Mineralogy and Petrology最新文献

Olivine represents the main constituent (40–50 vol%) of carbonate-rich olivine lamproites (CROL) and their xenocrystic cores offer great potential for characterizing the sub-continental lithospheric mantle (SCLM). We present electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data for olivine cores and rims from 11 on-craton CROL localities from the Kaapvaal craton. Olivine xenocrysts are predominantly Mg-rich [Mg# > 89; Mg# = 100 × Mg/(Mg + Fe)], constituting > 95% of the olivine core population. Cores with Mg# of 93–95 and low Ca (< 400 ppm) are unusually abundant in the CROLs, indicating sampling of highly refractory SCLM. Except for the localities Star and Marsfontein, peridotitic olivines from near the lithosphere-asthenosphere boundary (LAB) are rare, and the SCLM is sampled mainly up to ~ 180 km, consistent with the scarcity of deeply derived Cr-poor olivine megacrysts and sheared peridotites. CROLs that sample harzburgitic olivine from the deep lithosphere (> 160 km) feature high diamond grades. Assuming lamproite derivation from the bottom of the lithosphere or deeper, the 160–220 km depth interval beneath the localities Bellsbank, Klipspringer, and Roberts Victor, which show main olivine sampling modes at < 160 km, is likely dominated by eclogite lithologies which is in line with the record of xenoliths and diamond inclusions. Parental melts of the CROLs likely assimilated SCLM components, a process which influenced melt Mg# and the budget of Mn, Co, Zn, Li, and Ti in olivine based on the observed correlation between average core and rim compositions. Conversely, the concentrations of Al, Na, Ca, and Cu in magmatic olivine rims and, therefore, lamproite melts appear to be related to the thermal conditions (and entrainment depth) of the assimilated SCLM.

Present-day continental lithospheric mantle (CLM) heat production estimates vary considerably and likely overestimate heat generation due to the infiltration of the host magma (i.e., kimberlite), mantle metasomatism or variable heat-producing element (HPE) ratios. We present estimates of heat production in the CLM beneath Jagersfontein, from bulk rock reconstruction of 11 peridotitic xenoliths based on in-situ analyses of primary mineralogy, to avoid kimberlite contamination. Higher concentrations of Th and U are observed in the reconstructed bulk rocks at shallower depths (< 5 GPa) and decrease towards the deepest parts of the CLM (Th: 0.5–26 versus 1–5 ppb; U: 0.4–19 versus 1–3 ppb). Moreover, the reconstructed samples have a broad range of bulk K/U (~ 70-16500) and Th/U ratios (~ 0.2–3.8), outside the expected range of the modern convecting mantle. A crucial factor is garnet, as it can control the U budget, has Th/U < 1 and is present across the CLM in the garnet stability field. The differences of the CLM with the convecting mantle challenge the use of assumedly constant HPE ratios to calculate the heat production. Our estimates of present-day heat generation from reconstructed bulk data yield ~ 0.0002–0.008 µW/m³ at shallow depths, decreasing down to ~ 0.0005 µW/m³ near the lithosphere-asthenosphere boundary, lower than typical heat generation values used in most previous models. The variable heat production in the CLM derives from the metasomatism and re-fertilization near the base caused by rising asthenospheric melts, which react and fractionate as they ascend, potentially carrying most of the HPE in a fluid phase to shallower depths.

Correctly classifying primary diamondiferous volcanic rocks as kimberlites or lamproites is important because correct identification of a melt provides insight into the lithospheric thickness and thermal conditions during melt formation and because the economic potential of each rock type may be different. This study investigates the impact of crustal xenolith assimilation on host rock classification using samples from the Late Devonian Pionerskaya pipe located in the mined Lomonosov diamond deposit, part of the Zolotitsa cluster in NW Russia’s Arkhangelsk Diamond Province. Historically, the pipes in the Zolotitsa cluster have been classified as transitional or Group II kimberlites (now called Carbonate-rich Olivine Lamproites) due to the presence of groundmass clinopyroxene, high modes of phlogopite, and transitional Sr–Nd isotope systematics. We conducted detailed petrographic and mineralogical studies of the Pionerskaya pipe-fill samples using microbeam electron techniques, X-ray diffraction, X-ray fluorescence, and thermodynamic modelling with Perple_X. The pipe-fill at Pionerskaya consists of two units of pyroclastic kimberlite (PK-1 and PK-2), hypabyssal kimberlite (HK), and transitional hypabyssal kimberlite (HKt). Volcanologically, we classify Pionerskaya as a Kimberley-type pyroclastic kimberlite (KPK) due to the presence of HKt and the mineralogy and texture of PK-2, which contains distinct oval magmaclasts with thin melt selvages and microlitic fringes of clinopyroxene. The Pionerskaya pipe contains granitoid, diorite, and metabasite xenoliths that reacted with the kimberlite. An archetypal kimberlitic affinity for the Pionerskaya pipe is supported by: 1) the metasomatic origin of groundmass clinopyroxene and phlogopite, both modelled as subsolidus, low-temperature phases; 2) variation in phlogopite size, morphology, inclusion content, and compositional zoning trends near igneous and metamorphic crustal xenoliths; 3) the presence of monticellite and 4) modelling of Sr–Nd systematics of the Pionerskaya pipe showing contamination from mafic and felsic crustal xenoliths. The high degree of contamination by crustal xenoliths through metasomatic reactions between the xenoliths and the kimberlite resulted in the previous erroneous classification of the Pionerskaya pipe.

Trace element and Sr-Nd-Pb isotope compositions of high-density fluids (HDFs) trapped in diamonds provide key insights into mantle processes and diamond formation. This study focuses on diamonds containing different HDF types from the Voorspoed carbonate-rich olivine lamproite (CROL) in the Kroonstad cluster, South Africa. Their trace elements reveal signatures varying between primitive mantle-normalized incompatible enriched fractionated patterns mostly characterizing saline HDFs, and overall flatter patterns for silicic-carbonatitic compositions. The HDFs Sr-Nd-Pb isotope compositions vary markedly; ⁸⁷Sr/⁸⁶Sr = 0.70647–0.71556, ¹⁴³Nd/¹⁴⁴Nd = 0.5113–0.5122, ²⁰⁶Pb/²⁰⁴Pb = 17.36–18.77, ²⁰⁷Pb/²⁰⁴Pb = 15.41–15.71 and ²⁰⁸Pb/²⁰⁴Pb = 37.47–39.39. A Rb–Sr age of 780 ± 220 Ma recorded by the saline HDFs does not correspond with the timing of their host diamonds formation (~ 160–220 Ma; based on nitrogen aggregation estimates). The age records an earlier metasomatic event associated with formation of the silicic-carbonatitic HDFs and diamond (~ 330–730 Ma; based on nitrogen aggregation estimates), that likely took place during the Pan-African Orogeny. We suggest that Neoproterozoic subduction-related saline fluids infiltrated different lithologies in the Kroonstad lithospheric mantle. Upon interaction with eclogite, melting occurred and diamonds crystallized, forming the older silicic-carbonatitic HDF-bearing diamonds with lower alkalis and La/Nb, Th/Nb, La/Sm ratios. Concurrently saline fluids that penetrated harzburgite had little interaction with the host rock and were stored as metasomes. These metasomes were locally re-melted during subsequent thermal event/s, potentially the Karoo flood basalt volcanism (~ 180 Ma), to form saline HDFs and their host diamonds. Later metasomatism that involved high-Mg carbonatitic HDFs was smaller in scale than the previous diamond-forming events and took place at < 160 Ma (< 30 Myr before the Voorspoed CROL erupted). The similarities in trace element and isotope compositions between Voorspoed HDFs and Kroonstad CROLs, support some degree of shared lithospheric origin or similar metasomatic processes that controlled their compositions.

Rare earth elements are crucial for the global transition to a green and modern economy. Their economic occurrence is rare, and their production is concentrated geographically. Consequently, they are classified as critical minerals in many jurisdictions and require alternative sources to prevent potential disruption in the supply chain. Kimberlites, the main source of diamonds, can contain REE at levels comparable to some primary REE deposits such as the REE clay deposits of South China (Kynicky et al. 2012). In this study, we investigated whether processed kimberlites (tailings) at diamond mine sites could be potential secondary sources of REE. The research focused on REE deportment and their behavior during hydrothermal and supergene alteration of kimberlites, using samples of kimberlite core and tailings from the Snap Lake diamond mine (Northwest Territories, Canada). The study included petrography, mineral chemistry, and whole-rock geochemistry. Tailing samples are enriched in heavy (HREE) relative to the kimberlite core samples. In both core and tailings samples, REE are concentrated in monazite (up to 62 wt% REE₂O₃), apatite (up to 1.8 wt% REE₂O₃), and anatase (up to 3000 ppm REE₂O₃). Subordinate amounts of ancylite-(Ce) and allanite-(La) were found only in the tailings. Perovskite, a common mineral host of REE in kimberlites, was not observed in any of our samples. However, the size and shape of anatase-monazite intergrowths suggest they are possible pseudomorphs of perovskite. We propose a paragenetic sequence involving hydrothermal alteration of REE-rich primary perovskite by deuteric CO₂-rich fluids, subsequently forming anatase-monazite intergrowths and monazite infillings in veins. The transformation of perovskite to monazite is economically significant because REE enrichment in monazite allows easier extraction than oxides.

