Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Chemical Geology 160 Ž1999. 335–356 www.elsevier.comrlocaterchemgeo

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa R.P. Rapp a

a,)

, N. Shimizu b, M.D. Norman c , G.S. Applegate

a,d

Department of Geosciences, Center for High Pressure Research and Mineral Physics Institute, State UniÕersity of New York, Stony Brook, NY, 11794, USA b Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA c Key Centre for Geochemical EÕolution and Metallogeny of Continents, School of Earth Sciences, Macquarie UniÕersity, Sydney, 2109, Australia d Department of Geology, Middlebury College, Middlebury, VT, USA Received 24 August 1998; accepted 15 February 1999

Abstract Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of ‘‘unadulterated’’ partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the ‘‘slab melting’’ experiments with another set of experiments in which these same ‘‘adakite’’ melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the ‘‘effective’’ melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio ; 2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mga orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation Žbecause melt is consumed., trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios Ž; 1:1., slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mga orthopyroxene. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Subduction zone; Experimental petrology; Arc magmatism; Trace-element geochemistry

1. Introduction

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Corresponding author. Tel.: q1-516-632-8241; fax: q1-516632-8140; E-mail: [email protected]

In recent years, an array of experimental studies has been published describing the melting behavior of hydrous, natural basalts over a pressure range appropriate to subduction of oceanic crust in the sub-island arc regime Že.g., Winther and Newton,

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 0 6 - 0

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R.P. Rapp et al.r Chemical Geology 160 (1999) 335–356

1991; Sen and Dunn, 1994a; Wolf and Wyllie, 1994; Rapp and Watson, 1995; see Winther, 1994 and Rapp, 1995 for reviews.. These studies were important because they Ž1. established the pressure–temperature Ž P–T . conditions under which hydrous basalt in subduction zones Žwith water present either as a free fluid phase, or bound in hydrous minerals. would begin to melt Ži.e., the wet basalt solidus, or the dehydration solidus, respectively., and Ž2. gave an indication of the composition of crustal-derived, low-degree melts that might be coming off the subducting slab.

The experimentally-determined solidus curves were used in conjunction with finite-element thermal models to show that dTrd P slopes for subducting slabs were typically too shallow to intersect the wet basalt solidus, and that ‘‘slab melting’’ would be restricted to those rare cases in which subduction of very young, hot oceanic lithosphere was involved ŽPeacock et al., 1994.. Compositionally distinct, Naand Al-rich dacitic lavas had already been identified in several intraoceanic island arcs and continental margin arcs associated with ‘‘hot’’ subduction, and broad similarities between these lavas, termed

Fig. 1. Plot of Mga Žmolar MgrMg q Fe4. vs. wt.% SiO 2 for experimental melts of natural hydrous basalts at 1–4 GPa, using data from Sen and Dunn Ž1994a. at 1.5 and 2.0 GPa for a single basalt composition, Rapp and Watson Ž1995. at 1.2–3.2 GPa for four different basalt compositions, and Rapp Ž1995. and this study for basalt AB-1 at 1.2–3.8 GPa. The data point for the melt of basalt AB-1 Žsame as basalt a1 from Rapp and Watson, 1995. can be identified by referring to Fig. 3. The experimental slab melts are compared with adakites from the Philippines ŽSajona et al., 1994., Panama ŽDefant et al., 1991., NE Japan ŽMorris, 1995., and the Austral Andes ŽStern and Killian, 1996., adakite veins in peridotite xenoliths from the northern Kamchatkan arc ŽKepezhinskas et al., 1995., and adakites in the Cordillera Blanca derived by partial melting of underplated mafic crust ŽPetford and Atherton, 1996.. Also shown are data for high-magnesian andesites ŽHMAs. from the western Aleutians ŽYogodzinski et al., 1994. and the Setouchi Volcanic belt in Japan.

R.P. Rapp et al.r Chemical Geology 160 (1999) 335–356

‘‘adakites’’ ŽDefant and Drummond, 1990., and the experimental melts were recognized. The extent to which adakite lavas represent pristine melts of subducted slabs can be assessed by comparing them with the low-degree, Na-rich or tonalitic–trondhjemitic Ži.e., adakitic; see Defant and Drummond, 1990. liquids produced in basalt melting experiments at 1–3 GPa, a reasonable pressure range when considering slab melting in the arc regime, and where crystalline residues consist of garnetamphibolite or eclogite Že.g., Sen and Dunn, 1994a; Rapp and Watson, 1995.. In Fig. 1, we compare experimental liquids from those studies with adakites from the Philippines ŽSajona et al., 1994., the southern Andes ŽStern and Killian, 1996., Central America ŽDefant et al., 1991., and northeast Japan ŽMorris, 1995.. Although many of the samples for individual suites fall well within the field for the experimental melts, many others do not, and it is these high-Mga lavas Žwhere Mga s  MgrŽMg q Fet .4. which are inconsistent with a simple, slab melting origin for adakites. High-Mga adakites are distinct from, yet compositionally continuous with, highmagnesian andesites Žor HMAs. from both the western Aleutians, which possess a strong slab melt geochemical signature ŽYogodzinski et al., 1994, 1995., and HMAs from the Setouchi Belt in SW Japan, which possess a strong mantle geochemical signature ŽTatsumi, 1982; Tatsumi and Ishizaka, 1982.. This continuity between low-Mga adakites Ži.e., derived from a slab source?., high-Mga adakites Ži.e., derived from a hybrid source., and high-Mga adakites and HMAs Ži.e., derived from a hybrid andror mantle source?. is exemplified by: Ž1. adakites from Mindanao in the Philippines, with Mga’s ranging from 30 to 60, that erupted in response to subduction of the intra-arc Molucca sea plate beginning 5 Ma ago, and Ž2. adakitic andesites and dacites in the Andean Austral Volcanic Zone ŽAVZ; Stern and Killian, 1996., related to subduction of the young Ž12–24 Ma. Antarctic plate beneath southern Chile. These lavas comprise both high-Mga adakites at Cook Island ŽMga’ss 64– 69., and low-Mga adakites 300 km to the NW at Mt. Burney ŽMga’ss 48–53.. Directly north of the AVZ the actively-spreading Chile Rise is being subducted beneath the South American Plate at the Chile Rise–Trench triple junction ŽForsyth, 1975..

