Carbonate Melt Research Articles

The Effect of Addition Amount of Chromium Iron on the Bonding Strength between Alloy Steel Surfacing Layer and Steel Base Metal

Fe-C-Cr-Nb alloy steel surfacing layers with different contents of C and Cr were prepared on 45 steel base metal by selfshielded flux-cored wires with distinct amounts of high carbon chromium iron addition and melt arc surfacing. The composition and microstructure changes of the surfacing layer were tested and analyzed. The surfacing test plate was processed into a pulling specimen, and the bonding strength between the surfacing layer and the 45 steel base metal was tested with a self-designed pulling test method. The fracture location of the pulling specimen and fracture characteristics were observed by a metallurgical microscope and a scanning electron microscope. The result shows that with the increase of the amount of high carbon chromium iron added to flux-cored welding wire, the content of C and Cr in the surfacing layer increases, and the NbC hard phase disperses. The microstructure of the steel matrix changes from mixed martensite + residual austenite to high carbon martensite + residual austenite, and then independent austenite appears. The hardness of the surfacing layer first increases and then decreases. The bonding strength between the surfacing alloy and the 45 steel base metal first decreases and then increases, and the fracture location is at the bottom of the surfacing layer or the fusion zone with mostly quasi-cleavage characteristics. When the additional amount of high carbon chromium iron reaches 13%, thee pulling specimen exhibits significant deformation with the highest bonding strength, and the fracture is close to the fusion line, where there are numerous tearing edges and shallow dimples.

An experimental study on the role of F−, PO43−, Cl− and SO42− ligands in the natrocarbonatite-nephelinite system at 850 °C and 0.1 GPa

Carbonatites and their comagmatic silicate rocks related deposit provide significant resources of rare earth elements (REEs), niobium (Nb) and other elements such as U, Th, Mo, V, Ba, Sr, etc. However, the genesis of mineralization, especially for REEs and Nb, in carbonatite remains enigmatic. Previous liquid immiscibility experiments have demonstrated that both REEs and Nb are preferentially enriched in the silicate conjugate instead of carbonate melts under anhydrous conditions. Nevertheless, ligands other than carbonate ion appear to be abundant due to ubiquity of apatite, baryte, celestine, fluorite and sodalite in carbonate–silicate magmatic systems. Here, we experimentally investigate the trace element partitioning between natrocarbonate and silicate (nephelinite) melts in systems doped with varying amounts of additional F−, PO43−, Cl−, and SO42− ligands (0, 2, 4 and 6 wt%) to understand and constrain the role of ligands.The experiments were conducted at 850 °C and 0.1 GPa using rapid quench cold-seal pressure vessels (CSPVs). A comparison of experimental partition coefficients in this study reveals that the significant amounts F− and PO43− incorporated in the silicate melts can increase the D values for REE by influencing melt structure (DLaCM/SM = 0.85–7.42). In contrast, irrespective of the amount of added Cl− and SO42−, DCM/SM is not affected significantly by these species and the DREECM/SM values remain always lower than 1 (DLaCM/SM = 0.12–0.40). Notably, the DNbCM/SM values are all <1, with only one exception containing 6 wt% F. Besides, in all the investigated systems, Ba, Sr, Mo, V, Cs, Rb and Li preferentially partition into the conjugate carbonate melt. All the high field strength elements (Pb, Th, U, Zr, Hf, Nb, Ta), transition metals (Mn, Co, Cu, Zn) and common network formers (Ga, Ge) essentially partition into the silicate melt.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

Melting behavior of impure limestone under H2O-poor conditions: Implications for the contribution of carbonate-rich sediments to arc magmatic carbon output

