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The .sup.40Ar/.sup.39Ar age is determined for K-bearing minerals from high-K nepheline syenite (rischorrites), liebenerite and carbonatized nepheline syenite, and pseudoleucite syenite of the Bogdo alkaline massif (Arctic Siberia). The polychronous formation of alkaline complexes of the Tomtor type at the Paleozoic stage is revealed from summarizing and analysis of the .sup.40Ar/.sup.39Ar age data on the thermochronological diagram for minerals of rocks of the Tomtor massif, Udachnaya-Vostochnaya kimberlite pipe, and alkaline rocks of the Bogdo massif. The isotopic data indicate a complex three-stage evolution of rocks of the massif with the most productive Late Devonian-Early Carboniferous rare metal-rare earth element mineralization. The Devonian stage of the formation of the Tomtor and Bogdo massifs is related to the impact of the Vilyui plume on the eastern margin of the Siberian Craton. A similar age range is registered during the formation of rocks of the Kola alkaline province.

Despite over 400 occurrences of kimberlites and related rocks in India, mantle-derived xenoliths are known only from a few occurrences. This paucity of mantle-derived xenoliths in Indian kimberlites has hampered investigations of the subcontinental lithospheric mantle (SCLM). Using a valuable selection of the rare xenolith inventory, we here report Fe3+/ΣFe measurements for garnets using the electron microprobe (EPMA) flank method, targeting six mantle eclogite xenoliths (KL2 pipe) and fourteen peridotitic garnet xenocrysts (P9 and P10 hypabyssal intrusions) from the Wajrakarur kimberlite field (WKF) on the Eastern Dharwar craton (EDC). These data provide some of the first direct constraints on the oxygen fugacity (fO2) of the lithospheric mantle beneath the Indian subcontinent. The measured Fe3+/ΣFe ratios vary between 0.02 and 0.05 (± 0.01) for the eclogite xenoliths and between 0.02 and 0.10 (± 0.01) for the peridotitic garnets. Calculated ΔlogfO2 values for the KL2 eclogites show a wide range from FMQ-3.9 to FMQ-0.9 (± 0.6), straddling the boundary between the diamond and carbonate stability fields. In terms of redox compositions, it appears that the KL2 eclogites are able to host diamond, which is consistent with the diamondiferous nature of this particular WKF locality and the presence of eclogitic garnet inclusions in diamonds from the nearby TK4 kimberlite body. The peridotitic garnet xenocrysts from the P9 and P10 kimberlite bodies, which were entrained between ~ 125 and 170 km depth, reveal ΔlogfO2 values between FMQ-4.5 and FMQ-2.6 (± 0.9). Garnet xenocrysts with ‘normal’ REE patterns exhibit higher Fe3+/ΣFe ratios compared to garnets with ‘sinusoidal’ REE patterns. Importantly, the Fe3+/ΣFe ratios of garnet xenocrysts with ‘normal’ REE patterns (~ 125–160 km depth) correlate with metasomatic Ti–Y–Zr–V enrichment, which suggests metasomatism-driven oxidation of the cratonic mantle at mid-lithospheric depths. Such melt-related mantle metasomatism was probably diamond-destructive within the otherwise diamond-fertile lithospheric keel. The observed wide range of ΔlogfO2 values for the Dharwar cratonic mantle lithosphere allows for stabilization of various metasomatic phases (e.g., amphiboles, micas, carbonates) that may have formed (or concentrated in) distinctly different metasome assemblages within the continental root that underpins Peninsular India. Changing the relative contributions from such highly diverse volatile-rich metasomes may explain the spatiotemporal association of kimberlites and various diamond-bearing potassic magma types such as orangeites, ultramafic lamprophyres and lamproites, a scenario that is influenced by the redox composition of the Dharwar craton root.

