Earth and Planetary Science Letters 301 (2011) 275–284

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Earth and Planetary Science Letters

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Subduction erosion of forearc mantle wedge implicated in the genesis of the SouthSandwich Island (SSI) arc: Evidence from boron isotope systematics

Sonia Tonarini a,⁎, William P. Leeman b, Phil T. Leat c

a Istituto Geoscienze e Georisorse, CNR, Via Moruzzi1, 56124 Pisa, Italyb National Science Foundation, Arlington, VA 22230, USAc British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

⁎ Corresponding author.E-mail address: s.tonarini@igg.cnr.it (S. Tonarini).

0012-821X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.epsl.2010.11.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 July 2010Received in revised form 3 November 2010Accepted 3 November 2010Available online 3 December 2010

Editor: R.W. Carlson

Keywords:boron isotopesvolcanic arc magmatismsubduction zones

The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, ischaracterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonicsetting, and is ideal for assessing element transport through subduction zones. As a means of quantifyingprocesses attending transfer of subduction-related fluids from the slab to the mantle wedge, boronconcentrations and isotopic compositions were determined for representative lavas from along the arc. Thesamples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55)and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest sofar reported for mantle-derived lavas; these are highest in the central part of the arc (+15 to +18‰) anddecrease toward the southern and northern ends (+12 to+14‰). δ11B is roughly positively correlated with Bconcentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba,Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. +10‰) and elevated B contents(38–140 ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+5 to−13‰; average=−4.1‰), with negative values most common. The unusually high δ11B values inferred forthe South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slabmaterials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources ismore plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantlewedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11Band high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produceextremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantlewedge likely accounts for the unusual magma sources inferred for this arc.

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1. Introduction

The South Sandwich Islands (SSI) are the emergent parts of aclassic intra-oceanic volcanic arc constructed on young back-arc crustformed at the East Scotia Ridge (ESR). It represents an early stage offormation of primitive arc crust, with minimal complicating factors,and is optimal for evaluating variations in magma chemistry resultingfrom mantle processes. Compared to western Pacific intra-oceanicarcs (e.g., Izu–Bonin–Mariana, Tonga–Kermadec), the SSI arc ischaracterized by a simple tectonic setting (Larter et al., 2003; Leatet al., 2003). It is far from any known continental crust, there are nocollisions with seamount chains, oceanic plateaux or ridges, and intra-arc rifting is negligible. All sediment arriving at the trench issubducted, as there is virtually no accretionary prism. Moreover,there is no complexmixture of pelagic and volcanogenic sediments on

the down-going plate as is the case for most western Pacific arcs. TheSSI represents one of the simplest active island-arc systems in which avariably depleted mantle wedge is modified by subducted compo-nents (Pearce et al., 1995).

Volcanic arc lavas define generally coherent intra-arc correlationsbetween B-enrichment and isotopic composition (i.e., δ11B) thatprovide unique insights into material recycling processes at conver-gent margins. The fact that B-enrichment is decoupled from fluid-immobile elements (Ti, Zr, Nb) but parallels that of fluid-mobileelements (Pb, As, Sb) implies dominant transfer of the latter elementsinto arc magma sources via aqueous fluids rather than silicate melts(Leeman, 1996; Noll et al., 1996). Notably, in cross-arc transects B-enrichment is strongest at the volcanic front and diminishes towardback-arc regions (e.g., Ishikawa and Nakamura, 1994; Ryan et al.,1995; Tonarini et al., 2001). This observation, coupled with data formetamorphic suites, implies progressive B-depletion with increasingmetamorphic grade and suggests that B is selectively mobilized fromsubducting materials due to progressive slab dehydration (Bebout etal., 1999).

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Considering that the heavy boron isotope (11B) is preferentiallypartitioned into aqueous fluids (You et al., 1995a,b), progressivemobilization of boron is expected to induce isotopic fractionationbetweenfluids and residual solids. Several processes have been invokedto model boron isotopic fractionation during slab dehydration (Bebout,2007; Bebout and Nakamura, 2003; Benton et al., 2001; Marschall et al.,2006, 2007; Peacock and Hervig, 1999; Straub and Layne, 2002). Therelatively elevated δ11B signature in frontal arc lavas suggests that otherB reservoirs are involved in addition to altered oceanic crust (AOC) andsubducted sediments. For example, high δ11B values in Mariana forearcserpentinites (Benton et al., 2001) suggest that such rocks may bepotentially important fluid sources in this context. The SSI arc is an idealsystem to investigate geochemical contributions of this type of reservoirto the genesis of arc magmas because rates of subduction erosion havebeen qualitatively assessed and samples of dredged forearcmantle havebeen characterized and are available for study. This paper presents newdata for B contents and isotopic compositions (δ11B) of representativearc lavas, forearc peridotites, and incoming pelagic sediments (from theODP Site 701; 50.946 °S, 26.369 °W).

2. Geological setting and sampling

The South Sandwich Islands comprise a 400 km long arcuate arrayof eleven main islands situated on the Sandwich microplate (Fig. 1).The islands consist largely of basalt and andesite lavas; minor evolvedcompositions are present locally. All exposed volcanic rocks are of lateTertiary and Quaternary age. Some islands are active volcanoes andthere is intense fumarolic activity on several. Absolute motion of thesmall Sandwich plate is eastward at about 57 mm/yr (Barker, 1995). Itis bound on the east by the South Sandwich Trench, where it overridesthe South America plate at a rate of 67–79 mm/yr (Thomas et al.,2003). The Sandwich plate consists of oceanic crust generated at theeastern flank of the East Scotia Ridge (ESR), and magnetic anomaliesindicate that most of the arc is built on oceanic crust formed withinthe last 10 Ma (Barker, 1995; Barker and Hill, 1981; Larter et al.,2003). The ESR back-arc spreading center consists of nine mainsegments and is spreading at an intermediate rate of 65–70 mm/yr(Livermore et al., 1997).

The subducting South America plate oceanic crust ranges in agefrom about 80 Ma at the northern end of the trench to about 27 Ma atthe southern end (Leat et al., 2004). It carries a veneer ofdiatomaceous ooze, volcanic ash, and clay-rich pelagic sedimentsthat is approximately 400 m thick at ODP site 701 drill holes (Fig. 1;Plank and Langmuir, 1998). Geological and geophysical data have ledto the interpretation that the South Sandwich margin is dominated by

Fig. 1. (a) Geological sketch map of the South Sandwich Island (SSI) arc–East Scotia Ridge (Scotia and Sandwich plates. Filled square shows location of ODP Site 701. Filled triangles, presto Thule in the south. Shaded boxes indicate locations where peridotites were dredged (‘A’

subduction erosion with an average rate of removal of forearclithosphere near 40 km3 km−1 Ma−1 (Larter et al., 2003; Vannesteand Larter, 2002). Variably serpentinized peridotites and harzburgiteshave been dredged from the inner wall of the South Sandwich Trench(dredges 52–54) and at the intersection between the South SandwichTrench and the South America–Antarctic ridge (dredge 110); samplesfrom these localities (Fig. 1) have been studied by Pearce et al. (2000).

The present study is based on a subset of arc lavas first described byBaker (1978) and Hawkesworth et al. (1977); many of these sampleswere reanalyzed by Barry et al. (2006) and Pearce et al. (1995).Additional samples were included from more recent collections of Leatet al. (2003, 2004). We also present new data for representativeperidotites studied by Pearce et al. (2000), and for representativesediments from ODP Site 701 (Plank and Langmuir, 1998).

3. Analytical methods

Boron contents in the arc lavas and sediments were measured vianon-destructive prompt gamma neutron activation (PGNA). Themethod is described by Leeman (1988) who presents analyses fornumerous standard reference materials; based on this comparisonand subsequent calibrations, the accuracy and precision are estimatedas ±5–10% for concentrations above 5 ppm and no better than ±20–25% at concentrations below 1 ppm. Boron contents in peridotites andselected arc lavas were determined at Rice University (Houston, TX)using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The typical uncertainty for replicate analyses is ±4%; details ofthe analytical procedure are given in Agranier et al. (2007) andRodríguez et al. (2007).

Boron isotopic compositionsweremeasured inPisa (CNR-IGG)usinga VG Isomass 54E positive ion thermal ionization mass spectrometerfollowing boron extraction and purification procedures described byTonarini et al. (1997, 2003). The 11B/10B isotopic ratio is reported instandarddeltanotation aspermil (‰) deviation from themeanvalue forthe SRM-951 boric acid standard routinely passed through the samechemistry as the samples. Precision and accuracy are estimatedconservatively as±0.5‰ based on replicate determinations for samplesand for repeated analyses for standard rock JB-2 (δ11B=7.25±0.32‰(1 σ), n=33 analyses with independent chemistry).

Where Sr and Nd isotopic data were unavailable for the samplesanalyzed for B isotopes, thesewere determined in Pisa by TIMSmethodsusing a FinniganMAT 262multicollectormass spectrometer in dynamicmode. Measured 87Sr/86Sr ratios were normalized to 86Sr/88Sr=0.1194,and 143Nd/144Nd ratios to 146Nd/144Nd=0.7219. Over the course of thisstudy, themeanmeasured value of 87Sr/86Sr for NIST standard SRM-987

ESR) region after Leat et al., 2004. The ESR is generated by east–west divergence of theent-day subduction zone. (b) Detailed locations of the SSI, from Zavodovski in the north: forearc 671 sites DR52–DR54 and ‘B’: trench-fracture zone intersection site DR110).

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was 0.710242±0.000013 (2SD, N=25) and that of 143Nd/144Nd for theLa Jolla standard was 0.511847±0.000008 (2SD, N=25).

Table 1 presents all data for B contents in SSI samples as well as Bisotopic composition where determined. A set of six samples from

Table 1Boron content and isotope compositions.

Location Sample Rock type B ppm PGNA B ppm ICP δ11B‰

SSI arc lavasLeskov SSL 4.2 CA 6.2Zavodovski SSZ 83.2 LKT 3.7Zavodoski SS.19.2 LKT 4.8 4.8 12.2±0.6Zavodoski SS.19.5 LKT 4.4 8.1 12.1±0.3Visokoi SSW 2.1 TH 8.4 13.5±0.5Visokoi SSW 3.1 TH 7.9Visokoi SSW 6.3 TH 11Visokoi SSW 9.1 TH 13.7Vindication SSV 1.2 LKT 9.8Vindication SSV 2.1 LKT 8.9Vindication SS.12.7 LKT 7.8 8.3 14.6±0.7Candlemas SSC 24.2 LKT 3.9Candlemas SSC 35.1 LKT 3.2 15.6±0.5Candlemas SSC 35.4 LKT 5.7Candlemas SSC 41.1 LKT 8.5Candlemas SS.78.1 LKT 17.7 20 16.7±0.3Candlemas SS.82.2 LKT 15.9Saunders SSS 10.1 TH 18Saunders SSS 15.1 TH 18 15.6±0.2Saunders SSS 5.4 TH 19.5 16.0±0.4Saunders SS.18.3 TH 7.3 12.1 16.1±0.4Saunders SS.18.7 TH 5.6 13.1Saunders SS.18.11 TH 16.1 17.3 16.1±0.3Saunders SS.18.18 TH 4.7 13.4 15.4±0.2Saunders SS.21.4 TH 13.3 13.3 15.1±0.3Saunders SS.21.3 TH 13.2Montagu SSM 5.4 LKT 10Montagu SSM 1.3 LKT 6.6Montagu SS.24.5 LKT 5.5 6.4Bristol SSR 1.3 LKT 7.7Bristol SS.15.2 LKT 8.1 8.1 15.9±0.3Freezeland SSF 2.2 CA 10Bellingshausen SSB 11.1 TH 21Bellingshausen SS.17.10 TH 18.2 17.6±0.3Thule SST 4.1 TH 19.5Thule SST 6.3 TH 5.6Thule SST 7.1 TH 8.5 13.7±0.7Cook SSK 1.1 CA 28Cook SSK 1.2 TH 4.5

Dredged peridotitesForearc 52.11 Hz 71.4 10.23±0.3Forearc 52.12 Hz 103.6Forearc 54.1 Trans. 93Forearc 52.8 Trans. 139.9Forearc 52.4 Trans. 109.4Forearc 52.5 Dunite 93.4Forearc 52.6 Dunite 113.6Forearc 53.2 Dunite 116.7Forearc 53.3 Dunite 116.2TFI 110.1 Lhz 38.1TFI 110.2 Lhz 47.3TFI 110.3 Lhz 52.5TFI 110.4 Hz 48.9 9.0±0.4

ODP-701 (depth, m)5.9 1H-4 Diat + ash 148 5.4±0.453.7 6H-5 Diat + ash 97 1.1±0.5105.9 4H-5 Diat + ash 86 −6.3±0.4150 9H-5 Diat + mud 163 −8.7±0.3188.4 13X-3 Diat ooze 100 −11.8±0.6211.7 23H-4 Diat + clay 101 4.0±0.2284.7 31X-2 Silic clay 107 −13.1±0.4343.2 37X-3 Silic mud 110 −3.2±0.4387.7 42X-1 Clay 121 −3.4±0.3

CA, Calc-alkaline; LKT, low-K tholeiitic; TH, tholeiitic.Hz, harzburgite; Trans., transitional;Lhz, lherzolite.TFI = Trench-Fracture zone Intersection.

Saunders Island was analyzed to evaluate internal consistency andpossible correlations of δ11B with other geochemical parameters;these B isotopic data are essentially indistinguishable and define nosignificant intra-island trends. The remaining data comprise areconnaissance study to evaluate intra-arc compositional variability.Compiled major and trace element and Sr, Nd and Pb isotopic data forthese and other selected samples are presented in the Appendix(Table A). ESR basalts and submarine SSI lavas were not analyzed for Bconsidering their potential susceptibility to modification by seawaterinteraction.

3.1. General petrology and geochemistry

Most SSI lavas studied belong to the (low-K) tholeiite magmaseries, but include a few calcalkalic rocks (from Leskov and Cookislands). The majority are basalts or basaltic andesite (cf. classificationscheme of Peccerillo and Taylor, 1976), but more evolved lavas wereincluded. The petrography of the South Sandwich Islands (SSI) lavas issummarized by Pearce et al. (1995). Overall, the rocks follow a simplepattern of phenocryst assemblages (basalt: olivine, plagioclase ±augite; and the more differentiated products: plagioclase, augite,hypersthene, magnetite ± pigeonite) typical of the tholeiitic associa-tions of oceanic arcs (Wilson, 1989). Extensive fractional crystalliza-tion of mafic phases occurred before these magmas erupted, asevidenced by their relatively low MgO, Ni and Cr contents. Much ofthe intra-island compositional diversity may be ascribed to magmachamber processes. Pearce et al. (1995) suggested that the transitionfrom basalt to dacite requires the fractionation of about 80% of a solidmade up of plagioclase (50%), clinopyroxene (25%), olivine (13%),titanomagnetite (9%) and hypersthene (3%), although generation ofsilicic magmas within the arc by partial melting of mafic lithologies isalso possible (Leat et al., 2003, 2007). Trace element geochemistry(e.g., chondrite normalized REE plots, MORB-normalized traceelement patterns and trace element ratio plots) strongly suggeststhat SSI magmas were derived dominantly from a depleted N-MORBtype mantle source, with low-K tholeiites produced from the moststrongly depleted domains. It is likely that the source was firstdepleted by anhydrous melting at the early ESR spreading center,followed by rejuvenated hydrous melting beneath the arc due toingress of slab-derived fluids (Leat et al., 2003; Pearce et al., 1995;Woodhead et al., 1993). This hypothesis is supported by petrographicand geochemical studies of peridotites from the South Sandwichforearc region (Pearce et al., 2000).

Regional investigations suggest that subduction processes havehad a perceptible influence on parts of the Scotia Sea plate peripheralto the SSI arc. Though not part of this study, Kemp and Nelsonseamounts from the southern end of the arc have isotopic andelemental characteristics that are attributed to involvement ofsediment-derived melts or supercritical aqueous fluids (Barry et al.,2006; Leat et al., 2004). There is also geochemical evidence for influxof a subduction component (e.g., up to 2% sediment melt in themagmas) at the northern and southern ends of the ESR, albeit basalticlavas from the central ESR exhibit negligible subduction signatures(Fretzdorff et al., 2002). These distinctions are readily seen in Sr andNd isotopic data for SSI lavas ESR basalts (Fig. 2).Whereas both groupshave overlapping Nd isotopic compositions, significantly higher Srisotopic ratios in SSI lavas are attributed to selective input ofsubducted radiogenic Sr to the arc magma sources.

