Petrology, isotope geochemistry and chemical budgets of oceanic gabbros-seawater interactions in the Equatorial Atlantic

 Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ¹⁸O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (⁸⁷Sr/⁸⁶Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry. Received: 8 March 1999 / Accepted: 12 July 1999