South African Kimberlites Research Articles

Experimental modeling of the conditions of crystallization of subcalcium chromium pyropes

1062 Subcalcium chromium pyropes with a CaO concen� tration of ~3 wt % or lower and widely variable Cr2O3 concentration of ~5 wt % and higher are observed as inclusions in diamonds of peridotitic assemblage, xeno� liths of harzburgite (dunite) and lherzolite, and as indi� vidual grains in the heavy fraction of kimberlite [1–3]. Of special interest are chromium pyropes with a CaO concentration of <2 wt %. Independently of the method of analysis recalculation, these minerals con� tain a significant admixture of the MgCr component (knorringite). The main problem in the continuing, long discussion about the conditions of the formation of these pyropes is the composition of the protolith. The authors of one of the hypotheses [4] suggested that subcalcium chromium garnets, similarly to dia� monds, observed only within cratons are accessory minerals of restitic rocks from the komatiitic process of deep melting. This hypothesis was supported by the data on the Archean Sm–Nd model age of subcalcium pyropes included in diamonds of some South African kimberlites. However, experiments performed at high pressure [5] demonstrated that subclacium chromium pyropes of the diamond assemblage [1] could not be in equilibrium with the komatiitic melt being character� ized by too high a Cr 2

Analysis of hydrogen and fluorine in pyroxenes: II. Clinopyroxene

Studies of coexisting, nominally anhydrous minerals in mantle samples show that clinopyroxene is an especially important host for hydrogen. Recent experimental studies have also shown that clinopyroxene may contain significant amounts of fluorine, which has implications for the F budget of the mantle. More accurate quantification of H and F is therefore a desirable goal. We measured H in 13 natural clinopyroxenes using Fourier transform infrared (FTIR) spectroscopy. 16 O 1 H/ 30 Si and 19 F/ 30 Si were also measured in the samples using secondary ion mass spectrometry (SIMS). H data were compared between the two techniques and F was calculated with reference to F-bearing silicate glass standards. Four of the clinopyroxenes are used as standards for SIMS calibration in multiple laboratories, and three have been measured previously using hydrogen manometry and/or elastic recoil detection analysis. Compared to clinopyroxenes in previous surveys comparing FTIR and SIMS, the 13 samples cover a broader range in chemistry and band positions in the O-H vibrational spectrum. They also all lack detectable amphibole lamellae, which are otherwise commonly present in this mineral group. In contrast to orthopyroxene, the SIMS and FTIR data for clinopyroxene show significantly better correlations (r 2 = 0.96-0.98) when the frequency-dependent IR calibration of Libowitzky and Rossman (1997) is applied, as opposed to the Bell et al. (1995) calibration (r 2 = 0.92-93). We derive a frequency-dependent molar absorption coefficient with parameters different from those of Libowitzky and Rossman’s calibration, which was established using data on stoichiometric hydrous phases and gives poor agreement with the manometrically determined value for PMR-53. Comparison of data for PMR-53 to our SIMS calibrations for orthopyroxene and olivine suggests that the matrix effect among these phases is less than 20% relative. Fluorine concentrations vary depending on geological context, with the highest concentrations (up to 214 ppm) found in diopsides from crustal metamorphic environments. Mantle samples follow similar geographic trends as olivines and orthopyroxenes, with higher F in xenocrysts from Kilbourne Hole (46 ppm) and South African kimberlites (up to 29 ppm) compared to the Colorado Plateau (8 ppm). On the basis of chemical correlations, we propose two different incorporation mechanisms for F: (1) coupled subsititution with Al 3+ and/or Fe 3+ in tetrahedral sites; and (2) coupled substitution with monovalent cations (Na and K) in the M2 site. The second substitution is more relevant to mantle augites than crustal diopsides. Our measured F concentrations are much lower than those in some clinopyroxenes synthesized in recent high P-T studies. Nevertheless, our data support suggestions that the F budget of the mantle can be entirely accommodated by incorporation in nominally anhydrous/fluorine-free minerals.

Analysis of hydrogen and fluorine in pyroxenes: I. Orthopyroxene

Studies of coexisting, nominally anhydrous minerals in mantle samples show that clinopyroxene is an especially important host for hydrogen. Recent experimental studies have also shown that clinopyroxene may contain significant amounts of fluorine, which has implications for the F budget of the mantle. More accurate quantification of H and F is therefore a desirable goal. We measured H in 13 natural clinopyroxenes using Fourier transform infrared (FTIR) spectroscopy. ^(16)O^1H/^(30)Si and ^(19)F/^(30)Si were also measured in the samples using secondary ion mass spectrometry (SIMS). H data were compared between the two techniques and F was calculated with reference to F-bearing silicate glass standards. Four of the clinopyroxenes are used as standards for SIMS calibration in multiple laboratories, and three have been measured previously using hydrogen manometry and/or elastic recoil detection analysis. Compared to clinopyroxenes in previous surveys comparing FTIR and SIMS, the 13 samples cover a broader range in chemistry and band positions in the O-H vibrational spectrum. They also all lack detectable amphibole lamellae, which are otherwise commonly present in this mineral group. In contrast to orthopyroxene, the SIMS and FTIR data for clinopyroxene show significantly better correlations (r^2 = 0.96–0.98) when the frequency-dependent IR calibration of Libowitzky and Rossman (1997) is applied, as opposed to the Bell et al. (1995) calibration (r^2 = 0.92–93). We derive a frequency-dependent molar absorption coefficient with parameters different from those of Libowitzky and Rossman’s calibration, which was established using data on stoichiometric hydrous phases and gives poor agreement with the manometrically determined value for PMR-53. Comparison of data for PMR-53 to our SIMS calibrations for orthopyroxene and olivine suggests that the matrix effect among these phases is less than 20% relative. Fluorine concentrations vary depending on geological context, with the highest concentrations (up to 214 ppm) found in diopsides from crustal metamorphic environments. Mantle samples follow similar geographic trends as olivines and orthopyroxenes, with higher F in xenocrysts from Kilbourne Hole (46 ppm) and South African kimberlites (up to 29 ppm) compared to the Colorado Plateau (8 ppm). On the basis of chemical correlations, we propose two different incorporation mechanisms for F: (1) coupled subsititution with Al^(3+) and/or Fe^(3+) in tetrahedral sites; and (2) coupled substitution with monovalent cations (Na and K) in the M2 site. The second substitution is more relevant to mantle augites than crustal diopsides. Our measured F concentrations are much lower than those in some clinopyroxenes synthesized in recent high P-T studies. Nevertheless, our data support suggestions that the F budget of the mantle can be entirely accommodated by incorporation in nominally anhydrous/fluorine-free minerals.

Metamorphic re-equilibration and metasomatism of highly chromian, garnet-rich peridotitic xenoliths from South African kimberlites

A history of decompression and metasomatism is preserved in a suite of highly chromian, garnet-rich peridotitic xenoliths from the diamondiferous Newlands and Bobbejaan kimberlites, South Africa. A high proportion of the garnets and chromites in these rocks plot in the diamond-facies fields on Cr2O3–CaO and Cr2O3–MgO wt% plots, and Cr-rich compositions are found in both the harzburgitic and lherzolitic fields. Petrographic evidence suggests that the earliest known mineralogies were those of olivine-bearing, garnet-rich rocks. These were modified by a decompression event that caused recrystallization of garnets and led to orientated spinel and pyroxene inclusions in garnet. Chemical zonation within garnet is divided into (1) external re-equilibration between garnet and matrix; (2) internal re-equilibration between garnet and its inclusions; and (3) metasomatically induced zoning between garnet core and a metasomatic rim. The compositional trajectories associated with zonations (1) and (2) in Ca–Cr plots may be closely modelled by means of sliding, garnet–spinel transition reactions whose slopes vary with bulk Ca composition; at intermediate Ca compositions, the trajectories closely match the slope of the lherzolite line or harzburgite/lherzolite boundary. The decreasing Cr/(Cr + Al) of the garnet in these zonations is in agreement with the evidence for decompression given by the petrographic recrystallization features, and overall decompression of probably 10–20 kb is indicated. We speculate on the age of these events, and consider the possibility of their association with major orogenic events documented by South African crustal rocks at 2.9–2.7 Ga, and events evidenced by peridotite-xenolith Re–Os model ages at 2.8–2.7 Ga.

40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0–1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00–1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800–1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric mantle below southern Africa may have been frequently influenced by different episodes of fluid or melt migration during subduction of oceanic crust at active continental margins.

Sheared lherzolite xenoliths revisited

The microstructures of sheared lherzolite xenoliths from South African kimberlites are investigated using new microstructural analysis techniques and new rheological data. By applying electron backscatter diffraction (EBSD) methods and the M‐index technique for quantifying the strength of lattice preferred orientation (LPO), it is demonstrated that olivine and orthopyroxene, after an initial stage of dynamic recrystallization, deformed by different mechanisms: olivine deformed by dislocation creep, while orthopyroxene deformed by a grain‐size sensitive mechanism that randomized the preexisting LPO. New rheological data, which incorporate the effects of pressure on deformation, are applied to these xenoliths. The effects of pressure are shown to dramatically reduce the inferred strain rates of these xenoliths in comparison to previous estimates. The contrasting microstructural features of olivine and orthopyroxene are analyzed using the Avrami theory of recrystallization, coupled with microstructural observations and experimental data. This analysis suggests that (1) the difference in the recrystallized grain‐size and (2) the difference in the completeness of recrystallization, can be largely explained by the contrast in grain boundary mobility between olivine and orthopyroxene.

Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts

Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyroclasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite (Fo87^89) and NiOcontents (1300^2800 ppm), which precludes a restitic origin for the dunites. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite.Trace element compositions differ between porphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins.The dunites have high Os/Os ratios (0 11^0 15) that result in young model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyroclastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyroclasts are interpreted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode.The calculated parental magma for the most primitive olivine neoblasts in the Fe-rich dunites is similar to low-Ti Karoo basalts. Modelling the crystal fractionation of the inferred parental magma with pMELTS yields element fractionation trends that mirror the element variation of primitive low-Ti Karoo basalts.

Geochemistry and petrogenesis of tholeiitic intrusions of possible Bushveld-age in the Vredefort Dome, South Africa

The tholeiitic group of intrusions that occurs in the core and collar rocks of the Vredefort dome comprises three related intrusions, namely the Wittekopjes norite, the Parsons Rust dolerite-norite and the Reebokkop dolerite. The texture of these intrusions varies from hypidiomorphic-granular in the norite to the distinctive poikilitic character of the dolerite-norite to hypidiomorphic to subophitic texture of the dolerite. The major constituent minerals, pyroxene and plagioclase, are normally zoned. Chemical variations in the normally zoned minerals indicate a differentiating magma. The tholeiitic group comprises siliceous high-MgO tholeiitic rocks with a strong noritic character. The characteristic geochemistry of the group includes high MgO, SiO 2, Cr and Ni contents; low enrichment in light and heavy rare earth elements (REE) with regard to average chondrite composition; absence of significant fractionation of light REE with regard to heavy REE; and low TiO 2, P 2O 5, K 2O, Sr, Zr, Y, and Nb contents. Geochemical variation within the sills conforms to normal differentiation trends. To explain the relatively high SiO 2 content in these mafic intrusions two mechanisms can be proposed: incongruent melting of enstatite in a depleted mantle source will generate forsteritic olivine and progressively silica enriched melts; or alternatively, crystallisation of a high percentage of olivine could result in silica enrichment of an ultramafic magma. Two possible scenarios for the generation of a noritic magma with composition similar to that of the Wittekopjes norite are proposed. Firstly, derivation of a noritic magma by means of fractional crystallisation of approximately 50% olivine from an ultramafic Bushveld-type magma. Secondly, derivation of a primary noritic magma from depleted mantle material with near chondritic REE composition, by approximately 42% equilibrium partial melting. The mantle material is similar to the depleted peridotite nodules in South African kimberlite. It is illustrated that the Parsons Rust dolerite-norite and the Reebokkop dolerite can be derived from such a noritic magma by means of fractional crystallisation of olivine, pyroxene and plagioclase. The tholeiitic group is compared to other mafic intrusions in the Vredefort dome. It differs from the tholeiitic Annas Rust dolerite and associated rocks, and can, just like the epidiorite, be classified as calc-alkaline. Considering field, petrographical and geochemical evidence it is postulated that the tholeiitic group is of Bushveld age.

Why do kimberlites from different provinces have similar trace element patterns?

Analysis of the trace element contents in kimberlites from various provinces around the world, including South Africa, India, and Yakutia (Siberia, Russia), reveals remarkable similarity of the maximum abundances. In addition, we find that abundances of the rare earth elements (REE) in the South African kimberlites are highly coherent between individual elements. We suggest that the observed similarity of the trace element patterns may result from a common physicochemical process operating in the kimberlite source region, rather than from peculiar source compositions and magmatic histories. The most likely candidates for such a process are (1) partial melting at very low melting degrees and (2) porous melt flow and diffusive exchange with the host rocks. These two processes can produce the same maximum trace element abundances and similar undersaturated patterns. We argue that the porous flow, and the associated chromatographic enrichment, is preferred because it allows high saturations at relatively large melt fractions of ∼1%. Observations of enrichment of the xenolith grain rims due to an exchange with metasomatizing melts of quasi‐kimberlitic composition imply that the melt percolated beyond the source region, in agreement with basic assumptions of the percolation model. We demonstrate that the saturated REE patterns are in a good agreement with the maximum observed REE abundances in kimberlites from different provinces. The theoretical patterns are independent of the melt fraction and only weakly (if at all) depend on the source modal composition. Characteristic diverging fan‐like patterns of trace elements predicted by the percolation model are identified in kimberlites from South Africa. We propose that a high coherency of the REE patterns in the South African kimberlites results from a general dependence of all REE abundances on the calcium content. According to this interpretation, the overall depletion of the source rocks in REE with temperature (and depth) postulated by our model is a natural consequence of a decrease in the calcium content along the lherzolite trend.

Isotopic constraints on the cooling of the continental lithosphere

A new model of continuous diffusion of radiogenic isotopes was applied to mineral 147Sm– 143Nd and 176Lu– 176Hf data on low-temperature garnet-peridotite xenoliths from Cretaceous South African kimberlites. The radiometric ages are younger than the Archean whole-rock Re–Os and U–Pb ages and reflect that both the Sm–Nd and Lu–Hf chronometric systems remained open under the thermal conditions of the lithospheric mantle. The radiogenic character of Hf in garnets, however, indicates that even if essentially no pyroxene remained immune to the effects of metasomatic events, the core of many garnets may preserve memory of the long history of this mineral in the subcontinental lithosphere. The cooling rates deduced from the garnet Sm–Nd ages in the South African lithosphere are fairly low (40–105 °C Gy −1), but compare well with values obtained on similar samples from different regions. These unexpectedly low values imply that the heat flow at the base of the subcontinental lithospheric mantle has changed only very slowly through time. They further support the recent suggestion that, as a result of viscous dissipation by plate bending, convection vigor and heat flow are to some extent decoupled, which argues against a thermal feedback on geodynamics. Modern convection may still be mining fossil heat stored in the lower mantle.

Crystalline inclusions and C isotope ratios in diamonds from the Snap Lake/King Lake kimberlite dyke system: evidence of ultradeep and enriched lithospheric mantle

