A limited role for metasomatized subarc mantle in the generation of boron isotope signatures of arc volcanic rocks

Metasomatized subarc mantle is often regarded as one of the mantle reservoirs enriched in fluid-mobile elements (FMEs; e.g., B, Li, Cs, As, Sb, Ba, Rb, Pb), which, when subject to wet melting, will contribute to the characteristic FME-rich signature of arc volcanic rocks. Evidence of wet melts in the subarc mantle wedge is recorded in metasomatic amphibole-, phlogopite-, and pyroxene-bearing veins in ultramafic xenoliths recovered from arc volcanoes. Our new B and δ 11 B study of such veins in mantle xenoliths from Avachinsky and Shiveluch volcanoes, Kamchatka arc, indicates that slab-derived FMEs, including B and its character-istically high δ 11 B, are delivered directly to a melt that experiences limited interaction with the surrounding mantle before eruption. The exceptionally low B contents (from 0.2 to 3.1 µg g –1 ) and low δ 11 B (from –16.6‰ to +0.9‰) of mantle xenolith vein minerals are, instead, products of fluids and melts released from the isotopically light subducted and dehydrated altered oceanic crust and, to a lesser extent, from isotopically heavy serpentinite. Therefore, melting of amphibole- and phlogopite-bearing veins in a metasomatized mantle wedge cannot alone produce the characteristic FME geochemistry of arc volcanic rocks, which require a comparatively large, isotopically heavy and B-rich serpentinite-derived fluid component in their source.


INTRODUCTION
Direct observation of the processes of ele ment transfer and isotope fractionations associ ated with slab dehydration in subduction zones is not possible. However, the classic study of Tatsumi (1989) suggested that a hydrous compo nent released from dehydrating slabs in subduc tion zones is responsible for the depression of the wet solidus in depleted mantle wedge harz burgite, thus generating fluidmobile element (FME)-enriched arc volcanic rocks. Contrary to what is seen at midocean ridges, elevated water contents of the subarc mantle control the extensive melting in subduction zones (Kelley et al., 2006). Subsequently, it has been suggested that a slabderived hydrous fluid or melt perco lates through the subarc mantle via an intercon nected vein network (Pirard and Hermann, 2015;Plümper et al., 2016), comprising metasomatic mineral phases such as hornblende, phlogopite, and pyroxenes (GSA Data Repository Tables DR1 and DR2 1 ). Previous studies (e.g., Kepe zhinskas et al., 1995;Kepezhinskas and Defant, 1996) speculated that metasomatic veins could be mantle reservoirs of slabderived elements, which, upon melting, will generate the char acteristic FMErich signature of arc volcanic rocks. In this model, the role of the subducting hydrated oceanic plate is central to the genera tion of FMEenriched arc volcanic rocks, since both primitive mantle and midoceanicridge basalt (MORB) source mantle contain only traces of FMEs (McDonough and Sun, 1995;Marschall et al., 2017).
Boron and δ 11 B (the per mil difference be tween the 11 B/ 10 B of a sample and NIST [U.S. National Institute of Standards and Technology] 951 boric acid) have been widely used in studies of slabderived fluids in subduction zones (de Hoog and Savov, 2018, and references therein). Boron and its isotopic composition (as δ 11 B) are particularly sensitive tracers of slabderived metasomatic agents because of the highly fluid mobile nature of B (Hervig et al., 2002). Boron is enriched in subducting oceanic lithosphere relative to Bpoor mantle (e.g., Marschall et al., 2017), and a wide range of δ 11 B values (~70‰) is preserved in natural materials (e.g., de Hoog and Savov, 2018, and references therein). How ever, this versatile tracer has not been employed previously in the investigation of FME budgets in metasomatized (veined) subarc mantle xeno liths. Here, we report, for the first time, B and δ 11 B measurements demonstrating that meta somatic veins formed by the percolation of hy drous melts and fluids through the subarc mantle cannot play a significant role in the generation of arc magmas.

GEOLOGICAL BACKGROUND
The Kamchatka arc extends from the Kuril Islands in the south to northern Kamchatka, where it terminates at the Aleutian transform fault (Fig. 1). It is situated on the continental margin and consists of three volcanic belts: the Eastern volcanic front (EVF), the Cen tral Kam chatka depression (CKD), and the Sredinny Range (SR; e.g., Churikova et al., 2001;Portnyagin and Manea, 2008). For this study, we collected mantle xenoliths from the Avachinsky and Shiveluch volcanoes (for min eral majorelement abundances, petrology, and geothermometry, see the Data Repository), in addition to revisiting the Shiveluch mantle xeno lith suite of Bryant et al. (2007).

RESULTS
Boron contents and δ 11 B ratios of the hydrous vein minerals (amphibole and phlogopite) and nominally anhydrous mantle minerals (olivine, pyroxene, and plagioclase) were measured by secondary ion mass spectrometry (SIMS) using a Cameca 1270 ion microprobe at the University of Edinburgh (for analytical methods, see the Data Repository).

