Oceanic gabbro from Leg 153 records retrograde metamorphism caused by progressive unroofing in the footwall of a brittle-ductile normal fault. Decreasing temperature is reflected by amphiboles that evolved from pargasitic to actinolitic compositions. Highest degrees of water-rock interaction are associated with greenschist facies hydrothermal alteration that produced actinolite + chlorite ± talc ± epidote ± prehnite ± pyrite ± quartz ± titanite. Alteration is strongest near microfractures and veins that we interpret to be the main fluid conduits. Chlorite and actinolite make up 50% of highly altered samples. Chlorite typically occurs as zoned coronas around felted cores of actinolite, and reaction textures show that chlorite predominately replaces plagioclase and actinolite replaces olivine. Reaction textures and strong linear correlations in the enrichment of secondary chlorite and actinolite and depletion of primary plagioclase and olivine indicate that the two reactions operated simultaneously. Based on mass-balance calculations and lack of significant variations in whole-rock composition, we infer that calcium required to transform olivine to actinolite was derived from the alteration of plagioclase to chlorite; whereas iron and magnesium required to transform plagioclase to chlorite were derived from the amphibolitization of olivine. Mass balance of hydrothermal alteration of troctolitic gabbro from Hole 922A shows that the dominant metasomatic change was the addition of water, approximately 5110 mol/m3. The enrichment patterns of other elements are largely antithetic to patterns observed in submarine hydrothermal water relative to seawater. Therefore, alteration could have occurred in the root zone of such hydrothermal systems.
Date of initial receipt: 15 August 1995
Date of acceptance: 6 February 1996
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