Homogenisation of sulphide inclusions within diamonds: A new approach to diamond inclusion geochemistry

Abstract

Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth's crust and subsequent surface weathering. However, at temperatures below 1100°C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, 'flaking' of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187 Os/ 188 Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100°C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (mCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby widening the scope for multiple methods for quantitative analysis, even on 'flakes' of single BMS inclusions. Finally we show that the trace elements present in peridotite (P-type) and eclogitic (E-type) BMS are distinct, with P-type diamonds having systematically higher total platinum-group element (particularly Os, Ir, Ru) and Te and As concentrations. These distinctions suggest that the PGE and semi-metal budgets of mantle-derived partial melts will be significantly dependent upon the type(s) and proportions of sulphides present in the mantle source.

Sulphide Petrology and Contribution of Subducted Sulphur in Diamondiferous Garnet-Bearing Pyroxenites from Beni Bousera (Northern Morocco)

“…Our database provides some clues for a comparison with the well-known BMS-diamond association. Deines & Harris, 1995;Clemens et al, 2008;Thomassot et al, 2009;Aulbach et al, 2012 ;McDonald et al, 2017;Kempinnen et al, 2018). The two complete datasets for trace chalcophile/siderophile elements contents available for eclogitic diamonds are those of Aulbach et al predicted by models of diamond production involving interaction between oxidized carbonatebearing subducted slabs and reduced metal-saturated mantle rocks (see reviews by Cartigny, 2005 ;Stachel & Harris, 2008;Shirey et al, 2013) or between reduced oceanic crust and oxidizing CO 2 -rich fluids, if, as pointed out by Aulbach et al (2017Aulbach et al ( , 2019, the Archaean oceanic crust had highly reduced compositions, markedly below carbonate stability.…”

“…BMS are commonly included in eclogitic-type diamonds (E-type (e.g. Yefimova et al, 1983 ;Meyer, 1987;Harris, 1992;Deines & Harris, 1995;Chaussidon et al, 1987;Stachel & Harris, 2008;Thomassot et al, 2009;Aulbach et al, 2012;McDonald et al, 2017;Kempinnen et al, 2018). This association has received highly controversial explanation.…”

Supplemental Material: Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

“…This problem is more severe for Ni-rich sulfides in peridotitic lithotypes than for Ni-poor sulfides in eclogites, if up to 5% Ni is accomodated in pentlandite (Cabri, 1973). In fact, it is critical for isotopic analyses that the entire mass of the inclusion is analysed to ensure the quantitative reconstruction of the composition of the original bulk sulfide, given their differing Re-Os compositions (Taylor and Liu, 2009;McDonald et al, 2017). Inclusions in diamonds are commonly studied by destructive methods in which the diamonds are crushed or ablated using a laser.…”

“…Inclusions in diamonds are commonly studied by destructive methods in which the diamonds are crushed or ablated using a laser. However, it is impossible to guarantee 100% recovery of the inclusion, and for laser ablation analyses partial sampling is the default situation introducing serious bias to geochemical results (McDonald et al, 2017).…”

“…X-ray diffraction in-situ analyses are capable of quantifying the whole inclusion without compromising it via polishing, i.e., it provides a true "bulk" analysis. In fact, examination of polished sections to reconstruct the bulk composition of the sulfide results in higher uncertainties, as separate microcrystals are non-uniformly distributed and fractioned isotopes can be lost during sampling (Taylor and Liu, 2009;McDonald et al, 2017). Here, X-ray diffraction data were collected on single crystals and converted into a 2D powder patterns to perform Rietveld refinements of the diffraction pattern of the polycrystalline assemblage.…”