Presented at the 1998 Annual Meeting of the Geological Society of America

BEBOUT, G. E., Dept. Earth & Environmental Sci., Lehigh Univ., Bethlehem, PA 18015, geb0@lehigh.edu; NAKAMURA, E., and NAKANO, T., Institute for the Study of Earthís Interior, Okayama Univ. at Misasa, Tottori-ken 682-0193, Japan

    Boron isotopes are increasingly being used to trace material transfer at various depths in subduction zones and should, in general, be useful in studies of fluid-rock interaction and metasomatism because of the affinity of B for aqueous fluids. Relatively little is known, however, regarding the sources and nature of B-isotope fractionation in high-T fluid-rock settings. The Catalina Schist (CA) affords application of B isotopes in a system for which previous detailed petrologic, major and trace element, and stable isotope (O-H-C-N) work has provided specific hypotheses regarding devolatilization and metasomatic alteration.

   d¹¹B_SRM-951 of low-grade (lawsonite-albite, lawsonite-blueschist) metasedimentary rocks (-16 to -10 per mil) resembles that of the structurally bound component in seafloor sediments (-17 to -6 per mil). In metasedimentary rocks which experienced higher-T prograde paths (up to ~650 degrees C), B contents are decreased and d¹¹B is increased to values as high as -1.3 per mil approaching those of altered oceanic crust (-3 to +17 per mil). Shifts to higher d ¹¹B values with decrease in B content are consistent with the loss of B with relatively low d¹¹B to fluids during prograde devolatilization. Applications of Rayleigh and batch volatilization models to data for high-grade units, complicated by uncertainties in protolith B content, indicate D¹¹B_fluid-rock of ~-3 to -5 per mil. The d¹¹B of several vein and melange matrix samples in low-grade units is consistent with their inheritance of d¹¹B from sedimentary rocks. A pegmatite derived by melting of metasedimentary rocks has d¹¹B of -1.3 per mil identical to that of source rocks; this observation is compatible with the pegmatite having inherited source-rock d¹¹B previously increased by devolatilization. d¹¹B variations in the metasedimentary and metasomatic rocks closely mirror d¹⁵N and d¹³C systematics in the same lithologies.

    The d¹¹B of the low-grade metasedimentary rocks demonstrates that the relatively low-d¹¹B, structurally bound B component in sediments is retained to depths of up to 45 km in relatively cool subduction-zones. Data for the Catalina Schist provide a first suggestion that devolatilization in forearcs may produce shifts in the d ¹¹B of sediments subducted to subarc regions; sediment melts contributed to arc source regions would be expected to have d¹¹B higher than that of the structurally bound B initially subducted in seafloor sediments.