Petrology of basalts from Site 487, Deep Sea Drilling Project Leg 66, Middle America Trench area off Mexico.

Abstract

Basaltic rocks recovered at the Middle America Trench area off Mexico are typical plagioclase-olivine phyric abyssal tholeiites containing <0.2wt% K2O. Phenocrysts of plagioclase and olivine unsually make up the aggregate. Plagioclase phenocrysts are Ca- rich and up to An90. Olivine phenocrysts, which are always attached to plagioclase phenocrysts, are magnesian, Fo88-Fo89, and contain 0.2-0.3wt% of NiO. Plagioclase phenocrysts contain numerous glass inclusions with the Mg/Mg + Fe atomic ratio of 0.70-0.73, which is distinctly higher than the same ratio of the bulk rock (0.62-0.63). Olivine of Fo88 is equilibrated with the liquid with an Mg/Mg + Fe atomic ratio of approx 0.7, assuming the KDMg-Fe between liquid and olivine 0.3. Small droplets of glass within glass inclusions in plagioclase are more enriched in K2O and volatiles than the host glass. This enrichment may have been caused by the extraction of Al2O3 as plagioclase from the trapped liquid and implies its immiscibility. Aggregates of plagioclase with small amounts of olivine may have been floated from more primitive magma with an Mg/Mg + Fe atomic ratio of approx 0.7, judging from the chemical characteristics mentioned above. Flotation must have occurred at relatively high pressure. Large crystals of plagioclase and smaller crystals of olivine are xenocryst rather than phenocryst. Parental magma of Leg 66 basalt was high-MgO olivine tholeiite.-Author