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To assess the effects of peridotite assimilation on the composition of slab-derived melts, and to gauge the impact of migrating adakite melts on the minerTable 1 Composition of starting materials for basalt melting and peridotite assimilation experiments Wt.% oxide SiO 2 TiO 2 Al 2 O 3 FeOU MnO MgO CaO Na 2 O K 2O LOI Sum

Basalt AB-1a 51.19 1.18 16.62 11.32 0.23 6.59 5.49 4.33 0.82 1.53 99.30%

Trace elements (ppm) Nb 3.5 Zr 85 Y 30.7 Sr 359 Rb 17.0 Ba 413 Ni 46 Cr 77 V 325 Th 0.33 U 0.071 Pb 1.15 Rare earth elements (ppm) La 4.62 Ce 12.7 Pr 2.14 Nd 11.6 Sm 3.68 Eu 1.24 Gd 4.89 Tb 0.84 Dy 5.49 Ho 1.17 Er 3.35 Yb 3.22 Lu 0.47 a

Peridotite AVX-51b

Peridotite KLB-1c

44.48 0.00 0.69 7.43 0.12 43.85 0.91 0.31 0.04 0.00 97.95%

44.59 0.16 3.59 8.10 0.12 39.22 3.44 0.30 0.02 0.00 99.43%

0.040 0.16 0.09 142 0.040 3.2 2752 2903 36 0.005 0.004 0.25

0.022 0.049 0.006 0.029 0.007 0.002 0.008 0.0015 0.009 0.003 0.011 0.024 0.005

0.15 6.3 4.7 11.5 0.06 4.1 1866 3083 86 0.004 0.008 0.12

0.064 0.41 0.11 0.80 0.38 0.152 0.66 0.122 0.86 0.19 0.62 0.62 0.098

Alkali-rich, amphibolitized pillow basalt from Josephine Ophiolite ŽHarper, 1984.. b Harzburgite xenolith, Valvoyam Volcanic Field, N. Kamchatka Arc ŽKepezhinskas et al., 1995.. c Spinel lherzolite xenolith, Kilbourne Hole, NM, USA ŽTakahashi, 1986..

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Table 2 Run conditions for multi-anvil experiments conducted at ; 3.8 GPa. Melt in the phase assemblage consists of hybridized melt, except for experiment AB-1, which represents ‘‘pristine’’ melt of basalt AB-1 at 3.8 GPa. ŽRZ. denotes phaseŽs. present in the melt-peridotite reaction zone. Only ŽRZ. phases participate in the hybridization reaction. gt, Garnet; cpx, clinopyroxene; opx, orthopyroxene; ŽNa, K.-amphs Krichterite Experiment

Starting material

Temperature Ž8C.

Duration Žh.

Phase assemblage

AB-1 SPX-3 SPX-4 SPX-7 SPX-9 SPX-10

AB-1 only AB-1 q 30% AVX-51 AB-1 q 16% AVX-51 AB-1 q 10% AVX-51 AB-1 q 12% KLB-1 AB-1 q 15% KLB-1

1100 1150 1100 1100 1100 1100

30 42 43 37 35 44

melt, gt, cpx, rutile gt, cpx, ŽNa, K.-amph, opx ŽRZ. melt, gt, cpx, gt ŽRZ., opx ŽRZ. melt, gt, cpx, gt ŽRZ. melt, gt, cpx, gt ŽRZ. melt, gt, cpx, gt ŽRZ.

alogic and geochemical constitution of the overlying mantle wedge in subduction zones, we have conducted a series of slab melting and peridotite assimilation experiments at 3.8 GPa. These experiments allow us to make specific statements about the geochemical characteristics of pristine and mantle-hybridized slab melts, and the nature of melt:rock reactions taking place at the slab–wedge interface. Our results also provide geochemical criteria for distinguishing between true slab-derived adakites, more magnesian adakites possibly representing hybridized slab melts, and truly primitive Žmantle-derived., HMAs with adakite-like trace-element signatures. 2. Experimental methods 2.1. Starting materials Natural rock powders Ž- 5 mm. were used in the slab melting and peridotite assimilation experiments,

and the major and trace element compositions of the starting materials are listed in Table 1. Basalt AB-1 is an amphibolitized pillow lava from the Josephine ophiolite, and is the same material as basalt a1 of Rapp and Watson Ž1995., who reported its melting phase relations between 0.8 and 3.2 GPa. The two peridotite powders are from mantle xenoliths and are representative of both depleted ŽAVX-51. and fertile ŽKLB-1. mantle. Peridotite AVX-51 was kindly provided by P. Kepezhinskas and is one of a suite of peridotite xenoliths from the Valvoyam Volcanic field in northern Kamchatka, some of which appear to have been metasomatized by trondhjemitic Ži.e., adakitic., ŽKepezhinskas et al., 1995.. Peridotite AVX-51 is representative of relatively unmetasomatized mantle wedge in the Kamchatkan arc. A powdered split of peridotite KLB-1, a well-known and extensively studied spinel lherzolite composition, was kindly provided by Z. Zhang and C. Herzberg, who reported its high-pressure melting phase relations between 5 and 22 GPa ŽZhang and Herzberg, 1994;

Fig. 2. Back-scattered electron photomicrographs of crystal-melt phase assemblages in multi-anvil experiments at 3.8 GPa. Large arrows indicate orientation of photo relative to the top of the sample capsule Žarrow points towards top.. Ža. Melting experiment on basalt AB-1, showing top melt layer with pits from laser-ablation ICPMS analyses, and illustrating the effects of modest thermal gradient on distribution of melt and crystals; residual eclogite mineral assemblage at top Žtowards bottom of capsule.. Žb. Assimilation experiment SPX-7, melting of basalt AB-1 with 10% depleted peridotite AVX-51, showing eclogitic source, melt:rock reaction zone containing hybridized melt and new-formed garnet, and hybridized melt layer at top of capsule Žbottom of photo.. Note that none of the original peridotitic minerals remain in the reaction zone. Žc. Assimilation experiment SPX-4, melting of basalt AB-1 with 16% depleted peridotite AVX-51, showing newly formed orthopyroxene and garnet surrounded by hybridized slab melt. Again, none of the original peridotitc phases remain in the reaction zone Ži.e., no olivine or high-Mga orthopyroxene.. Žd. Assimilation experiment SPX-3, melting of basalt AB-1 with 30% depleted peridotite AVX-51, showing interface between orthopyroxene formed in the reaction zone Žon left., eclogitic garnet and minor clinopyroxene in the slab source, and Na-amphibole in between these zones. Orthopyroxene in the reaction zones consists of ‘‘unaltered’’ cores of high-Mga opx Ždark portion., and altered rims of low-Mga opx Žlight portion.. High-Mga ŽOPX. presumably represents unreacted, peridotitic orthopyroxene, and suggests that the melt:rock ratio or the temperature is too low to drive the reaction to completion.

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Table 3 a. Major element Ždetermined by electron microprobe. compositions of liquids in basalt melting and peridotite assimilation experiments. Data for AB-1 at 3.2 GPa are from Rapp and Watson Ž1995.. Oxide percentages are normalized to 100%, whereas the hydrous total Ž1. reflects the sum of oxides from raw microprobe data. ArCNK denotes aluminosity index s molar Al 2 O 3 rŽCaO q Na 2 O q K 2 O.; Mga s molar MgrŽMg q Fe.. n.a.s Not analyzed Source material

AB-1

AB-1

Experiment P ŽGPa. Temperature Ž8C. Time Žh.