Sedimentary carbon may be transferred from subducting slabs to the overlying mantle wedge as a component of sediment-derived melts and contribute to carbon output in subduction zones. To investigate the melting behavior of subducting carbonate-rich sediments at shallow depths and evaluate the consequent efficiency of carbon recycling, this study presents the phase relations of H2O-poor impure limestone at 1.5–4.5 GPa and 750–1300 °C. The onset of melting occurs between 750 and 800 °C at 1.5–2.5 GPa, between 950 and 1000 °C at 3.5 GPa, and between 1000 and 1050 °C at 4.5 GPa. The melting experiments yield silicate melts with CO2 contents below 5 wt% and H2O contents below 10 wt%, exhibiting compositional evolution from rhyolitic to dacitic and trachytic as the temperature increases. Combining these results with previous experimental studies on various sediment systems, we conclude that the position of the solidus and melt compositions are influenced by multiple factors, including pressure, bulk composition, and volatile concentrations. Calculations demonstrate that the subduction of carbonate-rich sediment leads to a near-surface recycling efficiency for carbon below 5%, notably lower than the recycling efficiency of water (>75%). This study also indicates that sediment melting in subarc regions is restricted to either hot subduction zones or to subducted slabs at subarc depths exceeding 150 km, which are uncommon among global subduction zones on the modern Earth.However, sediment diapirism may be an efficient way for subducting sedimentary carbon to migrate into source regions of arc magmas. Reaction experiments involving hydrous impure limestone and lherzolite were conducted at 2.5 GPa and 1000 °C, as well as 4.5 GPa and 1100 °C to constrain the melting behavior of carbonate-rich sediment diapirs in the mantle wedge. Silicate melts and a thin websterite reaction layer are generated at 2.5 GPa, while at 4.5 GPa carbonate melts are produced, resulting in the thorough metasomatism of lherzolite. These findings highlight distinct and diverse contributions that carbonate-rich sediment diapirs make to compositional heterogeneity in the mantle wedge at different depths, and also to carbon degassing in magmatic arcs.

Incipient carbonate melting drives metal and sulfur mobilization in the mantle.

We present results from high-pressure, high-temperature experiments that generate incipient carbonate melts at mantle conditions (~90 kilometers depth and temperatures between 750° and 1050°C). We show that these primitive carbonate melts can sequester sulfur in its oxidized form of sulfate, as well as base and precious metals from mantle lithologies of peridotite and pyroxenite. It is proposed that these carbonate sulfur-rich melts may be more widespread than previously thought and that they may play a first-order role in the metallogenic enhancement of localized lithospheric domains. They act as effective agents to dissolve, redistribute, and concentrate metals within discrete domains of the mantle and into shallower regions within Earth, where dynamic physicochemical processes can lead to ore genesis at various crustal depths.

The effects of local variations in conditions on carbon storage and release in the continental mantle.

Recent advances indicate that the amount of carbon released by gradual degassing from the mantle needs to be revised upwards, whereas the carbon supplied by plumes may have been overestimated in the past. Variations in rock types and oxidation state may be very local and exert strong influences on carbon storage and release mechanisms. Deep subduction may be prevented by diapirism in thick sedimentary packages, whereas carbonates in thinner sequences may be subducted. Carbonates stored in the mantle transition zone will melt when they heat up, recognized by coupled stable isotope systems (e.g. Mg, Zn, Ca). There is no single 'mantle oxygen fugacity', particularly in the thermal boundary layer (TBL) and lowermost lithosphere, where very local mixtures of rock types coexist. Carbonate-rich melts from either subduction or melting of the uppermost asthenosphere trap carbon by redox freezing or as carbonate-rich dykes in this zone. Deeply derived, reduced melts may form further diamond reservoirs, recognized as polycrystalline diamonds associated with websteritic silicate minerals. Carbon is released by either edge-driven convection, which tears sections of the TBL and lower lithosphere down so that they melt by a mixture of heating and oxidation, or by lateral advection of solids beneath rifts. Both mechanisms operate at steps in lithosphere thickness and result in carbonate-rich melts, explaining the spatial association of craton edges and carbonate-rich magmatism. High-pressure experiments on individual rock types, and increasingly on reactions between rocks and melts, are fine-tuning our understanding of processes and turning up unexpected results that are not seen in studies of single rocks. Future research should concentrate on elucidating local variations and integrating these with the interpretation of geophysical signals. Global concepts such as average sediment compositions and a uniform mantle oxidation state are not appropriate for small-scale processes; an increased focus on local variations will help to refine carbon budget models.

Stable Zirconium Isotopic Compositions of the Metasomatized Mantle

AbstractMantle metasomatism, which affects the geochemical and lithological properties of the Earth, is crucial to the evolution and modification of the Earth's interior. Variations in chemical proxy of metasomatism, including high‐field strength elements (HFSE, e.g., Zr‐Ti isotopes and isovalent Zr‐Hf), have been commonly utilized as a tool to unravel the processes associated with the evolution of the Earth. However, the impact of mantle metasomatism by diverse agents on the Zr isotopic compositions of metasomatized mantle remains poorly understood. Here we carried out high‐precision double‐spike δ94/90Zr measurements for a suite of basalts, which were derived from the mantle sources metasomatized by typical metasomatic agents of fluid, silicate melt and carbonate melt originating from the deep mantle or recycled crustal materials. Mantle metasomatism results in lithological diversities within the mantle, ranging from hybridized peridotites to pyroxenite/eclogite and carbonated eclogite, which can significantly fractionate the isovalent HFSEs. However, our data show that these basalts have primitive mantle‐like δ94ZrNIST values (−0.033–0.043 ‰), regardless of their superchondritic Zr/Hf. The primitive mantle‐like δ94ZrNIST, irrelevant to any proxy of metasomatism (e.g., Sr‐Nd isotopes, Zr/Hf and La/Sm), indicate that mantle metasomatism and subsequence melting produce no measurable Zr isotopic variations in metasomatized mantle‐derived basalts. The δ94ZrNIST of basalts thus can be representative of the mean metasomatized mantle compositions. Combined with previously reported komatiites and oceanic basalts, we show that the Earth's mantle displays consistent δ94ZrNIST (−0.013 ± 0.053 ‰), even if it has a secular dynamic evolution with the interaction between Earth's exterior and interior.

