TYPE IB DIAMOND FORMATION RELATED TO GONDWANA ASSEMBLY: EVIDENCE FROM RE-OS ISOTOPIC COMPOSITIONS OF WEST AFRICAN SULPHIDE INCLUSIONS
Initial 187Os/188Os obtained from age arrays are between 1.5 and 2.2, extremely radiogenic compared to the O-chondrite reservoir at 650 Ma (187Os/188Os = 0.123879). These radiogenic 187Os/188Os can only evolve in a source with high Re/Os ratios (50 to 100): achieved though long-term isolation from the convecting mantle and typical in MORB10. This suggests the sulphides were derived from older eclogitic protoliths that have mafic oceanic crust precursors, possibly Archaean low-Mg eclogites from the nearby Koidu kimberlite11. The sulphides were then encapsulated during Pan-African diamond growth from carbon-bearing fluids remobilised during continental collision.
Host diamonds for these sulphides are Type Ib: rare diamonds that still preserve nitrogen as single atoms (C centre), rather than more common nitrogen pairs (A centre) and nitrogen aggregates (B centre). C centres in Zimmi diamonds require that they did not experience temperatures above 700 °C for any extended period, suggesting that after formation the diamonds were rapidly exhumed to shallower depths in the lithosphere, likely through tectonic uplift following continent collision.
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