Boron isotopic composition of subduction-zone metamorphic rocks

Many are lavas contain material derived from subducted oceanic crust and se diments, but it remains unresolved whether this distinctive geochemical sig nature is transferred from the subducting slab by aqueous fluids, silicate melts, or both. Boron isotopic measurements have the potential to distingui sh between slab transfer mechanisms because B-11 fractionates preferentiall y into aqueous fluids whereas little fractionation may occur during partial melting. Previous studies have shown that delta(11)B values of island are lavas (-6 to +7) overlap the range of delta(11)B values for altered oceanic crust (-5 to +25) and pelagic sediments and turbidites (-7 to +11). Second ary ion mass spectrometry (SIMS) analyses of minerals in subduction-zone me tamorphic rocks yield delta(11)B = -11 to -3 suggesting that slab dehydrati on reactions significantly lower the delta(11)B values of subducted oceanic crust and sediments. In order to explain the higher delta(11)B values repo rted for are lavas as compared to subduction-zone metamorphic rocks, the B- bearing component derived from the metamorphosed slab must enriched in B-11 relative to the slab, favoring an aqueous fluid as the slab transfer mecha nism. (C) 1999 Elsevier Science B.V. All rights reserved.