Carbonated Silicate Melt Research Articles

The Origin of Carbonatites—Combining the Rock Record with Available Experimental Constraints

Abstract Carbonatites are rare igneous rocks that host the largest resources of REE and Nb, yet, their genesis and evolution are far from clear. The leading models of carbonatite formation are the direct melting of carbonate-bearing peridotites, silicate-carbonatite liquid immiscibility, and fractionation of carbonated silicate melts. The validity of these models has never been robustly addressed through combining the available experimental results with the natural rock record. We thus re-evaluate the presently 633 alleged carbonatite occurrences including carbonatite type, bulk composition, mineralogy, and field exposure, followed by a review of experimental data pertinent to carbonatite genesis and evolution. Based on the available data, 454 carbonatite occurrences are magmatic, of which 87 without and 338 with spatially associated alkaline magma, 9 with kimberlites, and 20 with ultramafic cumulates only. Eighty-four percent of the magmatic occurrences contain calcite carbonatite (of which 1/3 also contain dolomite carbonatite), only 9% have dolomite but not calcite carbonatite, the incidence of dolomite carbonatite being similar for occurrences with or without associated silicate magmas. Available experimental data show that crystallization of calcite, dolomite, ankerite, and siderite at crustal conditions requires moderately alkaline and/or hydrous carbonate melts with ≥20 to 25 wt % (Na,K)2CO3 + H2O. It follows that carbonatite rocks, poor in these elements, are at best magmatic cumulates (if not carbo- or hydrothermal) that lost these ephemeral components. Carbonatitic melts could form in the lithospheric mantle, but these are always dolomitic and cannot deviate from close-to-minimum compositions when rising, their strong adiabatic cooling keeping them on the solidus until they decompose to olivine, clinopyroxene (cpx), and CO2 when reaching <2.1 GPa, i.e. the carbonated peridotite solidus ledge, which renders their extraction from the mantle highly unlikely. Furthermore, dolomitic carbonate melts crystallize periclase + calcite at crustal conditions. Only when containing ≥15 wt % (Na,K)2O + H2O they may crystallize dolomite and form dolomitic carbonatites. This value is far above the 2 to 5 wt % (Na,K)2O of mantle-derived carbonatitic melts. Liquid immiscibility from CO2-bearing close-to-natural melilititic, nephelinitic, and phonolitic melts requires 10 to 15 wt % Na2O + K2O in the silicate melt, increasing with SiO2. Extensive differentiation of primitive alkaline melilititic or basanitic parents is hence required to achieve immiscibility. The experimental data show that evolved nephelinites and phonolites unmix calcic carbonatitic melts, while melilitites and undifferentiated nephelinites with >4 wt % MgO may also unmix dolomitic carbonatitic melts. The latter may hence arise from liquid immiscibility or develop through fractionation from calcic carbonatitic melts. Finally, carbonatites may also derive through fractionation of CO2-rich ultramafic melts, but a continuous increase in dissolved CO2 from a carbonated silicate melt to a carbonatitic melt requires ≥3 GPa. We conclude that the combination of the natural rock record with melting and crystallization phase relations excludes a direct mantle origin for almost all carbonatites found in the crust. Instead, their vast majority forms through immiscibility from an alkali-rich differentiated silicate melt that stems from a mantle-derived alkaline parent, consistent with the common spatial association with alkaline complexes and similar isotopic compositions of carbonatite and alkaline silicate rocks. Direct fractionation from silicate melts may occur for kimberlitic or ultramafic lamprophyric melts, but only at ≥3 GPa, i.e. within the lithospheric mantle. To make progress in this field, we suggest a more rigorous distinction of magmatic and carbo- or hydrothermal carbonatite rocks in each occurrence, and to focus on mineral compositions in the carbonatite and associated silicate magmas, as bulk rocks are at best cumulative in nature. Additional experimental work to understand the role of alkalis and H2O in the formation and evolution of carbonatites, in particular crystallization and fluid saturation at crustal conditions, will be essential to provide a more complete understanding of carbonatite petrogenesis.

