Osmium isotope geochemistry of terrigenous and marine sediments.

摘要

Osmium isotopic compositions and concentrations have been determined in marine and continental sediments to determine the sources of Os to the ocean. {dollar}sp{lcub}187{rcub}{dollar}Re decays to {dollar}sp{lcub}187{rcub}{dollar}Os with a half-life of {dollar}sim{dollar}42 Ga. Continental crustal rocks have low Os, high Re/Os, and high {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os relative to meteorites or the mantle. Osmium isotopic data allow the distinction of continental from meteoritic, scavenged from detrital, and pollutant from nonpollutant Os. Sediment Os was analyzed by ID-SIMS with errors of 1-2% (2{dollar}sigma{dollar}).; Continental crustal {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os was determined by analyzing North American loess and Amazon, Changjiang, and Mississippi shelf sediments. Sediments not containing anthropogenic or scavenged seawater Os have {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os ratios between 10 and 11 and model {dollar}sp{lcub}187{rcub}{dollar}Re/{dollar}sp{lcub}186{rcub}{dollar}Os ratios (determined using Nd model ages) of 380 {dollar}pm{dollar} 20. When preferential erosion of younger crust is considered, the upper continental crust has {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os = 13.9. Sediment Os concentrations are 15-90 pg/g with a mean of 50 pg/g.; In a Long Island Sound core, Os concentration is highest in surface sediment. In adjacent New Haven Harbor, Os is correlated with organic carbon derived from sewage sludge. Osmium concentrations are 40-770 pg/g; {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os is 1.6-8.5 and is correlated with 1/Os. If this correlation is due to mixing, the extrapolated anthropogenic endmember {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os is {dollar}sim{dollar}1 and is indistinguishable from analyzed sewage sludge.; Bulk pelagic clays from DOMES sites A and C in the equatorial Pacific have Os = 130 pg/g and {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os = 6.1-6.7. Oxalic-acid leaches of four manganese nodules and of one pelagic clay from these sites have {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os = 7.9-8.7. Bulk pelagic-clay Os is less radiogenic than leachable Os or continental crustal Os. As these pelagic clays are free of metalliferous or volcaniclastic phases, the nonradiogenic phase must be particulate meteoritic material with {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os = 1. Since continental detritus has {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os = 10-11, a chondritic meteorite flux of 3-4 {dollar}times{dollar} 10{dollar}sp4{dollar} tons/a can be calculated by mass balance and favorably compared to estimates using {dollar}sp{lcub}53{rcub}{dollar}Mn and Ir.