Boron isotopic fractionation in subducted oceanic crust

Subducted oceanic crust plays an important role in controlling the chemical budget of the crust and mantle and in the composition of arc lavas. Oceanic eclogites represent fragments of oceanic crust that have been through a subduction zone and were subsequently exhumed and exposed. A range of geochemical signatures in oceanic eclogites have been studied to unravel the fluid-rock interaction processes. A key tracer is the trace-element boron and its isotope ratio (¹¹B/¹⁰B) in oceanic eclogites with its great potential for quantifying mass transfer processes at convergent margins. Current models predict a strong decrease of δ¹¹B values in the subducting crust with progressive dehydration to values much below that of the depleted mantle (appr. -7 ‰).

We have analyzed elemental abundances and boron isotopic compositions of oceanic metamorphic rocks, from Zambezi Belt, Cabo Ortegal complex, Raspas Complex, Syros Island, and Tian Shan. Whole-rock B/Pr ratios were used to quantify the progress of dehydration. The boron isotopic composition of almost all samples (approximately -10 to +5 ‰) ranges from δ¹¹B values close to that of fresh MORB to that of typical altered oceanic crust. Also, the sample set shows no correlation between δ¹¹B values and B/Pr, as would be expected from current theoretical models. Our results, thus, demonstrate that B isotopic fractionation in subducted oceanic crust is much smaller than predicted. We suggest that this discrepancy can be resolved by accepting high pH values in high-pressure hydrous fluids, which show a much smaller boron isotope fractionation in equilibrium with B-bearing silicates.