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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/m. 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. INTRODUCTION METHODS Oceanic gabbros recovered in Leg 153 are exposed in the footwall of an east-dipping, brittle-ductile, normal fault, which makes up the western wall of the median valley of the Mid-Atlantic Ridge, immediately south of the Kane Fracture zone (MARK area) (Karson, 1990). Based on the metamorphic evolution documented in this study and the microstructural evolution documented by Fletcher, Ceuleneer, et al. (this volume), we can demonstrate that the gabbroic rocks in the MARK area record progressive tectonic denudation similar to that observed in Cordilleran metamorphic core complexes. The most extensive hydrothermal alteration of these rocks produced the greenschist facies assemblage of actinolite + chlorite ± talc ± epidote ± pyrite ± quartz ± titanite. The main goals of this study are to characterize mass transfer of this extensive alteration and to investigate the effects of fluid circulation on oceanic rocks that have occupied a wide range of crustal levels at a slow-spreading ridge. The occurrence of these rocks in a major fault zone suggests that pressure gradients created by the opening of fractures and veins may have been an important driving force of fluid flow. Therefore, we compare water:rock interactions in this setting with other well-documented, thermally driven systems. 'Karson, J.A., Cannat, M., Miller, D.J., and Elthon, D. (Eds.), 1997. Proc. ODP, Sci. Results, 153: College Station, TX (Ocean Drilling Program). Departamento de Geologia, Centro de Investigación Cientifica y Education Superior de Ensenada, P.O. Box 434843, San Diego, CA 92143-4843, U.S.A. jfletche@cicese.mx 3 Department of Earth Sciences, The University of Queensland, Brisbane, 4072, Australia. (Present address: School of Geology, Queensland University of Technology, 2 George Street, Brisbane, 4000, Australia.) "Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-1183, U.S.A. Samples were collected mostly from Core 153-922A-2R, which had particularly high recovery. It also contains a suite of troctolitic gabbros that show a wide range of degrees of alteration (0%-60%), but only small variations in protolith rock type. This study uses three main approaches to characterize mass transfer of hydrothermal alteration in the troctolitic gabbro. First, compositions and zoning patterns of phases were determined so that representative phase compositions could be used in system reactions. Special attention was given to phase compositions near disequilibrium reaction boundaries to characterize diffusive mass transfer in solid phases. Second, modal mineralogy was determined for the suite of variably altered troctolitic gabbros to define reaction paths or relative changes in abundance of primary and secondary phases. Third, whole-rock compositions were determined for the suite of variably altered troctolitic gabbros to identify chemical variations resulting from metasomatism. Based on reaction textures, a set of system reactions was generated using representative phase compositions. The system reactions were combined to produce a single reaction that fits changes in relative proportions of primary and secondary phases, as well as changes in whole-rock compositions. Modal mineralogy was determined from microprobe-generated point counts. This routine was largely developed during this study and a detailed description of it follows. Peak intensities (counts/s) from wavelength dispersive spectrometry were collected for eight elements (K, Na, Ca, Mg, Fe, Si, Al, and Ti) over a rectangular grid of points. Each grid was defined to sample the largest possible surface area in each sample. The number of points varied from 700 to 2000, and spacing between points was generally between 100 and 200 μm. To minimize analysis time, groups of four elements were measured in two different passes through the grid. This eliminated the time re-

Greenschist facies hydrothermal alteration of oceanic gabbros: a case study of element mobility and reaction paths