Mineralogy and Petrology最新文献

In the course of a systematic investigation of the ternary system Cs₂O-CaO-SiO₂, a phase with the chemical composition Cs₂Ca₄Si₆O₁₇ was discovered. Synthesis experiments were performed using a starting mixture with an oxide ratio of Cs₂O:CaO:SiO₂ = 2:1:3 contained in a lid-covered platinum crucible. The educts were heated to 1000 °C at 2 °C/min, annealed at maximum temperature for two hours, and then cooled to 700 °C at 0.3 °C/min before final quenching in air to ambient conditions. The crystal structure was determined from a single-crystal diffraction data set collected at 15 °C using direct methods. The previously unknown compound crystallizes in space group P 2₁/n and has the following basic crystallographic data: a = 11.1053(7) Å, b = 7.3138(4) Å, c = 22.2609(15) Å, β = 98.289(6)°, V = 1789.2(2) ų and Z = 4. The final refinement calculations converged to R1 = 0.045 and wR2 = 0.106, for 2967 observed reflections with I > 2σ(I). The silicate anions correspond to unbranched dreier double chains running along [010]. Charge compensation is achieved by the incorporation of cesium and calcium cations distributed over a total of six independent sites within the asymmetric unit. The calcium atoms are surrounded by six or seven oxygen atoms in the form of uncapped and monocapped trigonal prisms, respectively. The CaO₆- and CaO₇-polyhedra share common edges forming 7.5 Å wide sheets parallel (001), the presence of which is reflected in the pronounced platy morphology of the crystals. The cesium cations in turn have more irregular coordination environments involving seven and eight oxygen ligands. The crystal structure of Cs₂Ca₄Si₆O₁₇ shows a closer structural relationship with the MDO₁-polytype of clinotobermorite, which is discussed in detail. The structural investigations were complemented by the determination of the thermal expansion tensor for the temperature interval between - 80 °C and 15 °C.

Diamond exploration in the Slave Craton within the Northwest Territories of Canada has been ongoing since the first kimberlite discovery at Point Lake kimberlite in 1991. Thousands of sediment samples have been collected and hundreds, if not thousands, of geophysical surveys have been completed over the past three decades. The results of which have, at times, perplexed explorers, demonstrating that the search for kimberlites is complex and that no silver bullet exists for their discovery. Recently, a more systematic approach to exploration has shown that despite these challenges, there is still great potential to discover economic kimberlites in the Slave Craton. The Kennady North Project (a wholly-owned subsidiary of Mountain Province Diamonds Inc.) has established that a program that methodically incorporates drill targeting and drill hole results systematically with till sampling, airborne geophysics and a toolbox of ground-based geophysics is critical to kimberlite discovery. Applying this systematic approach has led to the discovery of the Kelvin and Faraday kimberlites, which had previously eluded explorers, at least partially due to the bias of searching for the classic “vertical carrot-style” emplacement model. The Kelvin and Faraday kimberlites instead demonstrate emplacement at inclined angles, which produce different geophysical responses, have much smaller surface expressions, yet still contain significant economic potential. A tailored ground geophysical toolbox comprising electromagnetics [horizontal loop (HLEM)], gravity, resistivity surveys [capacitively-coupled (CCR) and snowmobile-towed] and magnetics helped to guide drill targeting at the Kennady North Project. This drill targeting assisted with the identification and delineation of the Kelvin kimberlite into an 8.5 million tonne indicated resource with 13.62 million carats and the Faraday 2 and 3 kimberlites into an inferred resource of 3.94 million tonnes with 7.35 million carats (Vivian et al. 2019).

Thermobarometry of composite peridotitic mineral inclusions in De Beers Pool diamonds (Kimberley, South Africa) has yielded puzzling results. Most non-touching inclusions record higher temperatures than touching inclusions, but both types record conditions colder than the Kimberley xenolith geotherm. Scenarios previously proposed to explain this discrepancy (lithosphere cooling after diamond formation, cooling of discrete diamond-growth pathways by slab-derived fluids, and diamond formation under various thermal regimes) fail to fully account for the observed thermobarometric outcomes. We propose an alternative scenario based on elastic theory of inclusion–host systems, which reconciles the contrasting pressure–temperature (P–T) estimates. Forward model calculations show that P–T conditions similar to those estimated for the touching inclusions can result from the development of overpressures on the inclusions. Our model requires initial diamond formation under conditions colder than a 35-mW/m² geotherm, followed by mantle uplift (~ 60 km, possibly multi-stage) and reequilibration on the Late Cretaceous xenolith geotherm (~ 40 mW/m²). The initial cold conditions could be promoted by foundering of shallow lithospheric materials. Consequent development of exsolution textures could favor entrapment of composite orthopyroxene–garnet inclusions in these early forming diamonds. The subsequent large uplift may be the result of Archean and possibly, in part, later tectonic events. The diamonds with the ‘warmer’ non-touching inclusions belong to one or more generations of uncertain age, which formed on a relaxed geotherm that was distinctly colder (~ 37 mW/m²) than the xenolith geotherm. Our proposed scenario may offer a generic explanation for sporadic cases of ‘cold’ touching inclusions reported at other localities.

