Radiogenic isotope investigation of the St-Robert H5 fall

Abstract

The St-Robert H5 chondrite yields a mineral/whole-rock Pb-Pb age of 4565 ±  23 Ma (2σ) comparable to the accepted age of most chondrites. The regression of chondrule data give a similar age of 4566 ± 7 Ma (2σ). These results imply that no major perturbation affected the Pb-Pb systematics of this meteorites parent body within the first few billion years following its accretion. Re and Os concentrations along with Os isotopic compositions of whole-rock fragments, surface fusion crusts and metal phases are also reported. The whole rock measurements for this ordinary chondrite are characterized by high Re/Os ratio coupled with relatively high 187Os/188Os (compared to average ordinary chondrites), that we interpret as a long term Re enrichment. As for most chondrites, no precise geochronological information could be extracted from the Re/Os systematics, although most data plot near the IIIAB reference isochron (Smoliar et al. 1996). From the fusion crust results, we rule out the possibility that atmospheric entry caused the perturbations in the Re-Os system, since melted crust analysis yields among the most concordant data points. Evidence from metal phases suggests that a very recent process perturbed the isochron, relocating Re from kamacite toward troilite.