Boron Isotope Variation During Subduction, Melting, and Ascent of Arc Magmas

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9973040HervigExperiments to determine the behavior of boron isotopes under conditions occurring in subduction zones will be performed. These experiments are designed to measure boron isotope fractionation between a) mafic silicate melts and hydrous fluid and b) muscovite and hydrous fluid and to measure self-diffusion coefficients for boron isotopes in melts. The experiments will allow boron isotope analyses of subducted rocks, mantle wedge materials altered by fluids, and the arc magmas themselves to be considered in the context of isotopic fractionation that may be occurring during the devolatilization of subducted and erupted materials. When combined with analyses of natural samples the following hypotheses can be tested: 1) devolatilization is the dominant mechanism for mass transfer into the overlying mantle wedge, 2) degassing of magmas during ascent does not alter the boron isotope ratio of melts and thus boron isotopic composition of primitive and evolved melt inclusions are equally indicative of the source region. The analyses of the experiments will be obtained using the secondary ion mass spectrometer (SIMS, or ion microprobe) ideally suited to obtain precise boron isotopic microanalyses. Characterizing the boron isotopic composition of subducted and erupted rocks in the context of experiments showing how boron isotope ratios in these minerals and melts change will increase our understanding of the processes occurring in subduction zones and the crust-mantle recycling mechanisms that occur at convergent plate margins.