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Hole 396B penetrated 205.5 meters into basaltic basement about 160 km east of the Mid-Atlantic Ridge (22°59.14'N,43°30.90'W). The Leg 46 Scientific Party (this volume) has given general descriptions of the core recovered. They have divided the basement section into eight lithologic units. The samples discussed in this paper come primarily from lithologic Units 1 and 2 (the sparsely phyric pullow sequence), Unit 3 (the sparsely phyric cooling unit), and Unit 4 (the porphyritic pillow unit), although the shipboard chemical analyses to be used cover the entire hole. The objective of this paper is to use the shipboard bulk chemical analyses in conjunction with petrographic and microprobe analyses of the phenocryst and groundmass mineral phases to put some restrictions the processes which acted on the magma on the conditions under which the parental magma existed before eruption. The conclusions reached are that (1) the range of composition observed in any one of the chemical groups cannot unequivocally be explained by low-pressure differentiation of the observed plagioclase and olivine phenocrysts; (2) the variation from one chemical group to another, especially from the porphyritic to sparsely phyric groups, cannot be explained by this process; (3) many of the olivine phenocrysts in the porphyritic pillow unit are too magnesian to have grown from a liquid with the present bulk composition; (4) the plagioclase phenocrysts in the porphyritic pillow sequence cannot all have grown in the same thermal and chemical environment; and (5) the processes which resulted in the present rocks must be much more complex than simple crystal-liquid differentiation.

Petrology of Basalts: Hole 396B, DSDP Leg 46