TYPE IB DIAMOND FORMATION RELATED TO GONDWANA ASSEMBLY: EVIDENCE FROM RE-OS ISOTOPIC COMPOSITIONS OF WEST AFRICAN SULPHIDE INCLUSIONS

Re-Os analyses of sulphides in Type Ib alluvial diamonds from Zimmi (Sierra Leone) are the first-ever analyses on Ib diamonds and diamonds from West Africa, providing the opportunity to better understand West African craton assembly and modification processes. In many cratons worldwide, ages obtained from diamond inclusions can be linked to tectonothermal events in the lithosphere. Not only can diamond formation be related to early Archaean craton formation^1,2, but many younger lherzolitic and eclogitic diamonds track orogenic events along craton margins³, craton assembly^4,5, metasomatic reworking⁶ and keel and diamond destructive events⁷. Ten eclogitic sulphides from 3 diamonds, have Re-Os isotopic compositions that fall along Pan-African age arrays (~650 Ma), correlating with the assembly of Gondwana that is recorded in the Rokelide orogen along the SW margin of the Man shield⁸.

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.

[1] Richardson et al., 1984 [2] Westerlund et al., 2006 [3] Aulbach et al., 2009 [4] Richardson et al., 2001 [5] Aulbach et al., 2009b [6] Smit et al., 2010 [7] Smit et al., 2014b [8] Lytwyn et al., 2006 [9] Walker et al., 2002 [10] Gannoun et al., 2007 [11] Barth et al., 2001