The redox state of subduction zones: insights from arc-peridotites

Chemical Geology 160 Ž1999. 409–423 www.elsevier.comrlocaterchemgeo

The redox state of subduction zones: insights from arc-peridotites Ian J. Parkinson ) , Richard J. Arculus National Key Centre for Geochemistry and Metallogeny of the Continents, Department of Geology, Australian National UniÕersity, Canberra, ACT 0200, Australia Received 1 August 1998; accepted 12 November 1998

Abstract Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities Ž fO 2 values. calculated from the ferric iron content of spinels, measured by Electron Microprobe ŽEMP. using secondary standards wWood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, contents of lherzolite spinels by 57 Fe Mossbauer ¨ 53, 1277–1291.x, yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz ŽFMQ. buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between fO 2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between fO 2 and degree of depletion as indicated by spinel Cra, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between fO 2 and spinel Cra suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe 2 O 3. This amount of Fe 2 O 3 in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H 2 O is required which is considerably less than the 0.25% H 2 O envisaged by Stolper and Newman wStolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.x, suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in fO 2 of the source region. Although it is unclear which model is correct the maximum fO 2 of the fertile mantle wedge is unlikely to be above FMQq 2 and therefore some decompression melting in the mantle wedge is required to explain the higher fO 2 values of primitive arc lavas than arc-peridotites. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Redox state; Mantle wedge; Arc-peridotites

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Corresponding author. Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. Tel.: q44-01908-654296; fax: q44-01908-655151; e-mail: [email protected] 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 1 0 - 2

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I.J. Parkinson, R.J. Arculusr Chemical Geology 160 (1999) 409–423

1. Introduction The oxygen fugacity Ž fO 2 . of the mantle is an important parameter in determining such features as sub- and super-solidus phase relationships, the nature of volatile species, trace element partitioning, diffusivity, electrical conductivity and mechanical behaviour ŽArculus, 1985.. Moreover, oxygen fugacity varies by over nine orders of magnitude in terrestrial rocks, a greater relative variation than any other parameter such as temperature, pressure or composition. The last 10 years has seen advances in our understanding of mantle fO 2 because of improvements in the calibrations which are used to calculate fO 2 . For spinel-bearing peridotites, two calibrations, one by Wood et al. ŽNell and Wood, 1991; Wood et al., 1990. and one by Ballhaus et al. Ž1991., essentially yield the same calculated fO 2 for equilibria involving olivine, orthopyroxene and spinel. The only essential input for these calculations is an accurate and precise determination of the ferric iron content of spinel. Mossbauer spectroscopic determi¨ nations of Fe 3qrÝFe ratios in spinel or more conveniently electron microprobe ŽEMP. determinations of this ratio utilising well characterised spinel standards Žsee Wood and Virgo, 1989. allow accurate calculation of fO 2 in mantle peridotites and primitive lavas containing Cr–spinel. Detailed oxygen thermobarometric studies have been carried out on abyssal peridotites ŽBryndzia and Wood, 1990., continental lithospheric mantle samples ŽWood and Virgo, 1989; Ionov and Wood, 1992., peridotite massifs ŽWoodland et al., 1992. and ocean island xenoliths ŽAmundsen and Nuemann, 1992.. Ballhaus Ž1993. has also carried out a detailed overview of fO 2 values in a global data set of peridotites and primitive lavas. By contrast, only a limited number of studies have been carried out on mantle samples from subduction zones ŽWood and Virgo, 1989; Brandon and Draper, 1996; Johnson et al., 1996.. This in part reflects the general paucity of mantle samples from this tectonic environment. In this paper, we present calculated fO 2 values from an extensive collection of mantle peridotites from a wide variety of arcs. These include mantle xenoliths from the volcanic front, drilled peridotites from forearc regions and exposed peridotites in island arc systems.

Although it has long been known that subduction zones lavas are generally oxidised ŽOsborn, 1959; Wood et al., 1990; Carmichael, 1991; Arculus, 1994. little is known about the spatial distribution in the redox state of the mantle wedge or more importantly whether fO 2 can be related directly to melting processes in the mantle wedge. Water plays a key role in the generation of magmas in the mantle wedge ŽDavies and Bickle, 1991; Davies and Stevenson, 1992; Pearce and Parkinson, 1993; Arculus, 1994; Stolper and Newman, 1994.. Water has also been suggested as the agent for the oxidised nature of the mantle wedge ŽArculus, 1994; Farley and Newman, 1994; Brandon and Draper, 1996; Frost and Ballhaus, 1998. and as the carrier for so-called fluid mobile elements whose enrichment relative to midocean ridge basalts ŽMORB. is a characteristic of island arc basalts ŽIAB. ŽPearce, 1983; Hawkesworth et al., 1993.. Obviously, there may be a strong interdependence on the amount of melting, oxidation and fluid-mobile element enrichment as has been suggested by some workers ŽMattioli et al., 1989; Ballhaus, 1993.. In this study, we provide data on the relationship between redox state, melting and melt reaction. We also present some model calculations which may explain the redox state of the mantle wedge and provide insights into melting processes in the mantle wedge.