Crystallisation of trapped carbonate–silicate melts terminating at the carbonated solidus ledge: a record of carbon immobilisation mechanism in the lithospheric mantle

Orogenic peridotites in the crystalline basement of the northwestern Bohemian Massif contain multiphase solid inclusions (MSI), which are interpreted to be crystallisation products of trapped former carbonate–silicate melts metasomatizing their host rocks. We applied conventional thermobarometry and forward thermodynamic modelling to constrain the P–T evolution ranging from the peak metamorphic conditions of the investigated harzburgite and lherzolite, through entrapment of the melts in the outer parts of garnets, to the (re)-equilibration of the MSI assemblages. The peak conditions of c. 1100 °C/4.5–5.5 GPa are recorded by garnet cores and large pyroxene porphyroclasts. The melt entrapment, during which garnet outer parts grew, was associated with influx of the metasomatizing liquids and probably took place during the early stage of the exhumation. Thermodynamic model of amphibole-free MSI assemblage comprising kinoshitalite/Ba-rich phlogopite (approximated by phlogopite in the model), dolomite, magnesite, clinopyroxene, orthopyroxene, garnet and chromite provided robust estimate of P and T of its (re)-equilibration, c. 900–1000 °C, 1.8–2.2 GPa. Furthermore, the lack of olivine reflects co-existence of COH fluid with high X(CO2) = CO2/(CO2 + H2O) ≥ 0.6. Models employing identical P–T–X(CO2) parameters successfully reproduced the other two amphibole-bearing assemblages observed. The modelled stability fields show perfect alignment with a characteristic isobaric segment of the solidus curve of carbonated peridotite. This co-incidence implies that the (re)-equilibration corresponds to the termination of the melt crystallisation once the near-isothermal exhumation path intersected the solidus. Decreased solubility of silicates at the carbonated peridotite “solidus ledge”, inferred from the published experimental data, as well as concentric textures of some MSI indicates sequential crystallisation from the early silicates to late dolomite. The carbonated “solidus ledge” is a relatively narrow boundary in the lithospheric mantle capable of an abrupt immobilisation of fluxing or transported carbonated melts. The investigated rocks are estimated to store approximately 0.02 kg C/m3 (or 6 ppm C) occurring as carbonates in the MSI.

An almost universal CO2 - CO32− carbon isotope fractionation function for high temperatures

High-temperature experiments on the CO2 - carbonate melt and carbonatite - nephelinite melt pairs are used to quantify equilibrium carbon isotope fractionation at conditions of 1 atm, 750–950 °C and 0.3–0.8 GPa, 1170 °C, respectively. Together with available experimental data, all results are within error consistent with a universal CO2 – CO32− fractionation function103lnαCO2/carbonate=6.41(11)·106T2−2.54(8)·1012T4(T in K, valid for >650 °C) that encompasses solid and molten carbonates and also carbonate components in silicate melts. The experiments hint towards a small fractionation of carbonatite vs. silicate melt of +0.39 ± 0.35 ‰ (1 σ), which however remains within error indistinguishable from zero. A generalized single fractionation function suggests that the nearest neighbours of carbon (i.e., O) dominate isotope fractionation while the next nearest neighbours only have a minor role, which greatly facilitates the understanding of carbon transfer in the deep Earth.If there is small to negligible 13C/12C-fractionation between solid carbonate, molten (ionic) carbonate liquid and covalent carbonated silicate melt, then H2CO3- and HCO3−-species in fluids or gases should also have low 13C/12C fractionation factors relative to the carbonate-group in depolymerized silicate melts or carbonates. Speciation models predict that the H2CO3- and HCO3−-species dominate over the CO2 species in COH-fluids at high pressures or high temperatures, i.e. during MOR basalt degassing or subduction zone devolatilization. A consequent small 13C/12C fractionation (of 0.1–0.3 ‰) applies to (i) continuous degassing of COH-fluids from mid-ocean ridge basalts, consistent with an observed difference of δ13CMORB and δ13Cmantle ≤1 ‰ and (ii) C-transfer of a carbonatite or COH-fluid from the subducted crust to the mantle, which then has a near-identical δ13C as the source. Finally, a combination of our results with available experimental values allows calculation of a consistent set of high-temperature (>650 °C) carbon fractionation factors for CO2, CO32− (carbonate, carbonatite or carbonated silicate melt), CH4, CO, and graphite/diamond.

Two styles of melt-peridotite interactions in the upper mantle revealed by mantle xenoliths from northeastern China