R & W ’95 3.2 1100 131

Major elements, EPMA (wt.% oxide) SiO 2 65.93 TiO 2 1.31 Al 2 O 3 17.55 FeO b 3.15 MnO 0.12 MgO 1.02 CaO 1.47 Na 2 O 6.72 K 2O 2.55 Hydrous total 92.40% Mga 0.37 ArCNK 1.06 Other phases garnet cpx – – a b

AB-1 3.8 1100 30

AB-1 with 10% AVX SPX-7 3.8 1100 48

AB-1 with 16% AVX SPX-4 3.8 1100 37

AB-1 with 12% KLB-1 SPX-9 3.8 1100 40

AB-1 with 15% KLB-1 SPX-10 3.8 1100 65

67.52 1.53 15.67 3.07 0.06 1.36 2.27 4.56 3.68 89.05% 0.44 " 0.02 1.00 " 0.01 garnet a cpx a – –

65.43 1.98 12.91 4.05 0.08 2.75 2.15 6.25 3.83 88.16% 0.55 " 0.01 0.76 " 0.03 garnet a cpx a garnet b opx b

61.10 2.19 12.98 5.50 0.11 3.94 2.25 6.18 5.12 87.90% 0.56 " 0.01 0.66 " 0.02 garnet a cpx a garnet b opx b

65.63 1.99 12.60 4.02 0.04 2.36 2.00 3.69 6.99 88.81% 0.52 " 0.03 0.78 " 0.04 garnet a cpx a garnet b –

64.41 1.98 13.03 4.39 0.06 2.76 1.98 4.58 6.32 88.06% 0.53 " 0.01 0.73 " 0.02 garnet a cpx a garnet b –

Garnet and clinopyroxene in the eclogite residue; these phases do not participate in hybridization reaction. Reaction zone phases.

b. Trace element abundances in pristine melt of basalt AB-1 at 3.8 GPa, and in melts of AB-1 hybridized by assimilation of depleted ŽAVX-51. and fertile ŽKLB-1. peridotite, also at 3.8 GPa, determined by SIMS and laser-ablation ICPMS

Cs Rb Ba U Th Nb La Ce Sr Nd Zr Hf Sm Ti Gd Dy Ho Er

AB-1 melt at 3.8 GPa

AB-1 melt with 10% AVX-51

SIMS

ICPMS

SIMS

n.a. n.a. n.a. n.a. n.a. n.a. 42.1 " 2.0 31.2 " 2.6 851 " 21 21.7 " 1.3 320 " 6 n.a. 6.2 " 0.2 7252 " 506 n.a. 2.6 " 0.2 n.a. 1.8 " 0.1

3.3 63 1002 0.48 0.98 5.4 36.3 32.5 927 18.7 272 7.0 4.51 n.a. 3.14 1.74 0.27 1.10

n.a. n.a. n.a. n.a. n.a. n.a. – 48 " 1 829 " 158 29 " 1 – n.a. 8.3 " 0.1 8198 " 105 n.a. 3.8 " 0.1 n.a. 2.0 " 0.2

AB-1 melt with 16% AVX-51

AB-1 melt with 12% KLB-1

AB-1 melt with 15% KLB-1

ICPMS

SIMS

SIMS

ICPMS

SIMS

3.8 82 1670 – 2.52 9.7 – 57.7 1471 35.0 – – 7.71 n.a. 5.03 2.84 0.43 0.92

n.a. n.a. n.a. n.a. n.a. n.a. 35.6 48.8 815 " 27 39.5 246 " 8 n.a. 9.2 11814" 337 n.a. 4.8 n.a. 1.8

n.a. n.a. n.a. n.a. n.a. n.a. – 65 " 3.1 854 " 75 42 " 2 330 " 6 n.a. 9.7 " 0.2 9854 " 259 n.a. 4.3 " 0.1 n.a. 2.3 " 0.1

4.4 98 1861 0.63 1.87 11.4 33.4 60 1560 37.3 358 8.2 7.33 11954 " 186 4.83 2.39 0.40 0.71

n.a. n.a. n.a. n.a. n.a. n.a. 89.5 " 1.2 44 " 2.9 769 " 52 32.7 " 4.5 325 " 7 n.a 6.9 " 1.3 11732 " 370 n.a. 3.8 " 0.7 n.a. 1.5 " 0.3

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Table 3 Žcontinued.

Y Yb Lu Sc

AB-1 melt at 3.8 GPa

AB-1 melt with 10% AVX-51

SIMS

ICPMS

SIMS

6.7 " 0.6 0.9 " 0.1 n.a. n.a.

6 1.1 - 0.15 8.5

10.7 " 0.8 1.5 " 0.1 n.a. n.a.

AB-1 melt with 16% AVX-51

AB-1 melt with 12% KLB-1

ICPMS

SIMS

SIMS

ICPMS

SIMS

11 0.84 0.08 11.2

5.8 " 0.2 1.3 n.a. n.a.

11.0 " 1.2 2.0 " 0.1 n.a. n.a.

9 - 0.5 - 0.4 10.0

18.2 " 2.1 1.1 " 0.1 n.a. n.a.

see also Takahashi, 1986.. More recently, Hirose Ž1997. reportedly generated high-magnesian, andesitic liquids from low-degree partial melting of KLB-1 at 1 GPa. Major element compositions of AB-1 and KLB-1 were taken from the above references, and determined for AVX-51 by X-ray fluorescence at Middlebury College. All three starting materials were analyzed for trace elements by solution ICPMS at Macquarie University Žsee Norman et al., 1998 for a description of methods.. 2.2. Experimental technique Approximately 15–20 mg of AB-1 was packed into a thick-walled Ž; 0.7 mm. gold capsules Žlength s 2.5 mm, diameters 2.0 mm. for the single basalt melting experiment. This experiment is a prerequisite to the peridotite assimilation experiments, in which a thin layer of either the fertile or depleted peridotite was placed on top of the basalt ‘‘source’’ for the slab melt. The capsule lid in all the experiments consists of an oversized gold plug which deforms and seals as it is squeezed into the top of the sample cup using a tool steel pin and die set and a small hand vise. The sample capsule is then placed in a thin-walled MgO cup which sits in the center of the cylindrical graphite furnace, with the top of the capsule at the geometric center of the furnace. The rest of the furnace sub-assembly consists of sintered Al 2 O 3 spacer rods within machineable MgO sleeves, situated above and below the MgO cup containing the sample capsule. The full sub-assembly fits into a cylindrical ZrO 2 sleeve, which itself is inserted into a cylindrical hole drilled in the octahedron-shaped MgO pressure-medium Žwhich has an edge length of 14 mm. for experiments in the multi-anvil apparatus. The MgO octahedron containing the furnace assembly nests in the center of eight tungsten carbide

AB-1 melt with 15% KLB-1

cubes with truncated, 8-mm edge-length corners. A W97 Re 3 –W75 Re 25 thermocouple situated at the top of the sample capsule was used to continually monitor temperature during the multi-anvil experiments. From double-thermocouple calibrations, as well as garnet–clinopyroxene Fe–Mg exchange geothermometry, we estimate that a temperature gradient of approximately 30–408C exists between the top of the sample chamber and the bottom Žapproximately 1– 1.2 mm.. All experiments were conducted in a Walker-type, split-cylinder multi-anvil device ŽWalker et al., 1990., using a 1000-ton uniaxial hydraulic press in the Stony Brook High-Pressure laboratory. Sample pressure for our cell assembly was calibrated at room temperature using the ZnTe and Bi ŽIII–IV. phase transitions, and at high temperatures using the NaCl melting curve at 5 GPa and the coesite–stishovite transition in SiO 2 at 10 GPa. Samples in the basalt melting and assimilation experiments were slowly compressed to 3.8 GPa at room temperature, then heated over the course of 3–5 min to run temperature Ž11008C., where they were held for the duration of the experiment Ža minimum of 30 h.. Experiments were quenched to below 1008C in less than 5–10 s. After decompression, the gold sample capsules were recovered, mounted in epoxy, and sectioned and polished for microprobe analysis. Table 2 summarizes the experimental conditions and Fig. 2 shows back-scattered electron photomicrographs of representative cross-sections of the sample capsules and the resulting high-pressure phase assemblages Žmelt q crystals.. 2.3. Analysis The major element composition of quenched melt Žglass. and coexisting crystalline phases were deter-