2. Sample localities and data sources 2.1. Ichinomegata Peridotites from Ichinomegata are perhaps some of the best-studied arc-peridotites. Data used in this study includes analyses of two peridotites from our sample collection plus previous data described in ŽTakahashi, 1980; Wood and Virgo, 1989; Johnson et al., 1996.. All the samples are spinel lherzolites or harzburgites that contain amphibole" minor phlogopite and glass patches. 2.2. MarelaÕa Marelava is a volcano located in the northern section of the Vanuatu arc. Four xenoliths from this island were the subject of a previous study by Bars-

I.J. Parkinson, R.J. Arculusr Chemical Geology 160 (1999) 409–423

dell and Smith Ž1989.. These data are supplemented by new data for a refractory spinel harzburgite, and a wehrlite tectonite. 2.3. Grenada Grenada is the southern most island in the Lesser Antilles arc. Peridotite xenoliths are located in scoria cones at three localities across the island and our samples come from Queens Park. The peridotites consist of spinel harzburgite, dunite and reacted harzburgite that contain wehrlitic reaction patches. A detailed petrographic and geochemical study of these xenoliths is presented elsewhere ŽParkinson et al., 1999.. 2.4. Izu-Bonin–Mariana forearc Serpentinite seamounts are located along the length of the Izu-Bonin–Mariana forearc system. Two of these seamounts, Torishima Forearc Seamount in the Izu-Bonin forearc and Conical Seamount in the Mariana forearc were drilled by ODP Leg 125. The peridotites are highly depleted spinel harzburgites with subsidiary dunites. Harzburgites from Torishima Forearc Seamount contain amphibole whereas at Conical Seamount, amphibole and phlogopite is restricted to dunites. A detailed petrographic and geochemical study of these forearc-peridotites is presented elsewhere ŽParkinson and Pearce, 1998..

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2.6. Simcoe Simcoe is located at Lorena Butte, WA, USA, 65 km behind the main Cascade arc. Spinel harzburgites and phlogopite bearing-wehrlite xenoliths have been the subject of detailed petrographic ŽDraper, 1992. and oxygen thermobarometric studies ŽBrandon and Draper, 1996..

3. Analytical techniques Spinel and silicate phases were analysed by electron microprobe using an energy dispersive system on a Camebex Microbeam at the Research School of Earth Sciences ŽRSES. at the ANU, with an accelerating voltage of 15 kV and a sample current of 5 nA, following the techniques of Ware Ž1991.. Further analyses were undertaken on a JOEL microprobe at the Research School of Biological Sciences ŽRSBS.. Spinel standards were run at the beginning and occasionally at the end of each analytical session. No obvious difference in the analyses of the standards was observed over the period of each session but significant long term variations in the standards exist which relate to calibration and machine differences Žsee below..

4. Methodology 2.5. Solomon Islands Mantle peridotites are exposed throughout the Solomon Islands in the SW Pacific. These peridotites have been exposed because of the collision of the Ontong Java Plateau ŽOJP. with the Solomons arc. Contrary to some perceptions in the literature ŽJohnson et al., 1996. many of the peridotites in the Solomon Islands are not arc-related but rather are derived from obduction of the leading edge of the OJP. However, two suites of peridotites located on the islands of Santa Isabel and San Jorge have petrographic, mineralogical and geochemical features that indicate they are residues to melting in an arc-system. Peridotites from Santa Isabel are spinel harzburgite and clinopyroxene-poor lherzolites some of which contain pargasitic amphibole.

4.1. Presentation of data Oxygen fugacities throughout this paper are presented relative to the fayalite–magnetite–quartz ŽFMQ. buffer Ž Dlog fO ŽFMQ.. at a specified 2 temperature and pressure based on geothermobarometry. In practice, this renders the calculated fO 2 values virtually independent of pressure and temperature over the range of temperatures derived using the geothermometers. A variation of 1008C in temperature corresponds to a change in fO 2 of 0.2 log units whereas a variation in pressure of 1 GPa corresponds to a change in fO 2 of by 0.3 log units ŽWood et al., 1990.. Temperatures are calculated using the Fe–Mg olivine–spinel exchange ther-

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412

mometer of Ballhaus et al. Ž1991. although temperatures using the two pyroxenes thermometer of Brey Ž1990., and orthopyroxene–spinel– and Koehler ¨ olivine thermometer of Witt-Eickschen and Seck Ž1991. are calculated for comparison. For the xenolith samples, there is a good correspondence between these thermometers. By contrast, for the exposed peridotites, the thermometer of Ballhaus et al. Ž1991. always gives lower equilibration temperatures than the other thermometers because of the lower blocking temperature for this exchange reaction. However, these variations in temperature do not affect the conclusions we reach because during cooling, the fO 2 values follow a trend essentially parallel to the FMQ buffer Žsee Woodland et al., 1996.. The lack of a suitable barometer for spinel peridotites means that an arbitrary pressure of 1 GPa has been used in the calculations because many of the peridotites are depleted and exposed with island arc systems and hence will have equilibrated at low pressure. 4.2. Calculating oxygen fugacities and their errors Oxygen fugacities can be readily calculated from a variety of reactions between coexisting phases in peridotites. The most commonly used and best calibrated is the heterogeneous equilibria: 6Fe 2 SiO4 q O 2 s 3Fe 2 Si 2 O6 q 2Fe 3 O4 ol

opx

troscopy or alternatively use spinel standards which have well characterised Fe 3qrÝFe ratios Žby Mossbauer spectroscopy. to correct the EMP data. ¨ Wood and Virgo Ž1989. utilise the systematic error that variation in the Al 2 O 3 content of spinel has on the calculated Fe 3qrÝFe ratios as the basis for a correction scheme for EMP analyses. By plotting the Cra Žatomic CrrŽCr q Al.. vs. Fe 3qrÝFe Žtrue . minus Fe 3qrÝFe Žcalculated value by Mossbauer ¨ from EMP analysis. for a series of standards, they found that a linear relationship exists of the form: 3q Fe 3qrÝFe Moss ¨ y Fe rÝFe Probe

s A q B Crr Ž Cr q Al .