SSI arc and ESR back-arc lavas have comparable enrichmentpatterns for all but the most fluid-mobile elements (FME), suggestingsimilar source compositions other than selective enrichment of thelatter elements beneath the arc. This is particularly well illustrated indiagrams of Zr/Yb plotted against Yb-normalized ratios for otherselected elements in SSI and ESR lavas and SSI forearc peridotites(Fig. 3). Lower Zr/Yb ratios in SSI lavas compared to those in ESRbasalts are consistent with a more strongly depleted source for the arc

Fig. 2. Plot showing 143Nd/144Nd vs. 87Sr/86Sr for lavas from the South Sandwich Islandarc (SSI; Barry et al., 2006; Pearce et al., 1995, this work) and East Scotia Ridge (ESR;Fretzdorff et al., 2002). Note that the two ESR samples characterized by low Nd and highSr are high K2O basaltic andesites.

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lavas, and it has been proposed that the arc magmas form by meltingof residual NMORB-type mantle from which the ESR melts had beenpreviously extracted (Pearce et al., 1995). Even lower Zr/Yb ratios inthe forearc peridotites attest to the extremely melt-depletedcharacter of these rocks (Pearce et al., 2000). With regard to melt-incompatible elements that have different affinities for aqueous fluids,the following relations are noteworthy. La/Yb ratios in the arc lavaspartly overlap with but are often lower than La/Yb ratios in ESRbasalts. Because both groups define a strong linear La/Yb–Zr/Ybcorrelation, it is likely that the higher values for these ratioscorrespond to lower degrees of melting of otherwise similar sources(with respect to these elements). In contrast, Ba/Yb, U/Yb, and evenTh/Yb ratios in SSI lavas are comparable to and in some cases higherthan those observed in ESR basalts despite somewhat lower Zr/Ybratios in the arc lavas. Thus, Ba, U, and Th appear to be selectively re-enriched in SSI lavas, presumably due to addition of a ‘subductioncomponent’. Also, Pb is significantly enriched in the arc lavas (Pearce

Fig. 3. Plot showing Zr/Yb vs. (a) Ba/Yb, (b) La/Yb, (c) Th/Yb and (d) U/Yb for SSI lavas, ESR bPearce et al. (1995, 2000), this work.

et al., 1995), and high U enrichment in the serpentinites (Pearce et al.,2000) is noteworthy. Because most of these elements are consideredto be highly fluid-mobile, the dominant transport medium for theirselective enrichment is inferred to be aqueous fluid.

3.2. Boron data

Boron contents range between 3.7 and 28 ppm in SSI arc lavas,whereas much higher B contents are observed in the serpentinizedperidotites (38 to 140 ppm) and ODP 701 sediments (86 to 163 ppm).The arc lavas display a positive correlation between boron concen-tration and degree of differentiation (as indicated by SiO2 content;Fig. 4). However, the degree of B-enrichment relative to Zr appears tobe higher in the least evolved arc samples, and this may reflect somevariation in source composition. Extremely high B/Zr ratios (up to1000) observed in the highly melt-depleted serpentinized peridotitessuggest that portions of the shallow mantle have been stronglymodified by infiltrating of seawater. For comparison, residual MORB-source mantle has B/Zr near 0.01 (B=0.06 ppm; Zr=9.7 ppm; cf.Leeman et al., 2004).

Looking in greater detail (Fig. 5), SSI lavas define distinct arrays inplots of SiO2, B, and Zr vs. B/Zr ratio. The basaltic lavas (b53% SiO2)exhibit a verywide range in B/Zr (0.1 to 0.6) that cannot be ascribed todifferentiation processes given the strongly incompatible behavior ofboth elements. Rather, the observed correlations are more consistentwith variable source composition that can be attributed to selectiveenrichment of B in variably depleted sources via addition of B-rich, Zr-poor fluids (Leeman, 1996; Leeman and Sisson, 1996). In contrast,more evolved lavas from the southern and northern islands have amuch narrower range in B/Zr (0.12 to 0.25) that increases onlymodestly with increasing SiO2 and incompatible elements (e.g., B, Zr,Nb, Th, La, etc.). These relations suggest that the mantle wedge isheterogeneously depleted in Zr or enriched in B (and other FMEs), orboth, and that evolved lavas either form by differentiation of basalticparental magmas or by partial melting of crustal sources that are notextremely enriched in B. Mafic and slightly evolved lavas from thecentral SSI (Saunders Island) are intermediate between theseextremes, and could be differentiates of high-B/Zr (0.25–0.45)parental magmas. Unfortunately, data from the rest of the arc aretoo sparse to evaluate geographic variation in B-enrichment. We alsonote that a single back-arc andesite from Leskov Is. has a significantlylower B/Zr value (0.07; not plotted) compared to the volcanic front.

asalts, and forearc peridotites. Data sources: Barry et al. (2006), Fretzdorff et al. (2002),

Fig. 4. Plot showing SiO2 vs. (a) B (ppm) and (b) B/Zr ratio in SSI lavas (from north,central, and southern parts of the arc). Note that B contents generally increase while B/Zr remains roughly constant as SiO2 increases in SSI lavas, as expected for fractionalcrystallization (FC). Data sources: Barry et al. (2006), Pearce et al. (1995, 2000), thiswork. Strong B-enrichment is a common characteristic of abyssal peridotites, and isattributed to metasomatism by seawater or other brines (see text).

Fig. 5. Variation of B-enrichment (B/Zr) with (a) SiO2, (b) B and (c) Zr contents and (d) δ11B(SiO2b53%) from different sectors of the arc. The latter samples define distinct arrays interprsystematic geographic variation in B/Zr suggests the presence of small-scale heterogeneitieprimarily by fractional crystallization (FC) of SSI basalts. δ11B is not well correlated with B/Zr oSaunders Island. (C SSI). There, near-constant δ11B and varied B/Zr suggest a narrowly defineSSI is comparable to that for many Pacific subduction zones, significantly higher δ11B value

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Virtually all cross-arc transects reveal decreasing B-enrichmenttoward the back-arc (Ishikawa and Nakamura, 1994; Leeman et al.,2004; Ryan et al., 1995; Tonarini et al., 2001), and this is attributed toprogressive B loss from the subducting slab.

Lavas from other intra-oceanic subduction zones commonly exhibitB/Zr enrichments comparable to the SSI, yet none to our knowledgehave δ11B values approaching those of the SSI samples (Fig. 5d). Overall,SSI lavas display a wide range in δ11B between +12 and +18‰, forcomparison, δ11B in Pacific arc lavas seldom exceed 8‰ (Ishikawa andNakamura, 1994; Ishikawa and Tera, 1997, 1999; Ishikawa et al., 2001;Palmer, 1991), although higher δ11B values have been documented forIzu glasses (+4 to +12‰ based on less precise SIMS analysis; Strauband Layne, 2002). SSI lavas showpositive correlations between δ11B and87Sr/86Srwithin geographical sub-portions of the arc (Fig. 6), and δ11B isalso roughly positively correlated with B content (Fig. 7). The sixisotopically analyzed samples from Saunders Island display nearconstant δ11B (15.1 to 16.1‰) as boron concentration increases from12.1 to 19.5 ppm (using the more precise ICP-AES data), and 87Sr/86Srvaries from 0.70403 to 0.70419.

All SSI arc lavas have significantly higher δ11B than the valuesmeasured on serpentinites dredged from the South Sandwich Trenchand southern margin of the arc (9.5 to 10.2‰; this study). These δ11Bvalues are characteristic of many abyssal peridotites (+7 to +13‰;Bonatti et al., 1984; Spivack and Edmond, 1987) and serpentinites fromAtlantis Massif (+9 to +16‰; Boschi et al., 2008) and the VemaFracture zone (+5 to +15‰; Tonarini, unpublished data). Incomingsediments (ODP Site 701; Plank and Langmuir, 1998) proximal to theSouth Sandwich trench display a wide range in δ11B (+5 to −13‰;average−4.1‰) and generally decreasewithdepth below seafloor; thehighest values correspond todiatomaceousash from the shallowest partof the borehole.

in SSI lavas. Evolved lavas (SiO2≥53%; triangles) are distinguished from basaltic lavaseted as the result of variable B-enrichment in variably depleted mantle sources. Lack ofs in the mantle wedge. Evolved lavas define an array consistent with their derivationn a regional scale, but may be coupled in lavas on a local scale as evidenced by data fromd B isotopic composition for the subduction component. Although the range in B/Zr fors for SSI lavas signify a uniquely distinct source composition.