To better constrain the mechanisms of interactions between different types of melts and mantle peridotites in the upper mantle of the big mantle wedge, we studied petrology, mineral chemistry, and temperatures recorded by major element and rare earth element (REE) for mantle xenoliths from the Wangqing area in the eastern part of northeastern China, and compared these data with those for xenoliths from the adjacent Jiaohe and Shuangliao areas. The Wangqing xenoliths, including olivine clinopyroxenite, wehrlite, orthopyroxenite, and websterite, generally contain clinopyroxene that is surrounding or replacing olivine. Mineral compositions of the xenoliths from northeastern China vary along melt–peridotite reaction trends, where the Wangqing xenoliths have lower mineral Mg#’s (77–85 in olivine and 81–87 in clinopyroxene) than the Jiaohe and Shuangliao peridotite and pyroxenite xenoliths. In addition, clinopyroxenes in the Wangqing samples have moderately elevated rare earth element contents and (La/Yb)N and 87Sr/86Sr ratios (0.7035–0.7046), within the range of those for clinopyroxenes in the Jiaohe and Shuangliao samples. The REE temperatures of the Wangqing samples (1024–1253 °C) are higher than those of the Jiaohe and Shuangliao samples (844–1120 °C). Based on the differences in mineral chemistry and temperature, we conclude that the Jiaohe and Shuangliao peridotites and pyroxenites were mainly derived from lithospheric mantle accreted from carbonated regions of the asthenosphere, whereas the Wangqing wehrlites and pyroxenites were formed by reaction of lithospheric mantle peridotites with hot carbonated silicate melts. These results provide evidence for two different types interactions between of carbonate-related melts and mantle peridotite, which are important for understanding the chemical evolution of the upper mantle in the big mantle wedge of northeastern Asia.

Growing diamonds in the laboratory to investigate growth, dissolution, and inclusions formation processes

Replicating lithospheric diamonds experimentally helps reveal their histories at depth, which may be complicated by successive growth and dissolution episodes. Here, we present diamond growth and dissolution experiments to constrain the conditions of diamond formation and residence in the Earth’s lithosphere before their transport to the surface. Experiments were performed on mixtures of carbonates, natural lherzolite or MORB, water, diamond seeds, and with or without graphite using multi-anvil presses over a few hours to more than a day at conditions relevant to the lithosphere (7 GPa, 1300–1670 °C). We observed growth within a volatile-rich melt resulting from the miscibility of silicate-carbonate melt and aqueous fluid via the reaction: Carbonate minerals + Silicate minerals/glasses + H2O = [volatile-rich melt] ± minerals + Cdiamond + O2. In the absence of graphite, we observed diamond dissolution within the same hydrous-carbonate–silicate-bearing melts and under similar pressure and temperature conditions used to grow diamond, indicating that diamond formation requires an oxygen sink (graphite, metallic and/or sulphide melts). In dissolution experiments, we also observed resorption features similar to those described in lithospheric monocrystalline diamonds, which we thus attribute to mantle fluids and not to kimberlite-induced resorption during magma ascent. We show that dissolution and growth may alternate in the mantle, and that protracted periods are not necessary to explain natural diamond histories. During experimental growth, inclusions were trapped in diamonds as they are in nature. We observed both syngenetic (formed during growth, representing the parental fluid) and protogenetic inclusions (here particles from the capsule), indicating that both kinds of inclusions can be trapped within a single growth event. Our experiments confirm that inclusions trapped in natural diamonds do not necessarily represent remnants of their parental fluids and are not necessarily contemporaneous: two inclusions near each other within a single monocrystalline diamond may have different histories, and inclusions must be shown to have achieved chemical and isotopic equilibrium before being considered synchronous.

Calcium isotopes track volatile components in the mantle sources of alkaline rocks and associated carbonatites

The volatile components CO2 and H2O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event.The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle.

Experimental study of the interaction between garnets of eclogitic and lherzolitic parageneses and H2O-CO2 fluid under the P-T parameters of the lithospheric mantle

Experimental modeling of the interaction between garnets of mantle parageneses with H2O–CO2 fluid was performed in order to determine the characteristic features and trends in garnet compositions due to metasomatic alterations, and to study the patterns of formation of carbon and carbon-bearing phases in these processes. Experiments were carried out in the lherzolitic garnet–CO2–H2O–C and eclogitic garnet–CO2–H2O–C systems at 6.3 GPa in the temperature range of 950–1550 °C using double graphite+platinum ampoule on a multi-anvil high-pressure apparatus of a split-sphere type (BARS). The interaction of mantle garnets with H2O–CO2 fluid was found to cause garnet recrystallization and formation of new phases, namely magnesian carbonate, coesite, kyanite, corundum–eskolaite solid solution and silicate–carbonate melt, as well as metastable graphite (950–1550 °C) and diamond growth on seeds (1250–1550 °C). It was established that the garnet-fluid interaction was accompanied by the bivalent cations' redistribution between recrystallized garnet, carbonate and carbonate–silicate melt. In these processes calcium was preferentially partitioned in the melt, while magnesium was partitioned in garnet and carbonate. Recrystallization of garnets of lherzolitic and eclogitic parageneses under effect of H2O–CO2 fluid resulted not only in compositional changes, but in the formation of carbonate, kyanite, coesite, graphite, corundum–eskolaite solid solution and carbonate-silicate melt inclusions in these garnets. The revealed patterns of variations in garnet compositions, as well as the typical inclusions in them, can be regarded as indicators of metasomatic alteration of garnets of mantle parageneses involving H2O–CO2 fluid. The obtained results allow one to infer that H2O–CO2 fluids play a crucial role in redistribution of divalent cations under the P-T parameters of the lithospheric mantle and control over the composition of carbonate–silicate melts coexisting with the eclogitic and lherzolitic assemblages. Another important result of this study is the experimental demonstration that graphite capsules on their own fail to ensure reliable hermeticity with respect to the fluid and melt phases in the high-pressure, high-temperature experiments. It was substantiated that the procedure involving a hermetically sealed platinum ampoule with inner graphite capsule prevents uncontrollable loss of matter in long-term petrological experiments and is optimal for modeling systems containing melts and fluids.