Ž 2.

where A and B are constants. The senior author has used the same set of spinel standards Ž8311, 8315, 8316 and 79-1 from Wood and Virgo, 1989. on several different EMP over a period of 5 years. Using secondary standards allows an effective correction of the spinel ferric iron contents and the production of a consistent data set of spinel analyses. To give an estimate of the potential errors in fO 2 calculations a series of error propagation calculations were undertaken ŽFig. 1.. A series of model spinel

Ž 1.

sp

between coexisting olivine Žol., orthopyroxene Žopx. and spinel Žsp.. Calibrations by Nell and Wood Ž1991. and Ballhaus et al. Ž1991. give fO ’s within 2 0.2–0.3 log units of each other and are well suited to spinel peridotites containing Cr-spinel. Calculated fO 2 values using reaction Ž1. depend on accurate and precise determination of the ferric iron content of spinels. Wood and Virgo Ž1989. argue that although electron microprobe ŽEMP. analysis is potentially precise enough for fO 2 calculations it is not accurate enough. This is because the ferric iron content of spinel is calculated using charge balance and assuming AB 2 O4 stoichiometry and is therefore very sensitive to small errors in the elements which make up the bulk of the analyses particularly errors in Al 2 O 3 ŽWood and Virgo, 1989.. One way to circumvent these problems is to directly measure the ferric and ferrous iron content of spinels using Mossbauer spec¨

Fig. 1. Plot of Dlog fO 2 ŽFMQ. vs. Fe 3qrÝFe ratio in spinel for spinels in equilibria with Fo 90 olivine at 12008C and 1 GPa. The calculated fO 2 value is bound by an error curve which represents the propagated effect of a "1.5% error in the R 2 O 3 content of the spinels on calculated fO 2 . The calculation indicates that the same relative error in recalculating ferric iron produces large errors in the calculated fO 2 at reduced oxygen fugacities.

I.J. Parkinson, R.J. Arculusr Chemical Geology 160 (1999) 409–423

compositions with varying ferric iron contents were taken and their analyses recalculated assuming a "1.5% relative error in their R 2 O 3 contents. Oxygen fugacities are then calculated using fixed olivine and orthopyroxene compositions ŽFo 90 and En 91 .. The model calculation is presented in Fig. 1 as a plot of Dlog fO 2 ŽFMQ. vs. Fe 3qrÝFe ratio. A constant relative error in R 2 O 3 propagates to a wide range in errors in calculated fO 2 ŽFig. 1.. At reducing fO 2 values below FMQ-2 log units the propagated errors are large and dominate the calculation of fO 2 . However, at oxidizing fO 2 values propagated errors relating to the determination of ferric iron from stoichiometry are insignificant. Therefore, we can use literature data for more oxidised spinels with some confidence. An error of "0.5 log units is applied to these data which reflects errors in determining the composition of the silicate phases and the inherent error in the calibrations Žsee Wood et al., 1990.. 4.3. Model calculations We lack ferric iron contents for the silicate phases in our peridotites so that we do not have an accurate estimate of their bulk Fe 2 O 3 contents. Using the mineral data in ŽCanil and O’Neill, 1996., it is possible to make an estimate of the ferric iron content of clinopyroxene and orthopyroxene by using the well characterised Fe 3qrÝFe ratio of the spinels and deriving an empirical relationship with the Fe 3qrÝFe ratio of the pyroxenes. Canil and O’Neill’s data yield ŽFe 3qrÝFe cpx .rŽFe 3qrÝFesp . s 0.97 " 0.36 Ž2 s . and ŽFe 3qrÝFe opx .rŽFe 3qr ÝFe sp . s 0.31 " 0.20. This coupled with the rela3q tionship of Fe 3q opx rFe cpxs 0.62 " 0.04 derived by Canil and O’Neill Ž1996. allows a semi-quantitative calculation of the bulk Fe 2 O 3 contents of the peridotites. There are large uncertainties in the calculated Fe 3qrÝFe ratio of the pyroxenes. This problem is acute for orthopyroxene which can make up a significant part of the whole-rock ferric iron inventory. Therefore, although the calculations suggest that oxidised peridotites should have higher bulk Fe 2 O 3 contents than the mantle array defined by Canil et al. Ž1994. Žsee also Brandon and Draper, 1996., the errors on these calculations are too large to be used in any quantitative modelling.

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In order to make some predications about the redox state of the mantle wedge, we also undertook some model calculations using a simplified peridotite system. By utilising the Brey–Nickel–Koehler ¨ ŽBNK. two-pyroxene and Ca-in-opx thermometers of Ž1990., the orthopyroxene– Brey and Koehler ¨ olivine–spinel thermometer of Witt-Eickschen and Seck Ž1991., the Fe–Mg olivine–spinel thermometer of Ballhaus et al. Ž1991. and the oxybarometers of Wood et al. Ž1990. and Luth and Canil Ž1993. we can calculate the composition of minerals in equilibrium in the system SiO 2 –Al 2 O 3 –Cr2 O 3 –Fe 2 O 3 – FeO–MgO–CaO. To check the validity of these calculations, we also used the MELTS program ŽGhiorso and Sack, 1995; Hirschmann et al., 1998.. Our calculations predict very similar bulk Fe 2 O 3 contents Ža difference of 0.06 wt.% Fe 2 O 3 . for a given fO 2 to the MELTS program Žsee later.. The coexisting mineral compositions produced by our calculations are consistent with published data for mantle xenoliths at their given pressure, temperature and fO 2 . We can then predict the composition of melting residues at different fO 2 values and calculate the bulk Fe 2 O 3 content of the fertile mantle wedge.

5. Results 5.1. Oxygen fugacity data Results of our new spinel data are presented in Table 1. The new data along with literature data is presented in Fig. 2. To assess whether there is a relationship between the amount of melt extracted from the peridotites and oxygen fugacity, the calculated fO 2 values are plotted against spinel Cra Žmolar CrrŽCr q Al... Cra is accepted as an indicator of melt depletion in the peridotites with increasing Cra indicating that the peridotite has undergone a higher degree of partial melting ŽDick and Bullen, 1984. although it can also be an indicator of melt reaction. Also plotted in Fig. 2a is the field for abyssal peridotites from Bryndzia and Wood Ž1990.. The fO 2 values of the arc-peridotites all lie above the FMQ buffer with values ranging from 0.3 to 2 log units above FMQ. The data form a broad array which has a weak positive correlation with Cra and lies above the abyssal peridotite field.

