Unusual occurrences of aegirine-augite,fassaite and melanite in oceanic basalts (DSDP Hole 504B)

DSDP Hole 504B, located on the south flank of the Costa Rica Rift, shows the most complete reference section through the upper oceanic crust; 274.5 m into the sediment and 1075.5 m into the basement have been penetrated.Very unusual secondary parageneses occur in some basaltic massive flows recovered at DSDP Hole 504 B. Replacement of magmatic augite by secondary Ti- and/or Na-rich augite and aegirine-augite was observed in five veins between the 298 and 685 m sub-basement depth; one of these veins also contains melanite (Ti-rich andradite) and another one contains fassaite. These minerals seem independent of, and prior to, other alteration stages.The conditions of formation and origin of the fluids responsible for the crystallization of these unusual minerals are discussed. Several arguments suggest that their origin is not hydrothermal, but possibly deuteric: (1) these minerals probably formed at higher temperatures than hydrothermal minerals from DSDP Hole 504 B (300–400°C for melanite, 800–1000°C (?) for fassaite, >400°C for aegirine-augite); (2) the chemical composition of the studied minerals are not compatible with the chemical composition of hydrothermal fluids responsible for the alteration at DSDP Hole 504 B; (3) the distribution and chemical composition of these minerals seem to be independent of the alteration zonation and thus of depth; and (4) these minerals are anhydrous, in contrast to mostly hydrated minerals from DSDP Hole 504 B known to be hydrothermal in origin. Moreover, since these minerals occur in subophitic or coarse-grained basalts, their crystallization may have been enhanced by the long standing stability of the physico-chemical conditions existing in the veins.