simulating (1) fractional crystallization and (2) equilibrium crystallization, were applied. The results obtained with the two methods are in a good agreement and we find polybaric crystallization conditions (200^900 MPa) for the normal (N)-MORB magmas beneath segments A1, A2 and A4. In contrast, the geochemically enriched MORB magmas from segment A3 apparently experienced their last equilibration with the Ol þ Pl þ Cpx mineral association at nearly isobaric crystallization conditions and at lower pressures (100^300 MPa). GEOLOGICAL BACKGROUND AND SAMPLE LOCATIONS The 200 km long portion of the Southern Mid-Atlantic Ridge axis in the vicinity of Ascension is bounded to the north and south by the Ascension and Bode Verde Fracture zones, respectively (Fig. 1). Between these fracture zones, the slow-spreading (3 cm/year) mid-ocean ridge is divided into four second-order segments of contrasting character and magma output by three non-transform offsets. The two central segments (A2 and A3) have shallow depths and rift axial highs whereas the two marginal segments (A1 and A4) are characterized by deep axial valleys (Fig. 1). Seismic studies have shown segments A2 and A3 to be characterized by a relatively thick crust of 11km, whereas segments A1 and A4 have crustal thicknesses of 5 km (Minshull et al., 1998; Bruguier et al., 2003). The shallow depth and anomalous axial morphology (for a slow-spreading segment) of segment A3, associated with the spatial proximity of numerous seamounts (Ascension, Circe, Grattan, seamount D), the presence of a mantle Bouguer gravity anomaly (Minshull et al., 1998, 2003), as well as geochemical and isotopic data (Schilling et al.,1985; Hanan et al., 1986; Graham et al.,1992; Fontignie & Schilling, 1996; Bourdon & Hemond, 2001) have been used to suggest a localized anomaly in mantle composition (not necessarily related to a thermal mantle plume but responsible for excess mantle melting) in this region (Minshull et al., 1998; Bruguier et al., 2003) or the influence of a hot mantle plume (Schilling et al., 1985; Bourdon & Hemond, 2001) on the convective system of the MAR in the vicinity of the segment A3. In 1998 the Mid-Atlantic Ridge spreading axis between 7 and 118S was dredged during cruise M41/2 of the German research vessel Meteor (see Fig. 1 for dredge locations). The dataset of chemical compositions, including major and trace elements and Sr, Nd and Pb isotopic ratios, of the recovered basaltic glasses has been presented by Moeller (2002). Selected samples from this set were 26

3 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 1. Map of the Mid-Atlantic Ridge in the vicinity of Ascension (7^118S) showing the ridge segmentation (A1^A4 from north to south; Bruguier et al., 2003), the bathymetry of the axial zone, and the dredge location of the samples studied by Moeller (2002) () and in the present study (*). used here to provide input data for the geochemical modeling. METHODS AND DATA TREATMENTS Analytical techniques Electron microprobe Major element compositions of selected quench glasses, as well as melt inclusions hosted in olivine phenocrysts, were analyzed using a Cameca SX100 electron microprobe at the University of Hannover, at 15 kv acceleration potential. For major and minor elements, the beam current was 4 na and the counting time was set to 6 s for Na and K and 12 s for the other elements. These analytical conditions are considered as the most appropriate for the analysis of experimental hydrous basaltic to silicic glasses. All glass analyses were performed using a defocused beam of 20 mm, except for some small glass inclusions, for which we used a beam size of 5 mm. Each reported analysis (Table 1) is the average of 10 points for the quenched glasses (except one glass with 175 analyses; see below), and fewer than four points for the glass inclusions. Cl, P and Cr in MORB glasses were measured with a beam current of 30 na, and counting times varied between 30 and 120 s. Infrared (IR) spectroscopy Doubly polished plates of natural quenched glass (2^3 mm in diameter) and olivine crystals with glass inclusions (80^120 mm in size) with thickness of 60^80 mm were prepared for Fourier transform IR (FTIR) spectroscopy to investigate the H 2 O abundances. The thickness of each sample plate was measured with a digital micrometer (Mitutoyo; precision 2 mm). The H 2 O concentrations were determined using a Bruker IFS88 FTIR spectrometer coupled with an IR-Scope II optical microscope (operation conditions: MCT narrow range detector, globar light source and KBr beamsplitter). H 2 O contents were analyzed using the main OH-stretching peak of OH groups and molecular H 2 Oat3500 cm^1. Typically 50 scans were used for IR measurements. The spot size applied was 100 mm 100 mm for the glasses and 60 mm 60 mm for the glass inclusions. The analyzed area was checked optically before IR measurement to avoid the presence 27