Diamondiferous kimberlites of the Cretaceous Fort á la Corne (FalC) field erupted through the Sask craton. The Palaeoproterozoic age of its lithospheric mantle root provides an unconventional setting for a major diamond deposit. We report the first diamond formation ages for the Sask craton, using diamonds from the Star kimberlite. Sm-Nd dating of garnet and clinopyroxene inclusions of lherzolitic paragenesis yields an isochron of 1262 ± 37 Ma and an ɛNd_i value of -10.8 ± 1.2. The average initial ⁸⁷Sr/⁸⁶Sr at 1262 Ma is 0.70459 ± 0.00001. A single diamond-forming event is supported by the overall similar inclusion compositions (major and trace elements), host diamond carbon isotopic compositions, N-abundance and low N-aggregation states. A Monte Carlo mixing model to generate the initial Sr-Nd isotope compositions of the FalC diamond inclusion suite supports a scenario in which the diamond substrates acquired their geochemical characteristics through earlier infiltration of lithospheric lherzolite by variable amounts (8 to 10 wt%) of an incompatible element-enriched melt with isotopic characteristics resembling cratonic lamproite. We propose a model in which asthenosphere-derived melts produced during rifting or the Trans Hudson Orogeny formed a metasome in the deep Sask craton lithospheric root. This metasome evolved isotopically for ~ 0.6 to 0.8 Gyr, before being remobilized and refertilizing lherzolitic substrates, resulting also in diamond formation. Diamond formation was associated with minimal thermal disturbance, during mobilization of fluids triggered by either far-field effects from the Mackenzie dyke swarm (~ 1270 Ma) or the Grenville orogeny (1.3–0.9 Ga).

This study investigates the geochemical and isotopic characteristics of Jurassic kimberlites and ultramafic lamprophyres (UMLs) from four clusters within the Adelaide Fold Belt (AFB) and two within the Gawler Craton in South Australia. Petrographic analysis, including the occurrence of magmatic clinopyroxene in the groundmass, supported by a review of available mica and spinel compositional data, indicates that many previously classified kimberlites (Eurelia, Angaston and Terowie) are, in fact, ultramafic lamprophyres. New whole-rock major-, trace element and Sr-Nd-Hf isotopic results, augmented by in-situ perovskite and carbonate Sr isotopes, reveal that this sample suite exhibits extensive geochemical variability. These new data highlight the significant role of crustal contamination in modifying not only bulk-rock major, trace element and Sr isotope systematics, the latter being pristine exclusively in low-SiO₂ samples, but also Nd and Hf isotopic signatures. This is most evident for the Mount Hope (εNd_(i) = -5.1 to -1.5; εHf_(i) = -8.6 to -2.7) and Cleve (εNd_(i) = -3.7 to -0.2; εHf_(i) = -6.6 to +0.3) kimberlites of the Gawler Craton which display geochemically enriched compositions that are rarely seen in kimberlitic rocks. In contrast, the AFB samples exhibit less inter-sample isotopic variability and have compositions that are more typical of kimberlites and UMLs globally (εNd_(i) = +0.3 to +3.9; εHf_(i) = +0.7 to +6.6). There is no clear lithospheric thickness control governing the absence of UMLs on the Gawler Craton and their presence within the AFB. Similarly, there are no systematic differences in Sr–Nd-Hf isotopes between uncontaminated kimberlites and UMLs, arguing against obvious differences in their asthenospheric sources. We tentatively suggest that contribution by more pervasively metasomatised lithospheric mantle beneath the AFB compared to the Gawler craton (based on existing garnet xenocryst data) may facilitate the formation of ultramafic lamprophyres in this region. While subduction along the southern palaeo-margin of Pangea likely did not directly trigger magmatism, it may have facilitated deep mantle upwelling linked to the contemporaneous Karoo-Ferrar Large Igneous Province, with related formation of kimberlites and UMLs in South Australia.