Notes: [Auszug] Monomineralic inclusions of olivine, garnet and pyroxenes are common in diamonds, and separate inclusions of two minerals in a single diamond have been found in many specimens6. Equilibration temperatures for these inclusion sets can be estimated by application of the same methods of ...

Notes: [Auszug] Experimental work on the melting relations of model perido-titic systems charged with CO2 has shown that liquids of carbo-natitic and kimberlitic composition may be produced by a small degree of partial melting of a carbonated peridotite1'2. This melting is likely to be caused by a change in the ...

Notes: Abstract 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.

Notes: Abstract The lower sill at Benfontein, South Africa, shows a high degree of magmatic sedimentation to kimberlite, oxide-carbonate, and carbonate layers. The iron-titanium oxide minerals are similar in the carbonate-rich and silicate-rich layers and are represented by titaniferous Mg-Al chromite, Mg-Al titanomagnetite, magnesian ilmenite, rutile, and perovskite. The spinel crystallization trend was toward enrichment in Mg and Ti and depletion in Cr; this trend is similar to that observed in many kimberlites. The ilmenite has Mg and Cr contents within the range observed in kimberlites and lacks the Mn enrichment observed in ilmenites from carbonatites. Perovskite in silicate-rich and carbonate-rich layers shows similar total REE contents and LREE enrichment and lacks the remarkable Nb enrichment observed in perovskite from carbonatites. These new data on the iron-titanium oxide minerals in the lower Benfontein sill do not support a genetic relationship between kimberlites and carbonatites.

Notes: Abstract Bulk compositions and mineral analyses for forty-one, large, garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite in the central Siberian platform have many similarities to those of well-studied peridotites from the Kaapvaal craton in southern Africa. Coarse Mg-rich lherzolites and harzburgites with equilibration temperatures below 1000 °C are abundant and are believed to form the principal rock type in the Siberian lithosphere. The low-temperature Udachnaya peridotites have an average mg number [Mg/(Mg+Fe)] of 92.6 with a wide dispersion in modal enstatite, ranging to over 40 wt%. High-temperature peridotites are relatively richer in Fe and Ti and are commonly deformed, with porphyroclastic or mosaic-porphyroclastic textures, some of the latter having fluidized enstatite. The Udachnaya peridotites have experienced late-stage metasomatism before, during and after eruption. Garnets and pyroxenes in many of the high-temperature rocks are zoned, probably by reaction with melt prior to eruption. Virtually all the peridotites contain secondary diopside, inhomogeneous on a micron scale, that mantles primary orthopyroxene. It is believed to have crystallized along with lesser amounts of intergranular calcite and monticellite during eruption. Bulk analyses for total Fe in many specimens are higher than whole-rock Fe calculated from the electron probe analyses and the modes. The magnitude of the difference between the two measurements of total Fe correlates with loss-on-ignition, suggesting that Fe has been introduced during serpentinization following eruption. These late metasomatic processes have thus affected some major as well minor and trace element compositions. The similarities in bulk composition of peridotites from Udachnaya and the Kaapvaal are evidence of a common origin. Low-temperature cratonic peridotites differ from oceanic peridotites in having higher mg numbers (〉92) and in having relatively high but wide-ranging modal enstatite (Mg/Si = 1.06–1.49 weight fraction). The Udachnaya low-temperature peridotites have an inverse correlation between FeO (calculated from the probe analyses and modes) and SiO2. This correlation is also present in the Kaapvaal data but is complicated by a greater range in fertility that produces a positive variation of Fe with Si. A negative trend for Fe/Si can be seen within a portion of the Kaapvaal data, that for low-Ca harzburgites, in which the variation in fertility is restricted. The negative trends for Fe/Si can be interpreted as a consequence of either segregation of olivine and orthopyroxene by metamorphic differentiation or partial sorting during cumulate formation.

Notes: Abstract 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.

Notes: Abstract Abundant small xenoliths in the Mzongwana kimberlite dike, Transkei, southern Africa, are predominantly pyroxenites composed of ilmenite, pyrope, orthopyroxene, clinopyroxene, rutile, and phlogopite; two of the xenoliths contain small amounts of Ti-rich amphibole near kaersutite in composition. A majority of the pyroxenites have polygonal granoblastic textures, but many have fasciculate, acicular and skeletal growths. The latter are believed to be the product of rapid crystallization because of similarities to textures of lunar and terrestrial volcanic rocks and quenched experimental charges. Segregations of garnet or ilmenite and pyroxene are common, and these are believed to have originated by crystallization from supersaturated magma. Pyroxenes in the rocks that appear to have crystallized most rapidly are richer in Al and Ti and the garnets are richer in Ti than comparable phases in the granoblastic rocks. The Mzongwana kimberlite is estimated to have a minimum depth of origin of 150 km by application of pyroxene thermobarometry to bronzite discrete nodules. The depth of crystallization of the pyroxenite xenoliths is believed to be near 100 km on the basis of comparison with phase relations determined by experiment. The pyroxenites appear to have crystallized from Ti-rich, olivine-free magma that was probably derived from a kimberlitic parent. A basaltic source (Karoo?), however, is not ruled out. Rapid crystallization of the pyroxenites at depth in the mantle may have occurred by intrusion in thin dikes some days prior to inclusion in erupting kimberlite. Alternatively, the kimberlite may have incorporated a pyroxenitic liquid, either derivative or unrelated, that crystallized through loss of volatiles and heat in contact with the expanding kimberlite vapor phase. The compositions of the minerals in the Mzongwana pyroxenites are similar to those of Fe-rich discrete nodules that occur in many other kimberlites. Perhaps the minerals in the pyroxenites and the discrete nodules have similar origins except that the Mzongwana pyroxenites crystallized more rapidly at shallower depths in the mantle.