Carbonatite Melt Research Articles

Depletion and refertilisation of the lithospheric mantle below the Kapsiki plateau (Northern Cameroon Volcanic Line) deduced from trace element and H2O systematics in mantle xenoliths

Clinopyroxene bearing spinel harzburgites recovered from Cenozoic alkaline basalts from the Kapsiki plateau were investigated with the aim of understanding depletion and enrichment processes in the subcontinental mantle below a major tectonic line. Concentrations of major and trace elements (including H) were determined in-situ in olivine, orthopyroxene, clinopyroxene and spinel by EPMA, LA-ICP-MS and FTIR. The xenoliths are refractory with high olivine (0.91–0.92) and orthopyroxene (0.92–0.93) Mg# coupled with relatively high modal proportions of clinopyroxene (5–8%) and a remarkably wide range in clinopyroxene Mg# (0.91–0.94), interpreted to reflect high degrees of partial melting followed by modal and chemical metasomatic refertilization. A fractional partial melting model of orthopyroxene trace elements in a depleted mantle indicates 20–30% of melting. Clinopyroxenes show two groups based on their CaO contents (18.7–18.9 and 20.6–22.6 wt %) suggesting a secondary origin and modal metasomatism. The low Ca clinopyroxenes were the latest to be formed, and the wide range of clinopyroxene Mg# shows a lack of equilibrium with the coexisting minerals. The high Ca clinopyroxenes record ancient metasomatic events that are also documented in orthopyroxene. Cryptic metasomatism is shown by strong progressive enrichments in LREEs and MREEs of orthopyroxene and clinopyroxene. Th vs La and Sr vs Nd of clinopyroxene are used to identify different generations of metasomatism. Ti/Eu vs (La/Yb)N and Zr/Hf systematics indicate that the metasomatising agent has both a carbonatite and a silicate component. The structural hydroxyl contents of the different mineral phases are very low (<1 ppm for olivine, 40–48 ppm for orthopyroxene and 164–277 ppm for clinopyroxene). The very low water contents compared to typical sub-continental peridotites could be attributed to the depleted nature of the xenoliths or reflect lower water activities of carbonatitic melts relative to alkaline melts. Estimated fO2 values expressed as ΔlogFMQ, (where FMQ corresponds to the fayalite magnetite-quartz oxygen buffer) range from -0.42 to -0.26 and are consistent with minor OH stretching regions of 3520 cm-1 in orthopyroxene, related to a ferric iron defect.The combination of textural investigations, major, and trace element and H2O data suggests that a refractory subcontinental mantle was affected by several episodes of intraplate metasomatism dominated by silicate-carbonatite melts above the North-Eastern end of the Cameroon Volcanic Line.

Slab-derived melts interacting with peridotite: Toward the origin of diamond-forming melts

It was recently shown that partial melting of carbonated metapelites, subducted to a depth of 200 km, yields the formation of two immiscible melts, CO2-bearing phonolitic and K-rich carbonate. These melts resemble silicic and low-Mg carbonatitic melt inclusions in diamonds from kimberlites and placers worldwide. Here we studied the interaction of these melts with natural garnet lherzolite at 6 GPa. We found that the CO2-bearing phonolite melt reacts with peridotite consuming olivine to produce orthopyroxene and garnet, while K2O and CO2 enter carbonate melt. The latter has Ca# 24–29 and appears in equilibrium with garnet lherzolite. The SiO2 content in the carbonate melt varies from 2 to 18 wt% as temperature increases from 1200 to 1500 °C. Our results imply that the slab-derived immiscible silicic and low-Mg carbonatitic melts react with peridotitic mantle producing the high-Mg carbonatitic melt, which makes up the majority of carbonatitic inclusions in diamonds. Thus, the melt entrapped by diamonds may decipher genetic signatures of different mantle lithologies: silicic and low-Mg carbonatitic inclusions correspond to eclogite or recycled pelite, while high-Mg carbonatitic inclusions correspond to peridotite.

