John J. Gurney

Diamonds and the African Lithosphere

Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900� to 1200�C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface.

Archean emplacement of eclogitic components into the lithospheric mantle during formation of the Kaapvaal Craton

Re-Os data for selected whole-rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the Kaapvaal-Zimbabwe cratons yield ages around 3 Ga. The presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought. Overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization. Many of these eclogitic components have fractionated C, N, O, and S isotopic compositions which could have been created at low-temperatures, implying that some style of Archean subduction involving emplacement of oceanic lithosphere was part of craton keel development.

The discovery of garnets closely related to diamonds in the Finsch pipe, South Africa

Magnesium-rich, calcium-poor, lilac coloured garnets have been found in the heavy mineral concentrate of the Finsch kimberlite pipe. Some of these garnets contain sufficient chromium to place them within the compositional field of the garnets previously only reported as inclusions in diamonds.These lilac garnets are considered to have formed in equilibrium with the minerals found as inclusions in diamond and hence with the diamond itself. Their presence in the kimberlite should be diagnostic of the presence of diamond, but it is not known if there is any quantitative relationship. The garnets are considered to have a deeper provenance than the magnesian garnets commonly found as xenocrysts in kimberlite and in garnet peridotite xenoliths. The mantle composition at their depths of origin must be more refractory in nature than the peridotite xenoliths. The garnets having a higher magnesium and chromium content, a higher Mg/Fe ratio and lower calcium, aluminium and titanium than those found in the xenoliths.

The interpretation of the major element compositions of mantle minerals in diamond exploration

Abstract Interpretation of the major element compositions of the mantle macrocryst minerals recovered during stream sediment or soil sampling should be one of the corner-stones of any modern exploration programme for diamonds. Studies must be implemented in such a way as to elucidate the maximum information about the lithospheric mantle underlying the exploration area of interest. This can provide a qualitative or at best semi-quantitative estimate of diamond potential. The composition of a suite of peridotitic garnets from a region or locality is particularly useful in differentiating between diamond bearing and non-diamondiferous mantle samples. Assessments of macrocryst composition are a means of prioritising exploration efforts useful over an extensive period of an active exploration programme, and in a variety of different sets of circumstances. Differences in mantle mineral suite compositions may also be used to identify and isolate specific exploration targets for further attention in areas where numerous potential diamond sources exist.

Variations in trapping temperatures and trace elements in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites, and implications for lithosphere stratigraphy

The proton microprobe has been used to measure the trace element contents of Cr-pyrope, olivine and orthopyroxene inclusion (DI) in>60 diamonds from southern Africa, and in concentrate garnets from the host kimberlites. Olivine inclusions show a negative correlation of Ca with Fo content, but olivines coexisting with very subcalcic garnets are anomously depleted in Ca relative to Fo. The maximum and median values of Ti, Zn, Ga, Zr and Y in DI garnets decrease as Ca decreases relative to Cr, consistent with increasing depletion by removal of silicate melts. However, the most subcalcic garnets are anomously enriched in Sr (and LREE). The contrasting depletion in Zr, Y, Ti etc. and enrichment in Sr is not consistent with a single-stage depletion event. The extreme Ca depletion of the most subcalcic garnets and their coexisting olivines, and the Sr enrichment, are interpreted as the result of metasomatism by a carbonatitic fluid following depletion. Trapping temperatures of garnet DI, estimated by nickel thermometry range from 950 to > 1500°C. Most DI with T>1200°C are lherzolitic, rather than subcalcic. In most pipes studied, the trapping T of DI garnets is higher than the T range of equivalent garnets from concentrates. The high Ts are interpreted as reflecting the formation of diamonds during short-lived thermal pulses, followed by cooling toward a conductive geotherm. The T distribution of calcic and subcalcic garnets in concentrates from kimberlites suggests that lherzolite and harzburgite are intimately intermixed over the depth range 150–180 km beneath the Kalahari craton. The abundance of calcic garnets with T>1200°C suggests the presence of significant amounts of lherzolite at greater depths. This deeper lherzolite may be associated with eclogite, and may be the source region for some high-T lherzolitic DI garnets.