4 28 Table 1: Major element (wt%), H 2 O (wt%) and Cl (ppm) in selected basaltic glasses and glass inclusions from segments A1, A2 and A3 Sample: 139DS3 140DS1 145DS4 147DS3 149DS1 160DS1 161DS1 164DS1 169DS2 174DS1 190DS4 191DS2 GI-1 GI-2 GI-3 GI-4 GI-5 GI-6 GI-7 Segment: A1 A1 A1 A1 A2 A2 A2 A2 A3 A3 A3 A3 A1 A1 A1 A3 A3 A3 A3 No. of analyses: SiO (41) 5051 (37) 5141 (34) 5169 (38) 5195 (56) 5175 (30) 5086 (22) 5162 (24) 5118 (27) 5071 (35) 5078 (25) 5127 (45) TiO (3) 104 (6) 186 (8) 117 (2) 205 (7) 111 (4) 116 (4) 19 (6) 171 (9) 259 (6) 207 (8) 309 (12) Al 2 O (36) 158 (22) 1375 (19) 1576 (21) 1384 (20) 1603 (20) 1600 (32) 1460 (23) 1464 (29) 1452 (13) 1437 (19) 1355 (22) FeO 1056 (47) 965 (39) 1212 (53) 1043 (50) 1234 (52) 1022 (44) 1005 (25) 1059 (32) 1102 (35) 1168 (63) 1106 (42) 1396 (55) MnO 019 (5) 021 (9) 022 (5) 019 (8) 024 (7) 013 (11) 022 (8) 017 (11) 02 (9) 023 (9) 018 (10) 023 (7) MgO 807 (21) 926 (16) 703 (19) 821 (19) 61 (21) 867 (13) 842 (12) 707 (16) 715 (11) 572 (12) 666 (17) 469 (16) CaO 12 (24) 1211 (24) 1133 (28) 1058 (26) 1022 (38) 1073 (22) 1138 (34) 1062 (26) 1166 (19) 1000 (39) 1131 (18) 912 (17) Na 2 O 234 (20) 232 (18) 261 (24) 266 (20) 272 (24) 271 (12) 256 (24) 282 (17) 282 (25) 373 (6) 291 (28) 348 (23) K 2 O 012 (2) 006 (3) 015 (3) 004 (2) 031 (4) 007 (2) 01 (3) 03 (4) 022 (2) 069 (4) 034 (4) 064 (5) P 2 O (2) 013 (3) 023 (1) 011 (3) 035 (1) 01 (1) 014 (3) 03 (2) 021 (2) 042 (3) 036 (3) 050 (3) Cr 2 O (1) 005 (1) 003 (1) 004 (1) 003 (1) 005 (1) 005 (1) 003 (1) 003 (1) 001 (0) 001 (0) 000 Total Mg-no Cl Cl/K H 2 O (IR) H 2 O H 2 O calculated using equation (1). GI-1 to GI-3 are glass inclusions in olivine from sample 140DS1; GI-4 to GI-7 are glass inclusions in olivine from sample 169DS2; mg-number ¼ Mg/(Mg þ Fe 2þ ), Fe 2þ /Fe total ¼ 085. The 1s standard deviation (the last two digits) is given in parentheses. Downloaded from JOURNAL OF PETROLOGY VOLUME NUMBER 1 by JANUARY guest on January 2019

5 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB of microlites, fluid bubbles, cracks or impurities. The glass density was assumed to be 2800 g/l. Molar absorptivity used for all glasses was 67 l/mol per cm for e 3500 (Stolper, 1982). Estimation of crystallization conditions: forward and inverse modeling We developed and applied a new methodological approach to estimate intensive variables of crystallization for MORB glasses. Following the terminology of Myers & Johnston (1996), we performed numerical forward and inverse experiments to identify the conditions under which the given MORB composition could be generated. In our forward modeling we assumed a genetic relationship between all MORB lavas within a given segment. We checked if the compositions within one segment could be produced by fractional crystallization of the most primitive sample of this segment by varying pressure and initial water content. In the inverse approach we performed equilibrium crystallization calculations for each MORB composition, with the aim of determining the pressure and temperature at which multiple saturation (Ol þ Pl þ Cpx) occurs. As a model to simulate phase equilibria we used the COMAGMAT program (Ariskin & Barmina, 2004), as the effects of water and pressure on liquid lines of descent (LLD) can be modeled simultaneously. Comparisons with experimental data on dry MORBs show that this program gives consistent results with respect to mineral crystallization sequences, mineral proportions, and melt and mineral compositions (Yang et al., 1996; Ariskin, 1999; Almeev et al., 2004). Although COMAGMAT utilizes a simplified approach (Almeev & Ariskin, 1996; Ariskin, 1999) to quantify the effect of H 2 O on the crystallization temperatures of minerals, it has been shown that the phase equilibria in H 2 O-saturated high-alumina basalt studied experimentally by Sisson & Grove (1993) can be reliably predicted (Almeev & Ariskin, 1996). In our calculations the oxygen fugacity was assumed to be buffered by the quartz^fayalite^magnetite (QFM) assemblage. These conditions are slightly more oxidizing than those measured in MORB glasses. In recent determinations of Fe 3þ / P Fe in MORB glasses, Bezos & Humler (2005) showed that the average ratio is 012, indicating that the oxygen fugacity of most MORB is 04 log units below the QFM buffer. Forward modeling: fractional crystallization (FC) calculations Figure 2 illustrates an example of the forward modeling approach. Assuming that most of the glasses within one segment (e.g. A1) are genetically related to each other through differentiation of the same parental melt (e.g. sample 140DS1, one of the most primitive samples of segment A1), we performed a set of fractional crystallization calculations for this composition with initial H 2 O contents of 0, 02, 05, 07 and 10wt% H 2 O. Calculations were performed up to 60% crystallization, in the pressure range 01MPa^1 GPa with a small pressure increment of 10 MPa. For each pressure, COMAGMAT was used to identify the liquidus phase(s) (using a bulk crystallization increment of 1wt%) and the composition of the residual liquids. As a result, we obtained 100 isobaric LLDs for each initial water content (in total 500 LLDs because five initial H 2 O contents were tested). The full dataset of modeled MORBs consisted of liquid compositions for each parental melt (500 LLDs 1 composition for each of 60 crystallization increments per LLD). A typical dataset of the calculated MORB melts (only dry glasses are presented) is shown in Fig. 2a. In a second step, we performed a systematic comparison of the calculated residual melt compositions with each natural MORB glass. The following procedure was used to determine the modeled liquid that has the same composition as the natural MORB in terms of all major oxides. First, we selected glasses that have the same MgO