Fig. 6. Plot showing δ11B vs. 87Sr/86Sr for SSI lavas. There is a hint of correlation forsamples from different parts of the arc (dashed lines). However, analyses of multiplesamples from Saunders Island (C SSI) define a restricted compositional field with nearlyconstant δ11B and limited variation in 87Sr/86Sr.

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4. Discussion

Processes of subduction recycling are here evaluated considering Bsystematics for the SSI arc. Noteworthy observations to be explainedare as follows: [1] despite their depleted character, SSI lavas have highboron abundances (3 to 25 ppm) and the heaviest δ11B values (+12to +18‰) yet observed for arc-rocks; [2] δ11B values measured inboth serpentinized peridotites and pelagic trench sediments proximalto the arc are too low to account for compositions of the arc magmassimply by incorporation of such subducted materials into a normalmantle source; and [3] the most mafic SSI lavas exhibit the greatestmagnitude of B-enrichment and this is decoupled from contents offluid-immobile incompatible elements.

Fig. 7. Plot showing δ11B vs. B relations for SSI lavas, two representative forearcperidotites, and proximal marine sediments (ODP 701 site). These parameters areroughly correlated in the lavas, such that δ11B increases with overall B-enrichment(bold arrow). This trend is inconsistent with incorporation of materials similar to eitherthe peridotites or the sediments into the arc magma source or as assimilants in themagmas themselves. Mariana forearc (Benton et al., 2001), Atlantis Massif (Boschi et al.,2008), and ODP leg 209 (Vils et al., 2009) serpentinites are plotted for comparison.

Considering that subducting slabs undergoheating anddehydration,the thermal structure of subduction zonesmust dictate themechanismsand rates of fluid release from slab and sediments and sedimentmeltingas a function of depth. Recent thermal models (e.g., Leeman et al., 2002;Syracuse et al., 2010) suggest that the upper slab surface (i.e., thesediment and AOC veneer) can become substantially warmer than theslab interior, and superposition of this strong thermal gradient onlithologic and chemical gradients in the slab is likely to lead to a complexdehydration and metamorphic history. It is likely that the uppermostpart of the slab becomes extensively dehydrated and perhaps evenreaches melting conditions while the slab interior may be significantlyless dehydrated and well below melting conditions. Compositions offluids or melts evolved from the slab interior will be determined bymantle and/or lower oceanic crust protoliths, the degree to which theywere serpentinized prior to subduction, and their specific metamorphichistory. Although seawater has high δ11B (~40‰), its direct involvementin establishing inferred SSI magma sources is dismissed because of thedifficulties in subducting brines into the deep mantle and its low Bcontent (~4.6 ppm). Below, we evaluate several more realistic reser-voirs/processes that potentially could contribute to formation of SSImagma source compositions.

4.1. Assessment of sediment contributions

As noted earlier, there is Hf isotopic evidence for involvement of asediment component locally near edges of the Scotia plate. Moreover,samples from Nelson Seamount (southern end of the SSI arc) displayelevated Th/Nb and lower 143Nd/144Nd, consistent with significant(ca. 2%) involvement of sediment melt in that area (Barry et al., 2006;Leat et al., 2003). We note that SSI basaltic lavas have Th abundancesalmost in the same range as ESR basalts, whereas La, Zr and Nb showsignificant depletion with respect to ESR basalts. This implies that Th/La and Th/Nb ratios are significantly higher in the subarc mantlecompared to the expected back-arc mantle source (Barry et al., 2006;Leat et al., 2004; Plank, 2005). However, because this enrichment isnot accompanied by a decrease in 143Nd/144Nd isotope ratio, it isunlikely due to addition of a pelagic sedimentary (melt?) componentto the subarc mantle. Moreover, addition of a pelagic sediment-derived component should dilute B-enrichment (e.g., decrease B/Zrratio) and reduce δ11B (b to −4.1, average δ11B in sediments priorsubduction) in the subarc mantle; this is because B, and particularly11B, are selectively fractionated into fluids liberated by slab dehydra-tion (see below). Consequently, we see little compelling evidence forpelagic sediment contributions to SSI basalts, and for the purposes ofthis paper they are assumed to be negligible.

On the other hand, multibeam surveys indicate that huge volumesof volcaniclastic sediments are eroded from the islands andtransported toward the trench (Leat et al., 2010); much of thismaterial appears to be subducted and could contribute to modifica-tion of arc magma sources. Because these materials are expected tohave B, Sr, Nd and Hf isotope compositions similar to those of themodern lavas it is difficult to uniquely quantify the magnitude of theirinvolvement. Although they probably have little distinctive leverageon isotope ratios in themantlewedge, over time they could contributeto increasing δ11B and enhance enrichments of fluid-mobile and otherincompatible trace elements in the mantle wedge.

4.2. Dehydration of the upper slab (AOC+sediments)

Asignificant fraction (N ~75%; Leemanand Sisson, 1996) of the initialB inventory in subducting slabs is contained in the uppermost few km,consisting of sediments and altered oceanic crust. Progressive dehydra-tion of this reservoir was evaluated numerically for a simple scenarioassuming an initial compositionwith about 90% AOC and 10% sediment.Compositions of the subducting materials were approximated as26 ppm boron and δ11B value of +5.5‰ for AOC (Leeman et al., 2004;

Fig. 8. Diagram showing calculated δ11B and B contents for slab-derived fluids andcomplementary residual slab solids at successive stages during progressive subductionzone dehydration. The initial starting composition (B=35 ppm, δ11B=2.4‰) assumesthat the uppermost slab comprises only AOC (90%) and sediment (10%); deeper mantleportions of the slab are ignored in these calculations (see text). B-rich fluids with heavyδ11B are released during shallow metamorphic dehydration of the slab. Such fluids aremost likely to metasomatize the overlying forearc mantle wedge and crust or escapealong the subduction decollement. Slab-derived fluids directly underneath the arcfront, cannot realistically account for the high δ11B ratios in SSI arc lavas.

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Smith et al., 1995), and 115 ppm B and δ11B value of −4.1‰ forsediment based on our averaged data for ODP 701 sediments. Initialwater contents were assumed to average 7.29% in the sediments(GLOSS, Plank and Langmuir, 1998), 2.7% for altered igneous oceaniccrust, and 6% for adsorbed, interstitial and pore water (Staudigel et al.,1996). B contents and isotopic compositions were calculated forliberated fluids and residual slab materials as functions of depth andtemperature along the descending slab. Dehydration was modeledarbitrarily in six stages (corresponding to depths of 30, 50, 70, 90, 110and 120 km) using the equations of Marschall et al. (2006) to simulateboron release linked to water content in the subducting materials (cf.Rüpke et al., 2004; Syracuse et al., 2010), boron isotopic fractionationwas calculated using the temperature-dependent B fractionation factoraccording toHervig et al. (2002) and a partition coefficient of B betweenhydrous fluids and restite slab of 0.1 (Brenan et al., 1998). A principaloutcome of the modeling (Table 2; Fig. 8) is that virtually all theadsorbed, interstitial, or pore water, and most of the structural waterfrom low-grade minerals is lost at shallow depths (i.e. within the firsttwo steps). This is accompanied by release of a large fraction of the initialB, and particularly 11B. Moreover,fluid production from the slab is likelyto drop off sharply at Benioff zone depths (≥~100 km) beneath the arcproper, and such fluids are predicted to have very low δ11B (end-member ‘A’; Fig. 9, Table 2)— possibly lower than that in normalmantle(i.e., b−5‰). Thus, it appears that fluids released from the uppermostslab at depths directly underneath the volcanic front, cannot reasonablyaccount for the high δ11B ratios in SSI and many other arc lavas.