EVOLUTION OF THE VICTORIA PIPE ULTRAMAFIC ALKALINE MELT (ANABAR REGION, SIBERIAN CRATON): MELT INCLUSION WITHIN OLIVINE AND GROUNDMASS MINERALS

To provide new insights into evolution of the primary ultramafic alkaline melts, we present an investigation of the primary and secondary melt inclusions within olivine macrocrysts and groundmass minerals from the Victoria monticellite-nepheline damtjernite, Anabar diamondiferous province, Siberia craton. The primary melt inclusions within olivine macrocrysts shown that initial damtjernite melts were K–Na bearing carbonate-silicate melts by composition whereas the K and Na associated within only silicate phases like that phlogopite, nepheline, kalsilite. That distinguishes these inclusions from similar inclusions in olivines from aillikites and kimberlites and emphasizes a more alkaline character of the damtjernitic parental melts. Based on the composition of melt inclusions within spinel and monticellite from magmaclastic groundmass, under the further evolution of the damtjernitic melts, K and Na are not only included in silicate daughter phases, but they can also form alkaline phosphates, carbonates, sulfates, and halides. That led to form the alkaline carbonate and saline sulfate-phosphate-chloride-carbonate liquids. This composition of the evolved ultramafic alkaline melt is common for melt inclusions within different minerals from aillikite, kimberlite and some carbonatites highlighted uniform mechanism of evolution of alkaline-ultramafic melts. Further accumulation of the fluid phase led to its reaction interaction with olivine with the formation of monticellite and degassing processes.

Formation and evolution of lithospheric mantle beneath the Yilan-Yitong fault zone, Northeast China: Constraints from the Shangzhi peridotite xenoliths

Despite numerous studies, the formation and evolution of the lithospheric mantle beneath the Yilan-Yitong fault zone (YYFZ), Northeast China, are still poorly constrained. Here, we use in-situ trace elements and Sr isotopes of clinopyroxene, as well as whole-rock platinum group elements and Re-Os isotopes of the Shangzhi peridotite xenoliths (including spinel lherzolite and harzburgite) within the YYFZ to address the above important issues. Clinopyroxenes in one harzburgite characterized by high Zr/Hf (38.0–59.9), Ca/Al (9.3–13.4), and low Ti/Eu ratios (118–334) can be attributed to carbonatite metasomatism. Clinopyroxenes in some samples that is selectively enriched in light rare earth element exhibits lower Zr/Hf (17.0–46.9), Ca/Al (4.0–7.6), but higher Ti/Eu ratios (2219–4538), which may have been metasomatized by a carbonated silicate melt. The Sr contents and 87Sr/86Sr ratios for these clinopyroxene grains from the Shangzhi peridotites increase gradually from core to rim, suggesting the addition of radiogenic Sr from the metasomatic melts. Such metasomatic melts are likely derived from subducted carbonate-bearing oceanic crustal components. The 187Os/188Os ratios of two groups of the Shangzhi peridotites are positively correlated with Al2O3, forming two nearly parallel lines, which indicate that the lithospheric mantle beneath the Shangzhi area was formed by partial melting of a fertile mantle precursor with variable 187Os/188Os during the Mesoproterozoic time (∼1.4 Ga). The dominance of Mesoproterozoic mantle peridotites in the Shangzhi area suggests that asthenosphere upwelling and lithospheric mantle replacement are not common beneath the YYFZ.

