PII S0016-7037(01)00676-7 Rhenium–osmium systematics of calcium–aluminium-rich inclusions in carbonaceous chondrites

Abstract—The Re-Os isotopic systematics of calcium–aluminium-rich inclusions (CAIs) in chondrites were investigated in order to shed light on the behavior of the Re-Os system in bulk chondrites, and to constrain the timing of chemical fractionation in primitive chondrites. CAIs with relatively unfractionated rare earth element (REE) patterns (groups I, III, V, VI) define a narrow range of 187Re/188Os (0.3764–0.4443) and 187Os/188Os (0.12599–0.12717), and high but variable Re and Os abundances (3209–41,820 ppb Os). In contrast, CAIs that show depletions in highly refractory elements and strongly fractionated REE patterns (group II) also show a much larger range in 187Re/188Os (0.409–0.535) and 187Os/188Os (0.12695–0.13770), and greater than an order of magnitude lower Re and Os abundances than other groups (e.g., 75.7–680.2 ppb Os). Sixteen bulk CAIs and CAI splits plot within analytical uncertainty of a 4558 Ga reference isochron, as is expected for materials of this antiquity. Eight samples, however, plot off the isochron. Several possible reasons for these deviations are discussed. Data for multiple splits of one CAI indicate that the nonisochronous behavior for at least this CAI is the result of Re-Os reequilibration at approximately 1.6 Ga. Thus, the most likely explanation for the deviations of most of the nonisochronous CAIs is late-stage open-system behavior of Re and Os in the asteroidal environment