S concentrations and its speciation in Miocene basaltic magmas north and south of Gran Canaria (Canary Islands): Constraints from glass inclusions inolivine and clinopyroxene

We report S concentrations and relative proportions of SO42- and S2- in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene ba saltic hyaloclastites drilled north and south of Gran Canaria during ODP Le g 157. Compositions of glass inclusions and lapilli resemble those of subae rial Miocene shield basalts on Gran Canaria and comprise mafic to more evol ved tholeiitic to alkali basalt and basanite (10.3-3.7 wt.% MgO, 44.5-56.9 wt.% SiO2). Glass inclusions fall into three groups based on their S concen trations: a high-sulfur group (1050 to 5810 ppm S), an intermediate-sulfur group (510 to 1740 ppm S), and a low-sulfur group (<500 ppm S). The most S- rich inclusions have the highest and nearly constant proportion of sulfur d issolved as sulfate determined by electron microprobe measurements of SK<al pha> peak shift. Their average S6+/S-total value is 0.75 +/- 0.09, unusuall y high for ocean island basalt magmas. The low-sulfur group inclusions have low S6+/S-total ratios (0.08 +/- 0.05), whereas intermediate sulfur group inclusions show a wide range of S6+/S-total (0.05-0.83). Glassy lapilli and their crystal-hosted glass inclusions with S concentrations of 50 to 1140 ppm S have very similar S6+/S-total ratios of 0.36 +/- 0.06 implying that s ulfur degassing does not affect the proportion of SO42- and S2- in the magm a. The oxygen fugacities estimated from S6+/S-total ratios and from Fe3+/Fe 2+ ratios in spinel inclusions range from NNO-1.1 to NNO+1.8. The origin of S-rich magmas is unclear. We discuss (1) partial melting of a mantle sourc e at relatively oxidized f(O2) conditions, and (2) magma contamination by s eawater either directly or through magma interaction with seawater-altered Jurassic oceanic crust. The intermediate sulfur group inclusions represent undegassed or slightly degassed magmas similar to submarine OIB glasses, wh ereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degasse d magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts w ith strongly varying S concentrations and S6+/S-total ratios. Copyright (C) 2000 Elsevier Science Ltd.