Early carbonatite magmatism at Oldoinyo Lengai volcano (Tanzania): carbonatite–silicate melt immiscibility in Lengai I melt inclusions

Carbonatites are unusual C-rich alkaline magmas that have been reported throughout the geological record. Nevertheless, there is only one currently active carbonatite system on Earth: Oldoinyo Lengai stratovolcano in northern Tanzania (God’s mountain in Maasai culture). Present-day Lengai carbonatites are natrocarbonatites, peculiar Na-rich carbonatites that, under atmospheric conditions, alter and leach to compositions similar to the more common Ca-carbonatites within weeks, preventing any long-term geological record of such Na-rich magmas. It follows that the oldest report of natrocarbonatites at Oldoinyo Lengai dates to the 19th century. Here, by using samples from the Lengai I cone (>11 ka), we show that immiscible silicate–carbonatite melts were already present at reservoir conditions at that time. Measurements of three-phase (carbonatite + silicate + gas) melt inclusions from Lengai I highlight that their chemical compositions were similar to those of immiscible melts recently present in the reservoir. Alkaline carbonatites in melt inclusions from both Lengai I and historical explosive eruptions are enriched in Ca relative to those historically effused at the surface and likely record higher equilibrium temperatures (>1100 °C). We also report chemical maps that qualitatively document elemental partitioning between immiscible silicate–carbonatite melts. We show that at the melt inclusions’ entrapment conditions Si, Fe, K, Na, and Cl are compatible with the silicate phase when C, Ca, P, Sr, Ba, and F are compatible with the carbonate phase.

Crystallization of bastnäsite and burbankite from carbonatite melt in the system La(CO3)F-CaCO3-Na2CO3 at 100 MPa

Abstract Bastnäsite [REE(CO3)F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnäsite-like compounds were precipitated from aqueous solutions by many different methods, but previous attempts to model magmatic crystallization of bastnäsite from hydrous calciocarbonatite melts were unsuccessful. Here we present the first experimental evidence that bastnäsite and two other REE carbonates, burbankite, and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO3)F-CaCO3-Na2CO3 at temperatures between 580 and 850 °C and a pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO3, Na2CO3, La2(CO3)3, and LaF3 were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO2 and fluorides act as independent components. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnäsite, burbankite solid solution (Na,Ca)3(Ca,La)3(CO3)5, and lukechangite Na3La2(CO3)4F. Calcite and bastnäsite form a eutectic in the boundary join La(CO3)F-CaCO3 at 780 ± 20 °C and 58 wt% La(CO3)F. Phase equilibria in the boundary join La(CO3)F-Na2CO3 are complicated by peritectic reaction between Ca-free end-member of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L): N a 4 L a 2 ( C O 3 ) 5 ( P e t ) + N a F ( L ) = N a 3 L a 2 ( C O 3 ) 4 F ( L u K ) + N a 2 C O 3 ( N c ) . The right-hand-side assemblage becomes stable below 600 ± 20 °C. In ternary mixtures, bastnäsite (Bst), burbankite (Bur), and calcite (Cc) are involved in another peritectic reaction: 2 L a ( C O 3 ) F ( B s t ) + C a C O 3 ( C c ) + 2 N a 2 C O 3 ( L ) = N a 2 C a L a 2 ( C O 3 ) 5 ( B u r ) + 2 N a F ( L ) . Burbankite in equilibrium with calcite replaces bastnäsite below 730 ± 20 °C. Stable solidus assemblages in the pseudoternary system are: basnäsite-burbankite-fluorite-calcite, basnäsite-burbankite-fluorite-lukechangite, bastnäsite-burbankite-lukechangite, burbankite-lukechangite-nyerereitecalcite, and burbankite-lukechangite-nyerereite-natrite. Addition of 10 wt% Ca3(PO4)2 to one of the ternary mixtures resulted in massive crystallization of La-bearing apatite and monazite and complete disappearance of bastnäsite and burbankite. Our results confirm that REE-bearing phosphates are much more stable than carbonates and fluorocarbonates. Therefore, primary crystallization of the latter from common carbonatite magmas is unlikely. Possible exceptions are carbonatites at Mountain Pass that are characterized by very low P2O5 concentrations (usually at or below 0.5 wt%) and extremely high REE contents in the order of a few weight percent or more. In other carbonatites, bastnäsite and burbankite probably crystallized from highly concentrated alkaline carbonate-chloride brines that were found in melt inclusions and are thought to be responsible for widespread fenitization around carbonatite bodies.