4.3. Dehydration of the slab interior

Although the ultramafic portion of the slab is likely to have very lowB concentrations (b0.1 ppm) following extraction of MORB, its total Binventory could be significant because the overall volume is large. Thispotential reservoir could also be enhanced through fault-aided fluidingress and serpentinization on the seafloor prior to onset of subduction(Ranero et al., 2003; Rüpke et al., 2004). The extent of alteration and B-enrichment depends largely on the integrated fluid/rock ratio, andvariations in this parameter could contribute to heterogeneities in arcmagma sources. For example, the exhumed Feather River ophiolite(California) preserves original low-temperature serpentine mineralsand has significantly lower B contents (5 to 15 ppm) than peridotitesfrom the SSI trench. Agranier et al. (2007) suggest that suchconcentrations may be typical of vast tracts of oceanic lithosphere.However, the distribution and amount of slab serpentinization remainspoorly constrained at present, and may vary significantly worldwide.

Table 2Parameters for model in Figs. 8 and 9.

B (ppm) δ11B

Uppermost slab (AOC + Sed) [A]Starting comp. 35 2.4Fluid (at 90 km depth) 189 −0.7Fluid (at 120 km depth) 170 −3.9Residual slab (at 120 km depth) 16.8 −12.8

Ultramafic slab ± serpentine [B]Starting comp. 52 12.8Fluid (at 90 km depth) 283 9.7Fluid (at 120 km depth) 252 6.6

Recycled forearc serpentinites [C]Starting comp. 60 20Fluid (at 90 km depth) 325 16.9Fluid (at 120 km depth) 289 13.8Mantle 0.06 −5

Data from Benton et al., 2001 (Mariana forearc serpentinites); Boschi et al., 2008(abyssal serpentinites); Leeman et al., 2004 and Smith et al., 1995 (AOC); and this work(sed). The Nb content was assigned to obtained Nb/B ratios of 0.01 for fluids released byAOC + sed; and 0.001 for fluids released by serpentinites.

Slab interior initial δ11B is likely to resemble that ofMORB (ca.−5‰;LeRoux et al., 2004), but interaction with seawater can dramaticallyincrease δ11B on the scale offluid infiltration.Data for abyssal peridotites

Fig. 9. Plot of δ11B versus Nb/B for arc volcanic rocks and representative mantlematerials. The diagram shows mixing relations between mantle (taken as medianMORB and OIB composition) and fluids released beneath the arc from the followingreservoirs: [a] uppermost slab (AOC + Sed); [b] ultramafic part of slab ± serpentinites,and [c] recycled forearc serpentinites. The numbers indicate the percentage of fluidadded to the mantle source. Hypothetical “composite fluids” are obtained by mixing Aand C end-members. Binarymixing between themantle source and residual slab melt isalso illustrated. Fields for Syros tourmalines (Marschall et al., 2006) and Marianaforearc serpentinites (Benton et al., 2001) are shown for comparison. Diamonds areglobal arc lavas and squares are intraplate lavas (MORB + OIB). Data sources: Ishikawaand Nakamura (1994), Ishikawa and Tera (1997, 1999), Ishikawa et al. (2001), Leemanet al. (2004), Palmer (1991), Rosner et al. (2003), Smith et al. (1997), Tonarini et al.(2001, 2005, 2007) for arc lavas; LeRoux et al. (2004), Roy-Barman et al. (1998), Tanakaand Nakamura (2005), Turner et al. (2007) for intraplate lavas.

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provide an approximate upper limit for B isotopic composition. Locally,shallow hydrothermal metasomatism at mid-oceanic ridges hasproduced serpentinized peridotites with δ11B as high as 30 to 40‰(Vils et al., 2009). However, themajority of analyzed abyssal peridotiteshave lower δ11B values (ca. 10 to 15‰) similar to those of SSI forearcperidotites (Fig. 7; cf. Boschi et al., 2008; Spivack and Edmond, 1993).The initial composition of the bulk mantle portion of subducting slabsobviously depends on the extent of serpentinization; its δ11B is unlikelyto exceed the latter range but could be significantly lower. On the otherhand, devolatilization of this reservoir will largely involve the alteredportions of the slab, such that the serpentinized sub-domains willdetermine compositions of released fluids.

High δ11B boron and B concentrations (and up to ~13% ofstructural water) may be hosted in serpentine minerals that arestable to pressures up to 3–4 GPa and temperatures approaching650 °C (Ulmer and Trommsdorff, 1995). Thus, serpentine stabilityprimarily will control the sequestration and ultimate release of B fromultramafic portions of the slab although a large amount of boron maybe liberated by chrysotile breakdown to antigorite during early stagesof subduction (Scambelluri et al., 2004; Vils, 2009). Whereas fluidsmay be released progressively from the uppermost slab materials(AOC and marine sediments) as subduction proceeds, serpentine-hosted fluids could be retained up to P–T instability conditions forserpentine, above which these fluid and B inventories would bereleased. Minimal B isotopic fractionation is expected duringbreakdown of serpentine minerals given high T of the process andthe basic pH of the fluids (Savov et al., 2007). Consequently, δ11Bvalues of slab-derived fluids are expected to mimic those of the initialslab serpentinites (end-member ‘B’, Table 2). The effect of adding suchB-rich fluids to a depleted mantle wedge is similar to that of upperslab fluids, except that higher δ11B is predicted. Such a mixing curve isshown in Fig. 9 where it is assumed that δ11B is similar to that intypical abyssal peridotites (end-member ‘B’). Again, it is difficult toachieve the high δ11B values seen in SSI lavas, but this process couldcontribute significantly to sources of many other arc lavas.

Given that slab interior domains are likely to follow T–P trajectoriesoffset to lower temperatures relative to overlying parts of theuppermost slab, they will provide a relatively delayed fluid flux thatenables deeper subduction of hydrous components. For the SSI arc, andmany other subduction zone thermal structures, interior slab tempera-tures at subarc depths are predicted to be lower than the serpentinebreakdown conditions (Peacock et al., 2005; Syracuse et al., 2010). Incases where most of the slab section is below its melting solidus, aspectrum of fluid compositions might be released — providing anaggregate sampling of all slab lithologies if the fluids mix. Eventually,serpentine breakdown can release aqueous fluids that could flux theoverlying metabasalts and sediments, perhaps instigating water-saturated partial melting (Nichols et al., 1994). However, if upper slabrockswere previously dehydrated and B-depleted, theymay contributelittle B to fluids derived from the slab interior. If the uppermost slabreaches melting conditions (wet solidus) while the interior remainsbelow its solidus, bothmelt and fluid signatures could be transferred tothe mantle wedge. In other words, despite the onset of melting inuppermost parts of a slab, a cooler slab interior could continue toproducedistinctive aqueousfluids. Inprincipal, both types offluidfluxescould transfer a mixed or ‘decoupled’ signature to the mantle wedge.This may be an inconsequential scenario for SSI magma sources givenearlier geochemical arguments that involvement of slab-derived silicatemelts is insignificant inproducing theSSImagmasources (except locallyin southern SSI seamounts).

In summary, it appears that the upper and interior slab reservoirsare unlikely to produce fluids with sufficiently high δ11B to generatesuitable sources for most SSI magmas. If recent numerical models ofSyracuse et al. (2010) for the SSI slab thermal structure are accurate, itappears that serpentine breakdown in the slab interior could bedelayed until depths beneath the back-arc region.

4.4. Dehydration of subducted forearc material (multi-stage subduction)

In view of the inability of sediment melts or slab-derived fluids togenerate the B–δ11B relationships for the SSI lavas, we consideralternative processes in which the δ11B of the source and eruptedproducts may be enhanced by recycling of heavy B through thesubduction zone. Geological and geophysical data suggest that the SSIforearc is dominated by tectonic erosion (Larter et al., 2003), andgravimetric modeling is consistent with an average rate of subductionerosion of forearc crust of 31 to 47 km3 km−1 Ma−1 for the last 15 Ma(Vanneste and Larter, 2002). At the present convergent rate of 74 kmMa−1, a significant volume of forearc lithosphere apparently has beenremoved by subduction erosion and transported beneath the mantlewedge. Therefore, tectonic erosion appears to be an efficientmechanism for recycling of heavy δ11B via the trench. Recycledmaterial may consist of peridotites, volcanic basement and volcani-clastic sediments eroded from the volcanoes and transported to thetrench by density currents (cf. Goss and Kay, 2006). During subductionerosion, slivers of 11B-rich forearc serpentinites may be mingled withsediments and oceanic crust (i.e.,mélange) and subducted to depths ofat least 100–120 km. A notable advantage of this process is thatsubducted serpentinites follow a distinct, hotter subduction trajectory(following the upper slab surface) that allows them to reach thebreakdown conditions near subarc depths, whereas similar rocks inthe slab interior follow a cooler trajectory for which serpentineminerals likely persist to depths well behind the volcanic front.