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Table 1 Summary of new spinel, olivine and orthopyroxene data and oxygen fugacity values. Oxygen fugacities calculated using the Nell–Wood calibration ŽNell and Wood, 1991.. Site fractions of Fe on the M1 and M2 sites in orthopyroxene are calculated following the method outlined in Wood et al. Ž1990.. Temperatures calculated using the olivine–spinel thermometer of Ballhaus et al. Ž1991. Sample

Locality

Spinel Cra

Spinel Fe 3qrÝFe

Olivine Fo

Opx M1 M2 X Fe X Fe

Temperature Ž8C.

fO 2 Dlog FMQ

SI 65B SI 84A SI 146A SI 68A SI 161D SI 108G SI 41CA RLS 69r81 V3 V2 HK 640

Solomons Solomons Solomons Solomons Solomons Solomons Solomons Solomons Vanuatu Vanuatu Japan

0.518 0.381 0.271 0.398 0.434 0.379 0.543 0.370 0.695 0.725 0.528

0.248 0.202 0.181 0.164 0.213 0.205 0.226 0.187 0.470 0.318 0.304

0.9029 0.9031 0.8755 0.9041 0.9064 0.9063 0.9088 0.9033 0.8637 0.8932 0.9047

0.0089 0.0091 0.0142 0.0088 0.0077 0.0079 0.0075 0.0083 0.0150 0.0078 0.0064

747 689 659 681 614 613 647 657 1000 893 944

1.16 0.94 0.53 0.52 1.39 1.20 1.34 0.87 0.99 1.35 0.74

A key point to make about the peridotites in this study is how much of their geochemistry can be ascribed to their being melting residues within the mantle wedge and how much of their geochemistry reflects interaction with melts migrating through the wedge. From the brief sample description provided, it is clear that many of the samples have interacted with migrating melts to varying degrees. However, detailed trace element studies of these peridotites suggest that the geochemistry of the ODP Leg 125 samples from Torishima Forearc Seamount, the primary mineral assemblage of the Grenada xenoliths, the Solomon Island peridotites and some of the Ichinomegata xenoliths and one of the Vanuatu xenoliths reflect an origin as residues to partial melting with minimal melt interaction ŽParkinson and Pearce, 1998; Parkinson et al., 1999; Parkinson et al. unpublished data.. Although a detailed discussion of the trace element contents is beyond the scope of this paper a negative correlation between spinel Cra and the heavy rare earth elements ŽHREE. and the overall low abundance of the HREE suggest a residual mantle origin for these peridotites. By contrast the reacted Grenada and Vanuatu xenoliths some of the Ichonomegata xenoliths and the some of the ODP Leg 125 samples from Conical Seamount have trace element and spinel chemistry suggestive of more extensive melt interaction Žsee below.. To assess how much the melt has been extracted from the residual peridotites the spinel composition, coexisting silicate mineral chemistry and whole-

rockrclinopyroxene trace element contents can be used Že.g., Parkinson and Pearce, 1998.. Using these criteria, the sample suites used in this study record a range in partial melting from - 5% in the Ichinomegata samples to ) 25% for the depleted Vanuatu and ODP Leg 125 peridotites. This range is consistent with estimates of the amount of partial melting expected for subduction zone systems ŽPlank and Langmuir, 1988; Davies and Bickle, 1991; Pearce and Parkinson, 1993; Stolper and Newman, 1994.. The data for the ODP Leg 125 peridotites have been separated to illustrate the effects of meltrmantle interaction in the shallow upper mantle. Data from Torishima Forearc Seamount ŽHoles 783A and 784A. have been interpreted as residues to IAB generation ŽParkinson and Pearce, 1998.. Accordingly, these data plot within the array defined by the arc data. By contrast, the data from Conical Seamount ŽHoles 778A, 779A, 780C., especially the data from Hole 779A define a positive trend from reduced harzburgites with Cra of 0.4 and fO 2 values- FMQ to dunites with Cra of 0.75 and fO 2 values of ) FMQ. A positive correlation between fO 2 and Cra has been interpreted by Ballhaus Ž1993. as an intrinsic effect of decompression melting whereby increased amounts of melting produce a relative increase in oxygen fugacity because mantle fO 2 is controlled by a sliding Fe 3qrÝFe buffer system. However, in this case we have ample petrographic and mineralogical data to argue that oxidation in these peridotites is directly related to meltrmantle

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415

thopyroxene to leave a spinel-bearing dunite which contains minor amounts of orthopyroxene, clinopyroxene and occasional amphibole and phlogopite. Our data for Grenada also indicates that meltrmantle interaction can cause oxidation. Spinels in the Grenada xenoliths which have reacted with the host lava all have increased ferric iron, TiO 2 and Cra’s and a concomitant increase in fO 2 of up to 0.5 log units relative to the primary peridotite assemblage. Data from both suites of peridotites indicate the efficacy of silicate melts as oxidising agents a point made by Amundsen and Nuemann Ž1992.. They are especially efficient when reacted with depleted peridotites because of their limited buffering capacity. It should also be noted that oxidation in these examples is not by simple conversion of Fe 2q to Fe 3q but by a net transfer reaction involving other elements. 5.2. The role of hydrous phases