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mined using a Cameca CAMEBAX electron microprobe at Stony Brook, operating at 15 kV accelerating voltage, and using a 5-nA beam current for analyses of quenched melt Žglass., and 10 nA for crystalline phases. For analyses of the glass, a 22mm2 rastered beam was used in order to minimize volatilization of alkali elements, and sodium and potassium were measured first. Concentrations of selected trace elements in the experimental melts were also measured by secondary ion mass spectrometry ŽSIMS. using the Cameca 3f ion probe at Woods Hole using previously reported techniques Žsee Shimizu and Hart, 1982.. Basalt and andesite glass standards ŽSims et al., 1988. were used to correct for matrix effects in the glass analyses. The beam for the ion probe analyses was in general - 10 mm in diameter, and a pit of comparable depth was excavated over the course of a 30-min measurement. The same trace elements measured in the glasses

were also measured in some of the larger, co-existing garnet crystals. Several samples were also sent to Macquarie University, where a comprehensive analysis of the melt layer for trace elements was made by laser-ablation ICPMS ŽNorman et al., 1996, 1998.. The excavation pit for these analyses was much larger Ž50 mm in diameter, 100–150 mm deep; see Fig. 2a., so these measurements were restricted to the relatively thick Ž; 200 mm. melt layers formed in the basalt-melting and several of the peridotite-assimilation experiments.

3. Results 3.1. Melting and reaction assemblages Melting of AB-1 at 3.8 GPa and 11008C produces 25–30 wt.%, high-SiO 2 Ž‘‘adakitic’’. liquid coexist-

Fig. 3. Plot of Mga vs. wt.% SiO 2 for hybridized slab melts, compared with the pristine slab melts, adakite arc lavas, and HMAs from Fig. 1. Note the position of the pristine slab melt of AB-1 at 3.8 GPa relative to the other low degree melts of this basalt at 1.2–3.2 GPa ŽRapp, 1995; Rapp and Watson, 1995.. Error bars are 1 standard error of 3–8 glass analyses. Data for AB-1 melting at 3.2 GPa from Rapp and Watson Ž1995..

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Fig. 4. Ža. Enrichment or depletion of trace elements in partial melt of AB-1 at 3.8 GPa, 11008C, relative to the bulk composition of the starting basalt, based on laser-ablation ICPMS analyses of melt layer and whole-rock solution ICPMS analysis AB-1. Žb. Bulk distribution coefficients between slab melt at 4 GPa and eclogite crystalline residue, calculated assuming 30% melting by weight Ždegree of melting estimated by least-squares mass balance; mineral modes are as follows: garnet s 42%, clinopyroxenes 27%, rutile s 0.5%..

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ing with a residual crystalline assemblage consisting of garnet, clinopyroxene, and accessory rutile ŽTable 1 and Fig. 2a.. The major-element composition of the melt ŽTable 3a. is similar to other low-degree melts of AB-1 between 1.2 and 3.2 GPa Žfrom Rapp, 1995 and Rapp and Watson, 1995., that is, hydrous, high-SiO 2 liquids rich in sodium and aluminum, and it is this specific melt composition that will infiltrate and react with overlying peridotite in the assimilation experiments. In response to the modest but distinct axial temperature gradient, melt is concentrated towards the top of the capsule ŽFig. 2a. by a process of crystal dissolution Žat the top or ‘‘hot’’ portion of the capsule. and reprecipitation Žin the ‘‘cooler’’ portion of the capsule. ŽAgee and Walker, 1989.. This clearly acts to our advantage in the assimilation experiments, by driving melt transport into and through the peridotite layer and promoting melt-rock reaction. In the basalt melting experiment, the thermal gradient results in a melt layer of 100– 200 mm at the top of the capsule, garnet precipitation in a garnetq melt layer just below, and an eclogitic layer Žgarnet q melt q clinopyroxene. in the bottom half of the capsule ŽFig. 2a.. When melt of AB-1 reacts with the overlying layer of peridotite, a reaction zone forms that consists of interconnected melt distributed through a crystalline matrix consisting of garnet " orthopyroxene, with a thinner, crystal-free layer of hybridized melt at the hot end Žgeometric ‘‘top’’. of the sample capsule Že.g., Fig. 2b.. In all of the assimilation experiments in which some of the melt phase remains, no vestiges of the original peridotitic olivine or pyroxenes remain in the reaction zone, and the interface between the eclogitic residue and the reaction zone remains distinct. We should emphasize that we see evidence for only minor ‘‘back-reaction’’ of the hydridized melt with the eclogite residue, with

most of the eclogite residue from slab melting largely unaffected by the hybridization reaction going on above. Garnet and clinopyroxene in the upper third of the eclogitic layer reflect equilibration with increasingly more magnesian adakitic melts as the reaction zone is approached. However, melt pools at the bottom half of the capsules in the assimilation experiments are compositionally identical to the pristine melt of AB-1 at 3.8 GPa, as are their respective eclogitic residues. When depleted peridotite AVX-51 comprises 10% of the total sample weight Žexperiment SPX-7; melt:rock ratio approximately 3:1., melt:rock reaction completely consumes all the original peridotitic orthopyroxene and olivine in forming pyrope-rich garnet and Mg-rich, high-SiO 2 , hybridized melt ŽFig. 2b and Table 2a.. When slightly more peridotite is present Ž16%; experiment SPX-4; melt:rock ratio approximately 2:1., the reaction assemblage consists of orthopyroxene ŽMga s 0.79., pyrope-rich garnet, and hybridized melt similar in composition to melt in SPX-7 ŽFig. 2c.. When the melt:peridotite ratio is roughly 1:1 Ž; 30% AVX-51; experiment SPX-3., silicate liquid is fully consumed in modal metasomatic reactions that produce alkali-rich amphibole ŽNa-richterite., pyrope-rich garnet, and both adakite-altered, low-Mga ŽMga s 0.84, where Mga s molar MgrŽMg q Fe.. orthopyroxene, and unreacted Žoriginal., high-Mga Ž0.93. orthopyroxene ŽFig. 2d.. These effects are similar to the amphibole-forming metasomatic reactions observed between slab melts and spinel lherzolite in melt infiltration experiments conducted by Sen and Dunn Ž1994b. at 1.5 and 2.0 GPa. Assimilation experiments with 12–15% fertile peridotite Žexperiments SPX-9 and SPX-10; melt:rock ratios approximately 2:1. produced reaction assemblages similar to those of SPX-7, with high-Mga, silica-rich hybridized melts coexisting in the reaction

Fig. 5. Ža. The ratio of SrrY vs. Y for pristine slab melts of AB-1Ždata from this study at 3.8 GPa, and Rapp and Shimizu, unpublished data, for melts of AB-1 at 1.2 GPa and 10008C, ŽRapp, 1995., and at 3.2 GPa and 11008C ŽRapp and Watson, 1995., compared with data for adakites from the Philippines ŽSajona et al., 1994., Cook Island and Burney volcanoes in the southern Andes ŽStern and Killian, 1996., La Yegueda volcanic complex in Panama ŽDefant et al., 1991., and southwest Japan ŽMorris, 1995., and for HMAs from the western Aleutians ŽYogodzinski et al., 1994, 1995.. Note that Na-rich trondhjemites from the Cordillera Blanca Batholith, interpreted as partial melts of basaltic crust underplating the Andean arc, plot within the same field as high-SrrY adakites. Žb. Ratio of SrrY vs. Mga for this same data set. Note that this type of plot is better at distinguishing pristine slab melts ŽMga’s - 0.50. from mantle-hybridized slab melts ŽMga’s ) 0.50..

