Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

Rare and trace element analyses from DSDP Holes 31-291 and 31-294 from the Philippine Sea

Metal-rich sediments were found in the West Philippine Basin at DSDP sites 291 (located about 500 km SW of the Philippine Ridge or Central Basin Fault) and 294/295 (located about 580 km NE of the Philippine Ridge). In both cases the metalliferous deposits constitute a layer, probably Eocene in age, resting directly above the basaltic basement at the bottom of the sediment column. The chemistry of the major (including Fe and Mn) and trace elements (including trace metals, rare earth elements, U and Th) suggest a strong similarity of these deposits to metalliferous deposits produced by hydrothermal activity at oceanic spreading centers. Well-crystallized hematite is a major component of the metal-rich deposits at site 294/295. We infer that the Philippine Sea deposits were formed at some spreading center by hydrothermal processes of metallogenesis, similar to processes occurring at oceanic spreading centers. A locus for their formation might have been the Philippine Ridge (Central Basin Fault), probably an extinct spreading center. We conclude that metallogenesis of the type occurring at oceanic spreading centers can take place also in marginal basins. This has implications for the origin of metal deposits found in some ophiolite complexes, such as those in Luzon (Philippines), which may represent fragments of former marginal basins rather than of oceanic lithosphere.