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 isoto pic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alteration s ranging from slight hydrothermal alteration to complete amphibolitization . Five major kinds of alteration processes ranging from late-magmatic deute ric alteration (stage I) to low temperature (<150 degrees C) alteration (st age V) were identified. Water-rock interactions between 300 and 650 degrees C are the most dominant interactions resulting in the most prevailing seco ndary mineralogical assemblages which characterize the amphibolite and/or g reenschist facies (amphibole +/- plagioclase +/- epidote +/- titanite +/- c hlorite +/- piehnite). Hydrothermal alteration of these gabbroic rocks resu lts in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of grabbroic delta(18)O toward values as low as + 3.9 parts per thousand, and larger Sr isotopic variations than other ocea nic gabbroic rocks (Sr-87/Sr-86 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 hy drothermal fluids, In addition to metamorphic recrystallization and geochem ical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry.