Equilibrium boron isotope fractionation during serpentinization and applications in understanding subduction zone processes

Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (delta 11B) of serpentinite is used in studying watermediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The 11B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (beta-factor) of aqueous B(OH)3 and B(OH)4- are 1000ln beta B(OH)3 = -20.1 + 37.07(1000/T) + 8.2(1000/T)2 and 1000ln beta B(OH)4- = -13.94 + 24.43 (1000/T) + 9.12(1000/T)2 at the P-T range of 0-500 MPa and 298-1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Delta 11Blizardite-fluid = 15.89-22.88(1000/T) - 0.19(1000/T)2 between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the delta 11B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts.

Automated summary

This study investigates the boron isotope fractionation between minerals and aqueous fluids, revealing the order of boron enrichment in minerals and the limited fractionation between minerals formed during subduction and fluids released from subducted crustal lithologies. The findings provide insights into boron sources and pathways in subduction zones and the recycling of trace phases in the deep mantle and ocean island basalts.