Evidence for carbonatite derived from the earth's crust: The late Paleoproterozoic carbonate-rich magmatic rocks in the southeast Tarim Craton, northwest China

Carbonatites are generally accepted as derived from the mantle, whereas viewpoint of carbonatitic melt formed at crust level is considered marginal. Here we document large-scale (∼17 km2) igneous carbonate-rich rocks in the southeast Tarim Craton that were formed within the crust. These rocks exhibit clear intrusive contact with the wall-rocks and contain diverse xenolith, indicating an igneous origin. Zircon U-Pb dating reveals that they were emplaced at ca. 1.94–1.92 and 1.87–1.86 Ga, respectively. δ18O values in zircons (5.7–13.7‰) are higher than those crystallized in equilibrium with mantle melt. Total REE content is 1–2 magnitude lower than that of mantle carbonatite and shows weak fractionation of HREE. REE modeling reveals that the samples cannot be produced by partial melting of carbonated MORB at mantle conditions. The studied samples have positive δ13CV-PDB values (4.2–15.7‰), which are distinct from the mantle carbonatite but comparable to sedimentary carbonates. C-O-Sr-Nd isotope modelling indicates that the compositions of the studied samples cannot be produced by evolution of mantle carbonatite. Integrating these lines of evidence, we conclude that the studied carbonate-rich magmatic rocks were derived from partial melting of impure marble at crustal level via fluid-present melting. These carbonatites probably represent the initial magmatic record of tectonic extension of the late Paleoproterozoic collisional orogenic belt in the southern margin of the Tarim craton. The positive carbon excursion recorded by the high δ13CV-PDB values probably corresponds to the global Paleoproterozoic Lomagundi-Jatuli event. Our study implies that partial melting of sedimentary carbonates is more common than previously thought, which has significant impacts on crust rheology and global carbon cycling

Subducted sediment contributions to REE deposits recorded by alkaline mafic dikes in the Lizhuang REE deposit, Panxi area, southwest China

Carbonatite-associated rare earth element (REE) deposits are the most important source of REEs worldwide. Recycled subducted sediments are critical for REE mineralization, but it remains unclear how the subducted sediments contribute to the REE mineralization. This study presents new whole-rock and apatite geochemical data for alkaline mafic dikes from the Lizhuang REE deposit, southwest China. These alkaline mafic dikes were emplaced at ca. 28.4 Ma (monazite U-Pb age), broadly coeval with the carbonatite–syenite complex and REE mineralization in the Lizhuang REE deposit. The alkaline mafic dikes have enriched Sr-Nd-Pb isotopic compositions, and variable Ba/Th and La/Sm ratios, demonstrate they were derived from an enriched mantle source, which had been modified by both slab-derived fluids and sediment melts. The apatite grains in the alkaline mafic dikes have distinctive geochemical features in their irregular cores (apatite A) and rim overgrowths (apatite B). Apatite A has low REE contents (18.6–161 ppm) and apatite B has high REE contents (1869–4288 ppm). The different REE contents in these apatites could have been caused by the addition of REE-rich carbonatitic melts at lower crustal depths. We propose that the REE-rich carbonatitic melts could be formed by melting of a sediment-rich mantle source, and that collision between India–Asia triggered the melting.

SHRIMP U–Pb ages, mineralogy, and geochemistry of carbonatite–alkaline complexes of the Sillai Patti and Koga areas, NW Pakistan: Implications for petrogenesis and REE mineralization