We consider a model in which early stage fluids (high B and δ11B)released during shallow metamorphic dehydration of the slab variablymetasomatize the overlying forearcmantlewedge. Tectonic erosion andsubduction of such serpentinites provides an attractive means ofamplifying contents of B and other fluid-mobile elements and alsoincreasing δ11B in the subarc mantle wedge (e.g., Hattori and Guillot,2003; Hyndman and Peacock, 2003; Peacock andHervig, 1999). Indeed,very high enrichment in boron and δ11B is observed in highlyserpentinized peridotites from seamounts located at the outer edge ofMariana forearc (δ11B up to +25‰ in a position ~20–30 km above theslab; Benton et al., 2001), and in high pressure serpentinite mylonitesfrom Erro-Tobbio ultramafic rocks in the Alps (δ11BN20‰; Tonarini andScambelluri, 2010). Even higher δ11B values (N28‰) have beenestimated for fluids associatedwith subduction-relatedmetamorphism(ca. 450 °C, 50 km depth) of oceanic crust from Syros (Marschall et al.,2006). Such strong 11B-enrichment in the fluid is attributed to boronisotopic fractionation accompanying release of structurally boundboronfrom slab minerals during low-temperature (b300 °C) fluid–rockreaction. Alternatively, Straub and Layne (2002) suggest that heavyboron isotopic compositions of many arc magmas may result by mixingof low-δ11B (~1‰) “composite fluids” with metasomatized high-δ11Bmaterial tectonically eroded from the base of the mantle wedge andsubsequently down-dragged to subarc depths.

We evaluate a two-stagemodel for the SSI, inwhich slivers ofmantlealready metasomatized in the forearc are subducted and subjected toprogressive dehydration to generate high δ11Bmetasomatizing fluids asthey descend. Initial composition is assumed to be similar to medianMariana forearc serpentinites (ca. 60 ppm B, δ11B=20‰; end-member‘C’, Table 2). Accordingly, calculated compositions of released fluidsand residual solids have systematically higher δ11B than those obtainedin previously discussed models. Addition of such ‘second-stage’fluids to depleted MORB mantle (DMM) can successfully replicate theB/Nb–B-δ11B systematics of the inferred SSI magma source (Fig. 9). Forexample, at model depths between 90 and 120 km, predicted fluid δ11Bvalues bracket compositions of SSI lavas (~14 to 18‰). Thesecalculations admittedly are non-unique because, among other looselyconstrained variables, there is wide latitude in average composition ofsubductedmetasomatized forearcmantle. However, a useful conclusionis that it is difficult to produce fluids with such heavy B isotopiccompositions unless initial δ11B values are at least comparably high. The

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depth-dependent nature of slab and fluid δ11B values (i.e., heavier Bfluids derived from shallower depths) also can explain cross-arcvariation of this parameter in arc magma sources.

Subduction recycling of previously metasomatized forearc mantleis consistent with the elevated δ11B in SSI arc lavas with respect to thevalues of local serpentinized peridotites, and negligible enrichmentsin moderately fluid-immobile elements in these magmas. Lower 11B-enrichment at the ends of the SSI arc can be attributed to [a]diminished subduction erosion in those areas, [b] increased contribu-tions of AOC- and sediment-derived fluids in the southern andnorthern slab edges, or [c] differences in depth of fluid release (i.e.,fluid δ11B decreases with increasing depth).

Recycling of subducted volcanic crust/sediments is not very wellconstrained at present. Assuming that thesematerials have an averagecomposition similar to the modern arc lavas, this process couldgradually elevate B content and δ11B in themantlewedge— analogousto a “zone refining” process that could amplify effects of multi-stagetectonic recycling of forearc serpentinites.

5. Conclusions

Boron isotope compositions in SSI arc lavas are among the highestvalues reported for mantle-derived lavas. A suitably high-δ11B magmasource cannot be generated solely by addition of local subducted slabcomponents to typical mantle wedge material. Progressive metamor-phism of the subducted slab is expected to liberate fluids that areinitially rich in B and have elevated δ11B values due to selective releaseof 11B (Marschall et al., 2006), yet this process is unlikely to produceobserved δ11B levels in the volcanic arc. Rather, it is proposed that theexceptionally high δ11B compositions were produced by recycling ofheavy B through the subduction zone by subduction erosion. Themostsignificant B reservoirs are found to be frontal arc serpentinitesexposed in and sampled from the forearc. Other reservoirs may bevolcanic basement and sediments transported into the trench bydensity currents. Erosion of the SSI subaerial volcanoes is rapid whencompared to other arcs, and sediment transport is strongly directed tothe east, toward the trench. All these materials are subsequentlyeroded by tectonic processes and subducted to subarc depths wherethey eventually undergo dehydration. At serpentine breakdownconditions, virtually all serpentine-hosted fluids will be released. Itis unlikely that observed δ11B levels in SSI magma sources could beattained without such a multi-stage process.

The involvement of a serpentinite-reservoir in subduction zonescould explain how the high δ11B signature in arc volcanic rockssurvives if the uppermost part of the slab (AOC + sediment) isdepleted in 11B before it reaches the source regions of arc magmas.Subduction recycling of previously metasomatized forearc mantleprovides very high δ11B to arc lavas, whereas intra-slab serpentinizedlithospheric mantle appears to follow a T–P trajectory such thatserpentine breakdown occurs beneath the back-arc region in manysubduction zones. Further studies are required to flesh out the detailsof these processes. Important consequences of the scenarios describedabove, if correct, are that themantlewedge contributesminimally to Binventories in arcs, and that B systematics in arc lavas largely reflectsubduction zone processes.

Supplementarymaterials related to this article can be found onlineat doi: 10.1016/j.epsl.2010.11.008.

Acknowledgements

We thank Peter Baker, Julian Pearce, and Terry Plank for providingmany of samples on which this paper is based. We thank Gray E.Bebout and an anonymous Reviewer for constructive reviews andRichard Carlson for the editorial handling of this paper. This work was

supported by grants from C.N.R. (to Tonarini), the National ScienceFoundation (to Leeman), and Leeman also thanks NSF for time spentin continuing this research since moving there. This study is part ofthe British Antarctic Survey Polar Science for Planet Earth Programmeand was partly funded by The Natural Environment Research Council.

References

Agranier, A., Lee, C.-T.A., Li, Z.-X.A., Leeman, W.P., 2007. Fluid-mobile element budgetsin serpentinized oceanic lithospheric mantle: insights from B, As, Li, Pb, PGEs andOs isotopes in the Feather River Ophiolite, California. Chem. Geol. 245, 230–241.

Baker, P.E., 1978. The South Sandwich Islands: III. Petrology of the volcanic rocks. Brit.Antarct. Surv. Sci. Rept. 93, 1–34.

Barker, P.F., 1995. Tectonic framework of the East Scotia Sea. In: Taylor, B. (Ed.), BackarcBasins: Tectonics and Magmatism. Plenum Press, New York, pp. 281–314.

Barker, P.F., Hill, I.A., 1981. Back-arc extension in the Scotia Sea. Philos. Trans. R. Soc.London, Ser. A 300, 249–262.

Barry, T.L., Pearce, J.A., Leat, P.T., Millar, I.L., LeRoex, A.P., 2006. Hf isotope evidence forselective mobility of high-field-strength elements in a subduction setting: SouthSandwich Islands. Earth Planet. Sci. Lett. 252, 223–244.

Bebout, G.E., 2007. Metamorphic chemical geodynamics of subduction zones. EarthPlanet. Sci. Lett. (Frontiers) 260, 373–393.

