Subcratonic Lithospheric Mantle Research Articles

Reconstructions of mantle structure beneath the Anabar Shield kimberlites – Similarities and differences

Comparisons of mantle xenocrysts from Lower Triassic kimberlites in the Anabar Shield (Ary-Mastakh, Dyuken, Kuranakh and Orto-Yargyn fields) have shown essential differences from the xenocrysts in the Olenek River Basin (Chomurdakh field). Xenoliths in the Anabar Shield and its northern fields are very rare; they include mainly garnet dunites and harzburgites, and less commonly, pyroxenites and eclogites.PTXFO2 diagram reconstructions for the Boomerang pipe in the Ary-Mastakh field located in the suture zone of the Daldyn and Magan terranes have used monomineral thermobarometry to show that the Opx in rare lherzolitic pyropes formed between 6 and 7.5 GPa. Eclogites represent the mantle heated to the middle pyroxenite layer, and in these terranes the formation of Cr-less pyroxenites are linked to plume melt interactions with the eclogites. In the Dyuken, Kuranakh and Orto-Yargyn fields, the garnet advective trend starts from 7.5 GPa, while the asthenosphere – lithosphere boundary is found at 6 GPa, suggesting that the middle pyroxenite layer was heated and metasomatized. The lower and particularly mid-mantle parts of these fields also contain abundant eclogites. In the Chomur field, lherzolitic and pyroxenitic pyropes form from 7 GPa, while the captured materials mainly represent the upper mantle (4 GPa). All pipes show a similar mantle layering, consisting of seven parts and determined by the clustering of PT estimates for garnets, Cr-spinels, and pyroxenes. In the Boomerang pipe, the Cr-clinopyroxenes and pyropes show REE spectra with varying fan-shaped slopes, (La/Yb)n of 10–100 for pyropes, and HREE for garnets. Spider diagrams reveal peaks for Th, and troughs for U, Nb, Ta, and Pb. Eclogitic garnets and omphacites show minimum values of Eu and Nb, Ta, Zr, and Hf. REEs in ilmenites show a joint increase in LREE and HFSE for Mg to Fe-rich varieties with the degree of differentiation. Most depleted pyropes from Kuranakh have V-U shaped REE patterns, Ba and U peaks. The HFSE minima represents dunites from the arc and back-arc mantle, and the fertilization produces an increase in incompatible elements and sometimes large ion lithophile element (LILE) levels for lherzolitic pyropes. The pyroxenitic garnets display happed REE and the Cpx shows varying LILE and HFSE correlating with the (La/Yb)n.The Chomur pipe contains predominantly harzburgite-lherzolite garnets with minima Ba and Sr, in addition to various incompatible elements. Cpx shows similar variations with mainly depleted HFSE patterns. The marginal parts of the subcratonic lithospheric mantle (SCLM) of the Anabar Shield are extremely enriched in eclogitic deep-seated material. This is especially seen in the lower SCLM parts, demonstrating thermobarometric trends and features similar to the diamond inclusions from the Ebelyakh (Mayat) placers. The mantle column beneath several pipes (Los’, Universitetskaya, Kuranakh) contain Cr amphiboles distributed from the base of the lithosphere to the Moho. A comparison of the reconstructed SCLM beneath the Anabar and Chomurdakh fields reveal more fertile compositions of the mantle rocks and simpler structure for later field. The lower mantle lithospheric abundance and the PTX trends of the Kuranakh eclogites and the Orto-Yargyn kimberlites are similar to the eclogitic diamond inclusions from the Ebelyakh placers. This increases the possibility of finding diamonds deposits in the Anabar Shield kimberlites.

