Daniel J. Schulze

A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds

Abstract A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.

Extreme crustal oxygen isotope signatures preserved in coesite in diamond

The anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor. Recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons. Early crustal growth is thought to have been enhanced by the addition of slab-derived magmas, leaving an eclogite residuum in the upper mantle beneath the cratons. But the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean-floor basalt and intermediate-stage subduction-zone eclogites, and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean-floor basalts. We present here the oxygen isotope composition of eclogitic mineral inclusions, analysed in situ in diamonds using an ion microprobe/secondary ion mass spectrometer. The oxygen isotope values of coesite (a polymorph of SiO2) inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle, but are typical of subduction-zone meta-basalts, and accordingly provide strong support for the link between altered ocean-floor basalts and mantle eclogite xenoliths.

Coesite eclogites from the Roberts Victor kimberlite, South Africa

Abstract Coesite (or pseudomorphic quartz) is more common than previously recognized in the eclogite xenolith suite from the Roberts Victor Mine, South Africa. All coesite eclogites in this study (18 samples) are classified as group I, and have mineral compositions that span virtually the entire compositional range of group I eclogites described from this locality. Most of the Roberts Victor group I eclogite suite may be in equilibrium with free silica, thus weakening the case for these rocks representing residues of partial melting. Oxygen isotope compositions of garnets from 15 samples are in the range 5.32–6.95‰, similar to other Roberts Victor group I eclogites. These observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures, to variable extents, prior to subduction and metamorphism to eclogite facies, without suffering significant partial melting during or following subduction.

Trace element geochemistry of coesite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal craton

Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ18O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.

Diamondiferous garnet harzburgites from the Finsch kimberlite, Northern Cape, South Africa

Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has mineral compositions typical of low-T coarse textured garned peridotites, whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low-CaO content of garnet. Geothermobarometric calculations show both xenoliths equilibrated at temperatures above 1,100°C and pressures>55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different δ13C compositions between xenolith diamonds (-2.8 to-4.6‰) and diamonds in the kimberlite (generally<-4.3‰) suggest different origins or sources for the diamonds.

Oxygen isotope variations in Cr-poor megacrysts from kimberlite

Abstract As radiogenic isotope compositions of Cr-poor megacrysts from kimberlite demonstrate that the megacrysts and kimberlite hosts are genetically related (megacrysts are likely deep-seated liquidus phases), the oxygen isotope compositions of Cr-poor megacrysts are indicative of the δ18O of the host kimberlites and their mantle source regions. Oxygen isotope ratios of garnet megacrysts from Group I kimberlites (from slightly depleted mantle sources) worldwide (North America, southern Africa, Australia) are restricted (δ18OVSMOW = 5.24‰, SD = 0.15, SE = 0.01, n = 121) and typical of phenocrysts in magmas derived from upper mantle with “normal” oxygen isotope compositions. No significant involvement of subducted oceanic crust is indicated in the genesis of Group I kimberlites. Garnet megacrysts from Group II kimberlites (from “enriched” mantle, and restricted to southern Africa) have anomalously high 18O/16O ratios (δ18OVSMOW = 5.59‰, SD = 0.18, SE = 0.02, n = 55). Similarities in equilibration temperature and major element composition between garnet megacrysts from Group I and II kimberlites rule out roles for these parameters in producing the marked difference in 18O/16O between these two suites. Instead, the high 18O/16O values of the Group II garnets suggests incorporation of anomalously heavy oxygen from subducted ocean floor material in the source region of Group II kimberlites (as much as 30% if eclogite [with δ18O approximately 6.5‰] with an ocean floor protolith is the contaminant, less if the source of anomalous oxygen is pelagic or terrestrial sediment with higher δ18O). Actual samples of potential Group II kimberlite source rocks (e.g., mica/amphibole veined peridotites) or plutonic crystallization products of such kimberlites (e.g., mica-amphibole-rutile-ilmenite-diopside rocks) should have elevated δ18O values, but no such material has been previously described in southern African xenolith suites.