Interfacial shear strength and durability analysis of carbon fiber/rubber conductive composites for snow melting on bridge decks

ABSTRACT As a conductive material for snow melting on bridge decks, the interfacial shear strength of carbon fiber/rubber conductive composites is a prerequisite for engineering applications. The interfacial shear strength of the carbon fiber/rubber conductive composite was determined. The microstructure after the shear test was analyzed by scanning electron microscope (SEM). The interfacial stability was evaluated base on the extreme temperature on bridge decks. Finally, the interface durability was analyzed through 100 low-temperature cycles, 240 h high temperature and 50 electrification tests. The results show that the interfacial shear strength is 4.32 MPa. SEM results show that the shear force at the interface comes from three aspects. The interfacial shear strength is 13.45% higher than normal at −20°C, and 17.33% lower than normal at 60°C. The interfacial shear strength decreases by 7.48% after 15 cycles of low temperature and then stabilized. The effect of continuous high temperature and multiple energizations on the interfacial shear strength is small. SEM results confirm that low-temperature cycling reduces the internal bonding of rubber. The carbon fiber/rubber composite has good interfacial stability and durability. This study provides a mechanical foundation for the active snow melting of carbon fiber/rubber conductive composites on bridge decks.

Water Speciation and Storage Capacity of Olivine under the Reduced Fluid—Peridotite Interaction

The key features of the interaction between peridotites of the continental lithospheric mantle and reduced hydrocarbon-rich fluids have been studied in experiments conducted at 5.5 GPa and 1200 °C. Under this interaction, the original harzburgite undergoes recrystallization while the composition of the fluid changes from CH4-H2O to H2O-rich with a small amount of CO2. The oxygen fugacity in the experiments varied from the iron-wustite (IW) to enstatite-magnesite-olivine-graphite/diamond (EMOG) buffers. Olivines recrystallized in the interaction between harzburgite and a fluid generated by the decomposition of stearic acid contain inclusions composed of graphite and methane with traces of ethane and hydrogen. The water content of such olivines slightly exceeds that of the original harzburgite. Redox metasomatism, which involves the oxidation of hydrocarbons in the fluid by reaction with magnesite-bearing peridotite, leads to the appearance of additional OH absorption bands in the infrared spectra of olivines. The water content of olivine in this case increases by approximately two times, reaching 160–180 wt. ppm. When hydrocarbons are oxidized by interaction with hematite-bearing peridotite, olivine captures Ca-Mg-Fe carbonates, which are products of carbonate melt quenching. This oxidative metasomatism is characterized by the appearance of specific OH absorption bands and a significant increase in the total water content in olivine of up to 500–600 wt. ppm. These findings contribute to the development of criteria for reconstructing metasomatic transformations in mantle rocks based on the infrared spectra and water content of olivines.

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.

Low-Temperature Molten Salt Electrochemical CO2 Upcycling for Advanced Energy Materials.

One strategy for addressing the climate crisis caused by CO2 emissions is to efficiently convert CO2 to advanced materials suited for green and clean energy technology applications. Porous carbon is widely used as an advanced energy storage material because of its enhanced energy storage capabilities as an anode. Herein, we report electrochemical CO2 upcycling to solid carbon with a controlled microstructure and porosity in a ternary molten carbonate melt at 450 °C. Controlling the electrochemical parameters (voltage, temperature, cathode material) enabled the conversion of CO2 to porous carbon with a tunable morphology and porosity for the first time at such a low temperature. Additionally, a well-controlled morphology and porosity are beneficial for reversible energy storage. In fact, these carbon materials delivered high specific capacity, stable cycling performances, and exceptional rate capability even under extremely fast charging conditions when integrated as an anode in lithium-ion batteries (LIBs). The present approach not only demonstrated efficient upcycling of CO2 into porous carbon suitable for enhanced energy storage but can also contribute to a clean and green energy technology that can reduce carbon emissions to achieve sustainable energy goals.