Carbonatites and related alkaline igneous rocks are important hosts of rare earth elements (REEs). Carbonatite–alkaline complexes in NW Pakistan represent rare examples of carbonatite emplaced in a collisional orogenic belt. However, the precise age, magmatic source, and petrogenesis of carbonatites emplaced in collisional settings and associated REE mineralization remain uncertain. The present study focuses on the origin and REE mineralization potential of alkaline–carbonatite complexes exposed in the Koga and Sillai Patti (SP) areas, NW Pakistan.The Koga carbonatites occur as minor plug-like intrusions in syenites. SHRIMP U–Pb dating of zircons from the Koga syenites, which are spatially associated with carbonatites, yield weighted mean ages of 283.6 ± 1.7(1σ), 281.9 ± 1.6(1σ), 282.6 ± 1.6(1σ), and 306.5 ± 3.7 Ma (1σ). The Koga syenites are characterized by high SiO2 (62.79–65.58 wt%) and Al2O3 (16.92–18.55 wt%) and low TiO2 (0.10–0.59 wt%) contents and are enriched in alkalis (Na2O > K2O). Sr and Nd isotopes from the Koga carbonatite complex are characterized by positive εNd(t) values (+3.10 to + 3.47) and low (87Sr/86Sr)i values (0.70233–0.70382), which are consistent with rift-related carbonatites.The SP carbonatites yield high total REE contents (1768–1920 ppm) and are enriched in light REEs (LREEs) relative to heavy REEs (HREEs; LREE/HREE = 20.0–20.7). The results of Sr–Nd isotope analyses show that the SP carbonatite has high Sr and low Nd isotopic ratios with negative εNd(t) values ranging from − 1.71 to − 2.21, variable 207Pb/206Pb ratios, and anomalous Pb–Sr and Pb–Nd patterns, suggesting Pb contamination during ascent of the primary carbonatite melts. Values of δ18OSMOW and δ13CPDB (‰) for the SP carbonates vary from + 14.45‰ to + 18.12‰ and from − 6.57‰ to − 9.70‰, respectively, which can be attributed to low-temperature alteration.On the basis of data for bulk-rock trace elements and Sr–Nd–Pb–C–O isotopes, it is concluded that the Koga carbonatite–alkaline complex was formed during the rifting and break-up of the northern margin of Gondwanaland. In contrast, carbonatite of the SP carbonatite–alkaline complex was probably generated by partial melting of sub-continental lithospheric mantle in a post-collisional tectonic setting. Although the SP carbonatite contains abundant REEs, it does not host significant REE mineralization owing to the lack of a mature sequence of evolutionary phases and to possible phosphate mineral saturation in the primary magmas, which caused REE depletion in the later carbonatitic magmas through fractional crystallization. Therefore, we conclude that carbonatites that are relatively rich in apatite but poor in fluorite and barite may not be prospective for REE mineralization.

Isotopic fingerprints of recycled eclogite facies sediments in the generation of the Huanglongpu carbonatite, central China

Carbonatites are the most important sources of global niobium (Nb) and rare earth element (REE) resources, particularly the light REEs. The Huanglongpu carbonatite in central China is unique for the enrichment of molybdenum (Mo) and heavy REEs. Till now, the source nature of the Huanglongpu carbonatites is still poorly understood. In this contribution, we present elemental and C-O-Sr-Nd-Pb isotopic data of calcite to constrain the source signatures of the Huanglongpu carbonatite. Elemental geochemistry and C-O isotopes of the calcite vein-networks in the carbonatite reveal that the Huanglongpu carbonatites is of igneous genesis and originated from mantle-derived carbonatitic melts. The negative εNd(t) values (−8.03–−5.05) and elevated initial 87Sr/86Sr ratios (0.7049–0.7056) indicate considerable contribution of recycled crustal components in the mantle source. Combination of O-Sr-Nd-Pb isotopes suggests that over 10 wt% of recycled sediments might have been involved into the depleted mantle, accounting for the source of the Huanglongpu carbonatite. Considering the enrichment of heavy REE, REE distribution simulation indicates that the Huanglongpu carbonatite might be from the 30% partial melting of eclogite facies sediments. Accordingly, we infer that the Huanglongpu carbonatite might have been derived from the partial melting of a hybridized mantle source modified by recycled sediments of eclogite facies. The enrichment of Mo and heavy REEs plausibly resulted from the contribution of garnet/pyroxene from the eclogitic sediments within the subducted slab.

Carbonatitic Melts and Their Role in Diamond Formation in the Deep Earth

Carbonatitic high-density fluids and carbonate mineral inclusions in lithospheric and sub-lithospheric diamonds reveal comparable compositions to crustal carbonatites and, thus, support the presence of carbon-atitic melts to depths of at least the mantle transition zone (~410–660 km depth). Diamonds and high pressure–high temperature (HP–HT) experiments confirm the stability of lower mantle carbonates. Experiments also show that carbonate melts have extremely low viscosity in the upper mantle. Hence, carbonatitic melts may participate in the deep (mantle) carbon cycle and be highly effective metasomatic agents. Deep carbon in the upper mantle can be mobilized by metasomatic carbonatitic melts, which may have become increasingly volumetrically significant since the onset of carbonate subduction (~3 Ga) to the present day.