Trace Element Partitioning between Olivine and Melt: Analysis of Experimental Data

Knowledge of mineral–melt partition coefficients (D) is necessary for geochemical modeling of magma formation and evolution. The main source of these parameters is experiments on equilibrium between minerals and silicate melt. The database on mineral–melt equilibrium experiments has grown continuously, which allows one to refine partition coefficients and reveal the most important factors affecting them. This paper reports the analysis of available experimental data on trace element partitioning between olivine and melt (7000 experiments from 587 publications were used). Based on the statistical processing of the data array, the dependence of D on melt and olivine composition and P–T conditions was evaluated. It was found that most of incompatible elements are either insensitive or moderately sensitive to these parameters. Among the compositional parameters, CaO content in melt is most significant. It was shown that D estimates can be significantly improved by using ratios of D values for different elements. Such ratios are often independent of experimental parameters and much less variable than D values for particular elements. The obtained D estimates for basaltic compositions vary within six orders of magnitude, from 10–5 (U, Th, and La) to ~5–10 (Co and Ni). The low D values for most elements (0.1) indicate that many trace element ratios in melts do not change significantly even at high degrees of olivine crystallization. The obtained estimates were compared with data on element partitioning between high-pressure (Mg,Fe)2SiO4 phases (wadsleyite and ringwoodite) and silicate melt and between olivine and carbonate–silicate melts. In both cases, there is a close similarity to element partitioning between olivine and silicate melt. One exception is REE. DREE values for the wadsleyite/ringwoodite–melt and olivine–carbonate melt systems are approximately one order of magnitude lower than those for olivine–silicate melt partitioning.

Physicogeochemical Mechanisms of the Genesis of Matryoshka-Type Diamonds on the Basis of the Mantle-Carbonatite Theory

A physicochemical analysis of the genesis of a unique Matryoshka-type diamond from the Nyurbinskaya kimberlite pipe of the Nakyn kimberlite field, Yakutia, Russia, was performed. The specimen consists of a host diamond with a cavity containing a loose diamond inclusion; two through holes with a width of 0.1–0.4 mm emerge from the cavity. The analysis is based on the mantle-carbonatite theory of the genesis of diamonds and associated phases developed on the basis of consistent results of a physicochemical experiment and an analytical study of paragenetic inclusions in natural diamonds from kimberlite deposits. The published data of crystal morphological and physical studies of the Matryoshka diamond were used as well. As a result, the physicogeochemical mechanisms of nucleation and crystallization of the host diamond with a cavity and diamond inclusion under the conditions of mass genesis of diamonds in completely miscible carbonate–eclogite–carbon melts of the upper-mantle diamond-forming chamber are substantiated. The initially closed cavity was filled with a diamond-forming carbonate–silicate melt with dissolved carbon. In addition, an analysis of the conditions of etching and dissolution of the host diamond and diamond inclusion during the kimberlite transport of diamond-bearing material from the mantle chamber to the depths of the Earth’s crust was performed. The reasons for the explosive formation of cone-shaped through holes in the main diamond, which was accompanied by the ejection of the diamond-forming medium and the filling of the cavity with (С–О–Н)-fluid-containing kimberlite melt, are considered. The final episodes of partial dissolution of diamonds from the Matryoshka sample by kimberlite and assimilated melts continued during the formation of a cumulative chamber in the Earth’s crust and its solidification with the release of a highly compressed C–O–H fluid. “Fluid drilling” of the top of the cumulative chamber stimulated the explosive formation of the Nyurbinskaya pipe and its filling with kimberlite and assimilated diamondiferous matter. With prolonged compaction of this substance, the Matryoshka diamond was subjected to atmospheric and hydrothermal factors, as is evidenced by fine-grained sedimentary barite, a mineral of barium sulphate, found in the through holes of the host diamond.

Na-bearing bridgmanite: Synthesis, phase relations and application to the origin of alkaline melts in the uppermost lower mantle

Experimental studies of melting relations in the MgSiO3–Na2CO3 (±Al2O3, Fe2O3) multicomponent alkaline carbonate–silicate system were carried out at 23–24 GPa and 1100–1700 °C to shed light on the conditions and mechanisms of formation of Na-bearing bridgmanite. At 1100–1300 °C, an assemblage of low-Na bridgmanite (<0.6 wt% Na2O), ringwoodite, periclase (ferropericlase), alkaline carbonate with composition close to Na2Mg(CO3)2, and sodic carbonate–silicate melts is formed. With increase in temperature up to 1700 °C, carbonate disappears and the content of Na2O increases in all mantle phases (up to 1.6, 4.4, and 2.7 wt% in bridgmanite, ringwoodite, and ferropericlase, respectively), which indicates that Na-bearing bridgmanite may crystallize at moderate degrees of partial melting. The clear positive correlation between the contents of Na and Al, and negative correlation between Na and Fe in bridgmanite, as well as the preferable structural incorporation of Al and Na with temperature, provide evidence for the predominant crystallization of Na-rich bridgmanite via the vacancy mechanism Si4+B + Mg2+A + O2−O = Al3+B + Na+A + VO from Al-bearing alkaline carbonate–silicate melts, with composition similar to those detected as inclusions in lower-mantle diamonds.

