Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

摘要

Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 +/- 0.17 Ga (95 % conf. int.) reference line that is based on their mean T-MA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from similar to 50 to similar to 170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping Os-187/Os-188 at eruption age: Os-187/Os-188(EA), while group III samples have low Os concentrations, unradiogenic Os-187/Os-188(EA) and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of Os-187/Os-188 with degree of melt depletion and lack of radiogenic Os addit