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Legs 45 and 46 of the Deep Sea Drilling Project sampled a total of 351 meters of basaltic basement at Site 396. Several compositionally distinct eruptive units are found at this site, each having the characteristics of evolved mid-ocean ridge tholeiite. The minor and trace element abundances of the Site 396 basalts are also typical of LREE-depleted mid-ocean ridge tholeiite. Basalts sampled include aphyric and phyric basalts, the latter containing abundant Plagioclase and olivine phenocrysts. Microprobe analyses of olivine microphenocrysts and coexisting glass in both basalt types yield a KD = 0 . 2 7 for Fe-Mg partitioning between olivine and liquid. Application of this KD to phenocryst-bulk rock pairs indicates that many of the phenocrysts are more magnesian than the presumed equilibrium composition and must have been derived from a more primitive magma. One-atmosphere melting studies confirm this discrepancy. The diversity of zoning patterns in the Plagioclase phenocrysts in the phyric basalts similarly indicates a mixed parentage for this phase. In addition, both the olivine and Plagioclase phenocrysts contain Mg-rich, Ca-rich, Ti-poor melt inclusions which have compositional characteristics of more "primitive" magmas. Integration of these observations with major and trace element chemical data indicates that mixing of magmas and their attendant phenocrysts played an important role in the generation of these basalts. We suggest that mixing occurs as "primitive" picritic magmas are injected into zoned, fractionating magma chambers. The basalts recovered from Site 395 are very similar to the Site 396 lavas in mineralogy, mineral chemistry, and bulk composition. The nearly identical trace element systematics of basalts from the two sites indicate derivation of these LIL-depleted magmas from a similar, compositionally homogeneous mantle source.

Petrology and Geochemistry of Basalts from Site 396, Legs 45 and 46 of the Deep Sea Drilling Project