content (as a proxy of degree of differentiation) and the same CaO/ Al 2 O 3 ratio (as a proxy of location on the same isobaric mineral cotectic). Then, from these compositions, we selected the modeled liquids that are as close as possible to the natural composition in terms of other major oxides (e.g. CaO, FeO, SiO 2,Al 2 O 3,Na 2 O). If the correspondence between natural and modeled MORB was within the analytical precision of electron microprobe microanalyses (2s) for each oxide, the natural glass was considered to be successfully modeled, and the calculated intensive parameters of crystallization (e.g. temperature, pressure) and H 2 O in the melt (H 2 O that was accumulated in the melt as a result of its incompatibility), as well as mineral assemblage and mineral proportions were assigned as appropriate to produce the given natural MORB from the chosen starting composition (Fig. 2b^f). For example, in Fig. 2 the modeled sample (evolved MORB) is best reproduced assuming a LLD at 420 MPa with 02wt% H 2 O in the parental melt (black diamond on the 420 MPa LLD in Fig. 2b^d). This search procedure was applied to each natural MORB glass from segments A1^A4, although not all natural MORB samples were successfully reproduced (see below). As a result we obtained a representative set of crystallization parameters for MORB magmas from segments A1^A4 that are interpreted as the conditions existing in partly molten systems just before ascent of the magmas to the ocean floor. It is essential to appreciate that the method outlined above does not require a knowledge of the H 2 O concentration in the system; H 2 O concentration is determined in the course of the calculations. This requires that the crystallization model can correctly predict the role of small amounts of water on the 29

6 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 Fig. 2. MgO vs oxide (wt%) diagrams illustrating the forward approach used in this study to calculate crystallization conditions of MORB glasses from a parental melt. Star in open circle is parental MORB composition (140DS1); open circle with error bars shows the evolved MORB composition (139DS2) and the 2s analytical uncertainty. Isobaric simulations were completed in increments of 10 MPa. For each isobaric simulation, results were generated from the liquidus to 60% crystallization, in crystallization increments of 1%. (a) Results between 01 and 800 MPa at 10 MPa increments (01, 200, 400, 600 and 800 MPa isobars are highlighted by continuous lines), and from the liquidus to 30% crystallization, in 1% crystallization intervals. (b)^(f) Isobaric LLDs, calculated with different water contents in the parental melt. These LLDs were selected to examine the crystallization conditions of the given evolved MORB glass (139DS2). LLDs are calculated for different conditions as follows:, 370 MPa and 0 wt% H 2 O in the parental melt; *, 440 MPa and 0 wt% H 2 O; ^, 420 MPa and 02wt%H 2 O; i, 400 MPa and 05wt%H 2 O; œ, 410 MPa and 07wt%H 2 O; þ, 390 MPa and 1wt% H 2 O. Filled symbols indicate the modeled compositions, which have the same MgO and CaO/Al 2 O 3 ratio as the evolved natural MORB (139DS2). In this example, the natural evolved MORB is best reproduced at 420 MPa from the parental composition 140DS1 with 02wt% H 2 O. Grey crosses in (b)^(f) are results of 175 replicate microprobe measurements of the sample 140DS1 (see text for further details). crystallization temperatures of olivine, plagioclase and clinopyroxene. Inverse modeling: equilibrium crystallization (EC) calculations The second approach is based on the assumption that all MORB melts are (multiply) saturated with respect to Ol þ Pl þ Cpx prior to eruption. It is known that pressure and ah 2 O are the most important variables that significantly affect cotectic crystallization in the MORB system. Therefore, if the first parameter is known (pressure or H 2 O), the second parameter can be obtained for a given MORB composition if all three minerals (Ol þ Pl þ Cpx) are in equilibrium with this MORB liquid. In practice, 30

7 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 3. P^T diagram illustrating the inverse approach used in this study to obtain crystallization conditions for MORB glasses. Each line represents the crystallization temperature (pseudo-liquidus temperatures) for Ol, Pl and Cpx, calculated for a given MORB composition as a function of pressure at dry and water-bearing (03wt% H 2 O) conditions. The intersection of the pseudo-liquidus curves denotes the condition of multiple saturation. The conditions of multiple saturation were determined for each MORB glass [for estimation of water content, see text and equation (1)] and were considered as representative of pre-eruptive conditions. the pressure of partial crystallization is unknown, but the H 2 O contents of the glass can be measured or estimated from the H 2 O^Ce or H 2 O^K 2 O covariation (Michael, 1995). Then, for a given MORB composition (with known H 2 O), calculations simulating equilibrium crystallization can be performed to identify the pressure at which all three minerals Ol þ Pl þ Cpx coexist with a melt having the composition of the natural glass sample (the condition at which the natural glass composition is on the Ol þ Pl þ Cpx cotectic; Fig. 3). Thus, in these inverse calculations the exact H 2 O content is a required parameter for the modeling. In this study we used H 2 Ocontents estimated from the H 2 O^K 2 O relationship (see below). RESULTS Natural dataset: basaltic glasses from the MAR (7^118S) The chemical variations of lavas erupted along the spreading segments of the MAR (7^118S) are shown in Fig. 4 [data of Moeller (2002) and this study]. Roughly, four slightly overlapping compositional clusters can be noted: (1) MORBs of segments A1 and the majority of samples from segment A4; (2) MORBs of segment A2; (3) MORBs of segments A3; (4) evolved MORBs of segment A4. Basaltic glasses from segments A1 and most samples from A4 are relatively primitive (MgO contents between 67 and 97 wt% and 7 and 91wt%, respectively) and exhibit typical N-MORB characteristics (0035K/Ti502). Al 2 O 3 decreases with decreasing MgO; however, CaO does not vary systematically. The CaO/Al 2 O 3 ratio increases from 065 to 085 with decreasing MgO. Similar to A1 and A4 MORB, the glasses of segment A2 have a wide range of MgO contents (from 6 to 96 wt%). However, in contrast to A1 and A4 MORBs, for the same MgO content, they are characterized by lower CaO and tend to have slightly higher Al 2 O 3 contents (Fig. 4). Thus, the CaO/Al 2 O 3 ratio of A2 basalts is systematically lower than that of A1 and A4 MORBs. Segment A3 MORBs are more fractionated (MgO ranges from 44 to 71wt%) and are uniformly enriched in incompatible elements (025K/Ti 504). Although Al 2 O 3 and Na 2 O are slightly scattered, these lavas show an apparent trend in which CaO and CaO/ Al 2 O 3 decrease with decreasing MgO. The last compositional cluster is formed by the samples from two dredge stations collected within segment A4. These samples are characterized by lower CaO and MgO contents and higher Na 2 O content. They are also geochemically enriched (K/Ti 03). However, they are distinguishable from the A3 basaltic group in having lower CaO content at the same MgO content (Fig. 4). Although the range of K 2 O/TiO 2 illustrated in Fig. 4 and the range in trace element ratios reported by Moeller (2002) probably reflect mantle source variability, most of the major element variations can be explained by fractional crystallization processes. The Mg-number of all MORB glasses (Table 1) is below the value (70) that is assumed to be typical for primary melts of spinel lherzolite. This suggests that all MORBs have experienced significant amounts of fractional crystallization. The FeO content increases and Al 2 O 3 content decreases with decreasing MgO content, indicating that melt evolution is controlled by fractionation along the Ol þ Pl cotectic. The scatter in CaO at a given MgO content observed in A1, A2 and A4 MORB may be related to differences in the pressure at which Cpx begins to crystallize (see also CaO/Al 2 O 3 ratio in the range 065^085; Fig. 4). In contrast, the fairly linear positive dependence observed in the CaO vs MgO and the CaO/Al 2 O 3 vs MgO diagrams for segment A3 MORBs (Fig. 4) definitely reflects fractionation of the Ol þ Pl þ Cpx-bearing mineral assemblage. Petrochemical trends and experimental liquid lines of descent (A1 and A3 MORB as an example) The comparison of petrochemical trends from natural lavas with the compositions of experimental glasses is a widely used approach to characterize the conditions at which magma differentiation occurred. Compositional 31

8 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 Fig. 4. Major element variation diagrams for Al 2 O 3,FeO,CaO,Na 2 O, CaO/Al 2 O 3 and K/Ti vs MgO in basaltic glasses for different segments of the MAR near Ascension [data from Moeller (2002) shown by grey symbols (A1 to A3 MORBs) and open triangles (A4 MORBs)]. Filled symbols show samples in which the H 2 O contents were determined by FTIR (this study). The evolved (low-mgo) and geochemically enriched (high-k/ti) character of A3 MORB in contrast to typical depleted N-MORB from segments A1, A2 and A4 should be noted. similarity between petrochemical trends and experimental LLDs implies that conditions simulated in the experiments may be similar to those prevailing in nature [see review by Myers & Johnston (1996)]. Melting^crystallization relations in MORB-systems have been extensively studied experimentally (Bender et al., 1978; Walker et al., 1979; Fisk & Bence, 1980; Fisk et al., 1980; Stolper, 1980; Fujii & Bougault, 1983; Grove & Bryan, 1983; Fujii & Scarfe, 1985; Tormey et al., 1987; Juster et al., 1989; Grove et al., 1990, 1992; Kinzler & Grove, 1992; Gaetani et al., 1994; Thy & Lofgren, 1994; Yang et al., 1996; Thy et al., 1998, 1999; Berndt et al., 2005). However, only a few starting compositions were found to be suitable analogues for parental magmas relevant for segment A1 lavas and more evolved basalts from segment A3. These two segments are discussed in detail below, considering that they show the most contrasting compositions. Phase relations in systems relevant to the A1 most primitive composition were studied under anhydrous conditions at 01MPa (composition ; Yang et al., 1996) and 800 MPa (composition ALV ; Grove et al., 1992), and in a water-bearing system at 200 MPa (synthetic MORB B1; Berndt et al., 2005). Compositions similar to the most primitive A3 segment magmas have been investigated at 01MPa (East Pacific Rise basalt PROTEA ; Yang et al., 1996), and three nominally 32

