Petrography, chemistry and magnetic properties of ODP Leg 115 basalts

A detailed study of the Fe-Ti oxides in four basalt samples-one from each of the four holes drilled into basement on Ocean Drilling Program Leg 115 (Sites 706, 707, 713, and 715) has been performed. Ilmenite is present only in samples from Sites 706 and 715. In the sample from Site 715, Ti-magnetite intergrowths are characteristic of subaerial (?) high-temperature oxy-exsolution; Ti-magnetite in the other three samples has experienced pervasive low-temperature oxidation to Ti-maghemite, as evidenced by the double-humped, irreversible, saturation magnetization vs. temperature (Js/T) curves. The bulk susceptibility of these samples, which are similar in terms of major element chemistry, varies by a factor of ~20 and correlates semiquantitatively with the modal abundance of Fe-Ti spinel, as determined by image analysis with an electron microprobe. The variation in Fe-Ti oxide abundance correlates with average grain size: fine-grained samples contain less Fe-Ti oxide. This prompts the speculation that the crystallization rate may also influence Fe-Ti oxide abundance.

Petrology and geochemistry of sideromelane glass shards from Pleistocene as layers north and south of Gran Canaria

Major elements, S, F, Cl concentrations and relative proportions of S6+ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments drilled during ODP 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+/SumS (0.03±0.98) and calculated Fe2+/Fe3+ (2.5±5.8) ratios in the melt yield 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 temperatures (1030±1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife.