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

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.