BORON ISOTOPIC COMPOSITION OF SUBDUCTION-ZONE METAMORPHIC ROCKS

Abstract

Many arc lavas contain material derived from subducted oceanic crust and sediments, but it remains unresolved whether this distinctive geochemical signature is transferred from the subducting slab by aqueous fluids, silicate melts, or both. Boron isotopic measurements have the potential to distinguish between slab transfer mechanisms because fractionates preferentially into aqueous fluids whereas little fractionation may occur during partial melting. Previous studies have shown that δ values of island arc lavas (−6 to +7) overlap the range of δ values for altered oceanic crust (−5 to +25) and pelagic sediments and turbidites (−7 to +11). Secondary ion mass spectrometry (SIMS) analyses of minerals in subduction-zone metamorphic rocks yield δ=−11 to −3 suggesting that slab dehydration reactions significantly lower the δ values of subducted oceanic crust and sediments. In order to explain the higher δ values reported for arc lavas as compared to subduction-zone metamorphic rocks, the B-bearing component derived from the metamorphosed slab must be enriched in relative to the slab, favoring an aqueous fluid as the slab transfer mechanism.