Rhenium-osmium isotopes in pervasively metasomatized mantle xenoliths from the Bohemian Massif and implications for the reliability of Os model ages
Abstrakt
Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re-Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history.
Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (<1 ppb). Rhenium concentrations in the whole suite are below the primitive mantle value.
A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187Os188Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tectonic events to reset the original Os isotopic systematics.
The calculated mantle extraction ages (TMA) range from <0.1 to 2.1 Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion a/ges (TRD) vary between <0.1 and ~1.6 Ga.
However, the TRD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (TRDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite.
A prominent peak in the calculated TRDII ages ranges between 0.5 and 0.6 Ga which corresponds to the Cadomian orogenic cycle.