Carbon isotopic composition, nitrogen content and inclusion composition of diamonds from the Roberts Victor kimberlite, South Africa: Evidence for 13C depletion in the mantle

The C isotopic composition and N content of a suite of diamonds of known inclusion mineral composition from the Roberts Victor kimberlite have been measured. The mean 13C-content of diamonds containing peridotitic minerals (X = −5.4%.vs. PDB, s = ±0.9%., n = 65) does not differ significantly from those containing sulfides (X = −4.9%., s = ± 0.9%., n = 20). Diamonds containing eclogitic minerals can be subdivided into two groups based on their carbon isotopic composition: Group-A (X = −15.5%., s = ±0.4%., n = 10) and Group-B (X = −5.6%., s = ±0.6%., n = 4). The clinopyroxenes occluded by the Group-A diamonds are depleted in SiO2, MgO, and CaO and significantly enriched in Al2O3, FeO, and MnO compared to clinopyroxenes occluded by Group-B diamonds. Carbon in two graphite-diamond eclogites has a mean isotopic composition of −5.31%.; in both samples graphite shows a slight enrichment in 13C compared to the coexisting diamond. The range of N concentrations of diamonds containing peridotitic or eclogitic minerals is essentially the same (0 to 750 ppm), while it is considerably larger for sulfide-containing diamonds (0 to 1700 ppm). There is no difference in the C isotopic composition between Type I and Type II diamonds for sulfide and peridotitic minerals occluding diamonds. All Type II diamonds containing eclogitic minerals belong to Group-A. No correlation between N content and C isotopic composition could be established, although a large range in both variables is observed for the sample suite. The composition of eclogitic minerals included in diamonds of low 13C-content differs from that of eclogite xenoliths characterized by 18O-depletions, which have been related to subduction processes. Hence the data available do not suggest a common cause for the depletion of the heavy isotopes of the two elements. The chemical and isotopic characteristics of the suite of diamond samples reflect different mantle environments. Diamonds depleted in 13C (δ13C = −15 to −16%.) come from a region at greater depth than those of 13C contents of −5 to −6%.. The source region of the former is characterized by higher Fe, Mn, Al, and lower Mg, Ca, Si, and N contents than that of the latter.

Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa

Abstract Cubic diamonds from the Orapa kimberlite have a very restricted δ 13 C range with a mean of −5.98 ± 0.57‰ , a relatively high and constant nitrogen content 897 ± 171 ppm and low nitrogen aggregation state. They are thought to have been derived from shallow mantle depth. Diamonds with peridotitic inclusions (P-Type) range in δ 13 C between−4.2 and − 18.9‰ with a major mode between −5 and−7‰. Their nitrogen content is low and state of aggregation high. Pressure-temperature ( P - T ) equilibration conditions estimated for their inclusions are close to the wet peridotite solidus. Eclogitic inclusions in Orapa diamonds (E-Type) indicate higher P - T equilibration conditions than P-Type minerals. The δ 13 C values of their hosts vary from −2.6 to−18.0%. with a major mode between −13 and −15‰. The nitrogen content of E-type diamonds is significantly higher than that of P-Type diamonds. Comparison of the mineral and carbon isotopic composition of Orapa diamond eclogites with those of E-Type diamonds indicates that only part of the E-Type diamonds could have been derived from a physical breakup of diamond eclogites. 13 C-depleted E-Type diamonds and eclogite xenoliths with low δ 13 C diamonds were formed in a similar and restricted P - T range. An inclusion mineral paragenesis with compositions transitional between those of peridotitic and eclogitic minerals (websteritic, W-Type) has been recognized. It is characterized by a unique combination of a relatively high chrome and Fe content. The δ 13 C values of W-Type diamonds lie, with one exception (−6.9%.), in the range of −15.2 to −22.4‰. The nitrogen content of these diamonds is significantly lower than that of E-Type and is indistinguishable from that of P-Type diamonds. The P - T equilibration conditions estimated for them are similar to those of the peridotitic paragenesis. The collective data indicate a higher frequency of occurrence of 13 C-depleted diamonds, a decrease in the N content of diamonds, and an increase in the N aggregation state with increasing mantle depth. The decline in N content is not monotonic however, because P-Type diamonds tend to have lower N contents than E-Type diamonds. The combined high chrome and Fe content of the websteritic diamond inclusions are very difficult to reconcile with a subduction origin of the low δ 13 C host and indicate that 13 C-depleted C may be a primary mantle feature.

