THE ROLE OF AQUEOUS FLUIDS IN THE SLAB-TO-MANTLE TRANSFER OF BORON, BERYLLIUM, AND LITHIUM DURING SUBDUCTION: EXPERIMENTS AND MODELS

Abstract

The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island arc magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900°C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be (~2) exceeds that for either Li (~0.2) or B (~0.02) and values are 100× (B,Li) to 1000× (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium.