Geochemical aspects of hydrothermal sediments in the eastern Pacific Ocean; an update

Metalliferous sediments in the E central Pacific are represented mainly by an Fe-Mn-rich oxyhydroxide component, both in surficial sediments up to 2000 km from the East Pacific Rise and in sediments overlying basaltic basement as old as Palaeocene age. On a carbonate-free basis, these and other (non-nodule) metalliferous sediments in the E Pacific typically contain approx 25-35% Fe, 5- 10% Mn, 1000-2000 ppm Cu, 500-1000 ppm Zn and Ni, and <250 ppm Co and Pb. Fe/Mn ratios for the metalliferous component generally increase with distance from the palaeo-axis, whereas Fe/Al ratios and the mass accumulation rates of Fe, Mn and base metals decrease. These trends reflect decreasing fallout of particulate matter from dilute hydrothermal plumes. Much of the Pb in the sediments is of basaltic hydrothermal origin as far as 250 km from the axis, with mixed basaltic sea-water Pb beyond this, whereas the REE and Sr are of sea-water derivation. In some areas of the eastern Pacific, Fe-rich smectite is the main hydrothermal phase, and probably is a byproduct of early diagenetic reaction between biogenic silica and the hydrothermal Fe-Mn oxyhydroxides. In the Galapagos mounds field, nontronite is the main hydrothermal component. Here, nontronite has formed because a high rate of biogenic sedimentation allowed solutions upwelling from the basement to penetrate and react with the sediment cover; O isotope data for nontronite suggest that it generally formed at <30 degrees C. The REE in nontronite are interpreted to have been inherited mainly from sea-water, although Ce anomalies are less negative than that of sea-water. Granular nontronite has lower REE concentrations than nontronite in finer-grained transitional zones, suggesting REE expulsion as granules formed. Within the axial valley of the Southern Explorer Ridge in the NE Pacific, thin oxidized sediments overlie massive sulphide mounds. In situ oxidation of sulphides/sulphates produces an initial alteration gossan dominated by Fe oxyhydroxide phases. With continued exposure of the mound surface, it is suggested that Fe/Mn ratios decrease, sulphates break down with net loss of Sr, and the proportion of volcaniclastic material increases. Studies by other workers on ridge-proximal hydrothermal smoke and sediments in the eastern Pacific indicate that Fe-Mn oxyhydroxides become an important component of the hydrothermal plume soon after discharge either as direct precipitates or through oxidation of sulphide particles.