Origin and Significance of Ilmenite Megacrysts and Macrocrysts from Kimberlite

Ilmenite populations (megacrysts and macrocrysts) from 26 kimberlites in North America have been characterized by electron microprobe analysis to assist in the understanding of the origin and significance of ilmenite in kimberlites worldwide. Most belong to the Cr-poor megacryst suite. Geochemical trends in Cr-poor-suite ilmenites are consistent with a mantle fractional crystallization origin, with ilmenite forming only a minor proportion of the crystallizing assemblage. Coprecipitating magnesite is inferred to be an important host for Mg, with its crystallization causing Mg depletion in coexisting ilmenite. Decrepitation of magnesite megacrysts during kimberlite ascent may have enriched kimberlite hosts in Mg, contributing to the Mg increase characteristic of ilmenite rims. Ilmenite rims commonly have lower hematite contents than do cores, suggesting that the oxidation state of the kimberlite, and thus its potential for diamond resorption, can be overestimated if core compositions alone are considered. N...

Carbon Isotope Composition of Graphite in Mantle Eclogites

The carbon isotope composition (δ13C) of primary graphite from 28 mantle derived eclogite xenoliths from kimberlites in southern Africa and USA range from ‐14.31 to ‐2.84%PDB, with a mean value of ‐6.2%. Both Group I and Group II eclogites are represented by anomalously low carbon isotope ratios. There is no simple correlation between isotopic composition and depth of origin, demonstrating carbon isotopic heterogeneity in the shallow upper mantle not previously recognized, but known from the deeper eclogite diamond population. Interpreted in the light of recent geochemical evidence supporting a subduction origin for some mantle eclogites and their diamonds, low δ13C graphite‐bearing eclogites could represent some of the earliest material emplaced beneath the cratons by subduction.

Anticorrelation between low δ13C of eclogitic diamonds and high δ18O of their coesite and garnet inclusions requires a subduction origin

Diamond is essentially impermeable and unreactive under many conditions, and tiny mineral inclusions within natural diamonds can faithfully preserve information on the chemical and physical conditions during diamond growth. The stable isotope ratios of carbon, nitrogen, oxygen, and sulfur in diamonds and their mineral inclusions have been used to constrain models of diamond formation, but interpretations of the data have differed dramatically. The crux of the controversy lies in the interpretation of the carbon isotope ratios of eclogite-suite diamonds, which range well outside those expected for typical mantle materials such as peridotites, basalts, and carbonatites. Proposed explanations for these anomalous carbon isotope ratios include derivation from primordial mantle inhomogeneities, fractionated mantle fl uids, and subducted biogenic carbon. Working with samples from three continents, we have analyzed the carbon isotope compositions of eclogite-suite diamonds and the oxygen isotope composition of their mineral inclusions, primarily by ion microprobe methods. We have discovered a previously unrecognized, remarkably consistent anticorrelation between these two isotopic systems, in that virtually all diamonds with anomalously low carbon isotope ratios have silicate inclusions with anomalously high oxygen isotope ratios. This is a fundamental observation that can only be explained by formation of eclogite-suite diamonds through subduction of seafl oor altered basalt, admixed with marine biogenic carbon, into the fi eld of diamond stability.

Diffusion in diamond. I. Carbon isotope mapping of natural diamond

Abstract Recent advances in ion microprobe instrumentation and techniques have enabled the mapping of C isotope ratios across the whole of a polished plate of a natural diamond from Guaniamo, Venezuela. The resultant map of C isotope variation closely matches the cathodoluminescence image of the growth structure of the diamond and, therefore, indicates an extremely limited scale of diffusion of C atoms since the time of diamond formation. This result is compatible with the limite d mobility of N atoms shown by the IaAB aggregation state of the diamond. Inclusions in the diamond aree clogitic, in common with many Guaniamo diamonds with temperatures of formation of around 1200ºC. At such temperature the IaAB aggregation state indicates a mantle residence time on the order of 1 Ga. Such temperatures of formation and mantle residence times are common to many natural diamonds; thus the extremely limited diffusion of C isotopes shown by the mapping indicates that many diamonds will retain the C isotope compositions of their initial formation.