Geochemistry at DSDP Leg 70 Holes

A total of 32 holes at five sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November- December 1979) provide unique data on the origin of the hydrothermal mounds on the southern flank of the Galapagos Spreading Center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (< 100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with seawater and pelagic sediments above locally permeable zones of ocean crust. Mounds as high as 25 meters form in less than a few hundred thousand years, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

Geochemistry of pore water and sediments from ODP Leg 136 sites

During Leg 136 drilling was conducted at two sites in pelagic sediments of the north central Pacific Ocean. In this report, pore-water analyses for major seawater constituents, alkalinity, ammonia, nitrate, phosphate, silica, Ba, Fe, Li, Mn, and Sr are presented. Although concentration gradients are generally weak, resulting from slow sedimentation and concomitant diffusive communication with overlying water, there is evidence of sediment/pore-water interactions, associated sediment diagenesis, and formation of authigenic minerals. Bulk major and trace element compositions of the sediments are consistent with reactions inferred to occur within the sediments and with the lithology and mineralogy. Elemental compositions of the sediments are not strongly affected by diagenesis and are primarily related to the dominant mineralogy. Sediments are typical of deep ocean pelagic settings with a significant contribution from the alteration of volcanic ash and the formation of zeolites. Sedimentary rare earth element patterns also provide evidence of active scavenging processes by Mn and Fe oxide phases in the deeper sediments at Site 842.

Chemical analyses of ODP Hole 105-647A basalts (Table 2)

Basalt samples recovered from the lowermost 37 m of Leg 105 Hole 647A in the Labrador Sea are fine- to medium grained, have microphenocrysts of clinopyroxene, and show little evidence of alteration. Chemically, these rocks are low potassium (0.01-0.09 wt% K20), olivine- to quartz-normative tholeiites that are also depleted in other incompatible elements. In terms of many of the incompatible trace elements, the Labrador Sea samples are similar both to iV-type midocean ridge basalts (MORBs) and to the terrestrial Paleocene volcanic rocks in the Davis Strait region of Baffin Island and West Greenland. However, significant differences are found in their strontium and neodymium isotope systematics. Hole 647A samples are more depleted in epsilon-Nd (+9.3) and are anomalously rich in 87Sr/86Sr (0.7040) relative to the Davis Strait basalts (epsilon-Nd +2.54 to + 8.97; mean 87Sr/86Sr, 0.7034). We conclude that the Hole 647A and Davis Strait basalts may have been derived from a similar depleted mantle source composition. In addition, the Davis Strait magmas were generated from mantle of more than one composition. We also suggest that there is no geochemical evidence from the Hole 647A samples to support or to refute the existence of foundered continental crust in the Labrador Sea.