![]() ![]() Dehydration reactions as serpentinite is carried to sub-arc depths lead to release of 11B-enriched fluid, which is either released from the subducting slab beneath the arc 21, 22, or enriches the forearc mantle which is in turn dragged to subarc depths 23, 24. In contrast, hydration of oceanic crustal and mantle rocks by seawater generally leads to enrichment in and δ 11B, in particular serpentinisation of olivine-rich rocks of the upper mantle 16, 17, 18, 19, 20. ![]() Subducted sediments show high but usually negative δ 11B 15, and are therefore very unlikely to be the source of positive δ 11B values in volcanics. Accurate characterisation of the boron chemistry of the altered oceanic lithosphere entering subduction zones is an absolute pre-requisite if boron is to be used as a proxy either in modelling arc volcanism or in deep recycling of volatiles. While this has been rejected on the basis of global volcanic geochemistry 13 recent data on <300 Ma carbonatites suggest addition of isotopically heavy boron to the mantle in the past 14. Olivine formed by dehydration of serpentine can contain significant boron 10, 11, 12, leading to suggestions that seawater boron may be recycled in slabs into the deep mantle. Systematic decreases in B/Be, B/Nb and δ 11B across arcs suggest progressive release of boron from a subducted source rock 1, 2, 3, 4, 5, 6, 7, 8, while along-arc variations in B/Zr have been linked to enhanced release of boron from transform faults in the subducted slab 9. Both boron content () and boron isotope ratio (δ 11B) are higher in subduction zone volcanic rocks than in mid-ocean ridge or intraplate basalts, and this is generally seen as a signature of fluids derived from seawater ( = 4–5 p.p.m. Boron is a key element in tracking the fate of ocean-derived water in subduction zones. ![]()
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