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A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250[degrees]C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + [sup.VI]Al + [sup.VI][] = [sup.VI]Al + [sup.VI]Mg) and a chemical formula: [K.sub.1.01][([Mg.sub.2.45][Al.sub.0.19][[].sub.0.35]) [SIGMA]=3[([Si.sub.3.52][Al.sub.0.48])[SIGMA]=4[O.sub.10][[(OH,O) .sub.1.66] [Cl.sub.0.34])][SIGMA]=2. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 [cm.sup.-1] in the Raman spectra. Single-crystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1 M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) [Angstrom], [beta] = 100.06(4)[degrees], V= 489.1 (4) [[Angstrom].sup.3]. The structure refinement shows the presence of vacancies on the octahedral sites (15% for M 1 and 6.5% for M2). Chlorine occupies a position about 0.5 [angstrom] from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle [alpha] is 0[degrees]. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the [alpha] angle and expands its stability field at high pressure. High [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Al-celadonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds. Keywords: Phlogopite, Al-celadonite, chlorine, high pressure, solid solution, kimberlites, inclusions in diamonds

The mineral eskolaite ([Cr.sub.2][O.sub.3]) has been discovered in association with natural diamond from the Udachnaya pipe in Yakutia, where it is intergrown with an octahedral diamond, mostly as an inclusion in the diamond, but also exposed at its surface. A detailed study was performed on fragments extracted from the outer surface of the diamond, using single-crystal X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), analytical electron microscopy (AEM), including line-scan and elemental-mapping, EMP, and SIMS. These applied techniques confirmed the nature of the eskolaite with 86.8 wt% [Cr.sub.2][O.sub.3] and notable impurities of Ti[O.sub.2] (3.99 wt%), [Al.sub.2][O.sub.3] (2.00 wt%), [Fe.sub.2][O.sub.3] (5.83 wt%), and MgO (1.11 wt%). Trace elements, including V (4900 ppm), Mn (129 ppm), Zr (56 ppm), and Nb (32 ppm) were also detected. The entire range of REE is just at or below the limits of detection. A small picrochromite inclusion ([X.sub.Mg] 81.2; [Y.sub.Cr] 94.7) was detected in the eskolaite; its chemistry is typical of chromite diamond inclusions. It also contains minute inclusions of perovskite, corundum, and an unidentified Ti-phase. Nano-sized cavities in picrochromite were determined to consist of carbonate and quench products, including Si, Mg, Ca, P, K, and Cl. This may represent relics of the diamond-forming metasomatic fluids. The eskolaite, containing a picrochromite inclusion, was formed at high pressure within the diamond stability field from C-O-H-bearing fluids containing Ca, K, Cl, P, and possibly even peridotitic (U-type) oxides and silicates. Keywords: Diamond association, intergrowth, eskolaite, picrochromite, Udachnaya kimberlite pipe, Yakutia, Russia

Diamond oxidation experiments were undertaken in a piston-cylinder apparatus at 1150 to 1500[degrees]C and 1 GPa to understand the mechanism of diamond oxidation in kimberlite melts and to determine the main rate-controlling parameters for this process. Only surface graphitization, and no diamond resorption, occurs in melts that are fluid undersaturated (synthetic kimberlite, carbonate melt, alkaline basalt, CaO-MgO-Si[O.sub.2]-[H.sub.2]O-C[O.sub.2] melts). In contrast, fluid oversaturated conditions (as evidenced by the presence of bubbles) produce resorption features commonly seen in natural diamonds recovered from kimberlites. The diamond oxidation rate is the same in the melts with a free fluid phase, in a pure [H.sub.2]O or C[O.sub.2] fluid, suggesting that the process of diamond oxidation is its reaction with the fluid and not with the melt. Both C[O.sub.2] and [H.sub.2]O oxidize diamonds at a similar rate, but produce very different surface features. Therefore, the surface features of natural kimberlite-hosted diamonds may provide information on the relative proportion of [H.sub.2]O and C[O.sub.2] in the kimberlitic fluid. The common diamond morphologies imply significant amount of [H.sub.2]O. The absence of diamonds with surface graphitization and the abundance of resorbed diamonds in kimberlites suggest the presence of a free fluid phase in kimberlite magmas for hours or days. We found no correlation between the rate and character of diamond oxidation and the physical properties of diamonds (nitrogen content, color). Keywords: Diamond oxidation, resorption features, experiments, kinetics, fluid phase, kimberlite magmas, nitrogen content