Unique Amphibole-Bearing Mantle Column Beneath the Leningrad Kimberlite Pipe, West Ukukit Field, NE Yakutia

In the subcratonic lithospheric mantle (SCLM) beneath Leningrad pipe (West Ukukit field), Yakutia garnet thermobarometry allows us to identify seven horizons (paleo subduction slab). Microprobe data for Cr-bearing amphiboles >500 grains from mantle xenoliths and concentrates reveal a broad range of compositions changing from Cr- pargasitic hornblendes to pargasites, edinites, kataforites, К-richterites with increasing pressure determined with new amphibole thermobarometer constructed by the first author The low pressure (LP) Cr-hornblendes and pargasites compiles the high-temperature branch (90-60 mw/m2) from 3.5 GPa to Moho traced by basaltic cumulates. In the middle part of SCLM edinites mark 35 to 40 mw/ m2 geotherms. At high pressures kataforites also vary in thermal conditions. Richterites near the lithosphere base trace both low –and high temperature convective branches. The amphiboles reveal divisions into 9 chemical groups. The melts coexistion with amphiboles has concave patters typical for subduction related melts. LP varieties reveal Eu inflection U, Rb, Ba, Sr peaks and deep troughs in HFSE (except Zr), Pb. The pargasites show high U peaks and relativle less HFSE depressions. Encreasing in pressure for edinites, kataforites and richterise are accompanied byrise in U and Zr peak gentling of depressions in Th, Sr, Nb and Hf and less in Nb,Ta, Hf and peak in Zr and also decrease in LREE and REE. Clinopyroxenes and garnets show variable trace element patterns and divisions in groups with the plume and subduction signatures. The contrasting behaviour of Ta and Nb is regulated by the rutile partition coefficients likely for primary eclogites. A subduction and Na and K (siliceous) type of fluids percolated through the mantle with abundant eclogites possibly was accompanied by amphibolization at the different levels through all the mantle column. The plume melts produced hybridism with the mantle metasomatic assembleges which created smoother trace element patterns in reacted minerals, clinopyroxene. The new version of monomineral amphibole thermobarometry is suggested.

Mantle-Derived Cargo vs Liquid Line of Descent: Reconstructing the P–T–fO2–X Path of the Udachnaya–East Kimberlite Melts during Ascent in the Siberian Sub-Cratonic Lithosphere

Abstract Disentangling the physico-chemical evolution of the melts that give rise to kimberlites during their genesis, ascent through the sub-cratonic lithosphere and emplacement in the crust is challenging. This is because the extensive entrainment and assimilation of, and reaction with, mantle-derived material makes kimberlites mixtures of xenocrystic, magmatic components and alteration minerals, rarely preserving evidence of their original melt composition. Here, a detailed textural and compositional study of coherent and volcaniclastic kimberlite units from the Udachnaya–East pipe (Siberian craton) was performed to reconstruct the pressure–temperature–oxygen fugacity-compositional (P–T–fO2–X) path of kimberlite melts during their ascent through the sub-cratonic lithosphere. Routine and high-precision electron microprobe analyses of olivine, phlogopite and Fe–Ti oxides enabled to discriminate the mantle-derived cargo from the magmatic components, and thus make inferences on the liquid line of descent (LLD) of proto-kimberlite to kimberlite melts. Most of the olivine cores in Udachnaya–East kimberlite are xenocrystic and record conditions of last equilibration in the Siberian sub-cratonic lithospheric mantle at T–P ranging from 812–1227°C at 3.1–5.4 GPa, to 871–1170°C at 4.6–7.0 GPa, depending on the chosen model geotherm (35 mW/m2 vs 40 mW/m2). Based on their Ni, Cr, Mn, Al and P vs Mg/Fe systematics, olivine core populations were associated to the sheared, granular garnet-bearing or spinel-bearing cratonic peridotites. The occurrence of olivine Internal Zones (I) having the same composition as Fe-rich cores, as well as rare mantle-derived xenocrystic cores of phlogopite, bear witness of mantle metasomatism preceding kimberlite ascent. The assimilation of mantle material by initially H2O- and P-rich proto-kimberlitic melts is recorded by the cotectic formation of magmatic olivine Internal Zones (II) and phlogopite Internal Zones around resorbed xenocrystic cores. The LLD then evolved by inducing cotectic precipitation of olivine, phlogopite rims and Cr-spinel at P of 1.5–3.0 GPa, T of 1120–1250°C and fO2 from −2.8 to −1.6 ΔFMQ. Ilmenite–magnetite pairs in the groundmass record the later conditions of crystallization that occurred at P of ~1.0 GPa, T from 1133°C down to 1000°C and fO2 of +0.3 to +0.9 ΔFMQ. These results confirm the progressive oxidation of the melt that finally led to the formation of calcite, apatite and Mg-rich olivine and phlogopite rinds. The F-enrichment of phlogopite rinds reflects a late decrease of the H2O/CO2 activity of the melt that evolved toward alkali-carbonated composition. Our results showed that the crystal cargo of kimberlites can be used to track all processes acting in between the melt-rock reactions in the mantle and the emplacement in the crust.

