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Abstract: The origin of alluvial diamonds from the four main diamond mining districts in Kalimantan was studied through characterisation of their properties, and determination of PT and age of formation of representative collections of diamonds from four localities of the island. The diamonds are mostly colourless, yellow or pale brown, shiny surfaced, dodecahedroids, octahedron/dodecahedroids, and more rarely cube combination forms. They are intensively resorbed. They have surface radiation damage and show abrasion features indicative of fluvial transportation and crustal recycling. The diamonds were polished down to expose internal structures and mineral inclusions. The majority of the diamonds are internally homogeneous or have simple octahedral zonation and show plastic deformation. Analysis by Fourier transform infra red spectroscopy of their N content and aggregation characteristics shows that many diamonds are well-aggregated type IaB implying a long-term, mantle residence time and/or high temperatures of formation. Identified inclusion parageneses are 68% peridotitic and 32% eclogitic. The peridotitic inclusions are represented by olivine, chromite, garnet, orthopyroxene and pentlandite. Olivines (Fo92–93) belong to the dunite–harzburgite paragenesis, with one at Fo90 identified as lherzolitic. Chromite inclusions with 65–66 wt.% Cr2O3 and <1 wt.% TiO2 are typical of chromite diamond inclusions world-wide. Two garnet inclusions identified are a subcalcic high chrome harzburgite “G10” and a mildly subcalcic type transitional between “G9” and “G10”. The eclogitic inclusions are represented by omphacite, rutile, kyanite and coesite. Re/Os dating of a high Ni sulphide inclusion from one peridotitic diamond gave an Archean model age of 3.1 Ga±0.2 (2 sigma). In terms of their external and internal morphology, N aggregation characteristics and paragenesis the Kalimantan diamonds resemble those transported to surface by kimberlite or lamproite from sources in the subcontinental lithospheric mantle. Five distinctive genetic groups of diamond have been recognised but occur mixed in each of the four diamond mining districts due to a presumed long history of sedimentary recycling and alluvial transportation. Thermobarometry calculations from diamond inclusion chemistry (930 °C to 1250 °C; >4.2 GPa) are consistent with a paleo-heat flow of 38 to 40 mW/m2 and derivation from 120 to 160 km depth, i.e. subcontinental mantle lithosphere conditions similar to diamonds from African and Yakutian cratonic situations. [Copyright &y& Elsevier]

Diamonds from Dachine, French Guiana, are unique among worldwide diamond populations. The diamonds were transported to the surface in an unusual ultramafic extrusive magma with an affinity to boninite or komatiite, which was emplaced within an arc geological setting at ~ 2.2 Ga. Dachine diamonds have internal and external morphologies indicative of relatively rapid growth from carbon oversaturated fluids or melts, and exhibit internal features consistent with residence in a high-strain environment. On the basis of nitrogen (N) defects the diamonds are categorized as Type Ib–IaA. The unusually low aggregation state of N places severe constraints on the thermal history of the diamonds, effectively ruling out derivation in convecting mantle. The carbon and N isotopic compositions of Dachine diamonds are consistent with a sedimentary source of carbon, with the majority of diamonds having δ 13 C values < − 25‰ and δ 15 N values > + 4‰. The primary carbon was presumably deposited on an early Proterozoic seafloor. Sulphide inclusions have low Ni and Cr and are comparable to lithospheric eclogitic-type sulphide inclusions. Three garnet and one clinopyroxene inclusion are also eclogitic in composition, and one garnet inclusion has a majorite component indicating an origin around 250 km depth. The silicate inclusions are highly depleted in many incompatible trace elements (e.g. LREE, Nb, Hf, Zr), and modelling indicates an eclogitic source lithology that contained a LREE-enriched trace phase such as epidote or allanite, and an HFSE-rich phase such as rutile. Four of the five inclusions are unusually enriched in Mn, as well as Ni and Co, and modelling suggests a protolith with a bulk composition of subducted normal MORB plus about 10% ferromanganese crust component. We suggest a model wherein Dachine diamonds precipitated from remobilized sedimentary carbon at the slab–mantle interface from liquids derived ultimately by deserpentinization of slab peridotite at depths of ~ 200 to 250 km. These fluids may also trigger melting in wedge peridotite, resulting in a volatile-rich ultramafic melt that transports the diamonds rapidly to the surface. The process of diamond formation and exhumation from the slab–mantle interface occurred in a Paleoproterozoic subduction zone and over a very limited timespan, likely less than a million years. [ABSTRACT FROM AUTHOR]

