Mineralogy and Petrology最新文献

Published data for garnet and clinopyroxene described as “Cr-rich megacrysts” have been re-examined and compared to a new data set (> 350 samples) of megacrysts from the type locality of the Cr-rich megacryst suite, the Sloan kimberlite in Colorado. Other alleged occurrences of “Cr-rich megacrysts” in the literature include kimberlites from U.S.A., Russia, Democratic Republic of Congo, Canada, Botswana and South Africa. Many of these other studies included samples smaller than the accepted 1 cm minimum size for megacrysts. Furthermore, in most of these studies the mineral compositions do not match, or even approach, those of the Sloan Cr-rich suite. For example, in contrast to the range of Cr₂O₃ values of 6.0 to 13.8 wt% in Sloan Cr-rich garnet megacrysts, few garnets from other locations have > 6 wt% Cr₂O₃. In some studies even garnets with < 1.0 wt% Cr₂O₃ were termed Cr-rich. Garnets in most of these other suites are indistinguishable from garnets in peridotite xenoliths in kimberlite and thus are considered here to be peridotite xenocrysts. Compositions of many clinopyroxenes in the other studies also indicate an origin of these grains as peridotite xenocrysts, but others are probably members of the Granny Smith megacryst suite which, unlike the Sloan Cr-rich megacryst suite, lacks garnet but may contain ilmenite. In formulating models of megacryst formation, one must use well-characterized megacryst suites. In size, composition and/or paragenesis, however, virtually none of the other alleged “Cr-rich megacryst suites” meet the criteria established for the Sloan occurrence.

Diamonds mined from the Victor kimberlite (Ontario, Canada) grew at ~ 720 Ma in lherzolite-dominated substrates that themselves were generated ca. 400 My earlier by metasomatic overprinting of refractory harzburgites-dunites with proven Archean heritage. Since diamonds from Victor represent a restricted mantle sample (5.7 ± 0.2 GPa and 1129 ± 16 °C), we reanalyzed major- and trace-element compositions of 196 Cr-diopside xenocrysts from Victor to investigate the depth range, style and extent of ~ 1100 Ma-old overprinting in the lithospheric section. Our approach uses down-the-geotherm projection and visualization of geochemical variability. We find that a Si-Al-Ca-Na enriched andesitic to dacitic metasomatic agent, likely derived from eclogite, interacted with previously depleted high Cr/Al harzburgites-dunites, driving their bulk compositions towards lherzolites and pyroxenites. Our data provides support for (i) substantive preservation of (garnet-absent) refractory Cr-spinel harzburgites-dunites at T < 740 °C, (ii) profound overprinting of peridotitic substrates, thereby establishing a refertilized garnet ± spinel lherzolite ± pyroxenite assemblage over the lithospheric section from 600 to 1240 °C, (iii) relicts of partially overprinted refractory mantle at T ~ 1010 °C and T 1100–1150 °C, the latter coincident with lherzolite-hosted diamond mineralization, (iv) a discrete carbonatitic geochemical signal uniquely associated with diamond mineralization that resides, in part, in garnet wehrlite with refractory heritage, and (v) maximum metasomatic agent:substrate ratios over the 1000–1200 °C interval, producing high Na-Al, low-Cr clinopyroxene-garnet megacrysts. We infer that + 10 mm high-Cr clinopyroxene-ilmenite megacryst assemblages result from lower metasomatic agent:substrate ratios in the interval 860–1000 °C.

The petrography, mineral chemistry, geochemistry, and geochronology of 5 kimberlite bodies from the Victoria Island kimberlite field, Nunavut/Northwest Territories, Canada, are presented. Samples from dyke and pipe complexes from the King Eider and the Galaxy clusters were analyzed. Their geochemical (major and trace element, plus Sr-Nd-Hf systematics) and mineralogical (dominantly zoned phlogopite-eastonite mica and Trend 1 spinel) signatures are consistent with archetypal kimberlites. The majority of the samples reveal variable but typically limited crustal contamination, mainly of limestone xenoliths that are visible in drill core and hand specimen. Mildly depleted εNd_i (+ 1.4 to + 2.6) and εHf_i (+ 4.7 to + 8.4) isotopic signatures indicate an asthenospheric source and conform well to the Pre-Mesozoic temporal trend for global kimberlite sources. Variation in εSr_i (+ 4.7 to + 19.9) indicates the effect of contamination by local carbonate supracrustal rocks. Rb-Sr mica and U-Pb perovskite geochronology extends the age range for the Victoria Island kimberlites (286 to 241 Ma) into the Middle Triassic, yielding a ~ 45 Myr period of active kimberlite magmatism within the field. The Darnley Bay kimberlite, Parry Peninsula, 600 km southwest of the Victoria Island kimberlite field yields a Permian age 275 Ma, overlapping the ages of the Victoria Island field. We consider a number of potential triggers for both kimberlite fields, concluding that the early phase of the ascending Great Meteor mantle plume, that has been linked to kimberlite magmatism further southeast at Rankin Inlet during the Late Triassic, may have been responsible.

Mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths are unique examples of phlogopite-dominated mantle metasomatism hypothesized to correlate geochemically and petrologically to kimberlite and carbonate-rich olivine lamproite (formerly group II kimberlite) magmatism. Here, we investigate the linkage between MARID metasomatism and alkaline magmatism using multiple analytical techniques: in situ trace element major phase and zircon geochemistry, in situ Hf isotope and U–Pb zircon geochemistry and geochronology, and high-precision isotope dilution U–Pb zircon geochronology. We find that most zircons in the studied MARID suite from Kimberley (South Africa) crystallized over a protracted interval in the Late Cretaceous coeval with regional kimberlite volcanism, with one sample yielding Neoproterozoic and Mesoproterozoic dates that are broadly coeval with Proterozoic tectono-magmatic events. Notably, we observe that older metasomatic compositions and ages in zircon are not obscured by mantle storage or later metasomatic events. Our findings confirm that some MARID-like metasomatism occurred as early as the Proterozoic, evolving to form a viable lithospheric mantle source for carbonate-rich olivine lamproites. Zircon geochemistry and geochronology also document that MARID precursors are infiltrated and modified by later Cretaceous kimberlitic melts, resulting in their diverse geochemical and isotopic compositions.

Metamorphic manganese mineralisation recently studied at the Smolník - Malá Hekerová deposit is situated within the Early Paleozoic metamorphic volcano-sedimentary sequences of the Bystrý potok Formation (Gelnica Group, Gemericum), Spišsko-gemerské rudohorie Mountains, Slovakia. The manganese mineralisation is associated with metacarbonate bodies, within which Mn-rich calcite, spessartine, titanite, stilpnomelane, fluorapatite and pyrite have been identified. Stilpnomelane is enriched in Mn (up to 2.32 apfu) and Mg (up to 1.68 apfu), while being depleted in Ca, K, Ba and Na. It is considered a retrograde phase formed by the partial dissolution of spessartine. The carbonate-silicate bodies hosting the manganese mineralisation consist of rhodochrosite, kutnohorite, calcite, rhodonite group minerals, spessartine, tephroite, pyrosmalite-(Mn), magnetite, pyrophanite, clino-suenoite, clino-ferro-suenoite, actinolite, clinochlore, chamosite, caryopilite, greenalite, quartz, alabandite, pyrite, pyrrhotite, galena, sphalerite and chalcopyrite. This manganese assemblage is the result of multistage metamorphism during the Variscan and Alpine tectono-metamorphic evolution, which led to distinct mineral associations influenced by the release or incorporation of Fe-rich fluids, silicate alterations and recrystallisation. The multi-stage development is most evident in spessartine crystals, which exhibit chemically strongly distinguishable zones. The presence of significantly Fe-rich tephroite (with up to 31 mol% fayalite), pyrosmalite-(Mn) (with up to 2.39 apfu Fe), magnetite, pyrophanite, rhodochrosite and quartz inclusions in spessartine preserves the primary chemical composition of the later developing manganese ore. This suggests it formed concurrently with nearby magnetite lenses, as observed in the other Western Carpathian occurrences. Post-Variscan hydrothermal activity led to a greater accumulation of sulfides at the studied occurrence.

The sub-cratonic lithospheric mantle (SCLM) is known for its chemical buoyancy and mechanical strength, which have facilitated its preservation for billions of years. However, the thinning of lithospheric mantle beneath some cratons (e.g., North China, Sao Francisco) indicates that some cratons are not permanently stable. We have assembled a new compilation of SCLM thickness data based on previously published, robustly estimated palaeo-geotherms that are defined by mantle xenocrysts/xenoliths in kimberlites and lamproites globally. These data support a localised and episodic decrease in lithospheric thickness (e.g., Siberian craton) with thin lithosphere (< 200 km) only underlying some kimberlites/lamproites younger than 200 Ma. Some late-Phanerozoic thinning of the SCLM in areas of North America and southern Africa is supported by comparison of the xenocryst/xenolith-based estimates with present-day constraints from geophysical methods. To investigate the effect of lithospheric thinning on the compositional evolution of the SCLM and its implication for magmatism in cratonic settings, we have examined the compositions of olivine xenocrysts sampled by kimberlites and lamproites globally. The average composition of entrained lithospheric mantle olivine exhibits no correlation with kimberlite and lamproite eruption ages, nor with SCLM thickness, suggesting no simple relationship between lithospheric thinning and lithospheric composition over time. Our results further reveal a broad relationship between locally thinner cratonic lithosphere in the last 200 Ma and the frequency of kimberlite eruptions, which hints at more favourable conditions for the formation of kimberlite melts in the upper asthenosphere beneath thinned lithosphere (150–200 km) since the Mesozoic period.

