Rhenium-osmium systematics of calcium-aluminium-rich inclusions in carbonaceous chondrites

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 Re-187/Os-188 (0.3764-0.4443) and (OS)-O-187/(OS)-O-188 (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 Re-187/Os-188 (0.409-0.535) and (OS)-O-187/(OS)-O-188 (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. The Os-187/Os-188-Os systematics of CAIs are consistent with previous models that indicate group II CAIs are mixtures of components that lost the bulk of their highly refractory elements in a previous condensation event and a minor second component that provided refractory elements at chondritic: relative proportions. The high Re/Os of group H CAIs relative to other CAIs and chondrite bulk rocks may have been caused by variable mobilization of Re and Os during medium- to low-temperature parent body alteration similar to4.5 Ga ago. This model is favored over nebular models, which pose several difficulties. The narrow range of Os-187/Os-188 in group I, III, V, and VI bulk CAIs, and the agreement with Os-187/Os-188 of whole rock carbonaceous. chondrites suggest that on a bulk inclusion scale, secondary alteration only modestly fractionated Re/Os in these CAIs. The average of (OS)-O-187/Os-188 for group I, III, V, and VI CAIs is indistinguishable from average CI chondrites, indicating a modem solar system value for Os-187/Os-188 Of 0.12650, corresponding to a Re-187/Os-188 of 0.3964. Copyright (C) 2001 Elsevier Science Ltd.