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zone with pyrope-rich garnet. Note that at similar melt:rock ratios Ž; 2:1., hybridized melts produced in assimilation experiments with the depleted peridotite are in equilibrium with orthopyroxene-bearing reaction assemblages ŽSPX-4., whereas those from experiments with the fertile peridotite are not ŽSPX-9 and SPX-10.. 3.2. Composition of pristine and hybridized slab melts The hybridized slab melts can be compared with the pristine melt of AB-1 to assess the effects of peridotite assimilation on the composition of adakitic liquids. From Table 3a, it can be seen that assimilation causes decreases in the SiO 2 and Al 2 O 3 contents of the hybridized melts, with simultaneous increases in the contents of MgO, FeO, Na 2 O, K 2 O and TiO 2 . A sharp increase in the Mga of the melt and a sharp decrease in its alumina saturation index ŽArCNK s molar Al 2 O 3r CaO q Na 2 O q K 2 O4. are additional consequences of assimilation. When plotted with the data from Fig. 1, the hybridized melts extend the range of experimental liquid compositions towards the field for high-Mga adakites ŽFig. 3.. A strong argument can thus be made in favor of an origin for some high-Mga adakites by slab melt–peridotite interaction, based upon the displacement of the hybridized slab melts on Fig. 3 towards higher Mga, relative to the corresponding pristine slab melt. Abundances of trace elements in the pristine slab melt ŽTable 3b., measured by both SIMS and laserablation ICPMS, can be used in conjunction with the bulk composition to assess trace element fractionation during batch melting of subducted oceanic crust. In general very good agreement is found between trace-element abundances determined with the ion probe and with laser-ablation. Although the ICPMS techniques allows for analysis of a more comprehensive group of trace elements, much more damage to the sample is incurred, and the beam is too large to allow analyses of coexisting crystalline phase, so the ion probe continues to be our primary analytical tool for measuring trace elements in our high-pressure samples. The laser ablation data in Table 3b can be used in conjunction with the bulk composition of basalt AB-1

to gauge the extent of enrichment or depletion of a broad range of trace elements during slab melting ŽFig. 4a.: elements that are concentrated in the liquid during partial melting of AB-1 Žleft hand side of Fig. 4a. possess bulk meltrrock distribution coefficients Žwith respect to 20–25 wt.% melting of a hydrous, rutile-bearing, eclogitic mineral assemblage. less than unity ŽFig. 4b., and therefore these elements will be expected to behave incompatibly during melting of a hydrous eclogite source. On the other hand, elements that are relatively concentrated in the crystalline residue of melting Žright hand side of Fig. 4a. possess bulk meltrrock distribution coefficients greater than unity ŽFig. 4b. and will behave compatibly during slab melting. It is this bulk fractionation of elements that imparts to slab melts their characteristic geochemical signature, including enrichment in light-rare earths and large-ion lithophile elements ŽLILEs., depletion in heavy rare earths and yttrium, and high LarYb and SrrY ratios. The data represented in Fig. 4b can therefore be used to predict the trace element composition of any liquid derived from partial melting of hydrated, eclogitized basaltic crust, for any given Žtrace element. bulk composition, assuming ; 20–25% partial melting Žsee Table 5 and discussion below.. Trace element abundances in hybridized slab melts ŽTable 3b. are uniformly higher than those in the pristine slab melt, in response to the consumption of melt during reaction with the peridotite, but significantly, most element ratios Že.g., LarYb, SrrY, SrrNd, NbrLa, KrLa, etc.. remain nearly constant. High SrrY and LarYb ratios in adakite lavas are often cited as evidence for an origin by partial melting of an eclogitized slab source Že.g., Defant and Drummond, 1990., with fractionation of La from Yb and Sr from Y being attributed to the presence of residual garnet, which has high mineral-melt partition coefficients Ž K d’s. for Yb and Y, and low K d’s for Sr and La ŽGreen, 1994.. Our data for Sr and Y indicate that both pristine and mantle-hybridized melts have high SrrY ratios ŽFig. 5a., and that a better means of discriminating between wholly crust-derived adakites and adakites with a mixed crust–mantle lineage is by reference to their Mga’s ŽFig. 5b.. Limited assimilation of peridotite raises the Mga’s of slab-derived melts, but has little effect on SrrY, LarYb, and other key trace element ratios

R.P. Rapp et al.r Chemical Geology 160 (1999) 335–356

Žsee Fig. 7 and discussion below.. This suggests that the process of assimilation does not lead in any significant way to further element fractionation in

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adakite magmas, beyond that accompanying the initial melting of the slab. Thus, mantle-hybridized slab melts should possess mantle-normalized, trace ele-

Table 4 Electron microprobe analyses of crystalline phases in multi-anvil experiments at 3.8 GPa Experiment Žoxide wt.%. Garnet SiO 2 TiO 2 Al 2 O 3 FeOU MnO MgO Cr2 O 3 CaO Na 2 O K 2O Total Pyrope Almandine Grossular Mga Clinopyroxene SiO 2 TiO 2 Al 2 O 3 FeOU MnO MgO Cr2 O 3 CaO Na 2 O K 2O Total MgrŽMg q Fe q Ca. FerŽMg q Fe q Ca. CarŽMg q Fe q Ca. Mga

AB-1

SPX-4

SPX-7

SPX-9

SPX-10

38.71 1.63 20.82 20.48 0.45 10.26 0.02 5.75 0.37 0.02 98.67% 40 44 16 0.48

40.61 0.48 21.68 16.49 0.45 16.04 0.36 2.82 0.19 0.06 99.25% 59 34 07 0.63

40.63 0.80 22.32 17.34 0.39 15.46 0.32 3.34 0.19 0.06 100.54% 56 35 09 0.62

39.71 0.52 21.73 18.58 0.36 15.84 n.a. 2.68 0.24 0.07 99.76% 56 37 07 0.60

39.61 0.54 21.80 18.42 0.36 15.64 n.a. 2.80 0.25 0.07 99.54% 56 37 07 0.60

41.12 0.51 21.18 16.08 0.36 16.88 0.15 2.48 0.51 0.01 99.31% 61 33 06 0.65

54.90 0.74 13.79 5.16 0.14 7.76 0.05 9.91 7.30 0.05 99.80% 0.44 0.17 0.40 0.72

55.33 0.78 10.08 6.69 0.14 10.90 0.07 8.52 6.03 0.07 98.63% 0.53 0.18 0.29 0.75

55.54 0.99 11.08 5.62 0.13 10.14 0.03 7.47 6.85 0.13 98.11% 0.54 0.17 0.29 0.76

54.88 0.69 9.33 6.86 0.13 11.98 0.02 8.48 6.03 0.03 98.40% 0.55 0.18 0.28 0.76

55.44 0.68 10.05 6.25 0.13 11.36 0.02 8.41 6.31 0.03 98.67% 0.54 0.17 0.29 0.77

54.60 078 9.56 5.38 0.16 11.44 0.14 8.69 6.38 0.04 98.02% 0.55 0.15 0.30 0.79

SPX-3a 56.62 0.05 0.79 10.48 0.13 31.37 0.34 n.a. n.a. 0.15 100.34% 0.84 q 0.01

SPX-3b 57.99 0.06 0.33 5.34 0.08 36.28 0.17 n.a. n.a. 0.16 100.59% 0.925 q 0.005

K-Richterite SPX-3 53.76 0.40 2.39 4.75 0.04 20.26 3.93 4.21 5.67 n.a. 95.41% 0.88