Chemical analysis of kimberlite indicator minerals (KIMs) is quite robust for diamond exploration, but recent advances in crystallographic techniques offer additional insights. This work explores the structural-chemical trends in a large suite of mantle and crustal garnets using micro X-ray diffraction (µXRD) and electron probe microanalysis. Pronounced trends in garnets from kimberlites and mantle xenoliths are primarily driven by Al³⁺ ↔ Cr³⁺ and Mg²⁺ ↔ Ca²⁺ substitutions. Peridotitic garnets display unit-cell parameter (a) as small as 11.494 Å because Mg and Al have the smallest radii among substituting cations. Diamond associated harzburgitic garnets (G10D) generally have larger unit cells than G10 garnets due to higher Cr content at higher pressures. The largest unit cells among peridotitic garnets are for rare high-Ca, high-Cr wehrlitic green garnets. Unit-cell parameter alone cannot uniquely differentiate between mantle-derived garnets due to co-occurring substitutions. Broadly, a threshold of 11.780 ± 0.005 Å is determined to separate high-Ca, grossular-rich and other calcic garnets from peridotitic pyrope garnets (< 11.780 Å). Ti-rich garnets from mantle and crustal sources have noticeably larger unit cells compared to peridotitic and eclogitic garnets due to the large ionic radii of Ti and Fe. This study also highlights significant variations of strain-related mosaicity in garnet among coarse and sheared peridotites, associated with mineral chemistry, pressure, and mantle metasomatism. We conclude that µXRD is an effective tool for quantifying garnet deformation and it can potentially serve as a complementary tool to identify relevant garnet populations, and to flag these for subsequent chemical analyses.

Grospydites are a rare variety of eclogites composed of grossular–pyroxene – disthene (kyanite) occurring only in a few kimberlite pipes worldwide. The pressures, temperatures and redox conditions of formation for grospydites have been poorly investigated, even though such parameters are needed to better understand their origin and the scarcity of elemental carbon (graphite/diamond), a common feature of these rocks. In this study, we determined the chemical composition and Fe³⁺/∑Fe of coexisting garnet and clinopyroxene from nine C-free kyanite-bearing eclogite xenoliths from Zagadochnaya pipe (Yakutia). Five samples show garnet with Ca# >45 mol% [Ca# = Ca/(Ca + Mg + Fe + Mn)] and are classified as grospydites, while the remaining samples are high-Ca eclogites with Ca# of 23–38 mol%. Fe³⁺/ΣFe of garnet ranges between 0.03 and 0.09 (± 0.02), overlapping with values measured for eclogitic garnets equilibrated at mantle conditions. Fe³⁺/ΣFe in clinopyroxene spans from 0.21 to 0.46 (± 0.03), slightly higher than values measured in eclogites from other localities worldwide. The calculated equilibration pressures (3.7–6.7 GPa for grospydites and 4.3–5.6 GPa for high-Ca eclogites) and temperatures (920–1220 °C for grospydites and 1010–1060 °C for high-Ca eclogites) result in fo₂ values ranging from − 2.0 to − 0.2 log units for grospydites and − 1.9 to −1.7 for high-Ca eclogites relative to the fayalite-magnetite-quartz buffer. The δ¹⁸O values measured in two garnet-clinopyroxene pairs confirm that samples equilibrated at mantle conditions. Our study extends the knowledge of grospydites in terms of Fe^3+/ΣFe and P-T-fo₂ within the global eclogite database. Despite the finding that equilibration P-T-fo₂ values are compatible with the stability of graphite/diamond, the lack of elemental C in the investigated samples can be explained as either (1) these rocks formed from a slab that lost C during subduction, or (2) these rocks underwent metasomatism with C-poor fluids/melts or (3) a carbon-free layer was sampled by the ascending kimberlitic melts.