Water occurs in Earth’s interior mostly as trace hydroxyl in nominally anhydrous minerals. Clinopyroxene is known to be an important water carrier in the uppermost mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains some 50% of jadeite-rich clinopyroxene, making this potentially a significant H 2 O reservoir in the bulk lithospheric mantle. Mantle metasomatism, in particular by small-volume melts like kimberlite, is known to enrich the lithosphere in highly incompatible components, but its effect on H 2 O contents in cratonic eclogite remains unclear. We report H 2 O concentrations for clinopyroxene and garnet in eclogite and pyroxenite xenoliths from several African kimberlites, obtained by Fourier-Transform Infrared Spectroscopy (FTIR). Except one sample showing evidence for minor within-grain variability of H 2 O concentrations (< 15%), FTIR images demonstrate that H 2 O is homogeneously distributed in optically clear areas of clinopyroxene fragments mounted for this study. The samples were variably metasomatised by a kimberlite-like melt, as evidenced by elevated MgO contents and abundances of highly incompatible elements (e.g., Sr, Ce, Th). Although metasomatised eclogites and pyroxenites on average show higher H 2 O abundances than pristine ones, mantle metasomatism decreases the Al 2 O 3 content in clinopyroxene, which is known to enhance hydrogen incorporation in this mineral. As a consequence, hydrogen incorporation is inhibited, and c(H 2 O) becomes increasingly decoupled from other highly incompatible components, such as LREE. Thus, eclogite – metasomatised or not - does not significantly contribute to the H 2 O inventory in the bulk cratonic mantle.

This paper highlights published and new field and petrographic observations for late-stage (crustal level) deformation associated with the emplacement of kimberlites and other mantle-derived magmas. Thus, radial and tangential joint sets in the competent 183 Ma Karoo basalt wall rocks to the 5 ha. Lemphane kimberlite blow in northern Lesotho have been ascribed to stresses linked to eruption of the kimberlite magma. Further examples of emplacement-related stresses in kimberlites are brittle fractures and close-spaced parallel shears which disrupt olivine macrocrysts. In each of these examples, there is no evidence of post-kimberlite regional tectonism which might explain these features, indicating that they reflect auto-deformation in the kimberlite during or immediately post-emplacement. On a microscopic scale, these inferred late-stage stresses are reflected by fractures and domains of undulose extinction which traverse core and margins of some euhedral and anhedral olivines in kimberlites and olivine melilitites. Undulose extinction and kink bands have also been documented in olivines in cumulates from layered igneous intrusions. Our observations thus indicate that these deformation features can form at shallow levels (crustal pressures), which is supported by experimental evidence. Undulose extinction and kink bands have previously been presented as conclusive evidence for a mantle provenance of the olivines—i.e. that they are xenocrysts. The observation that these deformation textures can form in both mantle and crustal environments implies that they do not provide reliable constraints on the provenance of the olivines. An understanding of the processes responsible for crustal deformation of kimberlites could potentially refine our understanding of kimberlite emplacement processes.

The results of U–Pb Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) geochronological studies of perovskite from kimberlite rocks of the Manchary pipe (Khompu-May kimberlite field, Central Yakutia) are reported. The obtained U–Pb age (472 ± 1 Ma) indicates the Early Paleozoic age of the formation of the Khompu-May field kimberlites and allows us to identify a new Aldan kimberlite subprovince within the Aldan anteclise.

Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far. This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~ 100–145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160–180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed. Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas. Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO2–H2O-rich ultramafic melts.

This study aims to constrain the nature of kimberlite–xenolith reactions and the fluid origin for Kimberley-type pyroclastic kimberlite (KPK). KPKs are characterized by an abundance of basement xenoliths (15–90%) and display distinct pipe morphology, textures, and mineralogy. To explain the KPK mineralogy deviating from the mineralogy of crystallized kimberlite melt, we study reactions between hypabyssal kimberlite transitional to KPK and felsic xenoliths. Here, we characterize the pectolite–diopside–phlogopite–serpentine–olivine common zonal patterns using petrography, bulk composition, thermodynamic modelling, and conserved element ratio analysis. To replicate the observed mineral assemblages, we extended the thermodynamic database to include pectolite, using calculated density functional theory methods. Our modelling reproduces the formation of the observed distinct mineralogy in reacted granitoid and gneiss. The assimilation of xenoliths is a process that starts from high temperatures (1200–600 °C) with the formation of clinopyroxene and wollastonite, continues at 600–200 °C with the growth of clinopyroxene, garnet, and phlogopite finishing at temperatures < 300 °C when pectolite and prehnite join in. Critically, the majority of the new mineral growth occurs in the sub-solidus, at temperatures below 600 °C. The metasomatic origin of the xenolith mineralogy is best explained by gradients in the chemical potentials of Si, Al, Ca, and Mg across the xenolith–kimberlite contacts. The low-temperature mineralogy of the fluid-limited thermodynamic calculations, where H2O and CO2 are controlled by kimberlite concentrations, reproduces the observed mineralogy better than a fluid-saturated model with a meteoric fluid composition. Our findings imply the deuteric origin of the fluids in KPK pipes controlling the kimberlite mineralogy and texture.