DISCUSSION
Contrary to earlier predictions of meta somatized mantle wedge playing a fundamen tal role in generating the characteristic FME enriched arc volcanic rocks (e.g., Kepezhinskas et al., 1995;Kepezhinskas and Defant, 1996), the low B abundances and δ 11 B values of the metasomatized subarc mantle are unexpected. The majority of vein compositions can be repro duced by mixing of variable amounts of three components: (1) isotopically light composite slab fluid, (2) residual slab melt, and (3) the depleted mantle ( Fig. 2; for model input param eters, see Table DR5). Slabderived fluids can be generated either by dehydration of mélange diapirs in the subarc mantle under the arc front (Savov et al., 2007;Nielsen and Marschall, 2017, and references therein) and/or by ser pentine breakdown in the forearc, followed by dehydration of altered oceanic crust (AOC) by chlorite and amphibole breakdown under the arc front, as previously proposed in the Kamchatka subduction zone model (KonradSchmolke and Halama, 2014). Other hydrous minerals typi cally constituting the AOC, such as lawsonite and phengite, are absent in the top 10 km of the subducting slab in Kamchatka and are therefore not likely to contribute B to the subarc mantle (KonradSchmolke and Halama, 2014).
Dehydration of sediments and AOC, in response to rising pressure and temperature with ongoing subduction, leads to B isotopic fractionation between fluids and silicates, spe cifically, 11 B depletion in silicates. Trigonally coordinated 11 B preferentially partitions into fluids, and tetrahedrally coordinated 10 B parti tions into silicate minerals and melts in lowpH environments (Kakihana et al., 1977;Peacock and Hervig, 1999;Hervig et al., 2002;Wunder et al., 2005;Pabst et al., 2012;KonradSchmolke and Halama, 2014). Therefore, vein amphibole and phlogopite preserving low δ 11 B (i.e., <-7‰) may have equilibrated with slab fluid released by chlorite dehydration in the AOC (Rüpke et al., 2004;KonradSchmolke and Halama, 2014) or residual slab melt generated at ~90-120 km depthtoslab, assuming vertical transport of the released fluid or melt. In cold subduction zones, fully hydrated AOC and sediments dehydrate in several steps before they are subducted to 120 km (Rüpke et al., 2004), where they re lease isotopically light B upon their dehydration (Fig. 2). Isotopically light fluid, however, could also have been released by dehydration of ser pentinite that interacted with sediment ( Cannaò et al., 2015).
The higher δ 11 B (>-5‰) of some of the vein minerals requires at least some forearc serpen tinite fluid influx (δ 11 B = ~14‰; Tonarini et al., 2011) into the subarc mantle. Vein amphiboles with the highest δ 11 B require up to 15% of their B contents to be derived from serpentinite and 85% from a composite lithology comprising 99% AOC and 1% sediment (Fig. 2).
Our data demonstrate a negligible contribu tion to the otherwise large outfluxes of boron at volcanic arcs. The veins represent a volumetri cally minor mantle B end member with insuffi cient B concentrations to significantly skew the composition of the erupted arc volcanic rocks. Instead, a slabderived component enriched in 11 B must transit relatively rapidly through the mantle wedge (Fig. 3). In Kamchatka, the lim ited sedimentary pile (435 m of ashysiliceous clay; Plank, 2014) and the AOC are not likely to carry B deeper than the forearc, as more than 80% of their original boron content is released during shallow slab dehydration (Savov et al., 2007), and its further dehydration under the arc front releases isotopically light fluids (δ 11 B = -5.2‰; Fig. 2).
Several prior studies have established that serpentinite can host up to 80 µg g -1 B and retain a high δ 11 B signature of up to +25‰ in shal low subduction settings (Benton et al., 2001;Scambelluri and Tonarini, 2012;Harvey et al., 2014;de Hoog and Savov, 2018, and references therein). The results of our model suggest that fluids from dehydration of subducted forearc serpentinite and AOC, rather than metasoma tized veins in the subarc mantle, are responsible for the boron elemental and isotopic signature of Kamchatka arc volcanic rocks ( Fig. 2; Ishikawa et al., 2001;Churikova et al., 2007).
It has been shown that the initially high δ 11 B value of slab fluid rapidly decreases as it moves away from the dehydration site (Prigent et al., 2018), unless the fluid flow is focused in an inter connected vein network (Fig. 3; Pirard and Hermann, 2015;Plümper et al., 2016). The fluid flow through this vein network must be rapid for only limited chemical exchange to occur be tween the vein minerals and the percolating slab derived fluid (e.g., John et al., 2012). Large vari ations of δ 11 B in amphibole and phlogo pite in samples SHX0318, SHX0304, and SH98X16 ( Fig. 2; Table DR4) suggest that the veins inves tigated in this study sampled multiple pulses of slabderived fluids and melts originating from different depths. Alternatively, the slabderived fluids and melts could have been sourced by mélange diapirs in the mantle wedge (Nielsen and Marschall, 2017, and references therein) that are composed of a mixture of slab and hy drated forearc mantle lithologies with variable δ 11 B compositions.

CONCLUSIONS
The boron contents and δ 11 B values of vein minerals in Kamchatka arc xenoliths from Shiveluch and Avachinsky volcanoes are incon sistent with the interpretation that they provide a significant contribution to the boron budget of Kamchatka arc volcanic products. The veins re cord multiple pulses of fluids and melts percolat ing through the subarc mantle, ranging from iso topically light AOCderived fluids and melts to isotopically heavy forearc serpentinite derived fluids. The fluid flow appears to be focused in veins connecting either the slab dehydration sites or mélange diapirs with the magmagen eration region to facilitate the rapid transport of heavy B to arc magmas, with limited interaction with the vein minerals.