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Basalts from the upper 255 meters of the oceanic crust at Site 396B show moderately large variations in petrography, mineralogy, and chemistry. The upper part of the section comprises aphyric basalt with rare, rounded plagioclase phenocrysts and lesser amounts of olivine. The central part of the section is olivine and plagioclase phyric with modal abundances ranging from about 10 to 30 per cent. The lowest part of the sequence is moderately phyric with widely varying abundances of olivine and plagioclase phenocrysts. Clinopyroxene phenocrysts occur in only one rock. Co-existing olivine and plagioclase phenocrysts exhibit both normal and reverse zoning which suggests mixing of different magma batches prior to eruption. Major and trace element compositions of glass and whole rock basalt samples from Hole 396B suggest the following: a) The two major basaltic magma types are a Ti-poor, Ca-rich ("X") type and Ti-rich, Ca-poor ("Y") type. b) Different magma batches may represent separate fractions of the same differentiating parent liquid, or may reflect fractionation "events" acting upon similar or distinct liquid compositions. In either case, eruptive products of any one batch show a characteristic chemistry and lithology. c) Different magma batches appear to have been erupted in a contiguous time sequence except for minor amounts of extreme differentiates interlayered with lavas from unrelated batches. However, eruption of different batches deriving from a single liquid type may be separated by relatively large time intervals. d) "X"-type and "Y"-type liquids are characterized by different low pressure crystal-liquid phase equilibria. Plagioclase apparently precipitated at higher temperatures compared to olivine and clinopyroxene, for equivalent MgO content in "X"-type than in "Y"-type liquids. e) Leg 46 basalts generally have higher Na, Ti, and large ion lithophile (LIL) element abundances for equivalent MgO content than basaltic magmas from other parts of the Mid-Atlantic Ridge (e.g., FAMOUS). We tentatively suggest that these differences reflect lower degrees of partial melting at 22°N and are a reflection of ambient geothermal conditions.

Petrology and Geochemistry of Basalts from Hole 396B, Leg 46