Constraints on diamond formation and craton evolution from S isotopes and trace element contents in diamond sulfides
Final Report Abstract
Eclogitic sulphide-bearing diamonds in the central Slave craton are suggested to have formed in a gabbroic substrate during interaction with fluids ultimately derived from underlying serpentinite during 1.85 Ga subduction. Conventional S isotope compositions (δ34S) of the sulphide inclusions are consistent with a crustal origin (δ34S = -3.61 to 3.48 ‰, compared to the canonical mantle value of 0‰). However, because the mass-dependent fractionation evident in the δ34S signal is small and overlaps with that of mantle-derived melts, the new data do not unambiguously require a crustal input. By contrast, the presence of significant mass-independent S isotope fractionation, which so far has been documented only in Archaean or earliest Proterozoic materials, in two of the samples was completely unexpected. The origin of this signature cannot be constrained with the data at hand and resolution must await further analyses. It is planned to obtain N isotope data from the diamonds for further insights. Acquisition of trace-element contents in the same sulphide inclusions for which S isotope data were measured, plus additionally mounted samples, allowed the nature of the fluid to be constrained. Immobile behaviour of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Au and Re) can be deduced from overlapping concentrations of sulphide inclusions in diamond with modern MORB sulphide and indicates that the fluid was reducing and Cl-poor. The distribution of these and other elements that were immobile during subduction zone processing can be used to assess whether sulphide is the main host in oceanic crust, or whether additional host phases are required. This reveals that Cu, Mo, Se and Te, in addition to the HSE, are fully accounted for by sulphide in the oceanic crust. This work demonstrates the power of trace-element analysis of sulphide inclusions in eclogitic diamonds.
Publications
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(2011) C- and S-transfer in subduction zones: Insights from diamonds. 21st V.M. Goldschmidt Conference, Prague, Czech Republic
Aulbach S, Stachel T, Heaman LM, Creaser RA, Thomassot E, Shirey SB