Evolution of Carbonatite Magmas in the Upper Mantle and Crust

Carbonatites are the most silica-poor magmas known and are amongst Earth’s most enigmatic igneous rocks. They crystallise to rocks dominated by the carbonate minerals calcite and dolomite. We review models for carbonatite petrogenesis, including direct partial melting of mantle lithologies, exsolution from silica-undersaturated alkali silicate melts, or direct fractionation of carbonated silicate melts to carbonate-rich residual melts. We also briefly discuss carbonatite–mantle wall-rock reactions and other processes at mid-to upper crustal depths, including fenitisation, overprinting by carbohydrothermal fluids, and reaction between carbonatite melt and crustal lithologies.

Formation of Rare Earth Deposits in Carbonatites

Carbonatites and related rocks are the premier source for light rare earth element (LREE) deposits. Here, we outline an ore formation model for LREE-mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. The LREEs can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and LREEs concentrate in the residual melt due to their incompatibility in early crystal-lising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution of the residual liquid into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These primary carbonates are rarely preserved owing to dissolution by later fluids, and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.

Carbonatites: Contrasting, Complex, and Controversial

Carbonatites are unique, enigmatic, and controversial rocks directly sourced from, or evolved from, mantle melts. Mineral proportions and chemical compositions of carbonatites are highly variable and depend on a wide range of processes: melt generation, liquid immiscibility, fractional crystallization, and post-magmatic alteration. Observations of plutonic carbon-atites and their surrounding metasomatic rocks (fenites) suggest that carbon-atite intrusions and volcanic rocks do not fully represent the true compositions of the parental carbonatite melts and fluids. Carbonatites are enriched in rare elements, such as niobium and rare earths, and may host deposits of these elements. Carbonatites are also important for understanding the carbon cycle and mantle evolution.

A reversed redox gradient in Earth's mantle transition zone

The Earth's mantle hosts a variety of reduced and oxidized phases, including iron-bearing alloys, diamond, and sulfide and carbonate melts. In the upper mantle, increasing pressure favors the stabilization of reduced iron-bearing phases via disproportionation of ferrous iron into ferric and metallic iron. Pressure-driven disproportionation is thought to continue into the transition zone, based on the extrapolation of experiments conducted at lower pressures. To test this hypothesis, we performed high-temperature and high-pressure experiments on basaltic and peridotitic compositions at pressures of 10 to 20 GPa, buffered at different oxygen fugacities. Under these conditions, majoritic garnet is the dominant ferric-iron bearing phase. We analyze our experimental run products for their ferric iron concentrations with EELS and Mössbauer spectroscopy. Contrary to expectations, results show that at iron saturation, ferric iron content of majorite peaks in the upper transition zone and then decreases between 500 and 650 km depth, destabilizing and resorbing reduced phases. This peak can be explained by decreases in the effective volume of ferrous minerals in transition zone assemblages. We also show that natural diamond-hosted majorite inclusions that equilibrated in the sublithospheric mantle grew from variably reduced fluids. These results are consistent with the idea that these diamonds formed during progressive reduction of an originally carbonatitic melt.

Genesis of carbonatite and associated U–Nb–REE mineralization at Huayangchuan, central China: Insights from mineral paragenesis, chemical and Sr-Nd-C-O isotopic compositons of calcite