Diamonds entrained by kimberlites and olivine lamproites formed predominantly in peridotite and eclogite substrates within the lithospheric mantle. The main growth forms of monocrystalline diamonds are octahedral and cubic. However, many diamonds also exhibit a range of surface features derived during mantle residence and/or entrainment to surface. In pioneering research, Derek Robinson developed an interpretative catalogue of diamond morphologies and surface features, improving understanding of diamond growth, plastic deformation, oxidative etching and resorption processes, and the impacts of sedimentary transport and diamond recovery practices. He also established the sequence of events reflected in diamond physical characteristics. The etching and resorption surface features developed on diamonds provide important constraints on their exposure to oxidizing fluids (mainly CO₂, H₂O) in the mantle and/or kimberlite melt. There is broad consensus that common resorption features such as tetrahexahedroid (THH) forms result from interaction with H₂O-bearing kimberlite fluids. However, other surface features on trigonal octahedral faces (e.g., deep hexagonal pits, triangular plates) have been attributed to either pre-kimberlite mantle metasomatism or variations in kimberlite melt/fluid conditions. Evidence supporting mantle resorption includes cathodoluminescence (CL) imaging of internal diamond growth layers and rounded diamonds in some mantle xenoliths. As most diamonds in mantle xenoliths are typically sharp-edged with few etch features, the formation of specific surface etch features by pre- (or syn-) kimberlite mantle metasomatism is equivocal. Alternative explanations include limited ingress of kimberlitic fluids into host xenoliths during entrainment, ascent and/or emplacement, sampling of multiple diamond resorption groups from different pulses of kimberlite magma with distinct volatile compositions eruption/degassing histories.

Greenland is dominated by cratonic nuclei that provide conditions for diamond formation. Pre-1.6 Ga rocks are exposed over 43% of ice-free land and many basins in younger areas evidence underlying Archean basement. Studies of mantle xenoliths reveal thick mantle lithosphere up to 220 km. Kimberlites, ultramafic lamprophyres, lamproites and carbonatites are exposed abundantly in almost all regions, and span 1632 Ma of geological time. A total of 3029 discrete diamond-prospective bedrock occurrences are known, mostly occurring as sheets but with diatremes and evidence for volcaniclastic rocks in the south. Known diamondiferous bodies are well represented over 930 km of Greenland’s west coast. Recovered multi-carat diamonds and favourable mineral chemical data demonstrate the potential for diamondiferous bodies with large, good quality diamonds in potentially economic concentrations. Areas where pipes and diatremes are rare evidence extensive glacial erosion but retain the potential for better-preserved bodies to be discovered at high elevation and diamonds to be present offshore. Records of 120 334 good quality mineral chemical analyses, allow a regional diamond prospectivity analysis of Greenland to be conducted. Garnet, ilmenite, spinel, Cr-diopside and orthopyroxene all reveal mineral chemistries consistent with diamond-stable mantle sources. All geographic subdivisions overlap chemistries of indicators from diamond-producing areas of Canada. Quantitative prospectivity modelling incorporating geophysical data shows that further opportunities exist for diamond exploration in Greenland particularly in the North Atlantic and Rae Cratons of western Greenland, the Ketilidian Orogen of southern Greenland, as well as within less-explored areas such as the east and north, and off-shore.

