Kimberlite AT-56, discovered in February 2001, represents the most recent addition to the Attawapiskat kimberlite cluster, located in the James Bay Lowlands of Ontario, Canada. AT-56 is a small kimberlite body with a surface diameter of approximately 40 m and a steep southeastern plunge. It consists of a medium to coarse-grained matrix supported kimberlite with abundant olivine, clinopyroxene, garnet, ilmenite and mica macrocrysts in a green-black to orange-black matrix. The kimberlite is classified as a hypabyssal facies sparsely macrocrystic calcite kimberlite. Heavy mineral concentrates from two representative samples of AT-56 have been analyzed to characterize the mantle sampled by the kimberlite. Both samples yielded large heavy mineral concentrates comprised of roughly equal proportions of Mg-ilmenite, Cr-diopside, high-Cr garnet and low-Cr garnet. Mg-chromite is also present in quantities an order of magnitude less than the other constituents.
The high-Cr peridotitic garnet macrocrysts are only slightly more abundant than the low-Cr varieties, the population being dominated by G9 (lherzolitic) types with only a few (less than 10%) weakly sub-calcic G10 (probable harzburgitic) garnets present. Ni thermometry results for a representative selection of G9 and G10 garnets indicate that the majority equilibrated at temperatures ranging from 1000 to 1250 °C. A significant proportion of the low-Cr garnet population derived from AT-56 is characterized by relatively low-Ti (0.2 to 0.4 wt.% TiO2) and elevated Na (0.07 to 0.13 wt.% Na2O) contents characteristic of Group 1, diamond inclusion type eclogite garnets. These sodic garnets have elevated Cr2O3 contents (typically 1 to 2 wt.% Cr2O3), suggesting they may be websteritic in origin rather than eclogitic. Comparison of AT-56 garnet compositions with published data available for other Attawapiskat kimberlites suggests websteritic mantle has also been sampled by kimberlite bodies elsewhere in the Attawapiskat cluster and it may be an important diamond reservoir in this area. 相似文献
We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied. 相似文献
A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su-Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na-bearing garnet (up to 0.2 wt% Na2O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T =610–660 °C and P =2.5–3.5 GPa (for X CO=0.001). A reaction between dolomite and diopside to form talc under tremolite-unstable conditions indicates a temperature decrease under ultra-high-pressure conditions ( P >2.4 GPa, X CO<0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post-dating the ultra-high-pressure metamorphism. The temperature decrease under ultra-high-pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The P – T conditions of the amphibolitization stage are estimated at <0.9 GPa and <460 °C, and are similar to conditions recorded by the surrounding orthogneisses. 相似文献
Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix mélange. Peak lawsonite-eclogite facies mineral assemblages (garnet + omphacite + lawsonite + rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field.
Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths. 相似文献