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.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.

Speciation and dynamical properties of hydrous MgCO3 melt studied by ab-initio molecular dynamics

Understanding the effects of water on the geochemical behavior of carbonate melts in the Earth's interior requires thorough knowledge of their speciation and dynamical properties. However, the mechanisms of water dissolution in carbonate melts are unknown due to experimental challenges. Here we used ab-initio molecular dynamics simulations as a complementary approach to study the speciation and element diffusivities of MgCO3 melt containing 10 wt.% water at conditions up to 1765 K and ∼3.9 GPa. The simulations revealed a diverse COH speciation including CO32−, HCO3−, H2O, OH−, and CO2. The structural analysis was complemented by the calculation of vibrational density of states, and anharmonic infrared and Raman spectra. Good agreement between theoretical Raman spectra and experimental spectra from the literature supports the finding that HCO3− is an important species in hydrous MgCO3 melt. Mean square displacement analysis revealed rapid diffusional motion of hydrogen. However, the contribution of hydrogen to the total ionic conductivity is estimated to be relatively low compared to the already very high conductivity of anhydrous carbonate melts. In addition to fundamental insights into molecular structure and transport properties, our results provide a valuable basis for the development of thermodynamic speciation and solubility models, which are essential for a quantitative description of hydrous carbonate melts in the deep Earth.

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.

Caught in the moment: interaction of immiscible carbonate and sulfide liquids in mafic silicate magma—insights from the Rudniy intrusion (NW Mongolia)

The Devonian Rudniy intrusion is a composite magmatic body comprising two gabbroid units. Located in the Tsagaan-Shuvuut ridge in NW Mongolia, it is the only one known to contain disseminated sulfide Ni-Cu-PGE minerals out of numerous gabbroid intrusions surrounding the Tuva depression. The ore occurs as disseminated sulfide globules made of pyrrhotite, pentlandite, chalcopyrite, and cubanite, confined to a narrow troctolitic layer at the margins of a melanogabbro, at the contact with a previously emplaced leucogabbro. Globules generally display mantle-dominated sulfur isotopic signatures but show variable metallogenic and mineralogical characteristics, as well as notably different sizes and morphologies reflecting variable cooling and crystallization regimes in different parts of the intrusion. Sulfides from the chilled margin of the melanogabbro are surrounded and intergrown with volatile-rich (i.e., CO2-, H2O-, F-, and Cl) phases such as calcite, chlorite, mica, amphibole, and apatite. Based on the mineralogical and textural relationships of volatile-rich phases with sulfides, we argue that this assemblage represents the product of the crystallization of volatile-rich carbonate melt immiscible with both silicate and sulfide liquids. We put forward the hypothesis that volatile-rich carbonate melt envelops sulfide droplets facilitating their transport in magmatic conduits and that this process may be more widespread than commonly thought. The smaller sulfide globules, which are interpreted to derive from the breakup of larger globules during transport and emplacement, do not display an association with volatile-rich phases, suggesting that the original carbonate melt could have been detached from them during the evolution of the magmatic system. Variable rates of crystallization may have been responsible for the observed disparities in the mineralogical and metallogenic characteristics of different sulfide globules entrained in the Rudniy intrusion.

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.

Survived carbonates in orogenic dunites from recycling of subduction-related sediments

The carbon recycling between the Earth's surface and its interior is accomplished through plate subduction and magmatic processes. The orogenic mantle peridotites document multi-stage metasomatism in the subduction zone and play a significant role in revealing the material transportation and migration between the Earth's different spheres. However, how the migration process of carbon-bearing species influenced the subcontinental lithosphere mantle remains a mystery. To address these issues, we conducted detailed petrography, whole-rock and mineral geochemical compositions, and Sr–C–O isotope studies on two types of peridotites (silicate melt pocket-bearing and carbonate melt pocket-bearing dunites) of the Maowu ultramafic massif in the Dabie orogen. The silicate-melt pockets were captured earlier than the carbonate-melt pockets, and were derived from a large proportion of silicate melts during the formation of garnet pyroxenites. The subsequent carbonate metasomatism was mainly divided into two stages: the stage 1 reflects the reaction between dolomite melts and orthopyroxenes to generate clinopyroxenes with high CaO content and Mg# value (> 92); the stage 2 is the injection of Ca-rich fluids into dunites, forming dolomite veins and volatile-rich assemblages of tremolite, barite and monazite. The clinopyroxenes display high 87Sr/86Sr (0.7079–0.7097) and (La/Yb)N ratios, low Ti/Eu ratios, and different enrichment in Th, U, Sr and light rare earth elements (LREEs), indicating a carbonate metasomatic origin. The equilibrium melts of them are similar to sedimentary limestones with positive Sr anomaly and depleted high field strength elements (HFSEs). The Sr-O isotope mixing simulation shows a mixing of 80–90% carbonate sediments with the upper mantle peridotite. The C-O isotopes of the carbonate minerals have similar variation ranges (δ13CV-PDB: 19.7–15.6 ‰, δ18OV-SMOW: 18.1–28.1 ‰) within the range of sedimentary organic carbon. The 87Sr/86Sr ratios of limestone reservoir (0.708–0.709) suggest the addition of sedimentary carbonates into the mantle wedge at ∼370–500 Ma. Therefore, we propose that the sedimentary carbonates and organic carbon migrated into the deep mantle-wedge through the subducted Tethyan-Ocean slab in the Early Paleozoic, which experienced complex metasomatism and formed as carbonate minerals in the Maowu dunites to achieve the carbon recycling between the sedimentary carbon reservoir and the mantle wedge beneath the North China Craton.

