13. Major-element Chemistry of Basaltic Glasses Recovered during Deep Sea Drilling Project Leg

Fourteen microprobe analyses of basaltic glasses from three drill sites plus two dredged samples from the seafloor at the mouth of the Gulf of California suggest that magmatic liquids that cooled to form the volcanic crust in the Gulf of California have been fractionated, whereas other, more primitive submarine basalts from the Mid-Oceanic Ridge (MOR) have not. The limited chemical diversity among drill sites suggests that magmatic liquids, which erupted along the accretionary boundary in the Gulf of California, are grossly similar. Major-element variations in the glasses from Hole 474A may, in part, however, be attributed to simple crystal-liquid fractionation in a subridge magma chamber. Additionally, the analysis of a glass from Hole 475B, near the eastern continental slope of southernmost Baja California, suggests that magmatic liquids which produced submarine basalts associated with initial opening of the Gulf may have been chemically less evolved than present ones. Trace-element concentrations (Saunders et al., this volume, Pt. 2 [hereafter "this volume"]) in whole rocks from Hole 475B suggest that this magma may have experienced minor sialic contamination on its ascent through lower crustal continental material. Whole-rock and glass major-element concentrations, however, suggest that early Pliocene(?) basalts that erupted in Hole 475B were derived from a magmatic liquid of (deep) mantle origin. The liquid quickly rose to the surface with little shallow-level fractionation.