Orthopyroxene SiO 2 TiO 2 Al 2 O 3 FeOU MnO MgO CaO Na 2 O K 2O Cr2 O 3 Total Mga

SPX-4 54.56 0.19 2.25 12.82 0.12 27.85 0.68 0.61 - 0.01 0.15 99.26% 0.79 q 0.01

SPX-3

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Fig. 6. Ža. Cation proportions in garnet as a function of the amount of peridotite added to the sample capsule, for slab melting experiment Ž0% peridotite. and assimilation experiments with both depleted peridotite AVX-51 ŽDP and filled symbols. and fertile peridotite ŽFP and open symbols.. Žb. Cation proportions in clinopyroxene at the top of the residual eclogitic assemblage, as a function of the amount Žwt.%. of peridotite added to the sample capsule, for assimilation experiments with both depleted ŽDP and filled symbols. and fertile peridotite ŽFP and open symbols..

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ment abundance patterns Ži.e., spider diagrams. parallel to those of pristine slab melts, with the primary differences between the two being reflected in their major element chemistry Že.g., Mga, ArCNK, SiO 2 content.. 3.3. Composition of crystalline phases The major-element composition of crystalline phases in the slab melting and peridotite assimilation experiments are listed in Table 4. Garnet is present in the eclogitic residue of melting, and in the reaction zone of the assimilation experiments. Garnet produced during adakite melt-peridotite reaction, with either fertile or depleted compositions, becomes progressively more enriched in pyrope component ŽMg., and poorer in both grossular ŽCa. and almandine ŽFe. components as the proportion of peridotite added to the sample charge increases ŽTable 4 and Fig. 6a.. Thus the primary coupled substitutions involve exchange of Fe and Ca for Mg. Garnet in the assimilation experiments also has notably lower concentrations of minor components TiO 2 and Na 2 O relative to garnet in the slab melting experiment. Clinopyroxene in the assimilation experiments is restricted to the eclogitic residue of melting; clinopyroxene crystals in the upper portion of the eclogite layer show evidence for re-equilibration with the more magnesian liquids present at this level in the sample capsule, with slightly elevated Mga’s, higher MgO contents, and lower CaO, FeOU and Na 2 O contents relative to clinopyroxene in the straightforward slab melting experiment ŽTable 4 and Fig. 6b.. Clinopyroxene in the bottom half of the charge is compositionally equivalent to clinopyroxene in the basalt melting experiment. In contrast, clinopyroxene near the reaction zone becomes less ‘‘eclogitic’’ and more ‘‘peridotitic’’ in character as the proportion of peridotite added to the charge increases Ži.e., as the melt:rock ratio decreases; see Fig. 6b., and thus again the dominant substitution involves exchange of Fe and Ca for Mg, coupled with slight decreases in jadeite components Na and Al. As with garnet, only minor differences exist between residual clinopyroxene in assimilation experiments with the depleted peridotite, and clinopyroxene in assimilation experiments with the fertile peridotite. Orthopyroxene is present in the reaction assemblage of two experiments with the depleted peri-

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dotite: SPX-3, which originally contained 45% peridotite AVX-51, and SPX-4, which originally contained 16% AVX-51. Orthopyroxene in SPX-4 is enriched in ferrosilite ŽFe. component, with lower Mgas ŽMga s 0.79. and higher CaO Ž0.68 wt.%. and Al 2 O 3 Ž2.25 wt.%. contents ŽTable 4., than orthopyroxene in harzburgite and spinel lherzolite xenoliths from the upper mantle. Two distinct orthopyroxene phases are present in experiment SPX-3. The first is ‘‘metasomatic’’ orthopyroxene, with somewhat higher Mga’s Ž0.84., and lower CaO Ž0.34 wt.%. and Al 2 O 3 Ž0.79 wt.%. contents than orthopyroxene in experiment SPX-4, and which presumably formed by reaction of the original peridotite assemblage with adakitic melt. The second is orthopyroxene with original, ‘‘peridotitic’’ characteristics, including high Mga Ž0.93. and low CaO Ž0.17 wt.%. and Al 2 O 3 Ž0.33 wt.%. contents, which forms the cores of the metasomatic orthopyroxene Žsee Fig. 2d.. In addition to orthopyroxene, Na- and K-rich amphibole ŽK-richterite. also forms as a result of reaction between adakitic melt and depleted peridotite at low melt:rock ratios Ži.e., approximately 1:2, as in experiment SPX-3.. Similar results were reported by Sen and Dunn Ž1994b. in melt infiltration experiments at 1.5–2.0 GPa, in which pargasitic amphibole formed during modal metasomatism of a model depleted peridotite by adakitic melts. The presence of metasomatic amphibole and original, peridotitic orthopyroxene in the reaction zone of SPX-3 suggests that the melt:rock ratio in this experiment was too low for complete assimilation to take place. Thus, for the series of experiments with the depleted peridotite ŽAVX-51., a full range of melt:rock reactions is observed, from those dominated by assimilation and melt hybridization, to those involving modal Žand possibly cryptic. metasomatism of the peridotite, depending upon the melt:rock ratio.

4. Discussion 4.1. The ‘‘effectiÕe’’ melt:rock ratio Vague reference has been made to the term ‘‘effective’’ melt:rock ratio, which in our experiments

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we implicitly equate to the weight proportions of adakite liquid produced by melting of the basalt, and the amount of peridotite added to the charge. In subduction zones, defining the effective melt:rock ratio is much more problematic, since it will be dependent on such variables as the rate of melt generation and segregation in the slab, the rate and mechanism of melt infiltration and migration through the mantle, the nature and extent of the melt:rock reaction surface, and the thermal regime in the vicinity of the slab–wedge interface. Thus it is difficult to know how the interplay of these factors in nature will act to define the effective melt:rock ratio during slab melting. Our experimental results are representative of a range of possible effects, from near-complete dissolution of peridotite Žat high effective melt:rock ratios., to ‘‘arrested’’ modal metasomatic reactions in which the melt exhausts itself in consuming peridotitic olivine and orthopyroxene Žat low effective melt:rock ratios.. Since we are equating the melt:rock ratio in our experiments to the effective melt:rock ratio, the range of reaction assemblages observed in the experiments should be representative of the kinds of reactions that would be expected to take place at the slab–wedge interface in response to slab melting. The sequence in which minerals appear in the reaction zone of our experiments, and the evolution of the hybridized melt as more peridotite is added, are wholly in accord with the results of Sekine and Wyllie Ž1982a,b., who conducted a series of mixing experiments in the synthetic and natural systems granite–peridotite–H 2 O at 3.0 GPa, and Johnston and Wyllie Ž1989., who carried out similar experiments, also at 3.0 GPa, in the natural system tonalite–peridotite–H 2 O. Our results from assimilation experiments with both fertile and depleted peridotites suggest that the position of the orthopyroxene saturation surface described by Johnston and Wyllie Ž1989. for the system tonalite–peridotite–H 2 O is composition-dependent, since at the same melt:rock ratio, orthopyroxene is present in experiments with the depleted peridotite, but absent in experiments with the fertile peridotite. Presumably, reaction between slab melts and fertile peridotite would ultimately result in the precipitation of orthopyroxene from high-Mga, hybridized melts, if the melt:rock ratio drops below 2:1. By comparison, Johnston and