Eclogite Varieties and Their Positions in the Cratonic Mantle Lithosphere beneath Siberian Craton and Archean Cratons Worldwide

The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type “basaltic” eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.

The fate of delaminated cratonic lithosphere

Cratonic lithosphere delamination has been frequently suggested in recent studies. However, the fate of the delaminated Sub-Cratonic Lithospheric Mantle (SCLM) has not been thoroughly investigated. Here, we use 2D numerical models to study the evolution of initially delaminated SCLM whose density is initially larger than that of the ambient mantle. Our simulations reveal that after the dense lithospheric segments sink into the hot mantle, the increasing thermal buoyancy and/or removal of the dense components reverse their trajectory, and most of these segments eventually relaminate to the base of the above lithosphere. The time needed for the relamination process to complete is 100-300 Myr since initial delamination, with the exact value depending on the buoyancy of the SCLM and the mantle viscosity. Both delamination and relamination could generate a rapid hundred-meter to kilometer scale surface uplift. After the relamination, the subsequent cooling of the SCLM causes gradual subsidence by ∼2 km. This model provides a novel explanation for the observed Phanerozoic vertical motion of many cratons. According to our models, the delamination-to-relamination evolution mode could occur repeatedly during the past one billion years, as could reconcile the apparent long-term intactness of cratonic crusts and the temporal variations of cratonic topography. In addition, some delaminated lithosphere segments could relaminate to the base of the oceanic plate, providing a mechanism for the presence of ancient continental compositions within some oceanic basalts.

Clues from Garnet-Spinel-Bearing Peridotite and Pyroxenite Xenoliths about the Formation of the Subcratonic Lithospheric Mantle—a Case Study from the Orapa Kimberlite Cluster

Abstract A series of 18 ultramafic mantle xenoliths from the Orapa and Letlhakane mines containing both garnet and spinel were investigated petrographically and by means of classical and multi-equilibrium geothermobarometry in order to understand their P–T evolution and infer tectonic processes consistent with that evolution, assuming that samples from the shallow subcontinental lithospheric mantle (SCLM), may have remained cool enough to preserve memory of its formation. Samples broadly classify into peridotites, mainly harzburgites, and pyroxenites, mainly (olivine-) websterites with less than 15 vol% olivine. The websterites all show thin garnet coronas around xenomorphic spinel clusters and often coarse garnet and very fine spinel exsolving from pyroxenes. Classical geothermobarometry results vary consistently between the 40 mW/m2 geotherm at high pressures and the 50 mW/m2 geotherm at lower pressures but partly fail the reliability test indicating some degree of disequilibrium, particularly, as may be expected, toward lower P and T. Eleven samples were analyzed in detail for mineral modes, and pseudosections were calculated for the four compositionally most extreme samples. All four samples show systematic divergence of the mode and composition isopleths indicating overall chemical disequilibrium, but partial isopleth convergence in two distinct P–T regions that were interpreted as the main equilibration stage and a stage of late ‘freeze-in’ of a few composition isopleths at lower temperature and higher pressure. Specifically, the P–T changes between stage 1 and stage 2 were from 900°C and 13 kbar to 750°C and 23 kbar for a websterite and from 850°C and 24 kbar to 730°C and 33 kbar for a harzburgite. For an olivine-orthopyroxenite and -clinopyroxenite the change was from 910°C, 17 kbar and 770°C, 17 kbar for stage 1, respectively, to less well-constrained conditions for stage 2 of 650°C, 36 kbar and 630°C, 20 kbar, respectively. This interpretation of a protracted or stepwise freeze-in along a P–T path holds even after considering thermodynamic, analytical and, as far as possible, geological uncertainties. The result is consistent with a ‘subcretion’ or ‘buoyant subduction’ model for the formation of the SCLM, specifically with the down-drag of an already subcreted lithospheric platelet by a subsequently arriving platelet. In the framework of the crust–mantle evolution in the Orapa region this most likely occurred in the Neoarchean, between 2.8 and 2.5 Ga.