U-type paragenesis inclusions predominate (94.7%) among the crystalline inclusion suite of 115 diamonds (−4+2 mm) obtained from the recently discovered Snap Lake/King Lake (SKL) kimberlite dyke system, Southern Slave, Canada. The most common inclusions are olivine (90) and enstatite (22). Sulfide, Cr-pyrope, chromite and Cr-diopside inclusion are less abundant (15, 10, 5 and 1, respectively). Results of the inclusion composition study demonstrate the following. (a) The relatively enriched character of the mantle parent rocks of the U-type diamonds. The average Mg# of olivine inclusions is 92.1, and of enstatite inclusions average 93.3. CaO content in Cr-pyrope inclusions is relatively high (3.73–5.75 wt.%). (b) Four of ten U-type Cr-rich pyrope inclusions contain a majoritic component up to 16.8 mol.% which requires pressures of ∼110 kbar. Carbon isotopes compositions for 34 diamonds with U-type inclusions have a δ 13C range from −3.2‰ to −9‰ with a strong peak around −3.5‰. This is much heavier than the ratios of U-type diamonds from Siberia and South Africa (∼4.5‰). Diamonds with olivine inclusions can be divided into two groups based on their δ 13C values as well as the Mg# and Ni/Fe ratio in the olivines. Most show a narrow range of δ 13C values from −3.2‰ to −4.8‰ (average −3.72‰) and have olivine inclusions with Mg# less than 92.3 and relatively high Fe/Ni ratios. A second group is characterized by a much wider variation of C isotope composition ( δ 13C varies from −3.8‰ to −9.0‰, average −5.97‰), and the olivine inclusions having a higher Mg# (up to 93.6) and relatively low Fe/Ni ratios. This difference in the C isotope composition may have several explanations: (a) peculiarities of asthenosphere degassing coupled with an abnormal thickness of lithosphere; (b) the abnormal thickness and enriched character of lithospheric mantle; (c) involvement of subducted C of crustal origin in the processes of the diamond formation. The presence of subcalcic Cr-rich majorite (up to 17 mol.%) pyropes of low-Ca harzburgite paragenesis among the crystalline inclusion suite of SKL diamonds is strong evidence for the existence of diamondiferous depleted peridotite in lithospheric mantle at depth near 300 km beneath Southern Slave area and is postulated to be one of the main reasons for the much heavier C isotope composition of SKL U-type diamonds in comparison with those from Siberian and South African kimberlites.

Crystalline inclusions and C isotope ratios in diamonds from the Snap Lake/King Lake kimberlite dyke system: evidence of ultradeep and enriched lithospheric mantle

U-type paragenesis inclusions predominate (94.7%) among the crystalline inclusion suite of 115 diamonds (−4+2 mm) obtained from the recently discovered Snap Lake/King Lake (SKL) kimberlite dyke system, Southern Slave, Canada. The most common inclusions are olivine (90) and enstatite (22). Sulfide, Cr-pyrope, chromite and Cr-diopside inclusion are less abundant (15, 10, 5 and 1, respectively). Results of the inclusion composition study demonstrate the following. (a) The relatively enriched character of the mantle parent rocks of the U-type diamonds. The average Mg# of olivine inclusions is 92.1, and of enstatite inclusions average 93.3. CaO content in Cr-pyrope inclusions is relatively high (3.73–5.75 wt.%). (b) Four of ten U-type Cr-rich pyrope inclusions contain a majoritic component up to 16.8 mol.% which requires pressures of ∼110 kbar. Carbon isotopes compositions for 34 diamonds with U-type inclusions have a δ13C range from −3.2‰ to −9‰ with a strong peak around −3.5‰. This is much heavier than the ratios of U-type diamonds from Siberia and South Africa (∼4.5‰). Diamonds with olivine inclusions can be divided into two groups based on their δ13C values as well as the Mg# and Ni/Fe ratio in the olivines. Most show a narrow range of δ13C values from −3.2‰ to −4.8‰ (average −3.72‰) and have olivine inclusions with Mg# less than 92.3 and relatively high Fe/Ni ratios. A second group is characterized by a much wider variation of C isotope composition (δ13C varies from −3.8‰ to −9.0‰, average −5.97‰), and the olivine inclusions having a higher Mg# (up to 93.6) and relatively low Fe/Ni ratios. This difference in the C isotope composition may have several explanations: (a) peculiarities of asthenosphere degassing coupled with an abnormal thickness of lithosphere; (b) the abnormal thickness and enriched character of lithospheric mantle; (c) involvement of subducted C of crustal origin in the processes of the diamond formation. The presence of subcalcic Cr-rich majorite (up to 17 mol.%) pyropes of low-Ca harzburgite paragenesis among the crystalline inclusion suite of SKL diamonds is strong evidence for the existence of diamondiferous depleted peridotite in lithospheric mantle at depth near 300 km beneath Southern Slave area and is postulated to be one of the main reasons for the much heavier C isotope composition of SKL U-type diamonds in comparison with those from Siberian and South African kimberlites.

