Rare earth element and related chemistry of some drilled southern Indian Ocean basalts and volcanogenic sediments

Abstract

The rare earth elements (REE), selected incompatible elements, and overall compositions of some Deep-Sea Drilling Project (DSDP) leg 26 (southern Indian Ocean) basalts and volcanogenic sediments are reported. Variations in REE distributions are not always relatable to existing tectonic features that characterize the sampled sites. The olivine-rich basalt at site 250 is transitional between midocean ridge basalt (Morb) and alkali basalt and is tentatively considered to have been produced by hot spot activity. The age relationships of site 257 basalts are not clear, but the uppermost unit (flow?) has a distinct light REE and incompatible element enrichment and is apparently very much younger than the lower units, which show light REE depletion. Like the basalts, the volcanogenic sediments of Ninety East Ridge (sites 253 and 254) and Naturaliste Plateau (site 258) are enriched in light relative to heavy REE. Alteration seems to have little effect on the REE abundances except for the high La/Ce ratio. In the volcanogenic sediments it may result from an admixture of Ce-poor biogenic calcite. Site 256 and uppermost 257 basalts from the Wharton Basin may have been derived from the same single source as the rocks forming the Ninety East Ridge, Naturaliste Plateau, Broken Ridge, Kerguelen Plateau, and Amsterdam-Saint Paul islands.