The Huayangchuan deposit in the North Qinling alkaline province of Central China is a unique carbonatite-hosted giant U–Nb–REE polymetallic deposit. The mineralization is characterized by the presence of betafite, monazite, and allanite as the main ore minerals, but also exhibit relatively high budgets of heavy rare earth elements (HREE = Gd–Lu and Y). The origin of carbonatites has long been controversial, thus hindering our understanding of the genesis of the deposit. Here, we conducted an in-situ trace elemental, Sr–Nd isotopic, and bulk C–O isotopic analyses of multi-type calcites in the deposit. Two principal types (Cal-I and Cal-II), including three sub-types (Cal-I-1, Cal-I-2 and Cal-I-3) of calcites were identified based on crosscutting relationships and calcite textures. Texturally, Cal-I calcites in carbonatites display cumulates with the grain size decreasing from early coarse- (Cal-I-1) to medium- (Cal-I-2) and late fine-grained (Cal-I-3), whereas Cal-II calcites coexist with zeolite displaying zeolite–calcite veinlets. Geochemically, Cal-I calcites contain relatively high REE(Y) (151–2296 ppm), Sr (4947–9566 ppm) and Na (29–390 ppm) contents, characterized by right- to left-inclined flat distribution patterns [(La/Yb)N = 0.2–4.2] with enrichment of HREE(Y) (136–774 ppm), whereas Cal-II calcites display low REE, Sr and undetectable Na contents, characterized by a right-inclined distribution pattern [(La/Yb)N = 13.5, n = 16]. The U–Nb–REE mineralization, accompanied with intense and extensive fenitization and biotitization, is mainly associated with the Cal-I-3 calcites which show flat to relatively left-inclined flat REE distribution patterns [(La/Yb)N = 0.2–1.0]. Isotopic results show that Cal-I calcites with mantle signatures are primarily igneous in origin, whereas Cal-II are hydrothermal, postdating the U–Nb–REE mineralization. Cal-I calcites (Cal-I-1, Cal-I-2 and Cal-I-3) from mineralized and unmineralized carbonatites, displayed regular changes in REE, Na and Sr contents, but similar trace element distribution patterns and Sr–Nd-C-O isotopic signatures, indicating that these carbonatites originated from the same enriched mantle (EM1) source by low-degree partial melting of HREE-rich carbonated eclogites related to recycled marine sediments. The combination of trace elements and Sr-Nd isotopic composition of calcites further reveals that these carbonatites have undergone highly differentiated evolution. Such differentiation is conducive to the enrichment of ore-forming elements (U–Nb–REE) in the late magmatic–hydrothermal stages owing to extensive ore-forming fluids exsolved from carbonatitic melts. The massive precipitation of the U–Nb–REE minerals from ore-forming hydrothermal fluids may have been triggered by intense fluid–rock reactions indicated by extensive and intense fenitization and biotitization. Therefore, the Huayangchuan carbonatite-related U-Nb-REE deposit may have formed by a combination of processes involving recycled U–Nb–REE–rich marine sediments in the source, differentiation of the produced carbonatitic magmas, and subsequent exsolution of U–Nb–REE–rich fluids that precipitated ore minerals through reactions with wall rocks under the transitional tectonic regime from compression to extension at the end of Late Triassic.

Zr-Th-U MINERALS IN HIGH-MG DIORITE OF THE CHELYABINSK MASSIF (SOUTH URALS) – EVIDENCE FOR CRUST–MANTLE INTERACTION

Zr-Th-U minerals, namely baddeleyite, zircon and U-Th-oxide, were found in high-Mg diorite from the Late Devonian – Early Carboniferous synplutonic dyke in granodiorites of the Chelyabinsk massif, South Urals. Micron-sized minerals were investigated by electron microscopy and cathodoluminescence spectroscopy. Their chemical compositions were determined by electron probe microanalysis that was optimized to ensure more precise measurements of the composition of minerals. Baddeleyite grains are found as inclusions in amphibole crystals and reside in intergranular areas. The former retain their composition and show no traces of corrosion or substitution. In the intergranular areas, baddeleyite grains were replaced by polycrystalline zircon due to the reaction with an acid melt, and the U-Th-oxide precipitated inside baddeleyite simultaneously, which suggests the restite origin of baddeleyite. The main features of the baddeleyite composition are extremely high concentrations of ThO2 and UO2 (to 0.03 wt. % and 1.0 wt. %, respectively), which may be due to the metasomatic interaction between the mantle peridotite and the crustal or carbonatite fluid or melt.

Multiphase solid inclusions reveal the origin and fate of carbonate-silicate melts in metasomatised peridotite