Melt Composition and Phase Equilibria in the Eclogite-Carbonate System at 6 GPa and 900–1500 °C

Melting phase relations in the eclogite-carbonate system were studied at 6 GPa and 900–1500 °C. Starting mixtures were prepared by blending natural bimineral eclogite group A (Ecl) with eutectic Na-Ca-Mg-Fe (N2) and K-Ca-Mg-Fe (K4) carbonate mixtures (systems Ecl-N2 and Ecl-K4). In the Ecl-N2 system, the subsolidus assemblage is represented by garnet, omphacite, eitelite, and a minor amount of Na2Ca4(CO3)5. In the Ecl-K4 system, the subsolidus assemblage includes garnet, clinopyroxene, K2Mg(CO3)2, and magnesite. The solidus of both systems is located at 950 °C and is controlled by the following melting reaction: Ca3Al2Si3O12 (Grt) + 2(Na or K)2Mg(CO3)2 (Eit) = Ca2MgSi3O12 (Grt) + [2(Na or K)2CO3∙CaCO3∙MgCO3] (L). The silica content (in wt%) in the melt increases with temperature from &lt; 1 at 950 °C to 3–7 at 1300 °C, and 7–12 at 1500 °C. Thus, no gradual transition from carbonate to kimberlite-like (20–32 wt% SiO2) carbonate-silicate melt occurs even as temperature increases to mantle adiabat. This supports the hypothesis that the high silica content of kimberlite is the result of decarbonation at low pressure. As temperature increases from 950 to 1500 °C, the melt Ca# ranges from 58–60 to 42–46. The infiltration of such a melt into the peridotite mantle should lower its Ca# and causes refertilization from harzburgite to lherzolite and wehrlitization.

Petrogenetic history and melt inclusion characteristics of mantle plume-derived ijolites from NE India: implications for multistage crystallization and occurrence of “nano-calciocarbonatites”

The Sung Valley ultramafic–alkaline–carbonatite complex (UACC) of Meghalaya, NE, India, is a result of magmatic activity related to the Kerguelen mantle plume spanning from 101 to 115 Ma. In the present study, an integrated crystal size distribution (CSD), mineral chemistry, and melt inclusion analysis are carried out on the ijolites present within this UACC. The CSD analysis shows that these ijolites were formed in multiple stages through changes in the crystallization environment, such as cooling and nucleation rates. Raman spectroscopy of mineral inclusions of rutile, aphthitalite, apatite, carbonate–silicate melt inclusions, and disordered graphite within clinopyroxene and titanite, respectively, indicates a heterogeneous composition of the parental magma. These mineral and melt inclusion phases further suggest localized changes in oxygen fugacity (fO2) due to redox reactions in the lower crust. SEM–EDX analysis of the exposed melt inclusions reveals the presence of alkali-bearing diopside, phlogopite, and andradite, along with an unidentified carbonated silicate daughter phase. The studied melt inclusions are dominated by carbonate, whereas silicates are subordinate. The presence of this fully crystallized carbonate–silicate melt as calcite, diopside, phlogopite, magnetite, apatite, and andradite suggests the presence of “nano-calciocarbonatites” in these ijolites. Our study provides insights into different mechanisms of the loss of alkalies from initially entrapped alkaline carbonate melt in clinopyroxenes. The predominant occurrence of calcite as the only carbonate phase in the studied melt inclusions is a result of silicate–carbonate melt immiscibility, calcite-normative system in these inclusions, dealkalization of the alkaline carbonates in the presence of external fluid, and/or redistribution of the alkalies to the daughter alkali-bearing silicates.

New insights into deep carbon recycling and formation of nepheline-bearing alkaline rocks from Sr-Nd-Mg isotope compositions

Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 to −0.62‰) in nepheline syenites in the Tarim large igneous province (TLIP), NW China. To evaluate the significance of the light Mg isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petrogenesis. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating yields an age of 272.5 ± 1.4 Ma for the nepheline syenite. Petrographic and geochemical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr isotope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364−0.70399, εNd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348−0.70371, εNd(t) = +3.28 to +3.88 for nepheline syenite and nephelinite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystallization of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the extremely light magnesium isotopes of nepheline syenite to “genetic genes” of nephelinite (δ26Mg = −0.35 to −0.55‰) which were produced by the reaction between peridotite and carbonated silicate melt derived from the carbonated eclogite. We invoke a three-stage model for the genesis of the nepheline syenite in the TLIP. Initially, the subduction of oceanic crust delivered the sedimentary carbonate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbonated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline syenite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alkaline rocks.