Comment on "Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle".

Tschauner et al. (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds.

Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds.Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate‑carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.

Short-lived alkaline magmatism related to the Réunion plume in the Deccan Large Igneous Province: inferences from petrology,40Ar/39Ar geochronology and palaeomagnetism of lamprophyre from the Sarnu-Dandali Alkaline Igneous Complex

AbstractAvailable geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 ± 1.5 Ma and 68.17 ± 1 Ma. However, palaeomagnetic results show a VGP at 32.31° N and 298.52° E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 ± 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 ± 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB-type enriched (metasomatized) mantle source with an involvement of phlogopite.This finding suggests that the pre-Deccan ages of c. 68–69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67–65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.

Multi-stage garnet formation and destruction in Kimberley harzburgitic xenoliths, South Africa

Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major–trace element geochemistry and garnet Sr–Nd–Hf isotope data are presented. Equilibration pressures range from 3.8–6.1 GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115–185 km). High olivine Mg# (~93.4, n = 39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ≥90%, orthopyroxene ≤10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5–3.2 wt%) and SiO2 (43.1–49.1 wt%), however, indicate heterogeneous re-introduction of garnet (≤13%) and orthopyroxene (≤50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7–13% and ~ 9–30%; n = 6) are characterised by mildly sinusoidal garnet REE patterns (Tb-Dy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7–3.3 Ga; εHfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5–7.5% and ~ 25–50%; n = 19) are characterised by highly sinuous REE patterns (Ho-Yb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7–1.6 Ga; εHfe: −16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7 Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lu-Hf isochron age of 2702 ± 64 Ma (εHfi = +44, n = 31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nb-Ta anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Sr-Nd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217 ± 58 Ma, εNdi = +4; n = 34), consistent with recent G10 garnet transformation into G9 garnets (Ca + Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.

Zircon in mantle eclogite xenoliths: a review

AbstractVery few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.

Carajás Mineral Province - Example of metallogeny of a rift above a cratonic lithospheric keel

