THE RE-OS ISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

The Re-Os isotope sytem, based on the long-lived beta(-) transition of Re-187 to Os-187, has matured to wide use in cosmochemistry and high- temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most othe r long-lived radiogenic isotope systems. Magmatic iron meteorites (III AB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10-40 million years of Solar Sys tem evolution. Rocks from Earth's convecting mantle show generally cho ndritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core f ormation via late accretion. Oceanic basalts have Os-isotope systemati cs that improve the detailed geological interpretation of extant mantl e components. Some portions of ancient subcontinental lithospheric man tle are severely depleted in Re and have correspondingly subchondritic Os-187/Os-188, indicating long-term isolation from the convecting man tle during the Archean-Proterozoic. Magmatic ore deposits have differe nces in initial Os isotopic composition traceable to the crustal vs ma ntle sources of the platinum-group elements and base metals.