Bebout, G.E., Ryan, J.G., Leeman,W.P., Bebout, A.E., 1999. Fractionation of trace elementsby subduction-zone metamorphism — effect of convergent margin thermalevolution. Earth Planet. Sci. Lett. 171, 63–81.

Bebout, G.E., Nakamura, E., 2003. Record in metamorphic tourmalines of subduction-zone devolatilization and boron cycling. Geology 31, 407–410.

Benton, L.D., Ryan, J.G., Tera, F., 2001. Boron isotope systematics of slab fluids as inferredfrom serpentinite seamount, Mariana forearc. Earth Planet. Sci. Lett. 187, 273–282.

Bonatti, E., Lawrence, J.R., Morandi, N., 1984. Serpentinization of oceanic peridotites:temperature dependence of mineralogy and boron content. Earth Planet. Sci. Lett. 70,88–94.

Boschi, C., Dini, A., Früh-Green, L., Kelley, D.S., 2008. Isotopic and element exchangeduring serpentinization and metasomatism at the Atlantis massif (MAR 30 °N):insights from B and Sr isotope data. Geochim. Cosmochim. Acta 72, 1801–1823.

Brenan, J.M., Ryerson, F.J., Shaw, H.F., 1998. The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: experimentsand models. Geochim. Cosmochim. Acta 62, 3337–3347.

Fretzdorff, S., Livermore, R.A., Devey, C.W., Leat, P.T., Stoffers, P., 2002. Petrogenesis ofthe back-arc East Scotia Ridge, South Atlantic Ocean. J. Petrol. 43, 1435–1467.

Goss, A.R., Kay, S.M., 2006. Steep REE patterns and enriched Pb isotopes in southernCentral America arc magmas: evidences for forearc subduction erosion? Geochem.Geophys. Geosyst. 7, Q05016, doi:10.1029/2005GC001163.

Hattori, K.H., Guillot, S., 2003. Volcanic fronts form as a consequence of serpentinitedehydration in the forearc mantle wedge. Geology 31, 525–528.

Hawkesworth, C.J., O'Nions, R.K., Pankhurst, R.J., Hamilton, P.J., Evensen, N.M., 1977. Ageochemical study of island arc and back-arc tholeiites from the Scotia Sea. EarthPlanet. Sci. Lett. 36, 253–262.

Hervig, R.L., Moore, G.M., Williams, L.B., Peacock, S.M., Holloway, J.R., Roggensack, K.,2002. Isotopic and elemental partitioning of boron between hydrous fluid andsilicate melt. Am. Mineral. 87, 769–774.

Hyndman, R.D., Peacock, S.M., 2003. Serpentinization of the forearc mantle. EarthPlanet. Sci. Lett. 212, 417–432.

Ishikawa, T., Nakamura, E., 1994. Origin of the slab component in arc lavas from across-arc variation of B and Pb isotopes. Nature 370, 205–208.

Ishikawa, T., Tera, F., 1997. Source, composition and distribution of fluid in the Kurilemantle wedge: constraints from across-arc variations of B/Nb and B isotopes. EarthPlanet. Sci. Lett. 152, 123–138.

Ishikawa, T., Tera, F., 1999. Two isotopically distinct fluid components involved in theMariana arc: evidence from Nb/B ratios and B, Sr, Nd, and Pb isotope systematics.Geology 27, 83–86.

Ishikawa, T., Tera, F., Nakazawa, T., 2001. Boron isotope and trace element systematics of thethree volcanic zones in the Kamchatka arc. Geochim. Cosmochim. Acta 65, 4523–4537.

Larter, R.D., Vanneste, L.E.,Morris, P., Smyth,D.K., 2003. Tectonic evolutionandstructureof theSouth Sandwich arc. In: Larter, R.D., Leat, P.T. (Eds.), Intra-Oceanic Subduction Systems:Tectonic and Magmatic Processes: Geol. Soc. London Spec. Publ., 219, pp. 255–284.

Leat, P.T., Smellie, J.L., Millar, I.L., Larter, R.D., 2003. Magmatism in the South Sandwicharc. In: Larter, R.D., Leat, P.T. (Eds.), Intra-Oceanic Subduction Systems: Tectonicand Magmatic Processes: Geol. Soc. London Spec. Publ., 219, pp. 285–313.

Leat, P.T., Pearce, J.A., Barker, P.F., Millar, I.L., Barry, T.L., Larter, R.D., 2004. Magmagenesis and mantle flow at a subducting slab edge: the South Sandwich arc-basinsystem. Earth Planet. Sci. Lett. 227, 17–35.

Leat, P.T., Larter, R.D., Millar, I.L., 2007. Silicic magmas of Protector Shoal, SouthSandwich arc: indicators of generation of primitive continental crust in an islandarc. Geol. Mag. 144, 179–190.

Leat, P.T., Tate, A.J., Tappin, D.R., Day, S.J., Owen, M.J., 2010. Growth and mass wasting ofvolcanic centers in the northern South Sandwich arc, South Atlantic, revealed bynew multibeam mapping. Mar. Geol. 275, 110–126.

Leeman, W.P., 1988. Boron contents in selected international geochemical referencesamples. Geostand. Newalett. 12, 61–62.

Leeman, W.P., 1996. Boron and other fluid-mobile elements in volcanic arc lavas:implications for subduction processes. In: Bebout, G.E., Scholl, D., Kirby, S., Platt, J.P.(Eds.), Subduction Top to Bottom: AGU Monograph, 96, pp. 269–276.

284 S. Tonarini et al. / Earth and Planetary Science Letters 301 (2011) 275–284

Leeman, W.P., Sisson, V.B., 1996. Geochemistry of boron and its implications for crustaland mantle processes. In: Grew, E.S., Anovitz, L.M. (Eds.), BORON: Mineralogy,Petrology and Geochemistry in the Earth's Crust, Reviews in Mineralogy: Mineral.Soc. Am., vol. 33, pp. 645–707.

Leeman, W.P., Huang, S., Tonarini, S., 2002. Comparative thermal models for subductionzones: evaluation of slab contributions to volcanic arc magmas. Geochim.Cosmochim. Acta 66, A444.

Leeman, W.P., Tonarini, S., Chan, L.H., Borg, L.E., 2004. Boron and lithium isotopevariations in a hot subduction zone — the southern Washington Cascades. Chem.Geol. 212, 101–124.

LeRoux, P.J., Shirey, S.B., Benton, L., Hauri, E.H., Mock, T.D., 2004. In situ, multiple-multiplier, laser ablation ICP-MS measurement of boron isotopic composition(δ11B) at the nanogram level. Chem. Geol. 203, 123–138.

Livermore, R., Cunningham, A.D., Vanneste, L., Larter, R., 1997. Subduction influence onmagma supply at the East Scotia Ridge. Earth Planet. Sci. Lett. 150, 261–275.

Marschall, H.R., Ludwig, T., Altherr, R., Kalt, A., Tonarini, S., 2006. Syros metasomatictourmaline: evidence for very high-δ11B fluids in subduction zone. J. Petrol. 47,1915–1942.

Marschall, H.R., Altherr, R., Rüpke, L., 2007. Squeezing out the slab — modeling therelease of Li, Be and B during progressive high-pressure metamorphism. Chem.Geol. 239, 323–335.

Nichols, G.T., Wyllie, P.J., Stern, C.R., 1994. Subduction zone melting of pelagicsediments constrained by melting experiments. Nature 371, 785–788.

Noll, P.D., Newsom, H., Leeman, W.P., Ryan, J.G., 1996. The role of hydrothermal fluids inthe production of subduction zone magmas: evidence from siderophile andchalcophile trace elements and boron. Geochim. Cosmochim. Acta 60, 587–611.

Palmer, M.R., 1991. Boron isotope systematics of Halmahera Arc (Indonesia) lavas;evidence for involvement of the subducted slab. Geology 19, 215–217.

Peacock, S.M., Hervig, R.L., 1999. Boron isotope composition of subduction-zonemetamorphic rocks. Chem. Geol. 160, 281–290.

Peacock, S.M., van Keken, P.E., Holloway, S.D., Hacker, B.R., Abers, G.A., Fergason, R.L.,2005. Thermal structure of the Costa Rica–Nicaragua subduction zone. Phys. EarthPlanet. Inter. 149, 187–200.