Wyllie Ž1989., who used a mylonitic peridotite similar in composition to KLB-1 in their mixing experiments, found that hybridized tonalitic melts at 3 GPa and 11008C were saturated in orthopyroxene at melt:rock ratios between 2.5:1 and 2:1. Results of Sekine and Wyllie Ž1982a. schematic study of the natural system granite–peridotite–H 2 O also suggest that clinopyroxene Žandror phlogopite. will be present in the reaction assemblage Žalong with garnet and orthopyroxene. at lower temperatures Ž900– 10008C. than those in our experiments, but it is important to emphasize that the presence or absence of these phases will also depend upon the effective melt:rock ratio Ži.e., in the experiments of Wyllie and coworkers, the weight proportion of tonalite relative to peridotite.. All the experimental studies cited thus far are in accord with Sekine and Wyllie’s initial contention that olivine ‘‘does not appear among the hybrid products’’ resulting from reaction between peridotite and high-SiO 2 liquids. Saturation of hybridized slab melts in orthopyroxene is the first indication that either the melt:rock ratio or temperature is too low to sustain the assimilation reaction, and other phases such as sodic amphibole ŽSen and Dunn, 1994b; and this study., clinopyroxene, and phlogopite ŽSekine and Wyllie, 1982b. will precipitate. Our experiments suggest that slab melts will be fully consumed by melt:rock reactions if the effective melt:rock ratio is below approximately 1:1, unless additional heat is supplied to the system ŽJohnston and Wyllie, 1989; Kelemen, 1995.. 4.2. Hybridized slab melts and high-Mga adakites Our experimental results for basalt AB-1 can be taken to be representative of partial melting of hydrothermally-altered MORB, and as such they provide constraints on the nature and extent of element fractionation that would accompany slab melting in subduction zones. Assuming that a given MORB composition with a hydrous, eclogitic mineralogy undergoes 25–30% melting, the trace element characteristics of the liquid can be estimated by reference to Table 5 and Fig. 4a and b. Insofar as melts of basalt AB-1 are just as representative of slab melts as melts of any other MORB-like composition would be, in terms of element fractionation relative to the

R.P. Rapp et al.r Chemical Geology 160 (1999) 335–356 Table 5 Enrichment or depletion factorsa for generic slab melt and eclogite residue Element

Melt

Residue

Cs Rb Ba U Th K Nb La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Y Yb Lu Sc

3.51 3.71 2.43 3.57 2.97 4.49 1.54 7.86 2.56 2.58 1.61 3.20 2.97 1.23 0.936 1.30 0.640 0.317 0.230 0.330 0.195 0.340 0.320 0.203

0.023 0.020 0.445 0.020 0.233 0.050 0.789 0.380 0.390 0.277 0.762 0.140 0.235 0.913 1.02 0.885 1.14 1.27 1.30 1.26 1.31 1.26 1.26 1.31

a

Relative to bulk composition of any given basalt, assuming 30% melting Žby weight., and a residue consisting of 42% garnet, 27% clinopyroxene and 1% rutile.

bulk basalt, our pristine experimental slab melts, as well as their hybridized counterparts, can legitimately be used to make inferences about the origin of adakite arc lavas in modern-day subduction zones. This is so because we are emphasizing the relatiÕe effects of the hybridization–assimilation reaction on melt composition. The extent to which the experimental slab melts resemble adakites in modern subduction zones can be assessed by consideration of Fig. 7, in which the primitive mantle-normalized trace element abundance patterns of the experimental liquids are plotted along with those of: Ž1. adakites from the Philippines ŽMga’ss 30–60; Sajona et al., 1994. and HMAs from the western Aleutians ŽMga’ss 70–72; Yogodzinski et al., 1995. ŽFig. 7a.; Ž2. adakites from Panama ŽDefant et al., 1991., melt inclusions in peridotite xenoliths from the Philippines arc ŽSchiano et al., 1995., and adakite veins in peridotite xenoliths

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from the Kamchatkan arc ŽKepezhinskas et al., 1995. ŽFig. 7b.; Ž3. low-Mga Ž48–53, Mt. Burney. and high-Mga Ž64–69, Cook Island. adakites from the southern Andes ŽStern and Killian, 1996. ŽFig. 7c.. Although subtle differences in the composition, mineralogy, and mode, of any putative slab source should be expected, the overall shape of the patterns in Fig. 7 are remarkably similar, suggesting a common petrogenetic bond. Both the adakites and the experimental melts are, with few exceptions, marked by high ThrU, LarNb, KrLa, SrrNd, LarYb, and SrrY ratios Žsee also Fig. 5.. Significant discrepancies between the experimental melts and subduction-related adakites are limited for the most part to Zr and Ti ŽFig. 7a–c.. In the experimental melts, Zr is enriched relative to neighboring Nd and Sm ŽFig. 7a., but can be either relatively enriched ŽFig. 7b and c. or depleted ŽFig. 7b,c. in the adakite populations, which show a broad, continuous range of Zr abundances. The experimental melts are also slightly enriched or depleted in Ti relative to neighboring Eu and Gd, whereas almost all the adakites possess a prominent depletion in Ti. We attribute these depletions in Ti and Zr in adakites to residual accessory mineral phases in the melting slab. The relative depletion of Zr is likely to be due to buffering of the melt in accord with the saturation systematics of zircon in the melt ŽWatson and Harrison, 1983.. The distinct negative anomaly in Zr for some of the adakites in Fig. 7 is likely due to zircon saturation in the slab source, with the broad range of Zr abundances reflecting the strong temperature-dependence of zircon solubility in slab melts. Adakites with positive Zr anomalies were probably undersaturated with respect to zircon in the residual slab. In a related way, Ti abundances in slab melts at pressures above 2.0–2.5 GPa may be buffered by the presence of rutile in their crystalline residues ŽFig. 7c., or by amphibole at lower pressures ŽFig. 7c.. A strong temperature dependence to rutile solubility in partial melts of AB-1 and other basalts at 3.2 GPa was noted by Rapp and Watson Ž1995., and Ti concentrations in these liquids are at levels slightly less than three times the primitive mantle value ŽFig. 7c.. More significant is the fact that Na-rich, high-SiO 2 Ži.e., adakitic. melts in equilibrium with garnetamphibolite residues at 800–9008C and 1.5–2.4 GPa Že.g., Winther and Newton, 1991; Sen and Dunn,