Geochemical Characteristics and Emplacement Ages of the Mengyin Kimberlites, Shandong Province, China

New major- and trace-element compositional data and isotopic analyses of the Mengyin (MY) kimberlites are provided in this paper. Samples were selected to be contamination free and to represent intrinsic features and characteristics of the source. In comparison with the kimberlites of South Africa, MY kimberlites have moderate SiO2 and TiO2; higher Th, U, and Nb; and lower V, Co, Ni, Rb, Sr, and Ba. The characteristics of MY kimberlites are similar to Group IA kimberlites and are distinct from Groups IB and II kimberlites from South Africa. The initial values of 87Sr/86Sr and 143Nd/144Nd are 0.70364 to 0.70421 and 0.51193 to 0.51203, respectively. The ϵNd values of 0.25 to +2.1 are similar to Group I kimberlites, and represent a depleted mantle source that was slightly more enriched than that of Group I South African kimberlites. An Rb-Sr phlogopite age (493 ± 10 Ma) presented in this work represents the emplacement age of the MY kimberlites.

Applications of geophysics for the detection and exploration of kimberlites and lamproites

Geophysical techniques are effective in diamond exploration. They can in most cases accurately detect and map kimberlite and lamproite pipes, whether diatreme or crater fades, and may discriminate separate intrusive phases within an intrusion. The most cost-effective geophysical reconnaissance technique has been airborne magnetics. With evidence that a major contribution to kimberlite and lamproite magnetic anomalies is often remanent magnetisation, local anomalies may be of normal or reversed polarity compared to a non-magnetic background. It is common that Australian and Canadian lamproites and kimberlites have reversed magnetic polarity. Many South African kimberlites are however of Triassic to Cretaceous age, which coincides with a long period of mostly normal polarity, and hence these kimberlites would be of normal polarity even if remanence was predominant. Airborne electromagnetics (AEM) is extensively used in diamond exploration. While its cost is a factor of three greater than magnetics, it is particularly effective in detection of weathered or crater facies pipes. AEM assists selection of targets for ground follow-up in areas where kimberlite magnetic anomalies are small. It is also useful to discriminate targets where other sources of localised magnetic anomalies are common. Complementary surface mapping tools (for example airborne radiometrics, aerial photography, digital topography) have all been of assistance in the exploration process. Detailed ground geophysics can usually map out the surface extent of kimberlite or lamproite pipes, and may identify separate intrusions within one kimberlite or lamproite. Since separate intrusions may have very different diamond grades, this is of significant importance in the location of initial geological sampling. Ground magnetics together with either electromagnetics or resistivity are recommended for detailed mapping and multiple intrusion recognition over a potential kimberlite or lamproite. Seismic refraction and reflection may provide useful and accurate data but at a significant cost that is not usually justifiable in routine exploration. Determination of geometry, such as subsurface pipe shape or depth to top, may be possible in certain circumstances.

Determination of the platinum-group elements in South African kimberlites by nickel sulphide fire-assay and neutron activation analysis

Ten kimberlites from various localities in South Africa have been analysed for all of the platinum-group elements (PGEs) and gold using a nickel sulphide fire-assay preconcentration followed by neutron activation analysis (NAA). Problems encountered during the analysis of these samples prompted a radio tracer study to test the recovery of the precious metals during firing, and then the subsequent dissolution of the assay button. The results of this study suggest solutions to the potential problems of incomplete melting of MgO-rich and CO 2-rich during fire-assay, and minimising losses of Pt, Pd and Au during dissolution. Furthermore, this improved procedure offers lower limits of detection than previous methods which combined fire-assay and NAA. The concentrations of PGEs determined in this study of South African kimberlites are compared with previous partial analyses from the literature, indicating that earlier analyses may have seriously overestimated the concentrations of the some PGEs in kimberlites.

