Micro-scale Chemical Effects of Low Temperature Weathering of DSDP Basaltic Glasses

Abstract

Unaltered deep-sea basaltic g lasses are believed to be the best record of initial magma composition , and as such are important in the study of petrogenesis. However , these glasses are altered by their long contact with seawater, becoming hydrated and undergoing chemical exchange. This chemical exchange affects the composition of seawater and plays a role in the chemical equilibrium of t he oceans. A study of the trace metal and major element alteration of glasses from Deep Sea Drilling Project Site 396B has been conducted, using a selected area x - ray fluorescence technique (developed for this study) for the trace metal analyses, and the electron rnicroprobe for the major elements. The samples included sections of pillow basalt rinds, hyaloclastite s, and a few crystalline sections. The glasses were found to release a bout o ne- half the original Si and Al, two- thirds of the Mg and Na , and over 90% of the Ca originally present, during alteration to palagonite. Fe and Ti were found to be immobile, and K was increased 40-fold by concentration from seawater. For the trace metals, over one-quarter of the Zn, Cu and Ni were released, 40% of the Mn, and over 10% of the Cr. These changes apply only to the conversion of fresh glass (sideromelane ) to palagonite (smectite), and do not include the effects of authigenic phillipsite and calcite reprecipitated locally. Differences between the effects of low temperature weathering on the crystalline basalts and the glasses appear to be primarily a function of the susceptibility of the primary mineral phases to attack, with the glass, being the least stable phase, being the most altered.