LAUSR

Abstract Boron (B) and its isotopes are powerful geochemical tracers of subduction processes, enabling to understand the role played by fluids in large-scale mass-transfer at convergent margins. In such settings, oceanic and forearc serpentinites are the most important reservoirs for B and heavy B isotopes contributing to the genesis of arc lavas. Literature data support the release of significant amount of B during the chrysotile/lizardite to antigorite (atg) phase transition and during the partial to complete antigorite dehydration to form secondary peridotites. However, the information concerning the B-isotope fractionation associated to these reactions is limited. In this Letter, I attempt to model the B-isotope fractionation affecting subducted serpentinites through the ab initio B-isotope fractionation calculations provided for trigonal- and tetrahedral-coordinated B in both minerals and fluids. The obtained results mainly confirm serpentinites as the main vector of positive δ11B to depth as shown by natural high-pressure serpentinites analysed so far. However, a lack of information on pH conditions hampers the proper estimate of the fractionation of B in serpentinites during their prograde evolution in subduction zones. This may represent a limit in the approach proposed here, since poor-alkaline (close to neutral) pH conditions can result in low (potentially negative) δ11Batg signatures. Finally, the Letter try to address new research trends and major topics worth investigating to better unravel the B cycle in serpentinites and their dehydration products.