9 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 5. Comparison of natural major element trends for segment A1 (a^c) and A3 (d^f) with LLDs produced experimentally from MORB compositions at various pressures and H 2 O activities. Experimental LLD produced in primitive (a^c) and differentiated (d^f) MORBs are shown for the following starting compositions and run conditions: (1) composition ALV , dry, 800 MPa (Grove et al., 1992); (2) , dry, 01MPa (Yang et al., 1996); (3) B1, H 2 O-bearing, 200 MPa (Berndt et al., 2005); (4) PROTEA , dry, 01MPa (Yang et al., 1996); (5), (6) and (7) ALV , nominally dry isobaric crystallization LLD at 01, 200 and 800 MPa, respectively (Grove et al., 1990, 1992); (8) OB93, H 2 O-bearing, 500 MPa (Freise, 2004). The stars are the basaltic compositions 140DS1 (a^c) and 169DS2 (d^f) used as parental melts for segments A1 and A3, respectively (see text for further details). Numbers above the wet 200 MPa LLD denote concentration of H 2 Oin experimental glasses (Berndt et al., 2005). dry isobaric crystallization sequences (at 01, 200 and 800 MPa) were obtained on Serocki volcano tholeiitic basalt (sample ALV ; Grove et al., 1990, 1992). A hydrous tholeiite system was examined at 500 MPa (composition OB93, Kerguelen Plateau; Freise, 2004). All experimental glasses produced from these starting basalts are plotted together with natural quenched glasses of the A1 and A3 segments on the variation diagrams in Fig. 5. As shown in Fig. 5, the majority of A1 basaltic glasses are located within the compositional space defined by 01MPa and 800 MPa Ol þ Pl and Ol þ Pl þ Cpx cotectics at dry conditions. These cotectics probably bracket the possible pressure range of magma evolution beneath segment A1, as is shown on the CaO vs MgO diagram (Fig. 5a). Some of the A1 glasses, however, exhibit slightly Al 2 O 3 -enriched compositions, pointing to the possible role of H 2 O dissolved in the magmas. Their compositions 33

10 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 are located between the anhydrous and water-bearing LLDs determined by Berndt et al. (2005) at 200 MPa (Fig. 5). However, the H 2 O contents of the experimental glasses produced by Berndt et al. (2005) are in the range 1^4 wt%, a value that is too high to be realistic for magmas along the MAR to explain the observed difference in Al 2 O 3 content between natural lavas and the experimental LLD. Thus, the characteristics of most glasses from segment A1 are probably consistent with the nearly anhydrous experimental LLDs in the 01^800 MPa range. The basaltic glasses of segment A3 exhibit a welldeveloped trend on a CaO vs MgO plot (Fig. 5d). This trend is nearly parallel to the isobaric LLDs produced experimentally by Grove et al. (1992) and Yang et al. (1996), in contrast to A1 lavas (Fig. 5a). This observation may indicate crystallization at nearly isobaric conditions, although the estimation of absolute pressure values is not possible from this limited experimental dataset. Figure 5e and f shows that crystallization must have occurred in a H 2 O-bearing system for the A3 segment lavas. As shown in Figure 5e and f, the trend of A3 lavas is out of the experimental range obtained for dry MORB systems (01^800 MPa). The experimental results from anhydrous systems show that, with increasing pressure of crystallization, both FeO and Al 2 O 3 increase in residual melts with the same MgO content. Thus, the natural compositions cannot be reproduced by pressure variations only. Assuming that the anhydrous 500 MPa LLD trend is intermediate between the 01MPa and the 800 MPa trends, and using the experimental data for hydrous conditions (Freise, 2004), the effect of increasing water activity at constant pressure can be estimated. An increase in water activity causes a decrease of FeO and an increase of Al 2 O 3 in residual melts at a given MgO content (see below). Thus, the evolution of natural compositions can be explained only if water is present in the magmas. However, the experimental dataset of Freise (2004) cannot be used to estimate the exact water contents of basalts from segment A3 because the water contents in this study (23^93 wt% of H 2 O in the melts) are significantly higher than the maximum H 2 O abundances measured in MORB (51%; Danyushevsky, 2001). H 2 O and chlorine in MORBs H 2 O and chlorine concentrations were determined in 12 selected natural glasses, specifically chosen to represent the whole compositional range from the most primitive to the most evolved compositions for each of the segments A1, A2 and A3 (see all black symbols in Fig. 4). In addition, to characterize the H 2 O abundances in the parental magmas, we also measured H 2 O in several glass inclusions in olivines from the most primitive samples of segment A1 (sample 140DS1) and segment A3 (169DS2) (these samples were chosen as starting compositions in our forward calculations). Measured water and chlorine contents of MORB (7^118S) glasses are given in Table 1. The H 2 O concentrations range from 005 to 101 wt%. This H 2 O range is in agreement with those found in other studies: typical water contents measured in MORB glasses so far vary in the range of 005^06 wt% (Michael, 1995; Danyushevsky, 2001) and usually rarely exceed values higher than 1wt%. Enriched MORB glasses (segment A3) have the highest H 2 O contents (043^101wt%). N-MORB glasses from segment A1 are within a narrow range of H 2 O