Continental Thermal Structure and Carbonate Storage of Subducted Sedimentary Origin Control on Different Increases in Atmospheric CO2 in Late Ediacaran and Jurassic

AbstractCarbon release during continental rifting is thought to regulate atmospheric CO2 levels. Supercontinent dispersal‐induced extensional tectonics is similar during the Late Ediacaran and Jurassic, while they exhibit different increases in atmospheric CO2 concentration. The underlying mechanism of distinct CO2 emissions remains to be understood. Here, we conduct petrological‐thermomechanical modeling to show that metamorphic decarbonation and melting of carbonates that are derived from subducted sediments are ubiquitous during continental extension. We find that the hotter lithosphere and deeper storage of these carbonates cause more significant amounts of rift‐related CO2 release through volcanoes and faults. They may cause ∼12%–77% larger decarbonation efficiency, providing an efficient driving mechanism for a ∼31% larger increase in atmospheric CO2 levels during the Late Ediacaran than throughout the Jurassic. The rapid eruption and deposition of recycled carbonatite volcanic ash may contribute to the production of Late Ediacaran marine carbonates with the largest negative δ13C (−12‰).

Cristobalite Clinker and Paralavas of Ferroan and Melilite–Nepheline Types in the Khamaryn-Khural–Khiid Combustion Methamorphic Complex, East Mongolia: Formation Conditions and Processes

Abstract—Rock samples from the Khamaryn-Khural–Khiid combustion metamorphic (CM) complex, including cristobalite clinker, ferroan tridymite–sekaninaite and cristobalite–fayalite paralavas, which are rock types new to the complex, as well as clinker xenoliths in melilite–nepheline paralava, have been studied in terms of chemistry and mineralogy. The obtained data on rock-forming, minor, accessory, and rare phases (silica polymorphs, cordierite-group minerals, fayalite, Fe and Ti oxides, ferrosilite, etc.) have implications for the formation conditions and processes of the CM rocks. The Raman spectra of sekaninaite, indialite, ferroindialite, mullite, and anhydrous Fe–Ca–Mn phosphate, presumably from the graftonite group, have several specific features. The diversity of mineral assemblages in the CM rocks is due to heterogeneous lithology of the sedimentary protolith and to local effects in the multistage history of the Khamaryn-Khural–Khiid complex. According to geochemical data, all CM rocks of the complex are derived from the Early Cretaceous Dzunbain Formation, their protolith molten to different degrees. The cristobalite clinker and tridymite–sekaninaite and cristobalite–fayalite paralavas were produced by partial melting of pelitic rocks containing different amounts of iron in a wide temperature range. The formation of mullite developed from dehydration–dehydroxylation and incongruent partial melting of amorphous pelitic matter. Large-scale crystallization of mullite in clinker, occurred from the high-silica potassic aluminosilicate melt at &amp;gt;850 °C. Combustion of subsurface coal seams heated the overburden to &amp;gt;1050 °C or locally to &amp;gt;1300–1400 °C (melting point of detrital quartz) or even, possibly, to &amp;gt;1470 °C corresponding to the stability field of β-cristobalite. Melilite–nepheline paralava was formed by incongruent melting of silicate (pelitic) and carbonate (calcite) components of marly limestone under elevated CO2 partial pressure. Oxygen fugacity (fO2) during combustion metamorphism changed from strongly reducing conditions favorable for crystallization of Fe phosphides (barringerite, schreibersite) and metallic iron from silica-undersaturated melts parental to melilite–nepheline paralava to high fO2 values that can maintain the formation of hematite in Fe-rich CM rocks.