Hydrothermal activity in the Guaymas Basin occurs in a setting quite different from that on open ocean ridges, in that new oceanic basement forms underneath a thick blanket of rapidly deposited sediments. In this report we discuss the geochemical aspects of this system, and give particular attention to studies of interstitial waters, sediments, and the geochemistry of the overlying water column.

Evidence from Deep Sea Drilling Project sites allows us to distinguish two types of hydrothermal systems: one is associated with shallow sill intrusions, leading to hydrothermal activity of short duration and sediment temperatures not exceeding ~200°C; the other is associated with deep-seated massive intrusions or hot lavas, whose temperatures commonly exceed 300° C, and with high-temperature alteration of sediments to greenschist facies. The latter system is of long duration, long enough to allow alteration of substantial thicknesses of sediments. Hydrothermal fluids are channelled toward fault zones by overlying sill intrusions, as well as by near-surface layers of dolomite (as noted in piston cores) or cemented diatom frustules (observed in Alvin push cores), and r ach the sea floor at temperatures as high as 325°C. Finally, hydrothermal fluids exit the sediment directly or through hydrothermal mounds and chimneys, and constitute 0.1% of the basin subsill water mass; these fluids leave distinct imprints of anomalies in dissolved silica, excess He, barium and beryllium, and particulate manganese.

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