Mineral chemistry of primary and secondary phases in basaltic rocks, Leg 129

Abstract

Primary magmatic phases and secondary phyllosilicates were analyzed by electron microprobe in alkalic and tholeiitic dolerites and basalts from ODP Sites 800, 801, and 802. Aphyric alkalic dolerite sills. Alkalic rims to feldspars probably developed at the same time as quenched mesostasis feldspars and late-stage magmatic biotite, and represent the buildup of K-rich hydrous fluids during crystallization. Phenocryst phases in primitive mid-ocean ridge tholeiites from Hole 801C (Mg numbers about 70) numbers about 70) have extreme compositions with chrome spinel, Ni-rich olivine, and highly calcic plagioclase. Later glomerophyric clumps of plagioclase and clinopyroxene are strongly zoned and probably reflect rapidly changing melt conditions during upward transport, prior to seafloor quenching. In contrast, phenocryst phases in the Hole 802A tholeiites show limited variation and do not have such primitive compositions, reflecting the uniform and different chemical composition of all the bulk rocks. -from Authors