The origin of the alkaline magmatism, including kimberlites and carbonatites, is believed to be related to deep-seated mantle plumes. A chondritic Earth’s mantle contains very low amounts of alkaline elements, with Na prevailing over K. Consequently, the source of the alkaline rocks cannot be the ‘chondritic’ mantle and most likely a mantle modified by subducted crustal materials. Alkaline magmas and carbonatites appear first in the Mesoarchean (~ 3 Ga) and possibly coincided with the onset of plate tectonics. Melting and degassing of subducted slabs into the deep mantle caused widespread metasomatism and formation of reservoirs enriched in the alkaline and lithophile trace elements. These served as sources of alkaline and carbonatitic magmas, and from ~ 2 Ga onwards of kimberlite magmas. Theoretical and experimental modeling predict the lower mantle and transition zone to be largely composed of bridgmanite, ferropericlase, Ca-Si-perovskite, ringwoodite, wadsleyite, majorite, NAL (a hexagonal aluminous phase of the lower mantle containing Na, Al and K), breyite and carbonates. The alkaline elements, isomorphic in CaSi-perovskite, bridgmanite and NAL, can be released during the ascent of mantle plume and transferred to the melt/fluid-enriched reservoir of carbonatites and alkaline magmas. At ~ 600 km depth where majorite is stable, an extensive fractionation of K and Na occurs, as the partitioning coefficient of Na is an order of magnitude larger than that of K. This results in K enrichment of the metasomatic melt/fluid that contribute to prospective sources of kimberlitic and other deep-mantle K magmas.

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene aCaCrTscpx, and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:PGPa=11.03+-TKln(aCaCrTscpx)×0.001088+1.526×lnCr#cpxTKwhere Cr#cpx=CrCr+Al, aCaCrTscpx=Cr-0.81·Cr#cpx·Na+K, with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.

Juína is the second-largest diamond-producing municipality in Brazil and is globally known for its outstanding sublithospheric diamond occurrences in both placer and kimberlite-hosted deposits. However, the scarcity of petrological data for Juína kimberlite pipes hampers understanding the nature and mantle source of these primary diamond sources in this region. Here, we present a textural and mineralogical study of ten kimberlite pipes from the Juína area. Based on petrographic features and mineral compositions, we interpret the studied Juína pipes as archetypal kimberlites with pyroclastic emplacement styles filled with resedimented volcaniclastic kimberlite and Kimberley-type pyroclastic kimberlite variants. The composition and texture of the magmatic phases, particularly spinel and phlogopite, suggest crystallisation from kimberlite sensu stricto magmas. The presence of high-Na eclogitic garnets and the absence of high-Cr low-Ca G10 garnets within the mantle xenocryst suite suggest the likelihood of eclogitic diamonds among Juína's lithospheric diamond populations. The Zr and Y contents, Ti/Eu and Zr/Hf ratios in the peridotite garnets, and Zr contents, Ca/Al, LaN/YbN (primitive-mantle normalised), Ti/Eu, and Zr/Hf ratios in the clinopyroxenes suggest a solid connection to kimberlite melt-related mantle metasomatism. Thermobarometry calculations indicate a relatively narrow stability window (825–936 ºC and 32–36 kbar) for lithospheric diamonds in the Juína region. Our findings have important implications for regional diamond exploration programs, shedding light on the primary sources of Juína's diamonds and contributing to understanding the deep geological processes in the underlying lithospheric mantle beneath the Amazonian craton.