Gypsum and halite from the Mid-Atlantic Ridge,DSDP Site 395 |
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| Authors: | James I Drever James R Lawrence Ronald C Antweiler |
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| Institution: | 1. Geology Department, University of Wyoming, Laramie, WY 82071 U.S.A.;2. Lamont-Doherty Geological Observatory, Palisades, NY 10964 U.S.A. |
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| Abstract: | Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The δ34S value of the gypsum (+19.4‰) indicates a seawater source for the sulfate. The δ18O values of the saponite (+19.9‰) and phillipsite (+18.1‰) indicate either formation from normal seawater at about 55°C or formation from18O-depleted seawater at a lower temperature.The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca2+ released from basalt and SO42? in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite. |
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