Abstract: Inclusion-bearing diamonds, mantle xenoliths, and kimberlite concentrates from the Cambrian-aged Murowa and Sese kimberlites have been studied to characterise the nature of the lithospheric mantle beneath the southern Zimbabwe Craton. The diamonds are mostly octahedral, moderately rich in nitrogen with moderate to high aggregation, and contain mainly dunite–harzburgite mineral inclusions. Similarly, dunite xenoliths predominate over harzburgite and lherzolite and carry olivines with Mg/Mg+Fe (Mg#) values of 0.92–0.95, spanning the average signatures for Kaapvaal Craton peridotites. Eclogitic xenoliths are extremely rare, in contrast to the Kaapvaal mantle lithosphere. The Zimbabwe mantle assemblage has been only slightly affected by later silicic metasomatism and re-fertilisation with re-introduction of pyroxenes in contrast to the Kaapvaal and many cratonic lithospheric blocks elsewhere where strong metasomatism and re-fertilisation is widespread. Pyroxene, garnet and spinel thermobarometry suggests an ambient 40 mW m−2 geotherm, with the lithosphere extending down to 210 km at the time of kimberlite eruption. Whole rock peridotite Re–Os isotope analyses yield TRD model ages of 2.7 to 2.9 Ga, providing minimum estimates of the time of melt depletion, are slightly younger in age than the basement greenstone formation. These model ages coincide with the mean TRD age of >200 analyses of Kaapvaal Craton peridotites, whereas the average Re–Os model age for the Zimbabwe peridotites is 3.2 Ga. The Os data and low Yb n /Lu n ratios suggest a model whereby thick lithospheric mantle was stabilised during the early stages of crustal development by shallow peridotite melting required for formation of residues with sufficiently high Cr/Al to stabilise chromite which then transforms to low Ca, high Cr garnet. Sulphide inclusions in diamond produce minimum TRD model ages of 3.4 Ga indicating that parts of the lithosphere were present at the earliest stages of crust formation in this area. Published mineral analyses for mantle xenoliths, diamonds and concentrate macrocrysts from the Venetia, River Ranch, Mwenezi and Chingwizi pipes which intrude the Limpopo Mobile Belt show strongly depleted peridotitic mantle signatures matching with the mantle beneath southern Zimbabwe. Published seismic velocity imagery suggests the presence of a deep lithospheric mantle keel beneath the Limpopo Mobile Belt today and Re–Os analyses of peridotite xenoliths from Venetia have previously yielded an Archean age. The Limpopo Mobile Belt is therefore interpreted as a product of thin skin crustal tectonics with the underlying lithospheric mantle linked to that of the southern Zimbabwe Craton and unaffected by the overlying crustal events. [Copyright &y& Elsevier]

Sulphide inclusions are common features of natural diamonds. They can provide an insight into the nature of diamond-forming reactions and are especially important for Re–Os dating of diamond formation. A discrete molybdenite (MoS 2 ) phase has been identified for the first time by Raman spectroscopy in 73 out of 80 syngenetic sulphide inclusions in 7 eclogitic diamonds from the Mir kimberlite (Yakutia, Russia). The sulphide inclusions were chemically and texturally characterised by electron probe microanalyses (EPMA), focused ion-beam scanning electron microscopy (FIB-SEM) and synchrotron-based X-ray tomographic microscopy (SXRTM). Our observations suggest the molybdenite has unmixed from an original sulphide melt or monosulphide solid solution. It occurs as sub-micron sized grains, commonly in association with the chalcopyrite rims of the inclusions and sometimes, within surrounding decompression cracks. Molybdenite has also been identified by Raman spectroscopy in at least 50% of sulphide inclusions in preliminary studies of eclogitic diamonds from Argyle (NW Australia), Orapa, Letlhakane, Damtshaa (Botswana) and Dachine (French Guiana), and peridotitic diamond-hosted inclusions from Udachnaya (Yakutia, Russia) and Murowa (Zimbabwe). We have modelled the effects that different amounts of Re loss – through its segregation into an unrecovered molybdenite phase – could have on the radiometric ages of diamonds dated using the Re–Os system. In general Re loss through this process will lead to isochron ages older than the true age, and variable degrees of Re loss will lead to increased scatter around the apparent isochron. For model age calculations, the effects would depend on the 187 Re/ 188 Os ratio of the inclusions (if their compositions evolved above or below that of the chondritic mantle evolution curve) but Re loss could generate unrealistically old or future ages, particularly in eclogitic inclusions. [ABSTRACT FROM AUTHOR]

Fourier Transform Infrared (FTIR) spectroscopy is a commonly-used technique for investigating diamonds, that gives the most useful information if spatially-resolved measurements are used. In this paper we discuss the best way to acquire and present FTIR data from diamonds, using examples from Murowa (Zimbabwe), Argyle (Australia) and Machado River (Brazil). Examples of FTIR core-to-rim line scans, maps with high spatial resolution and maps with high spectral resolution that are fitted to extract the spatial variation of different nitrogen and hydrogen defects are presented. Model mantle residence temperatures are calculated from the concentration of A and B nitrogen-containing defects in the diamonds using known times of annealing in the mantle. A new, two-stage thermal annealing model is presented that better constrains the thermal history of the diamond and that of the mantle lithosphere in which the diamond resided. The effect of heterogeneity within the analysed FTIR volume is quantitatively assessed and errors in model temperatures that can be introduced by studying whole diamonds instead of thin plates are discussed. The spatial distribution of VN 3 H hydrogen defects associated with the 3107 cm − 1 vibration does not follow the same pattern as nitrogen defects, and an enrichment of VN 3 H hydrogen at the boundary between pre-existing diamond and diamond overgrowths is observed. There are several possible explanations for this observation including a change in chemical composition of diamond forming fluid during growth or kinetically controlled uptake of hydrogen. [ABSTRACT FROM AUTHOR]