Nitrogen and 13C content of Finsch and Premier diamonds and their implications

The nitrogen content and state of aggregation have been determined spectroscopically for diamonds from the Finsch (93) and the Premier (116) kimberlite, South Africa. The ranges of concentration for diamonds with eclogitic (E-Type) and peridotitic (P-Type) mineral inclusions are: Finsch: E-Type, 29 to 1639 ppm, P-Type, 7 to 1206 ppm; Premier: E-Type, 15 to 1359 ppm, P-Type, 7 to 1086 ppm. In both kimberlites nitrogen-free diamonds (Type II) contain less frequently eclogitic inclusions. Among E-Type diamonds there are none of Type II in the Finsch and 1% in the Premier sample suite; for P-Type diamonds the respective percentages are 29 and 25%. There is no significant difference in the nitrogen aggregation state between E-Type and P-Type diamonds within each kimberlite; however, there is a significant difference between the two locations. The nitrogen in the B form amounts to 26 ± 29% (n = 65) in Finsch and to 48 ± 26% (n = 101) in Premier diamonds. No correlation between δ13C and nitrogen content or aggregation state is observed. However, within the eclogitic and peridotitic paragenesis, groups of diamonds of very similar nitrogen concentration, δ13C, and mineral inclusion composition can be recognized. One of these may be common to both kimberlites. Some of the diamonds in the two kimberlites may have had a common origin but subsequent thermal histories, which differed for the sample suites from the two diatremes. The nitrogen content of the diamonds is a function of local nitrogen concentration, temperature and oxygen fugacity. The dependence on the last of these can cause complex relationships between δ15N, δ13C and nitrogen content of diamonds.

Program to study crust and mantle of the Archean craton in southern Africa

A multidisciplinary research program is studying the structure, composition, and tectonic evolution of the mantle and crust of one of the best preserved early Archean eratons: the Kaapvaal craton in southern Africa. Cratons, which have survived over 2.5 Ga of tectonic activity on the Earths surface, are more than just old crustal features. They form the stable nuclei of the present-day continents, comprising regions of thickened lithosphere extending deeper than 200 km.

The carbon isotopic composition and nitrogen content of lithospheric and asthenospheric diamonds from the Jagersfontein and Koffiefontein kimberlite, South Africa

Asthenospheric diamonds, from the Jagersfontein kimberlite, South Africa, have a δ13C range from −8.9 to −24.4%. vs. PDB. Deep diamonds from the nearby Koffiefontein mine have δ13C values between −2.9 and −6.1%.. At these two locations asthenospheric diamonds are more frequently nitrogen Type II than Type I. If they are Type I, the nitrogen content is low and its state of aggregation is high. Diamonds from the Jagersfontein kimberlite have a bimodal δ13C distribution. On the basis of the carbon isotopic composition of the hosts and their mineral inclusion composition, one can suggest that as many as six distinct diamond source regions were sampled by the kimberlite. In the mantle beneath Jagersfontein, the relative abundance of low δ13C diamonds may be higher in the asthenosphere than in the lithosphere. In the Koffiefontein sample suite, asthenospheric and lithospheric diamonds are indistinguishable in δ13C. The differences in the nature of the relationships between isotope and inclusion chemistry demonstrated by deep diamonds from two diatremes, which are very closely associated in space and time, indicate a high degree of complexity in mantle carbon geochemistry even within the asthenosphere.