Fig. 2. Ža. Plot of Dlog fO 2 ŽFMQ. vs. Cra in spinel for all the arc-peridotites described in the paper. A field for abyssal peridotites is also shown for comparison Žafter Bryndzia and Wood, 1990.. These data indicate subduction-zone mantle peridotites are ubiquitously oxidised relative to the oceanic mantle. The data set define a weak correlation between oxygen fugacity and Cra. Žb. Dlog fO 2 ŽFMQ. vs. Cra in spinel for peridotites from the Izu-Bonin–Mariana forearcs. These data indicate that samples from Torishima Forearc Seamount have fO 2’s comparable with their origin as residues to arc lavas. In contrast peridotites from the Conical Seamount define a trend of increasing Dlog fO 2 with increasing Cra in spinel with the most oxidised samples being dunites. These data are interpreted as the interaction and channelling of subduction zone melts through a reduced oceanic mantle.

interaction. The dunites are formed by focussing of melts into discrete channels. The melt dissolves or-

Hydrous phases have long been thought to be important in subduction zone magmagenesis because of the elevated water content of IAB. Amphibole occurs as a phenocryst phase in some arc-lavas ŽSisson and Grove, 1993. and cumulate nodules from arc-lavas ŽArculus and Wills, 1980.. As would be expected both amphibole and phlogopite occur in the some of the samples in this study. Amphibole and minor amounts of phlogopite are prevalent in the Ichinomegata peridotites. Amphibole was also found in some of the Solomon Island harzburgites and occasionally in the Izu-Bonin Forearc-peridotites. Phlogopite is a rare phase in the Simcoe samples and also occurs in one of the oxidised Mariana forearc dunites. By contrast no hydrous phases are present in either the Vanuatu or Grenada samples which have some of the highest calculated fO 2 values. Likewise, there is no obvious difference in fO 2 between Leg 125 peridotites which contain amphibole and those that do not. We can surmise that although hydrous phases are common in mantle peridotites from subduction zones there are not a prerequisite for elevated fO 2 values. Moreover, amphibole is highly uncommon in very depleted harzburgites suggesting that amphibole is unlikely to be a residual phase during mantle above subduction zones except possibly during the initial stages of melting Žsee Sisson et al., 1997..

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I.J. Parkinson, R.J. Arculusr Chemical Geology 160 (1999) 409–423

6. Discussion 6.1. Models of mantle wedge oxidation Although it is accepted that the mantle wedge above subduction zones is oxidised the exact process of relative oxidation are still controversial. Two models exist which are briefly summarised here and schematically illustrated in Fig. 3. Ballhaus Ž1993. and Ballhaus and Frost Ž1994. argue that the fO 2 of the asthenosphere is buffered at the CCO buffer. However when melting is initiated carbon is eliminated from the mantle and is controlled by a sliding Fe 3qrFe 2q buffer system. Ballhaus and Frost Ž1994.

calculate a value in the increase of fO 2 of 0.6 log units per GPa of decompression from the source region. Therefore, they argue that because arc lavas are have fO 2’s of FMQ q 2 log units the source region of IAB must lie between FMQ and FMQ q 1 if they are produced at pressures of ; 2.5 GPa. Importantly, this source region lies outside of the stability of graphite Žsee Fig. 3.. Although these observations seem logical in terms of phase equilibria, recent models for the generation of oxidised mantle in subduction zones have evoked a two stage process. Brandon and Draper Ž1996. argue that the moderately depleted harzburgites from Simcoe, western USA first lose ferric iron during partial melting Žbecause of its incompatibility. becoming reduced and are then re-oxidised by a water-rich component from the subducting slab. This model would suggest that the source region is relatively reduced Žand by inference has a low Fe 2 O 3 content. and that the oxidising agent traverses the mantle wedge to relatively shallow pressures in the wedge. We investigate these models and alternatives in light of our new data and model calculations. 6.2. Modelling constraints

Fig. 3. Schematic plot in pressure-Dlog fO 2 ŽFMQ. space of potential models for oxidation of the mantle wedge. The position of the graphite-saturated anhydrous ŽMcKenzie and Bickle, 1988. and hydrous ŽGreen, 1973. mantle solidii is determined by the CCO buffer at the requisite temperature and pressure ŽCCO buffer from Taylor and Green, 1989.. The two oxidation models shown are that of Brandon and Draper Ž1996. where initial melting involves reduction and is followed by post-melting oxidation Žopen arrows.. The second model involves source oxidation followed by melting Žthis study, solid arrows.. Note that the IAB source Žstippled. is outside of the stability field of graphite. The melting vector here is for some form of decompression melting Žsee Fig. 5, text and Ballhaus and Frost, 1994.. The vectors labelled melt, aFe 3O 4 and Fe 3qrÝFe indicate the effect of isochemically decompressing a melt Žsee Kress and Carmichael, 1991. or decompression melting of a the mantle with either the a Fe 3O 4 or Fe 3qrÝFe of the spinel held constant Žsee text and Ballhaus and Frost, 1994 for details..

Several key observations have to be reconciled in order to produce a consistent model for mantle oxidation and melting above subduction zones. Firstly, both the residues and melts produced in the mantle wedge have higher fO 2 values than peridotites from other tectonic settings. Secondly, the fertile mantle wedge has a bulk Fe 2 O 3 contents above that of the mantle trend defined by Canil et al. Ž1994.. Lastly, subduction melts have considerably higher Fe 2 O 3 contents than magmas from other tectonic settings consistent with their elevated fO 2 values. First, we shall consider the high ferric iron content of subduction zone magmas. The algorithms of Kilinc et al. Ž1983. and Kress and Carmichael Ž1988. can be used to calculate the ferric iron content of oxidised subduction zone lavas. Utilising good estimates of primitive arc lavas such as those from Vanuatu ŽEggins, 1993. yields calculated Fe 2 O 3 contents of 2.5–6.0 wt.% at 1 atm. This range of values is consistent with the measured ferric iron content of fresh primitive arc lavas ŽBallhaus, 1993.. If these