The Carajás Mineral Province (CMP) is located on the southeastern margin of the Amazonian Craton, in northern Brazil. It is the largest producer of high-grade iron ore and a major world supplier of copper, nickel, manganese and gold. This article presents the CMP as a product of oblique rift evolution (2.76–2.06 Ga) controlled by the Canaã and Cinzento strike slip system. The Carajás rift is hosted in the tonalite-trondhjemite-granodiorite (TTG)-greenstone Rio Maria terrains (3.0–2.85 Ga). It overlies a subcratonic lithospheric mantle (SCLM) keel as indicated by seismic tomography and is filled by the Carajás plutonic-volcanic sedimentary sequences (CPVSS) that host the CMP. The rifting stage (Grão Pará Group) began with intense tholeiitic fissure volcanism with basalt and rhyolite flows, coeval intrusions of alkali granites and mafic-ultramafic bodies, BIFs, shales and restricted tuff beds. The Azul and Águas Claras Formations consist mainly of shales and sandstones deposited during the sag stage. The calc-alkaline signature of the Grão Pará metavolcanic rocks was demonstrated to reflect the composition of the melted TTG basement. The high magnesium and platinum-group elements (PGE) contents of Puma and Onça mafic-ultramafic complexes indicate extensive partial fusion, conditions that are indicative of a plume origin. We are proposing that CMP's IOCG deposits were formed from saline carbonate fluids exsolved from deep-seated metasomatic mantellic magmas (MMM), channelled and concentrated along jogs and step-overs in strike-slip fault systems in the Carajás rift.The CMP comprises three main ore-forming stages: (1) a plume stage, ca. 2.76–2.73 Ga, creating supergiant exhalative sedimentary biogenic Fe deposits (Serra Norte, Serra sul and Serra Leste), syngenetic exhalative Cu–Au–Zn (Pojuca deposit), exhalative Mn (Antonio Vicente), Ni-PGE-reef deposits in a magma mixing zone of layered intrusions (e.g. Luanga deposit), and Siqueirinho IOCG deposit; (2) a metasome stage including crustal extension and mantle flash melting (60–100 km depth), ca. 2.68–2.50 Ga, forming the most important IOCG deposits like Salobo, Alemão and Igarapé Bahia. and (3) a sag stage, ca. 2.68–2.06 Ga, with sedimentary Mn deposited in a redox-controlled stratified oceanic environment. The deposits related to granitic intrusion, ca. 1.88 Ga, with Cu-(Au–Mo–W–Sn) greisen (Breves deposit) and Cu-(Au–Mo–F) veins (Gameleira and Alvo 118 deposits) are associated with the A-type magmatism that affected the entire central Amazonian Craton. These deposits are, in contrast, related to an intracratonic silicic large igneous province (SLIP) and have no relation to the Archean Carajás cratonic lithosphere keel rift evolution. The CMP is directly related to the thinning of the lithosphere above a mantle plume, beneath a sub-continental lithospheric mantle (SCLM) keel.

The Lu–Hf Isotope Composition of Zircon from Syenites of the Saharjok Alkaline Massif, Kola Peninsula

Zircon crystals from the alkaline and nepheline syenites of the Saharjok massif, which were formed during the magmatic (2645 ± 7 Ma), hydrothermal (1832 ± 7 Ma), and metamorphic (1784 ± 22 Ma) crystallization stages, were studied. Zircon from alkaline syenites displays lower eHf and eNd values than zircon from ocean island basalts (OIB); these values indicate their affinity to mantle-derived protoliths, which are similar to CHUR in isotope composition. It is assumed that the Neoarchean Keivy alkaline province formed as a result of the plume–lithosphere interaction, induced by the upwelling of a Neoarchean plume, which facilitated metasomatic alteration and the subsequent partial melting of rocks of the subcratonic lithospheric mantle and the crust. The parental melts of the Saharjok massif could have been formed during the resumption of plume activity 20–30 million years after alkaline granite emplacement in the province by the selective melting of the metasomatized lithospheric mantle rocks.

Sr–Nd–O isotopic evidence of variable sources of mantle metasomatism in the subcratonic lithospheric mantle beneath the Grib kimberlite, northwestern Russia

To provide new insights into the type and extent of mantle metasomatism in the subcratonic lithospheric mantle, we examined the Sr–Nd–O isotopic compositions of orthopyroxene, clinopyroxene, garnet, ilmenite and phlogopite from sheared garnet lherzolite, granular garnet harzburgites and lherzolites and clinopyroxene-phlogopite rocks from the Grib kimberlite in the Arkhangelsk diamond province in northwestern Russia.Clinopyroxene and orthopyroxene from sheared garnet lherzolite initially have a close value of 87Sr/86Sr(t) (~0.7034) and close weak positive εNd. Orthopyroxene and clinopyroxene are in oxygen isotope equilibrium with coexisting olivine. Clinopyroxene from a garnet harzburgite has a low 87Sr/86Sr(t) isotope ratio of 0.70266. Clinopyroxene from granular garnet lherzolites has a relatively narrow variation in 87Sr/86Sr(t) (0.70456–0.70582) and considerably larger variations in εNd (−4.3 − +1.0) isotope ratios. Garnet displays elevated initial 87Sr/86Sr(t) values (0.70540–0.70633). Ilmenite shows a narrow range in 87Sr/86Sr(t) (0.70497–0.70522) coupled with εNd values of +0.4 and +3.5. These isotopic data suggest granular garnet lherzolite of mantle metasomatism took place during the interaction of kimberlite melts with SCLM that contained mica-amphibole-rutile-ilmenite-diopside (MARID)-type metasomes. Clinopyroxenes from clinopyroxene-phlogopite (phlogopite wehrlite) xenoliths display a broader range in 87Sr/86Sr(t) (0.70486–0.70813) that is significantly higher than the kimberlite values and a circa-chondritic εNd (−0.1 − +1.3) with a restricted δ18O range (5.11‰–5.33‰). More radiogenic Sr isotopic composition decoupled from Nd isotopes could have been induced by metasomatic melt/fluid related to a subducted material.The isotopic compositions of mantle minerals preserve Sr–Nd isotopic evidence of pre-kimberlite metasomatic events that were probably due to incomplete reequilibration with ultramafic carbonated melt. Based on mineral pairs Rb–Sr isochrons and a clinopyroxene-based Sm–Nd errochron, these mantle metasomatic events correspond to ~550–600 Ma and ~1200 Ma episodes of magmatic–thermal activity.

