F. R. Boyd

Stabilisation of Archaean lithospheric mantle: A ReOs isotope study of peridotite xenoliths from the Kaapvaal craton

Os isotopic compositions of lithospheric peridotite xenoliths erupted by kimberlites in the Kaapvaal craton are almost exclusively less radiogenic than estimates of Bulk Earth (187Os/188Os as low as 0.106) and require long-term evolution in low Re/Os environments. Using Re depletion model ages which assume complete Re removal during formation, the data indicate that cratonic lithosphere stabilisation occurred at, at least, 3.5 Ga, when the lithosphere was over 200 km thick. This thick lithosphere persisted into the Phanerozoic to be sampled by kimberlites. Younger, Proterozoic and Phanerozoic Re depletion ages are interpreted to be largely the result of open system behaviour involving Re addition by metasomatic processes. Some of the younger ages may represent the addition of new lithospheric material during periods of major igneous activity. A mid-Archaean age for the Kaapvaal cratonic mantle concurs with Archaean ReOs ages found in samples of Siberian and Wyoming cratonic mantle. Both shallow (spinel facies) and deep (diamond facies) Kaapvaal peridotites have similar ages (3.3–3.5 Ga) suggesting that 150 km of mantle lithosphere may have accumulated very rapidly. Os isotope estimates for the timing of separation and stabilisation of Kaapvaal cratonic mantle overlap the main period of cratonic crust building and stabilisation (3.5-2.7 Ga). A similar overlap between crust and mantle stabilisation is evident for the Siberian craton. Archaean lithospheric mantle is compositionally different to that formed post-Archaean. The Kaapvaal peridotites have very low FeO compared to post-Archaean peridotites and show a large spread in Mg/Si. Some samples are anomalously Si-enriched compared with post-Archaean mantle samples. This compositional distinction and the varied NdOs isotope systematics are difficult to explain in terms of accepted models involving ancient melt depletion and subsequent metasomatism. Crystal segregation/cumulate processes have been suggested as a mechanism for forming the compositional range observed in Kaapvaal peridotites. This type of process may have occurred during harzburgite crystallisation from high-degree (> 50%) mantle melts associated with Archaean plume activity. A role for hot mantle plumes in generating the thick lithospheric keels beneath the Kaapvaal and Siberian cratons is supported by the possibility of their rapid formation and their thermal stability with respect to post-Archaean lithosphere. The coincidence of mid-Archaean cratonic mantle differentiation with periods of major crust building and stabilisation on the Kaapvaal and Siberian cratons suggests a link between crust generation and stabilisation and lithospheric mantle formation in the Archaean. Thermal energy from the plume may have been the impetus for major crust building at the time of lithosphere stabilisation, possibly by underplating of basaltic magmas. Direct involvement of mantle plumes in episodes of major mantle and possibly crust differentiation would imply that modern style plate tectonics may not have been the primary mechanism of planetary differentiation in the early Earth. Archaean ages for peridotites originating up to 200 km deep suggest that the mechanical boundary layer beneath continents is at least this thick.

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.

Low-calcium garnet harzburgites from southern Africa: their relations to craton structure and diamond crystallization