Although cratons owe their longevity to thick, depleted and consequently buoyant sub-continental lithospheric mantle, their extents are commonly indicated by the known distribution of crustal rocks. However, the limits of a mapped cratonic area often do not match the seismically imaged thick sub-continental lithospheric mantle roots. New mantle xenolith petrology and geochemistry, combined with continental scale geophysical data, confirm the interpretation that the São Francisco Craton extends beyond the extent of surface exposures of Archean rocks. Mantle xenoliths from outside the typically recognised São Francisco Craton boundaries from the Limeira-1 and Redondão kimberlite diatremes have refractory olivine [Mg# ≤ 93, with Mg# = 100× molar Mg/(Mg + Fe); Limeira], high-Cr pyrope garnets (Cr₂O₃ ≤ 7 wt% from Redondão), low bulk Al₂O₃ < 1 wt%, and ¹⁸⁷Os/¹⁸⁸Os_i = 0.11202–0.11916 (Limeira) and 0.10964–0.11576 (Redondão) that equate to Mesoproterozoic minimum T_RD ages. Geothermobarometry indicates that the lithospheric thickness from which the Redondão xenolith suite was exhumed, extended to > 150 km at the time of eruption, similar to the lithosphere thickness estimated from garnet pyroxenites from the ‘on-craton’ Braúna field. The occurrence of thick, refractory and ancient lithosphere extending outside the known surface extent of the São Francisco Craton suggests that this cratonic nucleus is significantly larger than recognised at the surface. Based on the evidence reported in this study coupled with existing literature, we propose that the current limits of the São Francisco Craton nucleus should be revised to consider deep lithosphere to the north and southwest edges, despite these areas having younger rocks at the surface.

The Dharma kimberlite, northeast of Great Bear Lake, is one of a few kimberlites erupted through Proterozoic cratonic regions west of the Slave Craton. The Dharma and nearby Dharma Uttar kimberlites consist dominantly of volcaniclastic kimberlite, with hypabyssal sills and dikes. Phlogopite, spinel, and ilmenite compositions plus whole-rock major and trace elements indicate the intrusions and volcanic bodies are archetypal kimberlites. A phlogopite Rb – Sr isochron yielded 225.3 ± 0.4 Ma, interpreted as the emplacement age, making it a rare example of Triassic kimberlite volcanism in Canada. This age helps define a corridor of magmatism that displays a broad younger age progression from northwest to southeast that could be interpreted as an extension of Great Meteor hot-spot, although the trend is scattered in the northwest. Whole-rock Dharma kimberlite samples have ⁸⁷Sr/⁸⁶Sr_initial 0.7039 to 0.7047, εNd_initial + 2.2 to + 2.5, and εHf_initial + 1.4 to + 6.8, with the isotopic characteristics of the least-contaminated rocks lying on the global temporal trend of mildly depleted source compositions defined by kimberlite. The compositions of peridotite micro-xenoliths and garnet xenocrysts indicate the presence of a heterogenous lithospheric mantle beneath Dharma. Garnet harzburgite compositions indicate a thick lithospheric mantle profile: minimum garnet Cr-saturation pressures are between 4.7 and 5.1 GPa. Ni-in-garnet thermometry for lherzolitic and harzburgitic garnets provide evidence for a stratified mantle, with a broad distribution of lherzolite and two harzburgite horizons. Extrapolation of these Ni-in-garnet thermometry temperatures to a regional geotherm indicates that approximately half of the mantle load sampled by the Dharma kimberlite originated within the diamond stability field.

Two diamond varieties are described in this review paper whose geological origin has only recently been illuminated. The first is large irregular Type IIa (nitrogen-poor) diamonds, which includes many famous, high-quality gems such as the Cullinan and the Koh-i-Noor. This variety has been named CLIPPIR (Cullinan-like, large, inclusion poor, pure, irregular, resorbed) diamonds based on their distinguishing overall characteristics. Metallic Fe-Ni-C-S melt inclusions, which are thought to represent the growth medium, are the most common material trapped within them, followed by Ca-silicates and low-Cr majoritic garnet. Heavy iron isotope signatures show the metallic melt evolves from a subducted protolith phase such as magnetite or metal alloys formed by serpentinization. The second diamond variety is Type IIb (nitrogen-poor and boron-bearing) diamonds, which are often blue in color, such as the Hope diamond. They tend to occur in deposits containing abundant CLIPPIR diamonds. Inclusions in Type IIb diamonds range from meta-basaltic to meta-peridotitic assemblages, similar to what has been documented previously in sublithospheric diamonds, including Ca-silicates, ferropericlase, retrogressed bridgmanite, stishovite, CF phase (a Na-rich aluminosilicate), low-Cr majoritic garnet, as well as metal alloys, sulfides, and oxides. Boron isotopes support a model whereby the hydrous phases in cold slab meta-serpentinites break down as the slab warms, releasing boron and hydrous fluids that contribute to diamond growth. CLIPPIR and Type IIb diamonds are both established as sublithospheric (superdeep) and their formation involves the subduction of cold, seawater-altered oceanic lithosphere to the mantle transition zone and uppermost lower mantle. These diamonds can contribute significantly to mine revenue but are difficult to detect and predict because they do not correlate with conventional indicator minerals or with other diamonds, including micro-diamonds. Geochronology suggests that sublithospheric diamonds ascend in buoyant packages of rock and reside at the base of the continents before being sampled by kimberlites. CLIPPIR and Type IIb diamonds at surface might be accompanied by distinct sublithospheric indicator xenocrysts or signatures, though these may evolve and re-equilibrate during upper mantle storage. Exploration for sublithospheric diamond potential should focus on indicator minerals from the base of the lithosphere and the possibility of an accreted layer.

