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

Abstract

We report S concentrations and relative proportions of SO 4 2− and S 2− in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene basaltic hyaloclastites drilled north and south of Gran Canaria during ODP Leg 157. Compositions of glass inclusions and lapilli resemble those of subaerial Miocene shield basalts on Gran Canaria and comprise mafic to more evolved tholeiitic to alkali basalt and basanite (10.3–3.7 wt.% MgO, 44.5–56.9 wt.% SiO 2). Glass inclusions fall into three groups based on their S concentrations: 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 dissolved as sulfate determined by electron microprobe measurements of S Kα peak shift. Their average S 6+/S total value is 0.75 ± 0.09, unusually high for ocean island basalt magmas. The low-sulfur group inclusions have low S 6+/S total ratios (0.08 ± 0.05), whereas intermediate sulfur group inclusions show a wide range of S 6+/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 S 6+/S total ratios of 0.36 ± 0.06 implying that sulfur degassing does not affect the proportion of SO 4 2− and S 2− in the magma. The oxygen fugacities estimated from S 6+/S total ratios and from Fe 3+/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 source at relatively oxidized f O 2 conditions, and (2) magma contamination by seawater 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, whereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degassed magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts with strongly varying S concentrations and S 6+/S total ratios.