Beryllium and boron in subduction zone minerals: An ion microprobe study

Abstract

Secondary ion mass spectrometry was used to measure the distribution of Be, B, Rb, Sr, Y, Nb, Ba, and Ce among minerals in rocks from the Franciscan Complex and Catalina Schist paleosubduction zone terranes. The rocks examined represent a wide range of metamorphic pressure-temperature conditions (6–15 kb, 300–750°C) and include metasomatized and migmatitic samples. White mica is the primary host of Be (2.5 ppm) and B (107 ppm) in the rocks examined (average concentrations). Amphibole (Be 0.7, B 15 ppm), lawsonite (Be 0.9, B 37 ppm), and biotite (Be 0.9, B 25 ppm) may also contain significant amounts of both Be and B. Omphacite may host Be (4.1 ppm in veins, 0.7 ppm groundmass) and chlorite is a minor host of B (22 ppm). Minerals in these subduction zone metamorphic rocks that lack measurable Be or B include albite, apatite, calcite, clinozoisite, epidote, garnet, ilmenite, sphene, stilpnomelane, plagioclase, quartz, rutile, and zoisite. White mica, biotite, and stilpnomelane are the major mineral hosts of Rb (90, 300, and 30 ppm, respectively) and Ba (2100, 1000, and 4700 ppm). Epidote and lawsonite are the major host minerals of Sr (600 and 300 ppm) and Ce (12 and 15 ppm). Garnet, epidote, lawsonite, and sphene host Y (100, 30, 40, and 60 ppm, respectively). Sphene (77 ppm, 1 analysis), ilmenite, and rutile host Nb. Evidence of veining and metasomatism is strongly correlated with Be and/or B enrichment of white mica, omphacite, amphibole, and lawsonite at the P-T conditions examined. Both Be and B appear to be transported in aqueous fluids at depths of 20–50 km in subduction zones. The transfer of Be, B, and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids. Transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure-dependent dehydration reactions or, alternatively, the Be and B in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis.