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Boron is an effective tracer of fluid processes in subduction zones. High B and δ11B in arc magmas require efficient B transfer from the slab to magma source regions. The Higashi-akaishi metaperidotite body in the Sanbagawa high-pressure belt, Japan, is composed of locally serpentinized mantle wedge peridotites exhumed in a subduction channel. Cores of coarse-grained primary mantle olivine have 1-4 µg/g B, enriched compared to typical mantle olivine, and δ11B of --10%c to --1%o, consistent with incorporation of fluids from dehydrating slab at --90-120 km depth. Rims of primary mantle olivine as well as olivine neoblasts have even higher B (5-20 µg/g) and higher δ11B (--8%c to +2%o) due to incorporating slab fluids at depths of --70-100 km. Antigorite, formed below 650 °C, shows comparable δ11B and B contents as olivine rims. The data show that olivine is capable of scavenging significant amounts of B from fluids by diffusion and recrystallization at sub-arc pressures and temperatures. Considering the large amount of olivine in the mantle wedge, transport of slab-derived material to magma sources requires processes with minimal interaction with mantle peridotite, such as intensely channelized fluid flow or ascent of mélange diapirs, and limited porous fluid flow. [ABSTRACT FROM AUTHOR]

This study demonstrates that significant matrix‐dependent bias occurs during the measurement of Li mass fractions by SIMS in silicate glasses. This bias is particularly evident for basaltic and ultramafic samples, where published SIMS data of reference materials show discrepancies up to 40% compared with other measurement techniques. Matrix effects can be successfully corrected by taking into account the SiO2 content of the analysed material. New matrix‐corrected SIMS data for a suite of silicate glass reference materials are presented (glasses from MPI‐DING, USGS, Smithsonian and NIST, with a range of 46–76 g per 100 g SiO2), which are on average within 4% of preferred values (PV) as published in the GeoReM database. As several PV for Li in GeoReM are biased by the inclusion of inaccurate SIMS data, updated reference values are proposed that include the new SIMS dataset presented here. Key Points: Significant matrix effects occur during SIMS analysis of Li contents of silicate glasses, which has affected SIMS data compiled in GeoReM. A correction procedure is presented which effectively removes matrix bias. New Preferred Values for Li are proposed for MPI‐DING and USGS glasses. [ABSTRACT FROM AUTHOR]

The Monviso ophiolite Lago Superiore Unit (LSU; Western Alps) constitutes a well-preserved, largely coherent fragment of eclogitic upper oceanic lithosphere subducted to c. 80 km depth (between 50 and 40 Ma) and exhumed along the subduction interface. Within-slab, 10–100 m thick, eclogite-facies shear zones cut this section; the Intermediate Shear Zone (ISZ) follows the boundary between gabbroic and basaltic eclogites (associated with minor calcschist lenses), and the Lower Shear Zone (LSZ) marks the contact between gabbroic eclogites and the antigorite serpentinite sole. Up to 10 m fragments of mylonitic gabbroic eclogites were transported within serpentinite schists from the LSZ during eclogite-facies deformation. Metasomatic rinds, formed on these fragments during peak to early retrograde lawsonite-eclogite-facies metamorphism (c. 550°C and 2·6 GPa), document episodic, prominent rock–fluid interaction along intra-slab, channelized fluid migration pathways associated with deformation. We present new petrological and geochemical data on hydrous eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and serpentinite-derived ultrabasic schists from block rinds. Bulk-rock compositions, laser ablation inductively coupled plasma mass spectrometry mineral analyses and X-ray Cr and Mg maps of garnet and clinopyroxene demonstrate that these samples underwent significant enrichments in Mg, Cr, Ni, ± large ion lithophile elements and prominent depletions in Fe and V during eclogite-facies metasomatism. Boron isotope data for phengite (δ11B = 0 to + 7‰; 80 < B < 130 µg g–1), clinopyroxene and chlorite (δ11B = –7 to + 4‰; B < 3 µg g–1), and antigorite (δ11B = –4 to 0‰; 20 < B < 30 µg g–1) suggest that the metasomatic block rinds formed during interaction with serpentinite-derived fluids. These compositional patterns point to focused, fluid-mediated element transfer through the subducted slab. Serpentinite-derived fluids, via antigorite breakdown some 15–20 km deeper than the maximum depth reached by these eclogites, thereby equilibrate with fluids derived from oceanic crust and/or sedimentary material. Although slab components of diverse flavour have long been recognized to be central in triggering island arc magmatism, this example is among the first to document synkinematic, long-range, large-scale, channelized dehydration fluid flow within subducted oceanic slabs. [ABSTRACT FROM AUTHOR]

Despite supporting productive ecosystems in the high latitudes, the relationship between macroand micronutrients in sea ice environments and their impact on surface productivity is poorly documented. In seawater, the macronutrient phosphate and the micronutrient cadmium follow similar distributions, which are controlled by biological processes in surface waters. We investigated cadmium and phosphate in sea ice brine, and the biological and physical processes controlling their distribution. Cadmium concentrations in sea ice brine ranged from 0.09-2.4 nmol kg-1, and correlated well with salinity. Our results show that micronutrients in sea ice are most probably sourced from the seawater from which it froze rather than external sources such as atmospheric deposition. The weak correlation between sea ice cadmium and phosphate, and the positive relationship between cadmium and biomass, suggests against biological uptake being a principal control over micronutrient distribution even in a highly productive setting. Instead, brine expulsion and dilution play a dominant role in cadmium distribution in sea ice. Nutrient dilution within brine channels during melting, and contrasting sea ice and open water phytoplankton populations, suggests that late spring sea ice is not a significant source of nutrients or biomass to seawater. We suggest that future changes in sea ice seasonality may impact nutrient distribution and Antarctic marine ecosystems. [ABSTRACT FROM AUTHOR]

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 δ11B study of such veins in mantle xenoliths from Avachinsky and Shiveluch volcanoes, Kamchatka arc, indicates that slab-derived FMEs, including B and its characteristically high δ11B, 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 δ11B (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. [ABSTRACT FROM AUTHOR]