Evolution of the lithosphere and its interaction with the underlying mantle as inferred from noble gas isotopes

Continental lithosphere is generally thicker than oceanic lithosphere and seems to have experienced different evolutionary processes in time and space. Based on studies of noble gas isotopes for ultramafic inclusions and related rocks, the evolution of the lithosphere and the interaction with the underlying mantle have been examined. In the 3He/4He‐40Ar/36Ar diagram, samples from oceanic areas indicate relatively high 3He/4He and 40Ar/36Ar ratios, which are similar to those of MORB. Those from the continental areas indicate similar or lower 3He/4HE ratios and generally lower 40Ar/36Ar ratios. The differing ratios appear to reflect differences in the average age and evolution of oceanic and continental lithosphere. Noble gas isotope signatures of ultramafic nodules in South African kimberlites imply that at least three isotopically different source materials might have contributed to the formation of the African lithosphere. The apparent differences in the variation of 3He/4He ratios with time among the Hawaiian, Icelandic and Reunion islands may be related to the speed of the moving plate and the supply rate of magmas from each hot spot. However, oceanic lithosphere seems to have formed with continuous addition of materials that are isotopically similar to those of MORB and probably came from the asthenosphere. Thus, noble gas isotopes suggest that the formation processes of the continental lithosphere seem to be more complicated than those of the oceanic lithosphere.

Argon isotope and halogen chemistry of phlogopite from South African kimberlites: a combined step-heating, laser probe, electron microprobe and TEM study

Argon isotopic analyses were undertaken on phlogopite from the Swartruggens kimberlite dyke and a Premier kimberlite (1200 Ma) peridotite xenolith. Groundmass phlogopite from Swartruggens yields a plateau age of 145.0± 0.4 Ma, consistent with previous age determinations. Phlogopite phenocrysts from Swartruggens and macrocrysts from the Premier xenolith yield complex age spectra, with anomalously old ages, attributed to incorporation of excess radiogenic argon. Laser probe analyses on single phlogopite grains reveal systematic zonations in excess Ar and CI concentrations across (001) cleavage surfaces. One Premier macrocryst exhibits ages in excess of 2.3 Ga and CI levels of 1600 ppm at its centre. These values decrease systematically to 1.2 Ga and 1300 ppm Cl along grain margins. Similar results were obtained from a single Swartruggens phenocryst, which exhibits a range in values of 340—800 Ma and 300—1300 ppm Cl. A second Swartruggens phenocryst is characterised by smaller variations in age (140–230 Ma) and CI content (390–470 ppm ). Fluorine concentrations, determined by electron microprobe, are relatively constant or increase slightly towards grain edges. The laser probe profiles cannot be reconciled with the step-heating results, probably due to phlogopite degradation during invacuo furnace heating. Transport of Ar and CI in kimberlitic phlogopite appears to be dominated by radial diffusion (cylindrical geometry). The variety of laser probe profiles obtained suggests that Ar and Cl diffusion is governed by factors such as lattice diffusion, diffusion anisotropy, and structural defects, which reduce the effective radii of diffusion and may impart a component of pipe diffusion. It is suggested that the xenolith phlogopite entrapped excess 40Ar and halogens in the mantle lithosphere, in response to elevated Ar and halogen fluid pressures. Swartruggens phenocrysts appear to have crystallised from a volatile-rich kimberlite melt. Subsequent magma devolatilisation prior to emplacement reduced Ar partial pressure and CI content. Possible reasons for enhanced F levels after devolatilisation include increased F solubility in the kimberlite melt, extraction of F from infiltrating hydrothermal fluids and local heterogeneities in fluid composition. The final distributions of Ar, Cl and F in kimberlitic phlogopite are variably dependent on several parameters, including local fluid composition, timing of melt devolatilisation, diffusion/ exchange mechanisms, and mineral composition.