Rifting of the Kaapvaal Craton during the early Paleoproterozoic: Evidence from magmatism in the western Transvaal subbasin (South Africa)

The Neoarchean to Paleoproterozoic (ca. 2.65–2.06 Ga) Transvaal Supergroup of the Kaapvaal Craton is dominated by clastic and chemical sedimentary rocks, with few strata-bound igneous units. Two ca. 2.2 Ga igneous units in its upper part (Pretoria Group), the Machadodorp Member of the Silverton Formation and the Mashishing dyke swarm, have so far only been recognized in the eastern part of the Transvaal subbasin. As these units are near-contemporaneous with an early Paleoproterozoic geochemical excursion to higher Th/Nb ratios compared to both older and younger igneous units, they probably mark a change in the geodynamic evolution of the craton. In the present contribution, we provide new data on the igneous units within the Silverton Formation (one lava and 5 sills) from a borehole (BB11) in the south-western part of the Transvaal subbasin. Two of these igneous units are particularly interesting because they show strong geochemical similarities to the Machadodorp Member and Mashishing dyke swarm, which are thus more spatially extensive than previously thought. In combination with existing data, our results further show that the Machadodorp Member corresponds to two separate, possibly not co-magmatic, igneous events, as documented by low- and high-Th basalts. The low-Th lavas show flat mantle-normalized REE and multi-element patterns, along with depleted Nd and Hf isotopic signatures, which are unusual among Precambrian magmatic rocks in the craton. This signature can be explained by partial melting of a depleted spinel peridotite incorporated into the subcratonic lithospheric mantle (SCLM) by a prior subduction event. A failed rifting event can have triggered melting of such a shallow source in a cratonic context. Even though comparable in age, the high-Th ca. 2.2 Ga Mashishing dyke swarm comes from a very different source, likely a part of the SCLM that had experienced subduction-related fluid metasomatism around the Archean – Proterozoic transition. About 150 million years later, the Bushveld Complex possibly exploited the same rifting structures during its intrusion.

Evaluating mechanisms for eclogitic diamond growth: An example from Zimmi Neoproterozoic diamonds (West African craton)

Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ13C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ13C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ13C values) and carbonates (high δ13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ13C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.