Pearce, J.A., Baker, P.E., Harvey, P.K., Luff, I.W., 1995. Geochemical evidence forsubduction fluxes, mantle melting and fractional crystallization beneath the SouthSandwich island arc. J. Petrol. 36, 1073–1109.

Pearce, J.A., Barker, P.F., Edwards, S.J., Parkinson, I.J., Leat, P.T., 2000. Geochemistry andtectonic significance of peridotites from the South Sandwich arc-basin system,South Atlantic. Contrib. Mineral. Petrol. 139, 36–53.

Peccerillo, A., Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocksfrom the Kastamonu area, northern Turkey. Contrib. Mineral. Petrol. 58, 63–81.

Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment andits consequences for the crust and mantle. Chem. Geol. 145, 325–394.

Plank, T., 2005. Constraints from thorium/lanthanium on sediment recycling atsubduction zones and the evolution of the continents. J. Petrol. 46, 921–944.

Ranero, C.R., Phipps Morgan, J.P., McIntoch, K., Reichert, C., 2003. Bending-related faultingand mantle serpentinization at Middle America trench. Nature 425, 367–373.

Rodríguez, C., Sellés, D., Dungan, M., Langmuir, C., Leeman, W.P., 2007. Adakitic dacitesformed by intracrustal fractionation of water-rich parent magmas at Nevado deLongaví Volcano (36.2 °S; Andean Southern Volcanic zone, Central Chile). J. Petrol.48, 2033–2061.

Rosner, M., Erzinger, J., Franz, G., Trumbull, R.B., 2003. Slab-derived boron isotopesignatures in arc volcanic rocks from Central Andes and evidence for boron isotopefractionation during progressive slab dehydration. Geochem. Geophys. Geosyst. 4(8), doi:10.1029/2002GC000438.

Roy-Barman, M., Wassemburg, G.J., Papanastassiou, D.A., Chaussidon, M., 1998.Osmium isotope compositions and Re-Os concentrations in sulfide globules frombasaltic glasses. Earth Planet. Sci. Lett. 154, 331–347.

Rüpke, L.H., Phipps Morgan, J.P., Hort, M., Connolly, J.A.D., 2004. Serpentine and thesubduction zone water cycle. Earth Planet. Sci. Lett. 223, 17–34.

Ryan, J.G., Morris, J.D., Tera, F., Leeman, W.P., Tsvetkov, A., 1995. Cross-arc geochemicalvariations in the Kurile island arc as a function of slab depth. Science 270, 625–628.

Savov, I.P., Ryan, J.G., D'Antonio,M., Fryer, P., 2007. Shallow slabfluid release across and alongthe Mariana arc-basin system: insights from geochemistry of serpentinized peridotitesfromMariana Forearc. J. Geophys. Res. 112, B09205, doi:10.1029/200JGR004749.

Scambelluri, M., Müntener, O., Ottolini, L., Pettke, T., Vannucci, R., 2004. The fate of B, Cland Li in the subducted oceanic mantle and in the antigorite breakdown fluids.Earth Planet. Sci. Lett. 222, 217–234.

Smith, H.J., Leeman, W.P., Davidson, J., Spivack, A.J., 1997. The B isotopic composition ofarc lavas from Martinique, Lesser Antilles. Earth Planet. Sci. Lett. 146, 303–314.

Smith, H.J., Spivack, A.J., Staudigel, H., Hart, S.R., 1995. The boron isotopic compositionof altered oceanic crust. Chem. Geol. 126, 119–135.

Spivack, A.J., Edmond, J.M., 1987. Boron isotope exchange between seawater and theoceanic crust. Geochim. Cosmochim. Acta 51, 1033–1043.

Spivack, A.J., Edmond, J.M., 1993. Boron isotope exchange between seawater and theoceanic crust. Geochim. Cosmochim. Acta 51, 1033–1043.

Staudigel, H., Plank, T., White, W.M., Schmincke, H., 1996. Geochemical fluxes duringseafloor alteration of the upper oceanic crust: DSDP Sites 417 and 418. In: Bebout,G.E., Scholl, D., Kirby, S., Platt, J.P. (Eds.), Subduction Top to Bottom: AGUMonograph, 96, pp. I19–I38.

Straub, S.M., Layne, G.D., 2002. The systematics of boron isotopes in Izu arc frontvolcanic rocks. Earth Planet. Sci. Lett. 198, 25–39.

Syracuse, E.M., Van Keken, P.E., Abers, G.A., 2010. The global range of subduction zonethermal models. Phys. Earth Planet. Int. 183, 73–90.

Tanaka, R., Nakamura, E., 2005. Boron isotopic constrains on the source of Hawaiianshield lavas. Geochim. Cosmochim. Acta 69, 3385–3399.

Thomas, C., Livermore, R.A., Pollitz, F.F., 2003. Motion of the Scotia Sea plates. Geophys.J. Int. 155, 789–804.

Tonarini, S., Pennisi, M., Leeman, W.P., 1997. Precise boron analysis of complex silicate(rock) samples using alkali carbonate fusion and ion exchange separation. Chem.Geol. 142, 129–137.

Tonarini, S., Leeman, W.P., Ferrara, G., 2001. Boron isotopic variations in lavas of theAeolian volcanic arc, South Italy. J. Volcanol. Geotherm. Res. 110, 155–170.

Tonarini, S., Pennisi, M., Adorni-Braccesi, A., Dini, A., Ferrara, G., Gonfiantini, R.,Wiedenbeck, M., Gröning, M., 2003. Intercomparison of boron isotope andconcentration measurements: Part I: Selection, preparation and homogeneitytests of the intercomparison materials. Geostand. Newslett. 27, 21–39.

Tonarini, S., Agostini, S., Innocenti, F., Manetti, P., 2005. δ11B as tracer of slabdehydration and mantle evolution in western Anatolia Cenozoic magmatism. TerraNova 17, 259–264.

Tonarini, S., Agostini, S., Doglioni, C., Innocenti, F., Manetti, P., 2007. Evidence forserpentinite fluid in convergent margin systems: the example of El Salvador (CentralAmerica) arc lavas. Geochim. Geophys. Geosyst. 8, doi:10.1029/2006GC001508.

Tonarini, S., Scambelluri, M., 2010. Boron and Strontium isotope systematics in deeplysubducted alpine-serpentinites: evidence of high-11B fluid-flow. Geophys. Res.Abstr. vol. 12 EGU 2010.

Turner, S., Tonarini, S., Bindeman, I., Leeman, W.P., Schaefer, B., 2007. Boron and oxygenisotope evidence for recycling of subducted components over the past 2.5 Gyr.Nature 447, 702–705, doi:10.1038/nature05898.

Ulmer, P., Trommsdorff, V., 1995. Serpentine stability to mantle depths and subduction-related magmatism. Science 268, 858–861.

Vanneste, L.E., Larter, R.D., 2002. Sediment subduction, subduction erosion, and strainregime in the northern South Sandwich forearc. J. Geophys. Res. 107 (B7), 2149,doi:10.1029/2001JB000396.

Vils F.A., 2009. Light elements in oceanic and ophiolitic serpentinites. PhD ThesisUniversité de Neuchâtel.

Vils, F., Tonarini, S., Kalt, A., Seitz, H.M., 2009. Boron, lithium and strontium isotopes astracers of seawater-serpentine interaction at Mid-Atlantic ridge, ODP Leg 209.Earth Planet. Sci. Lett. 286, 414–425.

Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, London, p. 466.Woodhead, J., Eggins, S., Gamble, J., 1993. High field strength and transition element

systematics in island arc and back-arc basin basalts: evidence for multi-phase meltextraction and a depleted mantle wedge. Earth Planet. Sci. Lett. 114, 491–504.

You, C.F., Chan, L.H., Spivack, A.J., Gieskes, J.M., 1995a. Lithium, boron and their isotopesin sediments and pore waters of Ocean Drilling Program Site 808, Nankai Troughimplications for expulsion in accretionary prisms. Geology 23, 37–40.

You, C.F., Spivack, A.J., Gieskes, J.M., Rosenbauer, R., Bischoff, J.L., 1995b. Experimentalstudy of boron geochemistry: implications for fluid processes in subduction zones.Geochim. Cosmochim. Acta 59, 2435–2442.