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melts are projected back to high pressures using the equations of Kress and Carmichael Ž1991. Fe 2 O 3 contents of 2.0–4.5 wt.% are attained. If such high values were generated by partial melting of a fertile mantle with a Fe 2 O 3 content of 0.3 wt.% and a bulk D-value of 0.1 as suggested by Canil et al. Ž1994. then subduction zone magmas would have to be generated by - 5% partial melting. This is considerably less than virtually every estimate of the amount of partial melting in arcs. Plank and Langmuir Ž1988. argue that similar degrees of melting occur in arcs as beneath MOR Ži.e., ) 15%., Pearce and Parkinson Ž1993. suggest 10–25% melting and Stolper and Newman Ž1994. argue for that 15–35% partial melting is readily accomplished in the mantle wedge. Even if ferric iron were partitioned perfectly into the melt phase this would still require - 10% partial melting. The most logical explanation for the elevated Fe 2 O 3 content of subduction zone melts is that the source has a higher Fe 2 O 3 content than 0.3 wt.%. If the a D-value of 0.1 is assumed approximately 0.8 wt.% Fe 2 O 3 is required in the source to produce the average Fe 2 O 3 content of oxidised subduction zone lavas Žsee also Arculus, 1994.. The effect of a decompression melting on the redox state of peridotites is now considered. Ballhaus and Frost Ž1994. calculate a value in the increase of fO 2 of 0.6–0.8 log units per GPa of decompression. Their calculation involves decompression of a fertile model peridotite with constant spinel Cra. We have performed calculations where spinel Cra and olivine Fo content are increased during decompression to model more accurately the effects of increasing partial melting. These calculations yield increases in fO 2 per GPa of decompression of 0.25 log units where a Fe 3 O 4 in the spinel is kept constant or 0.65 log units when Feq3 rÝFe of the spinel is kept constant during melting Žsee Ballhaus, 1993.. Utilising these two end member models, it is implicit that if the lowpressure depleted peridotites which are the basis of this study are projected back to depths of initial melting, then the fO 2 of their fertile source mantle is above FMQ and well above the CCO buffer. Ballhaus and Frost Ž1994. argue that decompression melting is a way of producing oxidised melts from a reduced source. However, this is only true if the melts stay in equilibrium with the source throughout the course of partial melting Ži.e., a pro-

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cess like batch partial melting.. If melts segregate from their source at depth and then ascent to the earth’s surface without significant interaction with the overlying mantle by a process such as melt channelling then what does the redox state of erupted magmas tell us about the source region of the magmas? Kress and Carmichael Ž1991. use their high pressure data on ferric–ferrous iron in magmas to calculate the effects of adiabatically and isochemically decompressing a magma. They find that magmas become ; 0.17 log units more reduced relative to the FMQ buffer for each GPa of decompression. Therefore magmas are very good probes of their sources or at least the oxygen fugacity of the peridotite they were in equilibrium with Žsee also Carmichael, 1991.. Utilising the above information, we can now make some useful predictions about the source region of subduction zone magmas. Firstly, it must contain higher Fe 2 O 3 contents than 0.3 wt.%. A range of 0.6–1.0 wt.% with an average of 0.8 wt.% is consistent with the Fe 2 O 3 content of arc lavas and the degrees of partial melting expected in the mantle wedge. Secondly, what is the fO 2 of the source region? This depends explicitly on the how melting in arc systems is envisaged. The alternatives are as follows and are illustrated in Fig. 4; Ž1.. Subduction zones lavas are produced at depth Ž; 2–3 GPa. and rapidly ascent through the mantle wedge with minimal interaction. Therefore the source region at depth is between 2 and 3 log units above the FMQ buffer because calculations indicate that isobaric melting has little effect of relative fO 2 . These values are higher than majority of the peridotites which are the basis of this study. Ž2. Melting is by decompression with the melt in equilibrium with the mantle. This will produce melts with fO 2’s similar to the residual peridotites at the end of the melting process. This implies the fO 2 of the source region will be between FMQ and FMQq 1 log unit. Ž3. Melting is by fractional decompression melting similar to that below mid-ocean ridges. Here, the source region will have a fO 2 between FMQ q 1 and FMQq 2 log units. The final fO 2 of the erupted magma depends on the pressure at which the melts are aggregated ŽFig. 4c.. Melting could also be a mixture of melting induced by water fluxed into the source region followed by a decompression melting event ŽPearce and

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Parkinson, 1993.. None of these models are compatible with that of Brandon and Draper Ž1996. because their model does not take account of the need to

have elevated Fe 2 O 3 contents of the source. We now discuss a variety of oxidation and melting models. 6.3. Mantle wedge oxidation and melting model Considerable evidence exists that water from the subducted slab plays an important role in the genesis of subduction zones related magmas ŽDavies and Bickle, 1991; Davies and Stevenson, 1992; Arculus, 1994; Stolper and Newman, 1994.. Therefore, water has been considered an obvious oxidising agent in subduction zones because water disassociates to form oxygen and hydrogen with the oxygen then oxidising ferrous iron to ferric iron, as long as hydrogen can escape the system. This reaction has previously explored as a possible redox reaction in the mantle by Canil et al. Ž1994. and specifically with respect to oxidation in the mantle wedge by Arculus Ž1994. and Brandon and Draper Ž1996. although its viability as an efficient oxidation reaction has been the focus of heated discussion ŽFrost and Ballhaus, 1998; cf. Brandon and Draper, 1998.. This debate focuses on whether hydrogen can efficiently diffuse away from reaction site. If it can, then water is potentially a very effective oxidising agent ŽBrandon and Draper, 1998., if it cannot, then water is an extremely poor oxidising agent ŽFrost and Ballhaus, 1998.. Surprising, only one study exists where a suite of subduction zone lavas has been analysed for water, ferricrferrous iron contents and trace elements. Farley and Newman Ž1994. analysed a suite of Lau Backarc Basin glasses and demonstrated that H 2 O contents and the concentration of fluid mobile large ion lithophile elements ŽLILE. correlate positively

Fig. 4. Plots illustrate three potential scenarios for oxidation and melting of the mantle wedge. Details of the solidus and melting vectors are the same as Fig. 3. In each model, the source is oxidised and then melted. The position of the oxidised source is set so that the melting model reproduces the range in oxygen fugacities of the IAB lavas. Ža. This model involves isobaric melting at depth followed by isochemical adiabatic decompression of the melt to the earths surface; Žb. This model involves decompressional batch melting with the melt and residue staying in equilibrium; Žc. This model involves decompressional fractional melting with separation of melt fractions throughout the melting column which are then aggregated at shallow pressures Žsee text for details..

