The osmium isotopic composition of the continental crust

Abstract

Osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers (the Amazon, Changjiang, and Mississippi) and for two loesses from the upper Mississippi River Valley. The loesses have heavy mineral and clay compositions characteristic of Mississippi river and delta sediment. Concentrations range from 15 to 120 pg Os/g, and 187Os/186Os ratios range from 8.2 to 10.9. Sediments which are polluted (surface sediment from the Changjiang Estuary) or which have scavenged metals from seawater (Gulf of Mexico slope surface sediment and distal Amazon Shelf sediment) have high osmium concentrations and less radiogenic isotopic compositions. Purely terrigenous sediments have 15–90 pg Os/g, and 187Os/186Os= 10.0–10.9. North American loess and Mississippi Delta sediment osmium isotopic compositions are indistinguishable. We believe that 187Os/186Os= 10–11 is a good average for currently eroding upper continental crust—these sediments have neodymium isotopic compositions close to the global mean for river sediment. Terrigenous 187Os/186Os is significantly more radiogenic than seawater 187Os/186Os(8.6), requiring a nonriverine source of osmium to seawater. Terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium. We use the improved estimate of terrigenous 187Os/186Os to recalculate the flux of extraterrestrial osmium to Pacific marine sediment. If the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region, a model-dependent upper crustal 187Re/186Os ratio can be calculated. For the model ages of the sediments analyzed in this study (Tdm = 1.5–1.7 Ga), model 187Re/186Os ratios are 340–360 and can be used to correct for bias resulting from the preferential erosion of young crust. The 187Os/186Os ratio of 2.2 Ga crust with 187Re/186Os= 400 is 14 ± 2. We have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg/g and 390 pg/g, respectively.