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Fig. 7. Mantle-normalized trace element abundance patterns of experimental pristine Žopen circles. and hybridized slab melts Žsolid circles for melts hybridized by the depleted peridotite; solid squares for melts hybridized by the fertile peridotite., compared with Ža. adakites from the Philippines, with Mga’s ranging from 30 to 60 Žcrosses., and HMAs from the western Aleutians Žopen squares.. Data from separate SIMS and laser ablation ICPMS analyses of pristine and mantle-hybridized melts of AB-1 at 3.8 GPa are shown for comparison and to demonstrate inter-laboratory correlation. Mantle normalizing values are from Sun and McDonough Ž1989.. Note the broad variation in the abundances of individual elements, but the general parallelism of the correspondingly broad band of patterns that emerge when the data is viewed as a whole. Note also the broad but continuous variation in the abundances of Rb and Ba in the Philippine adakites. Žb. Mantle-normalized trace-element abundance patterns of pristine and mantle-hybridized slab melts compared with patterns for adakites from Panama ŽDefant et al., 1991, solid lines with solid dots., adakite melt inclusions in peridotite xenoliths from the sub-arc mantle beneath the Philippines ŽSchiano et al., 1995, heavy dashed line., and an adakite vein in a peridotite xenolith from beneath the Kamchatkan arc ŽKepezhinskas et al., 1995, heavy dashed line with open dots.. Note the wide range of Zr abundances in the xenolith samples: melt inclusions in the sub-arc mantle possess a slight positive Zr anomaly relative to Nd and Sm, whereas the adakite vein possesses a strong negative Zr anomaly. A broad range of Ti abundances is also apparent and the data for adakite veins, melt inclusions, and arc lavas considered can be a continuum. Žc. Spider diagrams comparing trace-element abundance patterns of pristine and mantle-hybridized slab melts with low-Mga adakites ŽMt. Burney., and high-Mga adakites ŽCook Island. from the southern Andes ŽStern and Killian, 1996.. Note the very distinct negative Ti anomaly in both types of adakites: shown for comparison is the range of Ti concentrations in rutile-saturated slab melts at 3–4 GPa at temperatures from 1075 to 11008C Ždownward pointing arrow above the horizontal bar at approximately 3 = the primitive mantle abundance of Ti., and the range of Ti concentrations in slab melts coexisting with amphibole-bearing eclogitic residues at 1.5–3.2 GPa and 800–10008C Ždownward pointing arrow below horizontal bar. ŽTi data are from Winther and Newton, 1991; Sen and Dunn, 1994a; Rapp, 1995..

1994a; Rapp, 1995. have TiO 2 concentrations between one and three times the primitive mantle value. Thus, Ti concentrations in either rutile- or amphibole-buffered slab melts span the entire range encompassed by relatively Ti-depleted adakites. Slightly lower degrees of melting Ži.e., - 25–30%., at lower temperatures, of a rutile- andror amphi-

bole-bearing eclogitic source would account for ‘‘pristine’’ slab melts with trace element abundance patterns much the same as those for the melt of basalt AB-1 in Fig. 7, but with a significant relative depletion in Titanium would be concentrated in the residue due to its partitioning into amphibole ŽKlein et al., 1997., andror rutile ŽRyerson and Watson,

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1987.. Variable enrichment in alkali elements Cs, Rb and Ba in the natural adakite samples probably reflect variable extents of seafloor hydrothermal alteration of the basalt crust constituting the slab source Žsee discussion in Harper, 1984 on the spilitization of basalt AB-1.. Note, for instance, the correlation between adakites from Panama ŽFig. 7b. and our experimental slab melts in terms of these elements. 4.3. Adakite-induced mantle metasomatism The data for HMAs from the western Aleutians and Japan, and high-Mga adakites from the Austral Andes in Fig. 1 suggest that, because their Mga’s are too high, these lavas are not derived from melting of a basaltic, crustal source. In fact, Hirose Ž1997. has demonstrated that such HMAs can be derived from melting KLB-1 peridotite at 1.0 GPa and temperatures less than 11008C, although these results are presently under debate. Regardless, western Aleutian HMAs possess trace element abundance patterns that are virtually identical to those of

adakites, for example, from the Philippines ŽFig. 7a. and Burney volcano in the southern Andes ŽFig. 7c.. This suggests a hybrid process in which sub-arc mantle is metasomatized by adakitic slab melts, and the slab melt signature is transferred, modally or cryptically, to the future mantle source of HMAs. Although the mantle mineralogy adopted by metasomatizing adakite melts will control the overall composition of subsequent, mantle-derived melts, the incompatible element-rich nature of the slab-derived melt is retained, either in amphibole-forming, modal metasomatic reactions, as observed in experiment SPX-3, and in similar experiments at lower pressures Ž1.5–2.0 GPa. by Sen and Dunn Ž1994b., or by cryptic metasomatism of peridotitic mantle. An indication of the more subtle metasomatic effects that slab-derived, adakite melts might have on the composition of the sub-arc mantle is illustrated in Fig. 8, which shows the abundances of selected trace elements Žmeasured by SIMS., in garnet from both the slab melting experiment ŽAB-1., and from the melt-rock reaction zone of assimilation experiment

Fig. 8. Abundances of selected trace elements in garnet from slab melting experiment at 3.8 GPa ŽAB-1., and garnet from peridotite assimilation experiment SPX-7 Žwith 10% depleted peridotite AVX-51.. Also shown for comparison are the abundances of these elements in the ‘‘pristine’’ slab melt at 3.8 GPa.

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SPX-7. These data suggest that garnet formed in reactions between adakite melts and depleted mantle peridotite adopts the character of residual garnet from slab melting. In each case, garnet reflects the compositional characteristics of the melt with which it equilibrated.

5. Conclusions Our experimental results allow the following inferences to be made. Ž1. The hybridization of slab melts by reaction with peridotite produces high-Mga adakitic liquids, in reactions which form Mg-rich garnet and orthopyroxene at the expense of olivine. Some of the initial melt is also consumed in these reactions, causing abundances of incompatible trace elements to increase, but these assimilation reactions cause very little or no additional fractionation of trace elements. Many high Mga adakites may thus represent hybridized slab melts. Ž2. Only limited assimilation can occur without the input of additional thermal energy. If the ‘‘effective’’ melt:rock ratios drops below ; 1:1, slab melts will be completely consumed in metasomatic reactions with peridotitic mantle which form Na-amphibole and consume olivine. The ‘‘effective’’ melt:rock ratio will determine the nature of the assimilationr hybridization reaction and the mineralogy of the resulting reaction assemblage. Ž3. The sub-arc mantle, modally andror cryptically metasomatized by slab-derived melts, is the probable source of high-Mga andesites with traceelement abundance patterns parallel to those of adakites.

Acknowledgements The authors thank Steve Foley and Dana Johnston for their thoughtful and constructive reviews; the patience of the guest editors is especially appreciated. This research was supported by National Science Foundation grant aEAR89-20239 to the NSF Science and Technology Center for High Pressure Research and the Mineral Physics Institute at Stony

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Brook, and by National Science Foundation grant aEAR-9706517 to R.P.R. MDN acknowledges support from the Australian Research Council.

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