Geochemistry and effect of oxidation state of iron at DSDP Leg 69 Holes@en

Over 300 electron microprobe and wet chemical analyses were made of alteration minerals (mostly clays) in basalts from DSDP Sites 501, 504, and 505 in the Costa Rica Rift. These analyses are displayed using a three-dimensional graphical method. Several alteration minerals have been identified, of which saponites are by far the most common. In situ alteration of basalt glass has produced montmorillonite-like minerals rich in the less-mobile elements such as Al and Ti. Talc-like minerals commonly replace olivine and act as fillings to vesicles. Three other alteration minerals have been recognized chemically, rich in Na and Ca, but these are presently unidentified. There is no evidence for systematic change of mineral chemistry with depth.