PETROLOGY, GEOCHEMISTRY, S, CL AND F ABUNDANCES, AND S OXIDATION-STATE OF SIDEROMELANE GLASS SHARDS FROM PLEISTOCENE ASH LAYERS NORTH AND SOUTH OF GRAN-CANARIA (ODP LEG-157)

Major elements, S, F, Cl concentrations and relative proportions of S6 + to total S were analyzed with electron microprobe in sideromelane gl ass shards from Pleistocene volcaniclastic sediments drilled during OD P Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite and nephelinite through hawaiite , mugearite and tephrite to phonolitic tephrite. Measured S6+/Sigma S (0.03-0.98) and calculated Fe2+/Fe3+ (2.5-5.8) ratios in the melt yiel d preeruptive redox conditions ranging from NNO-1.4 to NNO + 2.1. The morphology of the glass shards, variations of S and Cl concentrations (0.010-0.127 wt% S, 0.018-0.129 wt% Cl), calculated preeruptive temper atures (1030-1200 degrees C) and oxygen fugacities suggest that glasse s deposited even within the same ash layers have diverse origin and ma y have resulted from both submarine and subaerial eruptions. Most vesi cle-free glasses are characterized by high concentrations of S and rep resent undegassed or slightly degassed submarine lavas, whereas vesicu lated glasses with low concentrations of S and Cl are strongly degasse d and can be ascribed to the eruptions in shallow water or on land. Si deromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 re present mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposite d at Site 956 resulted from submarine or explosive eruptions on Teneri fe.