Chemical and isotopic compositions of glass inclusions and their host clinopyroxenes from ODP Hole 157-956B

We report ion microprobe analyses of S and O isotopes in glass inclusions and their host clinopyroxenes (Cpx) from Miocene basaltic hyaloclastites drilled during Ocean Drilling Program (ODP) Leg 157. This is the first study where high-precision in situ d34S and d18O measurements were applied in order to constrain the isotopic variability of intraplate magmas and to have insights into establishment of isotopic equilibrium between the melt and crystallizing phenocrysts. Glass inclusions range from tholeiitic to alkali basaltic composition and contain varying to very high proportions of sulfur dissolved as sulfate [S(6+)/S(total) = 0.40 to 0.87] at S contents of 800 to 2100 ppm. The d34S values varying from -1.0 to +8.5 +/-1.5 per mil, exceed significantly the range reported for fresh submarine midocean ridge basalt (MORB) and oceanic island basalt (OIB) glasses, and positively correlate with Cl concentrations and S(6+)/S(total) ratios. The d18O values in glass inclusions and their host Cpx crystals analyzed within 50 µm distance away from inclusions are also strongly variable (inclusions: +5.0 to +8.2 +/-0.7 per mil; Cpx: +5.7 to +8 +/-0.7 per mil) and are in agreement with O-isotope equilibrium between Cpx and basaltic melt. A broad d18O range (+5.6 to +7.3 per mil) decoupled from that of the melt inclusions was also found in the Cpx crystals measured >500 µm away from inclusions. The variations of d34S and d18O in glass inclusions are interpreted as resulting from a combination of degassing and assimilation processes. Hydrothermally altered basaltic crust (d34S = +4.0 to +9.6 per mil, d18O = +9.0 per mil) mixed with oceanic sediments (d18O ~25 per mil) and containing up to 5 wt.% of seawater sulfate (d34S = +21 per mil, d18O = +9.5 per mil) represents the most probable contaminating end-member. The oxygen isotope heterogeneity found in most Cpx crystals and their glass inclusions implies that magma contamination and inclusion-trapping occurred during a few years only, i.e., too short for O isotopic homogeneity to be reestablished in the entire crystals.