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with calculated fO 2 values. This does not prove that water is the oxidising agent in the mantle wedge but is does strongly suggest that the oxidising agent is water-rich and carries many of the fluid mobile elements. Ballhaus Ž1993. also demonstrated that there is a positive correlation between the fluid mobile element Ba and calculated fO 2 for a dataset of primitive arc lavas. Therefore, the data from subduction zone lavas indicates that the oxidising agent in the mantle wedge is water-rich and carries the LILE. Recent geochemical studies ŽHawkesworth et al., 1997. suggests that the source of the LILE elements is from the dehydration of the subducting slab. Therefore, it is a reasonable to assume that the oxidising agent is also derived from the subducting slab. Using our modelling of a simplified peridotite system and the MELTS program, we can estimate how much oxidation the fertile mantle wedge undergoes to produce a bulk Fe 2 O 3 content of 0.6–1.0 wt.%. The results are presented in Fig. 5. The modelling suggests the source region has a f O 2 of 0.5–1.7 log units above FMQ. It is interesting to note that if disassociation of water is the means by which the mantle wedge is oxidised, then only 0.030–0.075 wt.% of H 2 O has to be added to the fertile peridotite

Fig. 5. Plot of calculated Dlog fO 2 ŽFMQ. vs. Fe 2 O 3 of the mantle wedge. The relationship is calculated following the methods outlined in the text. A calculated curve using the MELTS program ŽGhiorso and Sack, 1995; Hirschmann et al., 1998. is also presented indicating the similarities in the results.

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to produce 0.6–1.0 wt.% Fe 2 O 3 in source. This value is much less than the 0.25% H 2 O that Stolper and Newman Ž1994. calculate is needed to produce significant Ž25%. amounts of melting in the mantle wedge. The restriction of the source region to 0.5–1.7 log units above FMQ in this model also rules out the arc melting model illustrated in Fig. 4a whereby melting of a highly oxidised source at depth is followed by isochemical transport of the melt to the Earth’s surface. An alternative model to simple oxidation of the mantle wedge by water is the addition of a ferric-iron component from the slab. This could be as a melt from either subducted sediment or possibly the oceanic plate or by a solute-rich hydrous fluid. The complete miscibility between hydrous fluids and melts at high pressure means that a solute-rich hydrous fluid is the most likely candidate as an oxidising agent as has been suggested by Draper Ž1997.. The effect of adding a ferric-iron rich component to a peridotite would be to change the mineralogy of the mantle because it is net transfer reaction. The most likely minerals to be formed are either spinel if the melt is iron-rich or orthopyroxene or amphibole if the meltrfluid is silica and alkali rich. All of these minerals contain significant ferric iron contents but the formation of these minerals would not increase the fO 2 of the source by as much as simple oxidation by water. However, addition of only 0.3–0.7 wt.% Fe 2 O 3 will oxidise the mantle above the CCO buffer. Although it is difficult to determine which model is correct some firm conclusions can be reached from the arc-peridotite data and modelling calculations. Ž1. the source region has to be oxidised and have elevated Fe 2 O 3 contents before melting occurs in order to explain the elevated ferric iron contents of arc-lavas. Fig. 6 illustrates this schematically by plotting Fe 2 O 3 vs. MgO. Treating Fe 2 O 3 as an incompatible element with a bulk partitioning value of 0.1 ŽCanil et al., 1994. or by modelling oxidised melting more rigourously Žsee Section 4.3. predicts that the bulk Fe 2 O 3 content of arc-peridotites should be higher than the mantle array of Canil et al. Ž1994.. The limited data for Fe 2 O 3 contents of arcperidotites ŽBrandon and Draper, 1996. supports this assertion. Ž2. the mantle wedge is unlikely to be more oxidised than 2 log units above FMQ before melting. To produce the subduction zones melts with

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Fig. 6. Plot of Fe 2 O 3 vs. MgO for mantle peridotites and melting in the mantle wedge. The black dots define the data set of Canil et al. Ž1994. for continental lithospheric spinel and garnet bearing peridotites. These data can be modelled using an initial Fe 2 O 3 content of 0.3 wt.% and a bulk partition value for ferric iron of 0.1 ŽCanil et al., 1994.. MgO contents modelled using the expressions of Niu Ž1997.. The FMQ-1 melting trend using MELTS also plots through these data Hirschmann et al. Ž1998.. Also shown is the vector for oxidation of the source and calculated trends for partial melting using an initial Fe 2 O 3 content of 0.8 wt.%. A modelled partial melting trend following methods outlined in the text is also shown. Data for Simcoe are from Brandon and Draper Ž1996.. The plot indicates that the arc-peridotite can be explained by source oxidation followed by partial melting.

