Chemistry of basalt alteration from Leg 115

Abstract

Basalts recovered along the Reunion Hotspot track on Leg 115 range in age from 34 Ma at Site 706 to 64 Ma at Site 707. They have undergone various degrees of secondary alteration. Olivine appears to be the most susceptible to alteration and in some sections it is the only mineral altered. Clays are the predominant form of secondary mineralization. In addition to replacing olivine, pyroxene, glass, and ground-mass, clays have filled veins, vesicles, and voids. Minor amounts of calcite, zeolites, and K-feldspar were also detected. The electron microprobe was used to obtain chemical analyses of these veins as well as to characterize isolated clays that replaced specific minerals and filled voids and vesicles. Hole 707C clays are primarily saponites. In the upper high-MgO unit at Hole 707C, the clays have a higher MgO content than in the underlying low-MgO unit, possibly reflecting these chemical differences in the parent rocks. At Site 713 the few analyses that were obtained show that the clays are saponites and Al-saponites. -from Authors