Low-Ca garnet harzburgite xenoliths contain garnets that are deficient in Ca relative to those that have equilibrated with diopside in the iherzolite assemblage. Minor proportions of these harzburgites are of wide-spread occurrence in xenolith suites from the Kaapvaal craton and are of particular interest because of their relation to diamond host rocks. The harzburgite xenoliths are predominantly coarse but one specimen from Jagersfontein and another from Premier have deformed textures similar to those of high-temperature peridotites. Analyses for many elements in the harzburgites and associated iherzolites form concordant overlapping trends. On the average, however, the harzburgites are deficient in Si, Ca, Al and Fe but enriched in Mg and Ni relative to the lherzolites. Both the harzburgites and lherzolites are enstatite-rich with mg numbers [100.Mg/(Mg+Fetotal)] greater than 92 and in these respects differ markedly from residues generated by extraction of MORB. Equilibration temperatures and depths calculated for the harzburgites have the ranges 600–1,400°C and 50–200 km. Those of deepest origin overlap the interval between low-and high-temperature lherzolites that commonly is observed in temperature-depth plots for the Kaapvaal craton, suggesting that some harzburgites may be concentrated relative to lherzolites at the base of the lithosphere. The low-Ca harzburgites and lherzolite xenoliths have overlapping depths of origin, gradational bulk chemical characteristics and similar textures, and therefore both are believed to have formed as residues of Archaen melting events. The harzburgites differ from the lherzolites only in that they are more depleted. Garnets and associated minerals in harzburgite xenoliths differ from minerals of the same assemblage that are included in diamonds in that the latter are more Cr-rich, Mg-rich and Ca-poor. Coarse crystals of low-Ca garnet with the compositional characteristics of diamond inclusions commonly occur as disaggregated grains in diamondiferous kimberlites. Their host rocks are presumed to have been harzburgites and dunites. The differences in composition between the disaggregated grains that are similar to diamond inclusions and those comprising xenoliths imply some differences in origin. Possibly the disaggregated harzburgites with diamond-inclusion mineralogy have undergone repeated partial melting and depletion near the base of the lithosphere subsequent to their primary depletion and aggregation in the craton. Equilibration with magnesite may have reduced the Ca contents of their garnets and decomposition of the magnesite during eruption may have caused their disaggregation.

The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study

Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = − 12.3 to − −3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.

Geological Aspects of High-Pressure Research: High-pressure experimentation is providing a new look at problems in geophysics and petrology.

The low-density minerals that make up the bulk of rocks in the earths crust, such as quartz and the feldspars, are transformed by high pressure into much denser phases. In some cases the products of these transitions are new phases that were first discovered in the laboratory; in other cases they are minerals such as kyanite, jadeite, and pyrope, which have long been known as constituents of metamorphic rocks. Determinations of the stability fields of these high-pressure minerals show that either metamorphism of sedimentary rocks takes place at much greater depth than has hitherto been supposed or pressures generated by orogenic forces may have significantly augmented the hydrostatic pressure. The second alternative seems unlikely, but lack of information on the strength of rocks during metamorphism makes the matter uncertain. Geophysical and petrological observations indicate that the dominant rock type in the upper mantle is garnet peridotite. However, there is reason to believe that the mantle is inhomogeneous and that a variety of rocks ranging in bulk composition from eclogite to peridotite are present. Hydrous phases, such as amphiboles, are possible constituents in the upper 100 kilometers. The hypothesis that the Mohorovičić discontinuity is a dynamic equilibrium between basalt and eclogite seems improbable. The transition zone between the upper and lower mantle can be explained as a series of reactions in which silicates with the silicon ion in fourfold coordination are transformed into phases in which silicon is in six-fold coordination. This interpretation is supported by synthesis of stishovite, a polymorph of SiO2 with rutile structure, and by syntheses of germanate pyroxenes with ilmenite structure. Data on the melting of silicates at pressures up to 50 kilobars show that the initial dT/dP slopes of silicate melting curves are much steeper than those of metals but that they show considerable curvature. The increase of melting temperature with pressure should be much more pronounced near the top of the mantle than at greater depth.

Iron-Titanium Oxides and Olivine from 10020 and 10071

A new mineral (approximately Fe0.5Mg0.5Ti2O5) related to the pseudobrookite series has been discovered in section 10071,28. Electron-probe analyses for this mineral, a coexisting ilmenite, and a chromian ulv�spinel-ilmenite assemblage in section 10020,40 indicate crystallization under highly reducing conditions. Analytical and optical absorption studies of the olivine in 10020 show it to contain unusually high Cr (1400 parts per million) probably as Cr2+.