fO 2 values greater than 2 log units above FMQ requires that some decompression melting occurs in the mantle wedge Že.g., Fig. 4c.. Whether melting in the mantle wedge is by a combination of fluid-fluxed and decompression melting Že.g., Pearce and Parkinson, 1993. or solely by decompression melting Že.g., Sisson and Bronto, 1998. is still unresolvable but this conclusion means that models of the geodynamics of the mantle wedge may have to take into account decompressional melting. 6.4. Geochemical implications for subduction zones We have provided evidence that the mantle wedge above subduction zones is oxidised relative to mantle from other tectonic settings. This feature along with trace element contents and possibly the presence of amphibole andror phlogopite may be a useful indi-

cator of tectonic setting for peridotites found in the geological record. Careful analyses of fresh spinel cores allows oxygen fugacity to be calculated even in altered peridotites and clinopyroxene trace element concentrations are generally unaffected by serpentinisation ŽParkinson and Pearce, 1998.. Therefore, it is possible to assess whether peridotites are related to a subduction zone even for altered peridotites exposed in continental orogenic belts. The data can be used to assess whether elements that have variable redox states may behave differently during magmagenesis processes in the mantle wedge. The elements Uranium and Vanadium and many of the precious metals all have variable oxidation states which may only be effected at the oxidising conditions above subduction zones. U is enriched relative to Th in arc lavas and this feature had been widely attributed to the mobility of U 6q relative to U 4q and Th4q in hydrous fluids ŽBrenan et al., 1995.. Our data indicate that the fO 2 of the mantle is oxidised enough both in the source region and during partial melting for U 6q to be the prevalent species of uranium in both melt ŽCalas, 1979. and aqueous fluid ŽBrenan et al., 1995.. V potentially becomes significantly incompatible at the fO 2 values recorded in the mantle wedge. This feature has been explored for subduction zone melting process by Pearce and Parkinson Ž1993. and used to explain the low V contents of the oxidised ODP Leg 125 peridotites ŽParkinson and Pearce, 1998.. Recent experimental work on V partitioning verifies the assertion that V does become more incompatible with increasing oxygen fugacity with V 4q dominating over V 3q at fO 2 values above FMQ ŽHanson et al., 1996; Canil, 1997.. The behaviour of precious metals in subduction zone systems is still poorly understood. Recent experimental work indicates that all of the platinum group elements ŽPGE. have low valence states ŽM 0 or M 1q . at the fO 2 values prevalent in mantle in most tectonic settings ŽO’Neill et al., 1995.. The elevated fO 2 in the mantle wedge coupled with the fact that sulphide becomes unstable all point to the fact that the PGE’s may behave significantly differently during magmagenesis in subduction zones. An extreme point of view would be that the PGE’s become only slightly compatible or even incompatible. Recent Os isotopic work on arc-related peridotites suggests that radiogenic Os may be trans-

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ported from the slab into the wedge by hydrous fluids or melts indicating a different behaviour for these elements than was previously thought ŽBrandon et al., 1996.. Clearly, the study of PGE behaviour in primitive arc lavas and residual peridotites is a fruitful line of future research. A final interesting conclusion is that the net effect of producing highly oxidised melts and residual mantle in subduction zone is to subduct highly reduced material into the deeper Žultimately lower. mantle and leave an oxidised crust and oxidised buoyant depleted mantle which would be hard do subduct. This would keep the deeper mantle at low fO 2 values and having low ferric iron contents and produce the secular oxidation of the shallow upper mantle.

7. Conclusions Oxygen fugacity data for subduction zone-related peridotites from a variety of arcs record fO 2 values of 0.3–2.0 log units above FMQ supporting the contention that the mantle wedge is oxidised relative the oceanic and ancient cratonic mantle. There is no obvious correlation between fO 2 and the presence of hydrous phases, and in fact, the most oxidised peridotites do not contain any hydrous phases, although some of the peridotites analysed do contain amphibole or phlogopite or both. However, the ultimate source of oxygen that oxidises the mantle wedge is thought to come from the hydrated subducting plate. Model calculations and the elevated Fe 2 O 3 contents in subduction zone magmas both indicate that the source region of IAB must contain 0.6–1.0 wt.% Fe 2 O 3 . This increase in Fe 2 O 3 of the source produces a fO 2 of 0.5–1.7 log units above FMQ. The oxidation event must take place before melting of the wedge. If water is a viable oxidising agent, then only 0.030–0.075 wt.% of water is needed to produce 0.6–1.0 wt.% Fe 2 O 3 in the mantle wedge. However, our preferred model is that ferric iron is added to the wedge by melts or solute-rich hydrous fluids from the subducting slab. This would explain the oxidised nature of the mantle wedge, the elevated water content of subduction zone lavas and their enrichment in fluid-mobile elements relative to MORB. Although it is unclear which model is correct, the fO 2 of the

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fertile mantle wedge cannot be greater than 2 log units above FMQ. This in turn suggests that some form of decompression melting occurs in the mantle wedge. Meltrmantle interaction and oxidation is recorded in some of the Grenada xenoliths and the Leg 125 peridotites. These data provide evidence that silicate melts are efficient oxidisers of the depleted upper mantle because of its limited buffering capacity and such interaction may progressively oxidise the shallow upper mantle in subduction zones. The elevated redox state of the mantle wedge of subduction zones has important ramifications for the behaviour of elements with multiple oxidation states. The redox state of the mantle wedge is high enough that Vanadium, Uranium and the PGE’s all may exist in higher valence states than they do in other tectonic settings. Therefore U can be transported in aqueous fluids and melts, V will become highly incompatible during mantle melting and the PGE’s may have radically different properties being both incompatible and fluid soluble. Experiments at FMQ q 2 log units are needed to verify the partitioning of these elements between mantle and aqueous fluid and melt to fully understand their behaviour in subduction zones.

Acknowledgements Steve Eggins provided the Marelava xenoliths. Nick Ware at RSES ŽANU., Frank Brink and David Voles at RSBS ŽANU. have provided continued assistance in maintaining EMP at their respective institutes and for coping with my data collection. D. Canil and B.R. Frost provided thoughtful reviews which clarified many of the ideas presented here. IJP was supported by a Royal Society Anglo-Australian Exchange Fellowship during the initial stages of this work and later by the an Australian Research Council Grant to RJA. This is GEMOC publication number 140.

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