concentrations (01^024 wt%). Segment A2 glasses cluster within these two groups: a few A2 samples have H 2 O contents similar to those in the primitive samples from segment A3 (044^053 wt%), and the remaining samples have even lower H 2 Ocontents(005^012 wt%) than the A1 glasses. H 2 O contents determined in glass inclusions in olivines from the most primitive A1 and A3 samples also cluster within these two groups (grey symbols in Fig. 6). Water contents in glass inclusions (A1: 004^009 wt% H 2 O; A3: 032^055 wt% H 2 O) are slightly lower than the water contents determined in primitive quenched glass samples (Fig. 6). They may represent the water contents in the parental magmas. Figure 6a demonstrates that H 2 O is positively correlated with the K 2 O content of the basaltic glasses. Excluding one sample, the compositional trend of the quenched glasses can be described by a simple linear equation H 2 O ¼ 1612 K 2 O 0008 ðr 2 ¼ 0983Þ where H 2 O and K 2 O are given in wt%. Glass inclusions in 169DS2 glass (segment A3) are slightly off this trend and were not included in the regression. Simple crystal fractionation modeling indicates that the depleted MORBs of segment A1 and the enriched MORBs of segment A3 can evolve from two parental magmas with low (e.g. 005^01wt%) and high (e.g. 03 wt%) concentrations of H 2 O, respectively (Fig. 6b). In both cases the whole range of H 2 O in natural samples can be produced from the H 2 O-bearing parental melts by crystallization of up to 60^70%. This suggests that the relatively high H 2 Ocontents of the A3 MORBs are pristine and result from the enriched nature of the A3 parental magmas. The samples of segment A2 cannot be modeled by fractional crystallization of one parental composition with a given water content (Fig. 6b). However, A2 melts with high water contents also have high K 2 O contents and high K/Ti ratios, indicating that these samples have geochemical similarities to segment A3 samples. The calculated water abundances from the correlation between water in glasses and K 2 O contents [equation (1)] are assumed to represent magmatic H 2 O. In principle, however, the regression may be affected by assimilation of seawater, because the water contents are determined ð1þ 34

11 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 6. (a, b) H 2 O (determined by IR spectroscopy) vs K 2 O (a) and Mg-number (b) in MORB from segments A1, A2 and A3. g, þ, *,data from quenched glasses. Data from glass inclusions are shown by grey circles (sample 169DS2, segment A3) and diamonds (140DS1, segment A1). (c) Cl ppm vs K 2 O wt%; (d) Cl ppm vs Mg-number in the melt. The range of mantle-derived Cl/K values in (c) is from Michael & Cornell (1998). Continuous lines are calculated LLDs for depleted and enriched MORB with different amounts of initial water (b) and chlorine (d) contents. They illustrate how H 2 O (b) and Cl (d) vary with crystal fractionation. Crosses along the lines indicate 10% increments of crystallization. in erupted MORB glasses (and not in glass inclusions, which may be trapped before contamination occurs). The presence of a seawater component is apparent from the Cl contents determined in A1^A3 glasses (Table 1). The Cl/K ratio is useful to trace the presence of hydrothermally altered material (Michael & Cornell, 1998) and the overall Cl/K in the studied samples ranges from 007 to 017 (Fig. 6c, Table 1). These values are similar to or higher than the upper limit of the Cl/K range (001^007) proposed for MORBs that are unaffected by seawater contamination (Michael & Cornell, 1998). In addition, the Cl contents in differentiated A3 lavas significantly exceed the range of Cl enrichment allowed by crystal fractionation (Fig. 6d). The effect of seawater contamination on the estimation of magmatic H 2 O using equation (1) is difficult to quantify. If the seawater contamination effect was strong, the predicted water contents at a given magmatic K 2 O content would be higher than those from glass inclusions in the same sampleçthis is not observed (Fig. 6a). The good correlation between H 2 O and K 2 O as well as between H 2 O and melt Mg-number for different geochemical groups (e.g. segment A1, segment A3) is a further indication that variations in H 2 O concentrations are inherently magmatic and more probably due to source heterogeneity rather than seawater assimilation. RESULTS FROM FORWARD AND INVERSE MODELING Forward modeling at anhydrous conditions In our fractional crystallization calculations (forward modeling) we used four starting compositions, representing the parental melts (most magnesian samples) of the segments A1 (140DS1), A2 (161DS1), A3 (169DS2) and 35

