HOMOGENEOUS 187OS/188OS AND HIGH OS CONCENTRATIONS IN THE STEENS BASALT, COLUMBIA RIVER FLOOD BASALT GROUP, SOUTHEAST OREGON

The Steens Basalt (16.7 Ma) is the earliest and most primitive member of the Columbia River Flood Basalt. Sr, Nd, Hf and Pb isotopic ratios support involvement of two distinct mantle sources for the generation of these magmas, one similar to the depleted upper mantle source that generates mid-ocean ridge basalts (MORB), and the other similar to the enriched mantle sources sampled by ocean island basalts (OIB). The waxing, recharge-dominated stage of magmatism, represented by the informally named lower B Steens Basalt, has a small range of ¹⁸⁷Os/¹⁸⁸Os (0.1292 to 0.1311) that overlaps values typical of the mantle (MORB ~0.123-0.126; OIB ~0.124-0.150). However, Os concentrations of the lower B Steens Basalt range from ~0.3-0.9 ppb, significantly higher than most MORB (0.001 to 0.05 ppb) and most OIB (0.01 to 0.5 ppb). Plausible mechanisms for elevating Os concentrations above typical basalt values mainly involve accumulation or consumption of Os rich mineral phases, such as sulfides, olivine, and/or chromite. Among these phases, sulfides have the highest partition coefficients and Os concentrations. Whole rock and melt inclusion data indicate that the lower B Steens Basalt is sulfide undersaturated, and the slightly elevated δ¹⁸O signature for most samples (5.8 to 6.0‰) suggests that sulfide undersaturated melts generated in the mantle stall deep in the crust, fractionate abundant clinopyroxene, and evolve to sulfide saturation. Sulfides that precipitate from the more evolved basaltic melts are then consumed by hotter, sulfide undersaturated recharge melts. Consumption of very minor amounts of sulfide during mantle melting cannot be ruled out, but is difficult to invoke given uncertainty regarding the presence of sulfides in the mantle sources and the scarcity of mantle derived magmas with such high Os concentrations. Entrainment of cumulate olivine or chromite fractionated by earlier magmas in the magmatic system could produce the elevated Os concentrations, but would result in MgO or Cr₂O₃ wt.% well above the observed whole rock compositions. The data thus suggest that repeated use of the same magmatic system can result in erupted lava compositions strongly influenced by deep crustal fractionation and assimilation of the resulting cumulates precipitated throughout the development of the system.