Rhenium-osmium isotope systematics and platinum group element concentrations: Loess and the upper continental crust

[1] We investigate the use of loess as a proxy for the concentration and isotopic composition of highly siderophile elements, specifically Os, in the upper continental crust. The Os-187/Os-188 platinum group element, and Re concentrations of 16 loess samples from China, Eur+ope, and South America, previously analyzed for major, trace element, and Sr and Nd isotope composition, reveal subtle differences between loess provinces. Despite those differences, the Os-187/Os-188 of 1.05 +/- 0.23 is surprisingly homogenous. Average Os-187/Os-188 as well as average Os (31 pg/g) and Ir (22 pg/g) concentrations are similar to the lower limit of previous estimates for average upper continental crust, whereas Ru, Pt, and Pd concentrations are intermediate between previous estimates. We argue that hydrogenous enrichment of Os in riverine sediments led Esser and Turekian [1993] to overestimate the Os concentration of upper continental crust (50 pg/g). On the basis of this argument and correlations with major and trace elements we propose that average platinum group element concentrations of loess (i.e., 31 pg Os/g, 22 pg Ir/g, 210 pg Ru/g, 510 pg Pt/g, 520 pg Pd/g) are a proxy for the upper continental crust. We further suggest that the nonchondritic average Os/Ir of 1.4 reflects the combined effects of radiogenic ingrowth of Os from Re decay over the mean lifetime of the upper continental crust and preferential return of Os to the crust during subduction. Rhenium concentrations scatter significantly, with highest values in loess derived from organic-rich sedimentary rocks. Low median Re concentrations most likely reflect depletion of loess in organic matter, an important sink for Re in the upper continental crust. An average Re-187/Os-188 of 34.5 was calculated on the basis of the measured Os-187/Os-188 and Nd model ages. This value corresponds to a Re concentration of 198 pg/g. Correcting measured Os-187/Os-188 = 1.05 and inferred Os-186/Os-188 = 0.119871 (from Pt-190/Os-188 = 0.0176) for the older mean age (2.2 Gyr) of upper continental crust compared to loess (1.6 Gyr) yields average upper crustal Os-187/Os-188 of 1.40 and Os-186/Os-188 of 0.119885.