12 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 A4 (199DS2). The compositions of these starting MORBs are given in the Electronic Appendix (Table S1), which may be downoaded from rnals.org. The compositions of the modeled MORB liquids that have similar composition to their natural counterparts are also given in this table, together with conditions [P, T, H 2 O (wt%)] at which they can best be produced from the parental melt (H 2 O in the parental melts is also given). The compositions of the natural counterparts (MORB glasses) have been given by Moeller (2002). Sample numbers in this study are identical to those of Moeller (2002). Figure 7 shows the results of anhydrous fractional crystallization calculations at different pressures for MORB glasses from segments A1 and A3. The pressuresensitive CaO/Al 2 O 3^MgO diagram is widely used to discriminate pressure of differentiation, as Cpx crystallization occurs earlier at higher pressure when compared with plagioclase and olivine. This affects the CaO/Al 2 O 3 of the residual melts. Figure 7a indicates that A1 glasses are the products of differentiation occurring over a wide pressure range. Other diagrams, such as CaO^MgO or Al 2 O 3^MgO plots (Fig. 7a^c), also indicate a wide range of pressure, varying from 200 MPa to 1GPa. It should be noted that all calculated isobars are within the uncertainty of the calculations on the FeO^MgO plot (Fig. 7d) and that this diagram can, therefore, not be used for pressure estimations. However, natural A1 glasses have slightly lower FeO concentrations than would be predicted from modeled anhydrous LLDs. The compositional evolution of the A3 glasses indicates nearly isobaric crystallization, considering that the compositional trend is parallel to the calculated isobaric LLDs on all plots (Fig. 7e^h). However, the absolute values of the modeled pressure vary, depending on the major oxide considered. Natural compositions cluster within the range 200^400 MPa on the CaO/Al 2 O 3^MgO plot (Fig. 7e), and along the 100 MPa and 600 MPa isobars on the CaO^MgO and Al 2 O 3^MgO plots, respectively (Fig. 7f and g). The reason for these discrepancies has been formulated by Michael & Chase (1987) and has been thoroughly discussed by Danyushevsky et al. (1996) and Danyushevsky (2001), who addressed the problem of the effects of small amounts of H 2 O dissolved in MORB magmas. Although H 2 O is present in trace abundances, it can significantly suppress the plagioclase crystallization relative to olivine and clinopyroxene (Danyushevsky, 2001). This implies that if the water content in the system is higher than 03^04wt% H 2 O, it should be taken into account to model MORB magma differentiation accurately. The delay in Pl crystallization means that the strong FeO enrichment and Al 2 O 3 depletion of the residual melts, which is observed in dry tholeiitic systems in which plagioclase predominates over the Fe^Mg silicates in the crystallizing mineral assemblage (e.g. Toplis & Carroll, 1995; Yang et al., 1996), does not occur (see also Figs 5e,f and 7d,h). To summarize, the results of these preliminary fractional crystallization calculations at anhydrous conditions are acceptable for most of the H 2 O-poor MORBs of segment A1 (Table 1) but are not satisfactory for H 2 O-rich MORBs from segment A3. The crystallization model should lead to consistent pressure estimates (pressure intervals) for all major oxide (Fig. 7f^h), which is not the case for segment A3. Forward modeling: fractional crystallization with different initial H 2 O contents General remarks Following the procedure described above, we assumed that, within each segment, a single parental magma could give rise, by fractionation, to the entire suite of MORB. For segments A1, A2 and A3, approximately 60% of the basaltic glasses could be numerically reproduced from one parental melt, implying that the chemical diversity of most glasses can be explained by a fractional crystallization process occurring at various pressures, temperatures and initial melt H 2 O content. The calculations have been successful for 25 glasses from segment A1 (from a total of 40), 26 glasses from segment A2 (from a total of 40), and 20 glasses from segment A3 (from a total of 32). For the remaining basaltic glass compositions, which could not be modeled by crystallization of the chosen parental liquid, we emphasize that only small changes in the starting parental melt composition would lead to an overlap between natural and modeled derivative compositions. Although most of the basalts from segment A4 are very similar to A1 basalts (Fig. 4), only 12 glasses (glasses with MgO ranging from 75 to85 wt%) from a total of 32 could be reproduced from the chosen parental melt (199DS2). This is not surprising, as segment A4 samples do not show clear compositional trends. For example, A4 samples with MgO474 wt% show no negative correlation between Na 2 O and MgO contents (Fig. 4), contrary to what would be expected for melts related by a process of crystal differentiation. Instead, the samples appear to fall into two compositional clusters with lower Na 2 O and MgO and higher Na 2 O and MgO (see Fig. 4). Within these clusters, the Na 2 O and MgO contents of the glasses are positively correlated. All these features probably indicate a variety of parental magmas, as does the existence of groups of A4 magmas with low and high K/Ti ratios (Fig. 4). The high K/Ti segment A4 glasses display small compositional variations and cannot be modeled using the forward approach without reliable determination of the parental melt. 36

13 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 7. Major element oxides and CaO/Al 2 O 3 vs MgO (wt%) showing LLDs calculated at anhydrous conditions for a primitive MORB glass 140DS1 from segment A1 (a^d) and for a primitive sample 169DS2 from segment A3 (e^h). The trends defined by natural samples (g, *) are given for comparison. Comparison between modeled and natural MORBs The compositions of the MORBs and the modeled residual liquids that best reproduce these natural compositions are compared in Fig. 8. The correlation between MgO and CaO/Al 2 O 3 is not shown in this figure, as these values are identical by definition (see also the Methods and data treatment section). There is a good agreement for CaO, Na 2 O, Al 2 O 3 and SiO 2, whereas systematic inconsistencies are observed for FeO and TiO 2.Thisis because COMAGMAT systematically produces slightly 37