Formation of carbon-bearing silicate melts by melt-metacarbonate interaction at convergent plate margins

The incorporation of carbon into slab-derived silicate melts is a vital process that boosts carbon release and transfer at convergent plate margins. The available models mostly suppose the generation of carbonated silicate melts through partial melting of carbon-rich rocks. Here we show that the carbon-bearing silicate melts can also form by the interaction of slab-derived, carbon-free, silicate melts with eclogite-facies metacarbonate rocks in a subduction-collision environment. We examine the marble – reaction zone – migmatite systems from the Sulu ultra-high pressure (UHP) terrane (China). Field, petrological, geochemical, and Sr-O-C isotopic studies demonstrate that carbon-free silicate melts produced by the anatexis of gneisses infiltrate the marbles, leading to the formation of silicate reaction zones around the melt channels and the synchronous release of CO2 at ∼820–860°C. The occurrence and textural characteristics of calcite-bearing leucosomes in the reaction zones indicate an enrichment of CO2 in silicate melts after the melt-marble interaction. Compositional analyses and mapping of carbonates indicate that these carbonated silicate melts contain >0.8–1.6 wt.% CO2, in accordance with the solubility of carbon in silicate melts at similar conditions. In addition to the variation in carbon contents, the interaction causes a decrease in SiO2 (from 72–73 to 58–63 wt.%) and an increase of CaO (from 0.6–0.8 to 3–8 wt.%) contents in the infiltrating melts. Estimation of carbon budget suggests that less than 14% of released carbon was retained in the carbonated rocks in the form of calcite, whereas the rest was removed from the system through the long-distance transport of the melts or other processes.This study presents compelling evidence for the petrological transition from carbon-free to carbon-bearing silicate melts by melt-marble interaction and therefore provides a new mechanism for the enrichment of carbon in slab-derived melts and an important pathway for the massive extraction of carbon from subducted slabs. Addition of CO2 and reduction of SiO2 as the interaction progress results in a lower viscosity and larger buoyancy of the reacted melts compared to the unreacted felsic melts, which facilitates the rapid transport and rising of these carbonated melts and thus helps a more efficient cycling of carbon in Earth's interior.

Carbonatite-melilitite-phosphate immiscible melts from the aragonite stability field entrained from the mantle by a Pliocene basalt

A plagioclase megacryst containing composite crystal-glass inclusions was ejected in a Pliocene basaltic diatreme in the Carpathian back-arc basin (Pannonian Basin). The megacryst grew from phonolitic melt, relics of which are preserved in the crystal-glass inclusions. Most of the pristine melt has undergone substantial compositional resetting by interaction with several batches of a low-viscosity carbonated, P-rich melilitite melt, which infiltrated and largely replaced the original inclusion content. The melilitite melt also caused partial resorption of the host megacryst and crystallisation of new calcic plagioclase forming stringers and palisades. A P-rich calcic carbonatite melt exsolved from the melilitite and later crystallised to aragonite at ~ 800 °C and 1.9 GPa. The phosphate melt fraction exsolved from the carbonatite and solidified as CO32−-rich A-B type apatite. At a very late evolutionary stage, K- and Si-rich fluids caused potassic and silicic alteration of the solidified melilitite glass along cracks and interfaces between calcic carbonate globules and glass at temperatures below 680 °C. The oxygen isotope composition of the plagioclase megacryst (6.2 ‰ V-SMOW; Vienna Standard Mean Ocean Water) and the 87Sr/86Sr isotope ratio of carbonates in the inclusions (0.7034) are consistent with a mantle-derived melt. 87Sr/86Sr isotope ratios (0.7047–0.7051) in interstitial carbonates from associated syenite and carbonatite xenoliths indicate a metasomatised mantle source contaminated with radiogenic crustal material or altered marine carbonate. The O-isotope ratios in the carbonates, 22.7 ± 0.6 ‰ V-SMOW in calcite and 23.6 ± 0.7 ‰ V-SMOW in aragonite, are also consistent with a sedimentary precursor. Contrasting δ13C values in the calcite, -12.7 ± 0.5 ‰ V-PDB (Vienna PeeDee Belemnite), and the aragonite (-4.6 ± 0.5 ‰ V-PDB) indicate low-temperature modification of calcite assisted by delta13C-depleted CO2 and preservation of primary magmatic delta13C values in aragonite. The microstructural and geochemical evidence points towards heterogeneous silicate-carbonate melt fractions generated during the metasomatism and partial melting of a supra-subduction mantle wedge.