Orogenic peridotite in ultrahigh-pressure ultrahigh-temperature terranes provides increasingly strong evidence of material transfer in a subduction-zone setting, causing the metasomatism and melting of mantle rocks. Multiphase solid inclusions (MSI) trapped in garnet can provide clues to these processes occurring at great depth. The MSI enclosed in garnet in pyroxenite, lherzolite and harzburgite from a drillcore in the Saxothuringian basement in the Variscan Bohemian Massif are dominated by hornblende, barian mica and carbonate (dolomite and magnesite), and they contain a large variety of other phases, including spinel, clinopyroxene, orthopyroxene, garnet II, apatite, monazite, thorianite, graphite, norsethite and barian feldspar, scheelite and sulfides (e.g., pentlandite). We demonstrate, that the bulk chemical composition of the MSI evolves from the MSI trapped in garnet pyroxenite, which was the source of the metasomatising liquids, to the MSI in the host peridotite. Garnet pyroxenite has a carbonate-silicate composition comparable to kimberlite, and represents a rare example of a low-degree supercritical melt derived from a mixed crust-mantle source frozen in the mantle. Its high-pressure fractional crystallization produced a residual melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO2, Cl, F, P), which is now trapped in the MSI in pyroxenite garnet. The MSI in peridotite reflect changes in the physical and chemical characteristics of the metasomatising medium during its reactive infiltration into peridotite through silicate crystallization and interaction with mantle minerals reflecting the type of rock (lherzolite vs. harzburgite). A change from a more silicic, solute-rich melt to a more diluted carbonate-rich liquid gradually enriched in LILE (K, Ba) and volatiles (CO2, Cl) and LREE fractionation with an increasing degree of fluid evolution coincides with the evolution of kimberlitic melts to carbonatitic melts. The low-viscosity carbonate-rich liquids can migrate far and metasomatise a considerable volume of the rock. Our work demonstrates that the MSI trapped in garnet represent a unique tool to trace the chemical and physical characteristics of the liquids that metasomatise the mantle wedge and their evolution.

Pyroxenite as a Product of Mafic-Carbonate Melt Interaction (Tazheran Massif, West Baikal Area, Russia)

Pyroxenite and nepheline-pyroxene rocks coexist with dolomite-bearing calcite marbles in Tazheran Massif in the area of Lake Baikal, Siberia, Russia. Pyroxenites occur in a continuous elongate zone between marbles and beerbachites (metamorphosed gabbro dolerites) and in 5 cm to 20 m fragments among the marbles. Pyroxene in pyroxenite is rich in calcium and alumina (5–12 wt% Al2O3) and has a fassaite composition. The Tazheran pyroxenite may originate from a mafic subvolcanic source indicated by the presence of remnant dolerite found in one pyroxenite body. This origin can be explained in terms of interaction between mafic and crust-derived carbonatitic melts, judging by the mineralogy of pyroxenite bodies and their geological relations with marbles. According to this model, the intrusion of mantle mafic melts into thick lower crust saturated with fluids caused partial melting of silicate-carbonate material and produced carbonate and carbonate-silicate melts. The fassaite-bearing pyroxenite crystallized from a silicate-carbonate melt mixture which was produced by roughly synchronous injections of mafic, pyroxenitic, and carbonate melt batches. The ascending hydrous carbonate melts entrained fragments of pyroxenite that crystallized previously at a temperature exceeding the crystallization point of carbonates. Subsequently, while the whole magmatic system was cooling down, pyroxenite became metasomatized by circulating fluids, which led to the formation of assemblages with garnet, melilite, and scapolite.

Geochronology, petrography and Sr-Nd-Hf isotopes of Mbalizi carbonatite, southwestern Tanzania

The Mbalizi carbonatite is located in the middle of the Paleoproterozoic Ubendian Mobile Belt and the western branch of East Africa Rift, southwestern Tanzania. Calcite, dolomite, phlogopite, pyrochlore and apatite are found in the sample. Mineral chemistry studies have shown that the carbonatite phlogopite is linked to mantle-derived magmatism. The apatite is fluorapatite, means they are of magmatic origin. The analyses on two crystals of pyrochlore show high concentrations of Nb2O5, and therefore the Nb-oxide is classified as pyrochlore subspecies. Three types of zircon have been obtained from the Mbalizi carbonatite, including xenocrysts zircon, igneous zircon and metamorphic zircon. Zircon in-situ LA-ICP-MS U–Pb dating in this contribution indicates that the Mbalizi carbonatite was crystallized at ca. 116.0 ± 1.8 Ma. The εHf(t) values of igneous zircon ranging from −13.9 to +5.7, indicates that the carbonatite parental magma was originated from the sub-continental lithospheric mantle, and evolves toward HIMU and EM. The whole-rock Sr–Nd isotopic data suggest more contribution of the HIMU and EMⅠ material. We propose that the complex evolutionary history of the Ubendian Mobile Belt has stored the subduction oceanic crust which has the EMⅠ and HIMU components, forming the compositional heterogeneity mantle beneath the Ubendian Mobile Belt. At 116.0 ± 1.8 Ma, with the extension stress field, deep faults cause the pressure reduction, resulting in reactive of the upwelling of the HIMU and EMⅠ components. This provides the metamorphic conditions to induce the isotopic resetting and may result in large scatter of initial 176Hf/177Hf ratios of carbonatite melts.