14 JOURNAL OF PETROLOGY VOLUME 49 NUMBER 1 JANUARY 2008 Fig. 8. Comparison of the compositions of natural basaltic glasses and modeled residual melts (forward calculations). Crystallization conditions obtained for these residual melts are assumed to represent the crystallization conditions of their natural counterparts in Fig. 9a and b. Error bars represent the analytical precision (2s) of electron microprobe analysis adopted in this study. (See text for further details.) FeO-enriched and TiO 2 -depleted compositions, especially in highly differentiated samples. In previous studies (Ariskin, 1999; Ariskin & Barmina, 2004), a similar behavior of FeO in modeled melts was attributed to the lack of parameterization for spinel crystallization in the COMAGMAT model. The difference in K 2 O and TiO 2 between calculated and natural melts may be attributed to variations of these oxides in the parental melts as discussed above. Both elements are not expected to influence the calculated LLDs significantly. Although Yang et al. (1996) noted that the combined effect of TiO 2 and K 2 OonOlþ Pl þ Cpx-saturated melts shifts the composition of melts coexisting with Ol þ Pl þ Cpx to higher Al and lower Ca and Fe contents, we emphasize that these differences (501wt% for FeO, 5002 wt% for CaO and Al 2 O 3 ) are significantly lower than the analytical uncertainties of electron probe microanalysis (see below). Yang et al. (1996) noted that the effect of TiO 2 and K 2 O on calculated LLD is definitely more pronounced in Na-rich alkaline basalts, but such compositions were not considered in the course of this study. Crystallization conditions Figure 9a demonstrates that basalts from the northern A1 segment experienced crystallization over a range of pressures varying from 600 to 200 MPa and in the temperature interval 1250^11708C. Almost all numerically reproduced A1 basalts have been modeled at anhydrous conditions. It was necessary to assume water-bearing conditions (02wt% H 2 O) for a few samples (Electronic Appendix, Table S1) to obtain chemical similarity to the natural samples. The degree of fractionation for the majority of A1 MORB varied from 11 to 38 wt%. Segment A2 MORBs show the largest range of calculated pressures (900^200 MPa) and temperatures (1250^11308C). According to our calculations, 30% of modeled MORB can be reproduced at dry conditions (H 2 Oin parental melt ¼ 0) and 70% in the presence of small amounts of water. Among these latter samples, the best fits are obtained with 02wt% H 2 O (for 30% of MORB) or 05wt%H 2 O (for 70% of MORB) in the parental melt. The degree of fractionation varies from 2 to 40 wt%. Twelve samples from the southern segment A4 appear to have crystallized in the pressure range 480^230 MPa and temperature range 1220^11758C (Fig.9a).Only two samples have been modeled in dry conditions; the remaining MORB compositions can be simulated assuming 02 wt% of H 2 O in the parental melt (Electronic Appendix, Table S1). The degree of fractionation varies in therangefrom7to20%. The calculations suggest that the A3 MORBs have the highest parental magma H 2 O contents (05^10wt%; Electronic Appendix, Table S1). Crystal fractionation of between 7 and 40% leads to residual melts with water contents up to 16wt% H 2 O. Crystallization appears to have occurred at nearly isobaric conditions within a pressure interval between 300 and 200 MPa (Fig. 9a and b). The basaltic melts evolved in the temperature interval 38

15 ALMEEV et al. CRYSTALLIZATION OF HYDROUS MORB Fig. 9. Pre-eruptive conditions of MORB magmas computed in the course of the forward (a, b) and inverse (c, d) modeling (see text). 1170^10708C, which is significantly lower than that calculated for A1, A2 and A4 MORBs. The low temperatures obtained for A3 MORBs result from the combined effects of lower crystallization pressures, more evolved character of the parental melt (7 wt% MgO instead of 8^9 wt% MgO in A1 and A4) and the relatively high amount of H 2 O. Inverse modeling: equilibrium crystallization with known H 2 O contents In our equilibrium crystallization calculations (inverse modeling) all natural A1 to A4 MORB glasses were considered. The conditions [P, T, H 2 O(wt%)]atwhich each MORB composition is in equilibrium with Ol þ Cpxþ Plag (conditions of multiple saturation) are presented in the Electronic Appendix (Table S2). General remarks The option of equilibrium crystallization in COMAG- MATwas implemented to simulate the conditions of multiple saturation (Ol þ Pl þ Cpx) for the studied basaltic glasses. Calculations were carried out in two steps. First, for the given MORB composition with the estimated H 2 O content [see equation (1)], calculations were performed in the pressure range 01^900 MPa with a pressure increment of 100 MPa to check for the stable mineral assemblage within the first 3^5% of crystallization. These preliminary calculations allowed us to bracket a pressure interval in which the mineral crystallization 39