Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks

Mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the East Pacific Rise and the Mid-Atlantic Ridge provide information about the temperatures of alteration and seawater/rock ratios for various hydrothermal regimes within the oceanic crust. A systematic increase in alteration temperature is evident for the glassy rocksin the sequence: (1) nontronite and celadonite vesicle fillings (35°C), (2) saponite-rich pillow breccias ( 130–170°C), (3) calcite-rich greenstone breccias and epidote-rich greenstone (200–350°C). Theseresults include the highest temperatures thus far reported for saponite formation.

The “seawater-dominated” hydrothermal alteration process that formed the saponite-rich pillow breccias is characterized by high water/rock ratios (>0:1), low to moderate temperatures, a seawater origin of most of the carbon in vein calcites (δ¹³C≈0) and the predominance of Fe-rich saponite and calcite as secondary phases. Greenstones (chlorite-quartz-epidote) and greenstone breccias (chlorite-quartz-albite-calcite) are altered in a “rock-dominated” system with lower water/rock ratios (50:1 to < 1:1), higher temperatures, and vein calcites with carbon that is principally of magmatic origin (δ¹³C≈−4). The crystalline rocks (diabase, gabbro, and metagabbro) are affected to varying degrees by pervasive high-temperature seawater interactions that commence soon after solidification, producing varying proportions of fine-grained secondary minerals including talc, smectite, chlorite, vermiculite, actinolite, and sodicplagioclase. Hydrothermal solutions, derived from alteration of the crystalline rocks, are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal-rich deposits associated with the oceanic spreading centers.