Gurenko, Andrey A., Chaussidon, Marc and Schmincke, Hans-Ulrich (2002) Magma ascent and contamination beneath one intraplate volcano: Evidence from S and O isotopes in glass inclusions and their host clinopyroxenes from the Miocene basaltic hyaloclastites southwest of Gran Canaria (Canary Islands). Geochimica et Cosmochimica Acta, 65 (23). pp. 4359-4374. DOI 10.1016/S0016-7037(01)00737-2.

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Abstract

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 δ34S and δ18O 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 (S6+/Stotal = 0.40 to 0.87) at S contents of 800 to 2100 ppm. The δ34S values varying from -1.0 to +8.5 ± 1.5‰, 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 S6+/Stotal ratios. The δ18O 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‰; Cpx: +5.7 to +8 ± 0.7‰) and are in agreement with O-isotope equilibrium between Cpx and basaltic melt. A broad δ18O range (+5.6 to +7.3‰) decoupled from that of the melt inclusions was also found in the Cpx crystals measured >500 μm away from inclusions.

The variations of δ34S and δ18O in glass inclusions are interpreted as resulting from a combination of degassing and assimilation processes. Hydrothermally altered basaltic crust (δ34S = +4.0 to +9.6‰, δ18O = +9.0‰) mixed with oceanic sediments (δ18O ≈ 25‰) and containing up to 5 wt.% of seawater sulfate (δ34S = +21‰, δ18O = +9.5‰) 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.

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