Petrology of lamprophyre dykes in the Kola Alkaline Carbonatite Province (N Europe)

The study reports petrography, bulk major and trace element compositions of lamprophyric Devonian dykes in three areas of the Kola Alkaline Carbonatite Province (N Europe). Dykes in one of these areas, Kandalaksha, are not associated with a massif, while dykes in Kandaguba and Turij Mys occur adjacent (< 5 km) to coeval central multiphase ultramafic alkaline‑carbonatitic massifs. Kandalaksha dyke series consists of aillikites - phlogopite carbonatites and monchiquites. Kandaguba dykes range from monchiquites to nephelinites and phonolites; Turij Mys dykes represent alnöites, monchiquites, foidites, turjaites and carbonatites. Some dykes show extreme mineralogical and textural heterogeneity and layering we ascribe to fluid separation and crystal cumulation. Melt evolution of the dykes was modelled with Rhyolite-MELTS and compared with the observed order and products of the crystallization. Our results suggest that the studied rocks were related by fractional crystallization and liquid immiscibility. Primitive melts of aillikites or olivine melanephelinites initially evolved at P = 1.5–0.8 GPa without a SiO2 increase due to abundant clinopyroxene crystallization controlled by the CO2-rich fluid. At 1–1.1 GPa the Turij Mys melts separated immiscible carbonatite melt, which subsequently exsolved late carbonate-rich fluids extremely rich in trace elements. Kandaguba and Turij Mys melts continued to fractionate at lower pressures in the presence of hydrous fluid to the more evolved nephelinite and phonolite melts. The studied dykes highlight the critical role of the parent magma chamber in crystal fractionation and magma diversification. The Kandalaksha dykes may represent a carbonatite – ultramafic lamprophyre association, which fractionated at 45–20 km in narrow dykes on ascent to the surface and could not get more evolved than monchiquite. In contrast, connections of Kandaguba and Turij Mys dykes to their massif magma chambers ensured the sufficient time for fractionation, ascent and a polybaric evolution. This longevity generated more evolved rock types with the higher alkalinity and an immiscible separation of carbonatites.

Stable (C, O) and radiogenic (Sr, Nd) isotopic evidence for REE-carbonatite formation processes in Petyayan-Vara (Vuoriyarvi massif, NW Russia)

A study of radiogenic (Sr, Nd) and stable (C, O) isotopic data for rare earth carbonatites from the Petyayan-Vara field of the Devonian Vuoriyarvi alkaline-ultrabasic massif is presented. The cumulative evidence indicates that the primary igneous rocks of the Petyayan-Vara area are burbankite-bearing magnesiocarbonatites having isotopic signatures of the depleted mantle (εNd365Ma = 5.0, 87Sr/86Sr(i) = 0.7031, δ13C ca. –4‰, and δ18O ca. 11‰). Interaction of the primary carbonatite melt with the host silicate rocks produced high-Ti carbonatites with a mantle δ13C (ca. –4‰) and isotopically heavy δ18O (ca. 20‰). These rocks trapped K, Na, Mg, CO2, and rare earth elements (REEs) (mainly heavy REEs) from the melt and Si, Al, Fe, Ti, and P from the host rocks. Early post-magmatic exposure of burbankite-bearing carbonatites to a mixture of fluids of crustal and orthomagmatic carbonatite origin caused redistribution of REEs, Ba, and Sr and formation of REE-rich carbonatites with abundant ancylite mineralization. This effect did not disturb the SmNd system but induced radiogenic Sr accumulation and a change in C and O isotopic composition towards heavier values. Later, but most likely before denudation, the Petyayan-Vara rocks underwent another metasomatic event involving crustal fluids infiltrating through fracture systems. This event triggered formation of bastnäsite-rich carbonatites with fewer REEs at the expense of ancylite-rich carbonatites, and changed all the isotopic systems in the affected rocks. This model successfully accounts for the evolution of all the carbonatite varieties discovered to date in the Petyayan-Vara field.