Rhenium-osmium isotope geochemistry of the mantle

Os isotopic compositions (reported as {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os) and Re and Os concentrations have been determined on ultramafic and mafic rocks derived from the mantle in order to place new constraints on the processes which have affected the mantle and crust over geologic time. {dollar}sp{lcub}187{rcub}{dollar}Re decays to {dollar}sp{lcub}187{rcub}{dollar}Os with a half-life of about 42 Ga. The geochemical behavior of the Re-Os system is unique because Os is an exclusively siderophile/chalcophile element whose concentration is virtually unaltered in the mantle as a result of crust extraction and recycling.; After chemical separation and purification the Re and Os fractions are analyzed by secondary ion mass spectrometry for {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os and Re and Os concentration by isotope dilution. Typical analytical uncertainties are 1 to 2% for Os concentration and {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os and 3 to 10% for Re concentration (both at the 2{dollar}sigma{dollar} level).; The G-chromitite and J-M Reef horizons of the 2.7 Ga Stillwater Complex, Montana, have initial {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os ratios which vary significantly (0.895 to 1.353). The data are consistent with contamination of a mantle melt with isotopically enriched older crustal materials. The lowest {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os provides a maximum value for the source region of the magma and therefore places constraints on the {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os evolution of the mantle during the Archean.; Modern samples have a large range in {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os (1.0032 to 1.173). This variability is proof that Re depletion related to ancient melt extraction has occurred in some regions of the mantle. Ocean island (Hawaii and Iceland) basalts considered to be representative of a distinct plume-source mantle have {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os of about 1.10. Other basalts may have been contaminated with old oceanic crust and have high {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os of {dollar}geq{dollar}1.151. Ultramafic samples from the Southwest Indian Ridge, Zabargad Island (Red Sea), and Zambales Ophiolite (Philippines) range in {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os from 1.083 to 1.0032. Combining all results of this study with other available Os isotopic data, the {dollar}sp{lcub}187{rcub}{dollar}Os/{dollar}sp{lcub}186{rcub}{dollar}Os of the bulk silicate earth is estimated to be close to the ocean island basalt value of 1.10. (Abstract shortened with permission of author.)...