マントルの化学的非均質構造

Isotopic compositions of MORBs and OIBs indicate presence of large scale chemical heterogeneities in the earth's mantle. More than at least four independent source materials must be present in order to account for Rb-Sr, Sm-Nd and Pb-Pb isotopic variations in MORBs and OIBs. Distribution of different source materials in the mantle, however, is highly debatable. Because current geochemical discussions on mantle heterogeneities are based on isotopes and trace elements, direct comparison with the 3-D seismic tomography is unwarranted. In order to make such comparison, a theory must be developed which will connect the mantle heterogeneity in isotopes with the compositional heterogeneity in major elements and mantle mineralogy. Seismic tomography shows that Archean cratons have deep roots up to 200km which are characterized by anomalously high P and S wave velocities yet have densities lower than ambient mantle. Petrologic studies on mantle xenoliths in south African kimberlites show that the sub-continental lithosphere beneath the Archean craton consists of very depeleted garnet peridotites. These depleted peridotites may have been formed as residues after extensive extraction of komatiitic magma in the Archean mantle, and they may have been separated from convective mantle materials for a long geologic time due to their low intrinsic densities.

Speculations on the Archean Mantle: Missing link between komatiite and depleted garnet peridotite

Like in the modern Earth, thermal and tectonic regime of the planet in the Archean (≥2.5 Ga) may have been dominated by those activities on the ocean floor. Because of the continuous operation of plate tectonics, however, the geologic record of the Archean ocean floor has been eliminated a long time ago. Using high‐pressure melting experiment data on a mantle peridotite and some key observations on Archean rocks (DGPs and PKs), a model for mid‐oceanic ridge in the Archean (AMOR) is constructed. High magnesian peridotitic komatiites (PKs, MgO ≥ 33 wt %) which are limited only on the oldest cratons (≥3.3 Ga) are considered to have been produced by partial melting of mantle peridotite at ≥7 GPa and ≥ 1800°C. Less magnesian PKs (MgO ≥ 25 wt %) which are characteristic of the latest Archean (≥2.5 Ga) may have been produced at ≥ 4 GPa and ≥ 1650°C. Potential mantle temperatures (PMTs) of the Earth were estimated from the above two constraints on PKs and that for the genesis of mid‐ocean ridge basalts (MORBs) in the modern Earth; PMT = 1750°C at 3.5 Ga, 1600°C at 2.5 Ga, and 1280°C at present. If estimated cooling rate of the Earth's mantle (1.3°C/107 years) is valid for another 5×108 years or so, PMT will become lower than mantle solidus, and then plate tectonics would cease. Very depleted garnet peridotites (DGPs) found as xenoliths in south African kimberlites show distinct chemical trends compared with spinel and garnet peridotites found in young orogenic terrains and oceanic regions. Re‐Os isotopic analyses on bulk xenoliths and Sm‐Nd model ages of garnets in diamonds in them indicate that DGPs have undergone extensive partial melting and melt extraction in the early Archean. The bulk chemical trends of DGPs can be explained by extraction of PK magma from primitive mantle peridotite. A 40–60 wt % extraction of PK magma ranging from 25 to 35 wt % MgO would suffice to yield entire spectrum of DGPs. A hypothesis for their origin by global melting (magma ocean) of chondritic source mantle is not preferred because the KD(Fe/Mg) values between majorite and magma postulated from this model is 2–3 times greater than experimental determination. On the other hand, KD values between olivine in DGP (residue) and PKs (magma) are consistent with experiments. If DGPs (which are supposed to be major constituents of the lithospheric mantle beneath the Kaapvaal craton in southern Africa) are complementary residue for PK magma, total volume of extracted PKs should be at least the size of the craton. In order to allow such high degree of partial melting in a steady state, existence of MOR in the Archean (AMOR) is postulated. At the AMOR, where PMT was ∼300°C higher than today, extensive partial melting of adiabatically rising mantle material would have initiated at ∼200 km depth. Net production of partial melt at the AMOR axis was equivalent to yield ∼80‐km‐thick oceanic crust (roughly 10 times greater than today). Total latent heat dissipation from the Archean mantle by generation of the ∼200‐km‐thick chemically differentiated oceanic lithosphere may have been comparable or even greater than total heat conduction through oceanic and continental lithospheres. The average Archean geotherm in the area away from the AMOR, therefore, may have been even lower than modern Earth.

Volatile contents of phlogopite micas from South African kimberlite

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H2O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H2O, F and Cl. Substitution of H+ for tetrahedral and possibly octahedral cations may be responsible for the excess H2O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K2O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.