It has been proposed that, rather than the lithosphere, cratonic lamproites may be derived from convecting mantle sources like those of kimberlites, but with extensive subsequent melt modification via melt-rock reaction with metasomatized, phlogopite-rich sub-continental lithospheric mantle (SCLM). Here we explore this model using samples from kimberlite (Camp Alpha) and olivine lamproite (Weasua) localities in Liberia, West Africa. U–Pb dating of perovskite, performed using in-situ Pb isotope compositions of coexisting (low U/Pb) mica, provides broadly coeval Neoproterozoic ages for Camp Alpha (762 ± 9 Ma) and Weasua (779 ± 6 Ma and 754 ± 7 Ma), indicating emplacement during break-up of the supercontinent Rodinia. The mineralogy and mica compositions along with bulk-rock geochemistry of Camp Alpha kimberlites are consistent with derivation from a sub-lithospheric mantle source. The Weasua lamproite contains perovskite with trace element concentrations (e.g., Sr < 3000 µg/g), trace element ratios (e.g., Th/U, Th/Nb, and La/Nb), and ⁸⁷Sr/⁸⁶Sr values (0.7029 to 0.7030) that overlap those of perovskite in the Camp Alpha kimberlites (⁸⁷Sr/⁸⁶Sr = 0.7028 ± 0.0002). These data are also similar to those of perovskite in worldwide Neoproterozoic to Cambrian-aged kimberlites but distinct from typical cratonic lamproites, which exhibit perovskite with high Sr contents (> 4000 µg/g) and Sr isotope signatures typical of the enriched lithospheric mantle (bulk-rock ⁸⁷Sr/⁸⁶Sr_i generally > 0.7050). A possible petrogenetic model for the Weasua lamproites entails derivation from a sub-lithospheric source similar to that of the Camp Alpha kimberlites with mineralogical, and hence major-element, variations between these two proximal localities driven by variable assimilation of heterogeneous SCLM material.

One of the most distinctive features of Kimberley-type pyroclastic kimberlites (KPK) is the occurrence of elliptical serpentinized melt-bearing pyroclasts (also known as pelletal lapilli) with microlitic clinopyroxene rims. We propose a new model for their formation based on petrographic observations in hypabyssal transitional kimberlites (HKt) from Renard 65 and 5034 pipes (Canada). We show that subsolidus metasomatic reactions between the kimberlite and entrained crustal silicate xenoliths create serpentine segregations and pseudoclastic textures around the xenoliths. A preferential serpentinisation of the fine HKt groundmass between coarser olivine grains makes oval “pseudoclasts” resembling pelletal lapilli. Radial outward-oriented microlites of fibrous clinopyroxene form reactive coronas on the xenoliths and on the serpentinized olivine. Petrographic evidence for the metasomatic development of the pseudoclastic texture matches the Perple_X phase equilibria calculations that model the serpentinization in the subsolidus, driven by Si ingress across the xenolith–kimberlite contacts. The serpentine-producing reactions constrained by the observed mineralogy expand the volume by 3–44%. The pseudoclastic texture in HKt forms when silicate xenoliths are abundant, adding enough Si to ensure voluminous serpentine and clinopyroxene production and decomposition of primary kimberlitic calcite. The model of metasomatic development of pseudoclasts may be relevant to KPKs as i) there is a gradual transition from HKt to KPK textures correlated with the silicate xenolith modes, and ii) KPK textures are exclusively associated with pipes in competent crystalline silicate rocks. We propose that Kimberley-type pyroclastic kimberlites include assemblages formed in several intrusive and volcanic emplacement episodes, and some pelletal lapilli in KPK may be recycled from the previous magmatic HKt phases.