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 C_PDB = ? 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.     

The significance of groundmass ilmenite and megacryst ilmenite in kimberlites

Criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite. For instance, in contrast to groundmass grains, ilmenite xenocrysts usually are larger, have reaction rims of leucoxene and perovskite, exhibit strong magnesium enrichment outward, and sometimes have exsolution lamellae and deformation features. Most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite. On the other hand, kimberlitic groundmass ilmenite is rare, but consistently more magnesian than the cores of macrocrysts. Strong Mg-enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian, fractionating kimberlitic liquid. The hypothesis of extensive reaction of ilmenite with kimberlite melt/ fluid has implications with regard to the following: (1) the degree of differentiation of kimberlite melts; (2) the genesis of mantle megacrysts; (3) the reactivity of kimberlite; and (4) the usefulness of groundmass ilmenite as a petrogenetic indicator.     

Zambales ophiolite,Philippines

The Acoje massif is part of a mafic-ultramafic complex, the Zambales ophiolite, and is a fragment of Mesozoic oceanic crust. This paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the Acoje massif. The Acoje critical zone (ACZ) forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone. Two distinct sulfide associations have been identified: (1) a troilite (±pyrrhotite)-dominated group hosted by the mafic zone and (2) a pentlandite-dominated group hosted by the ultramafic zone. Troilite-dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence. The pentlandite-dominated group appears to have evolved from the primary magmatic sulfides during low-temperature re-equilibration. The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage appears to have proceeded as follows: (1) S-rich hexagonal pyrrhotite+pentlandite+chalcopyrite (or cubanite)+magnetite, (2) S-poor hexagonal pyrrhotite+pentlandite+intermediate solid solution (iss) phase (and/or cubanite)+magnetite, (3) troilite (or mackinawite)+pentlandite+iss+magnetite, (4) troilite (or mackinawite)+pentlandite+iss+native Cu+magnetite, (5) pentlandite+native Cu+magnetite, and (6) pentlandite+native Cu+Fe-Ni alloy+magnetite. This evolutionary trend, in conjunction with the observed textural, chemical, and sulfur-isotopic relations, indicates that the native metal and alloy phases in the ACZ were produced by low-temperature reduction of the primary magmatic sulfides. Correlations between sulfide assemblages and coexisting silicate-hydrosilicate-oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the Acoje massif. Two end-member models that could explain the inferred low-temperature mineralogic evolution of the ACZ sulfides are described: (1) an isothermal reduction model and (2) a non-isothermal equilibration model. Both isothermal and non-isothermal effects apparently were involved in the development of variably reduced sulfide-oxide-metal assemblages from the initial magmatic sulfides.     

Causes of the uniformly high crystallinity of graphite in large epigenetic deposits

In low-pressure environments, precipitation of graphite is hindered at low to moderate temperatures by the high solubility of carbon in C-O-H fluids and by kinetic barriers to nucleation. Those low-temperature fluids that do attain saturation tend to precipitate graphite continuously during flow and cooling, thereby producing widely dispersed films of low-crystallinity graphite. In contrast, at high temperatures, particularly when combined with high pressures, the precipitation of graphite is enhanced by decreased solubility of carbon in C-O-H fluids and by improved nucleation under those conditions. The longevity of fluid systems in high-temperature, high-pressure terranes permits efficient, long-term scavenging of dispersed carbon from the crust. The latter may be redistributed in a much more concentrated form as fluids rise, cool and decompress, and as the carbon is finally precipitated as highly crystalline graphite in fracture systems. The combined effects of the thermochemical controls on carbon solubility and the geological controls on fluid generation, movement and P – T  pathways are the reason that large, epigenetic graphite deposits form dominantly at high temperatures and pressures. Those high-temperature, high-pressure conditions, in turn, account for the uniformly high crystallinity of the fluid-deposited graphite.     

Early Tertiary fluid flow and pressure–temperature conditions in the Shimanto accretionary complex of south-west Japan: constraints from fluid inclusions

The pressure and temperature history of the Tertiary Shimanto belt of south-west Japan has been elucidated by analysing fluids trapped in quartz crystals which grew syn-kinematically along late-stage brittle faults. The samples come from three areas that span the Paleogene exposures on the Muroto Peninsula of Shikoku Island. Applying microthermometric and laser Raman microsampling techniques to coeval water-rich and carbonic fluid inclusions, we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation. The results suggest elevated geothermal gradients during late-stage deformation, conditions that are in disaccord with previous plate reconstructions that have depicted old, thermally mature Pacific crust subducting beneath Eurasia during the early to middle Tertiary. These conditions can most easily be accounted for by including an additional plate boundary in the western Pacific during Paleogene time. Plate reconstructions that include the Kula plate in this region are therefore consistent with our findings. In addition, our results provide clues to the conditions that likely accompany seismogenic deformation at active convergent plate boundaries.