Extended Deep Earth Water Model for predicting major element mantle metasomatism

Abstract Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H 2 CO 3 0 . Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe, Arkhangelsk province, Russia: Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite, from mantle-derived xenoliths (garnet peridotite, eclogite and clinopyroxene–phlogopite rocks) and for megacryst, macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite. Based on the analysed xenoliths, phlogopite is characterized by several generations. The first generation (Phl1) occurs as coarse, discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene–phlogopite xenoliths. The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate, serpentine, chlorite and spinel. In garnet peridotite xenoliths, phlogopite occurs as overgrowths surrounding garnet porphyroblasts, within which phlogopite is associated with Cr-spinel and minor carbonate. In eclogite xenoliths, phlogopite occasionally associates with carbonate bearing veinlet networks. Phlogopite, from the kimberlite, occurs as megacrysts, macrocrysts, microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts. Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains, which indicates that they are the disintegrated fragments of previously larger grains.Phl1, within the garnet peridotite and clinopyroxene–phlogopite xenoliths, is characterised by low Ti and Cr contents (TiO2 < 1 wt.%, Cr2O3 < 1 wt.% and Mg# = 100 × Mg/(Mg + Fe) > 92) typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences. They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene–phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas. One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H2O. Rb–Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma. Compared with peridotite xenoliths, eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr2O3 despite a wider range of TiO2 concentrations. The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite. Phl2 has high Ti and Cr concentrations (TiO2 > 2 wt.%, Cr2O3 > 1 wt.% and Mg# = 100 × Mg/(Mg + Fe) < 92) and compositionally overlaps with phlogopite from polymict breccia xenoliths that occur in global kimberlite formations. These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phl1 grains or crystallized directly from stalled batches of kimberlitic magmas. Megacrysts, most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths. Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts. Based on the results of this study, we propose a schematic model of SCLM metasomatism involving phlogopite crystallization, megacryst formation, and genesis of kimberlite magmas as recorded by the Grib pipe.

Lu-Hf изотопный состав циркона из сиенитов Сахарйокского щелочного массива, Кольский полуостров

This study examines zircon from the alkaline and nepheline syenites of the Sakharjok massif magmatic (2.645 ± 7 Ma), hydrothermal (1.832 ± 7 Ma) and metamorphic (1.784 ± 22 Ma) stages. Zircon from the alkaline syenites shows lower ɛHf and ɛNd values compared to that from ocean island basalts (OIB) suggesting their mantle-derived prololiths, are close to the CHUR in terms of isotopic composition. It is assumed that the formation of the Neoarchean alkaline Keivy province resulted from the plume-lithospheric interaction triggered by the uplift of a Neoarchean plume that stipulated the processes of metasomatic alterations and subsequent partial melting of the subcratonic lithospheric mantle and crustal rocks. The initial melts of the Sakharjok massif could probably form during the reactivation of the plume activity that occurred 20—30 million years after the emplacement of alkaline granites of the province by selective melting of the metasomatized lithospheric mantle rocks.

Phase equilibria and trace element modeling of Archean sanukitoid melts

Sanukitoids are Mg-rich, Late Archean granitoids, which are interpreted to have formed by the interaction of crustal and mantle melts possibly in a subduction zone setting. Geochemically and chronologically they seem to represent the transition between the tonalite-trondhjemite-granodiorite series rocks predominant in the Archean and the modern-style, subduction related magmatic suites. Studying their formation conditions can therefore provide important insights into the geodynamic processes shaping the cratonic crust in the Late Archean.We present a combination of phase equilibria and trace element modeling to study the melting conditions and protolith sources, which best reproduce the characteristic major and trace element concentrations of sanukitoid melts (high Mg#, V, Cr and Ni but enriched in LILE). After comparing our model to results from experiments for validation, we explore the formation conditions and melt sources for sanukitoids of two localities: the Amazonian craton and the Superior Province. Mixtures of 70% crustal melt (TTG) and 30% of relatively undepleted peridotite are most favorable to reproduce major and trace element characteristics of granodioritic sanukitoids (62–67wt.% SiO2). The P-T conditions of melting vary significantly with the composition of the crust–mantle mixture but range within 1100–1250°C and 2.0–3.2GPa generating relatively high melt fractions between 50% and 65%. Less differentiated sanukitoid melts (49–53wt.% SiO2) can result from either 50/50 or 70/30 crust–mantle mixtures, depending strongly on the composition of the TTG source. The addition of small proportions of carbonatite (∼5%) can account for the elevated concentrations of Ba, Sr, P, Eu and LREE in most sanukitoids of the Superior Province. In contrast to mere trace element models where the degree of melting and the P-T conditions are poorly constrained, the combination with phase equilibria calculations allows for a better-defined model that is consistent with formation conditions in the subcratonic lithospheric mantle or possibly a mantle wedge.