Mantle xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, possess a large range of mineralogical and chemical compositions, from both group A and B eclogites. Major-element contents of the group A eclogites exhibit transitional features between the group B eclogites and peridotite. The Mg# of clinopyroxenes is 0.86-0.94, with 0.60-0.84 for garnets. Differences in concentration of LREEs exist between the Obnazhennaya group A and the well-studied group B eclogites from the Udachnaya kimberlite pipe. In general, garnets in the group A eclogites contain lower LREEs than those from the group B eclogites; however, the trend for clinopyroxene is reversed. High [[delta].sup.18]O (5.46-7.81) values, and the positive Eu anomalies in the garnets and clinopyroxenes (Eu/[Eu.sup.*] 1.2-1.4) demonstrate the involvement of an oceanic crustal component in the formation of the group A eclogites. The group A eclogites formed between 21.0 and 37.6 kbar, and 711 and 923 [degrees]C, in a time interval of 1,071-1,237 Ma. An innovative model is proposed to explain the formation of the group A eclogites and websterites. It involves the reaction of a depleted mantle peridotite with TTG and carbonatite melts closely related to the subduction of oceanic crust.

Mafic rocks representative of the alkaline magmatism of the Alto Paranaiba Province in southwestern Minas Gerais, Brazil were studied by means of petrography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes examined in this research and the paucity of detailed studies of these Brazilian rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fifty-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leucite) and mafurite (with kalsilite) end members have been characterized. Mineral compositions were found to be efficient in distinguishing between kimberlites, mafurites and ugandites in the province, primarily on the basis of phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the magmatism in the region. Kimberlites, mafurites and ugandites have different [sup.187]Os/[sup.188]Os ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, respectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate first-order differences between kamafugites and kimberlites, whilst [sup.206]Pb/[sup.204]Pb ratios for the kimberlites are higher than those for the other rock types. Kimberlite and kamafugite isotopic compositions appear to be related to the mixture of at least two dominant mantle components: one with an isotopic signature similar to that of lithospheric peridotites, i.e., with [sup.187]Os/[sup.188]Os ratios of the order of 0.118, similar to those observed in mantle-derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with higher [sup.187]Os/[sup.188]Os ratios of the order of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to explain the isotopic characteristics of the Alto Paranaiba Alkaline Province alkaline rocks.

Garnets in xenoliths from the Wesselton kimberlite show significant zoning in major and trace elements. The garnets were studied using room temperalture Mossbauer spectroscopy with high spatial resolution, and show an increase in [Fe.sup.3+]/[summation over (Fe)] from core to secondary rim. Temperatures and pressures were determined using the garnet-olivine, garnet-orthopyroxene and Ni in garnet formulations, and indicate conditions close to 1,000 [degrees]C and 37 kbar for most of the garnets. Oxygen fugacities calculated using the garnet--olivine--orthopyroxene oxybarometer show an increase of approximately one log-bar unit from garnet core to secondary rim, relative to the quartz-fayalite-magnetite buffer curve. Combined with reanalysis of literature data from unaltered material from the same locality, there was an increase in relative oxygen fugacity of approximately two log-bar units during the course of metasomatism. Existing data from other South African garnet peridotites were recalculated using the same thermo-barometers and oxybarometers, and indicate relative oxygen fugacities that lie at least two log-bar units below the diamond-carbonate equilibrium in peridotitic systems, which defines the maximum limit of diamond stability in peridotite. Diamond would hence be preserved during the initial stages of metasomatism, but in later stages fluid would react with the diamonds, leading to their resorption and eventual destruction.

New experimental data in CaO-MgO-Si[O.sub.2]-C[O.sub.2] at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 [degrees] C, with liquid composition (by weight) 72% CaC[O.sub.3] (CC), 9% MgC[O.sub.3] (MC), and 18% CaMg[Si.sub.2][O.sub.6] (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 [degrees] C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-C[O.sub.2] systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaC[O.sub.3] with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaC[O.sub.3] (90 [+ or -] 1 wt%) in terms of only carbonate components. For C[O.sub.2]-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about ~70 km. If it percolates through metasomatic wehrlite from 70 km toward the Moho at 35-40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11-18% (70-40 km) on the silicate-carbonate boundary.