Role of Volatiles in the Evolution of a Carbonatitic Melt in Peridotitic Mantle: Experimental Constraints at 6.3 GPa and 1200–1450 °C

Reconstruction of the mechanisms of carbonatitic melt evolution is extremely important for understanding metasomatic processes at the base of the continental lithospheric mantle (CLM). We have studied the interaction between garnet lherzolite and a carbonatitic melt rich in molecular CO2 and H2O in experiments at 6.3 GPa and 1200–1450 °C. The interaction with garnet lherzolite and H2O-bearing carbonatite melt leads to wehrlitization of lherzolite, without its carbonation. Introduction of molecular CO2 and H2O initiates carbonation of olivine and clinopyroxene with the formation of orthopyroxene and magnesite. Partial carbonation leads to the formation of carbonate–silicate melts that are multiphase saturated with garnet harzburgite. Upon complete carbonation of olivine already at 1200 °C, melts with 27–31 wt% SiO2 and MgO/CaO ≈ 1 are formed. At 1350–1450 °C, the interaction leads to an increase in the melt fraction and the MgO/CaO ratio to 2–4 and a decrease in the SiO2 concentration. Thus, at conditions of a thermally undisturbed CLM base, molecular CO2 and H2O dissolved in metasomatic agents, due to local carbonation of peridotite, can provide the evolution of agent composition from carbonatitic to hydrous silicic, i.e., similar to the trends reconstructed for diamond-forming high density fluids (HDFs) and genetically related proto-kimberlite melts.

Experimental Modeling of Diamond Resorption during Mantle Metasomatism

The morphology of resorbed diamond crystals is a valuable source of information on the composition and ascent rate of kimberlite magmas, as well as on possible redox conditions in protolith. Previously, diamond resorption was thoroughly investigated at P–T–fO2 parameters of the kimberlite magma ascent. In this study, we investigated diamond resorption using unaltered group I kimberlite and model carbonatite at P–T–fO2 parameters that are typical of the peridotite source of kimberlite magmas in the subcontinental lithospheric mantle. An analysis of previous studies made it possible to determine the rate of diamond octahedron transformation into a spherical tetrahexahedron depending on the composition of the carbonate–silicate melt. It was shown that the rate of diamond resorption at 6.3 GPa increases in all the investigated systems as fO2 and temperature rise. There is a steady decrease in the diamond resorption rate as pressure increases from 1 GPa to 6.3 GPa. The morphology comparison of the experimentally produced samples with natural diamonds is indicative of the significant contribution of metasomatic alteration of protolith by the oxidized agent and at the initial stages of kimberlite magma ascent to the resorption of natural diamonds.

Mount Mather Creek, British Columbia – a new sodalite-bearing carbohydrothermal breccia deposit including a new Canadian occurrence for the rare minerals edingtonite and quintinite

AbstractThe Mount Mather Creek sodalite-bearing carbohydrothermal breccia dyke is located ~35 km northwest of Golden, British Columbia, Canada, within the Foreland Belt on the eastern side of the Mississippian-to-Devonian British Columbia Alkaline Province. The dyke occurs in situ on the western side of the Mount Mather Creek gully, cropping out over a distance of ~80 m with a thickness of up to 10 m, intruding a syncline of Middle and Upper Cambrian Chancellor Group carbonate rocks. To date, no parental alkaline complex has been found adjacent or proximal to the dyke. The breccia dyke is flow-banded and matrix-supported and consists of host rock clasts, medium- to coarse-grained, anhedral-to-poikilitic carbonate-rich syenite segregations that occur as veins and pods, and fine-grained banded sodalite plus carbonate fragments in a carbonate-rich matrix. The sodalite-carbonate segregations consist dominantly of sodalite, ferroan dolomite, calcite and microcline, with an extensive suite of trace minerals enriched in rare earth elements (REE), Na, Ba and Sr, including albite, analcime, ancylite-(Ce), chabazite-Na, fluorapatite, baryte, barytocalcite, cancrinite, galena, goethite, gonnardite, harmotome, edingtonite, a potentially new Mg-bearing edingtonite-like mineral, natrolite, nordstrandite, pyrite, quintinite and sphalerite. Alteration of the primary silicate–carbonate assemblage has resulted in a secondary assemblage of hydrothermal or carbohydrothermal REE–Ba–Sr–Na phases including albite, analcime, cancrinite, gonnardite and nordstrandite after sodalite, together with complex intergrowths of ancylite-(Ce), barytocalcite, edingtonite, and a potentially-new Mg-bearing edingtonite-like mineral. Remobilisation of Ba and Sr from barytocalcite resulted in crystallisation of late-stage baryte and Sr-rich calcite. Mount Mather Creek is only the fifth Canadian occurrence of the Ba-zeolite, edingtonite, and the second Canadian occurrence of the rare layered double hydroxide quintinite. The Mount Mather Creek breccia is a carbohydrothermal deposit, the product of a low temperature (&lt;450°C), highly evolved, alkaline, SiO2-undersaturated, Na–Ba–REE–Cl-rich, residual carbonated silicate melt whose parental origins remain unknown.