Metacarbonate Rocks Research Articles

Sr and C Isotope Chemostratigraphy of the Paleoproterozoic Metacarbonate Rocks of the Sortavala Group: Fennoscandian Shield, Northern Ladoga Area

The first Sr isotope data and the new C isotope signatures have been obtained for the metacarbonate rocks of the Sortavala Group in the Fennoscandian Shield. The metacarbonate rocks of the lower carbonate horizon of this group represent the sediments of shallow partly isolated paleobasins, whereas the carbonate deposits of the upper horizon represent the sediments of the distal part of an extensive marine paleobasin. The initial 87Sr/86Sr ratios of the calcite marbles in the lower carbonate horizon vary from 0.70761 to 0.71015 (Ristiniemi area) and from 0.71274 to 0.71292 (Impilahti area), whereas the same ratios of the marbles in the upper carbonate horizon vary from 0.70482 to 0.70489 (Ruskeala calcite marble), from 0.70463 to 0.70522 (Ruskeala dolomite marble), and from 0.70483 to 0.70493 (Yuven’ Island calcite marble). Samples retaining the initial Paleoproterozoic seawater signatures have been revealed as a result of the geochemical assessment of the retentivity of the isotope systems of the metacarbonate rocks. Substantial differences in δ13С values between the marbles of the lower (from +3.8 to +7.7‰, Impilahti and Ristiniemi) and upper (from –2.2‰ to +2.1‰, Ruskeala and Yuven’) carbonate horizons of the group have been established. The isotope composition of carbon from the marbles of the Sortavala Group is consistent with the deposition of the lower carbonate horizon during the terminal stages of the Jatulian Epoch and the upper horizon during the Ludicovian Epoch. The change in the Th/U ratio from 4.3–7.1 in the metacarbonate rocks of the lower horizon to 0.1–0.32 in the upper carbonate horizon reflects transition from the weakly oxidizing sedimentation environment during the Jatulian to the reducing environment during the Ludicovian. An 87Sr/86Sr isotope signature (from 0.70463 to 0.70493) was for the first time obtained for the Svecofennian Ocean during the Ludicovian time (1.9–2.06 Ga). The increase in the 87Sr/86Sr ratio in the ocean water during the post-Jatulian period reflects the global growth of the continental crust ca. 2 Ga.

Mineral-geochemical characteristics Cu-U-Au-manifestations in the Kon Ra region of the Kon Tum province, Vietnam

Background. Complex Au-Cu-U mineralization is found in the metamorphic rocks of central Vietnam in the zones of their contact with Triassic granite intrusions. It is localized in tectonic breccias and cataclasites, which affected Triassic granitoids and Precambrian metaskarn rocks. The greatest differentiation is observed for tectonically disturbed granites of the Hai Van complex and the metasomatically reworked zones of contacts of this complex with metacarbonate and metaultramafic rocks of the Kham Duc complex.Aim. To determine the mineral and elemental composition of complex occurrences of Cu, U and Au in Proterozoic rocks of the Kon Tum province of central Vietnam.Materials and methods. The work was based on the authors’ data obtained during fieldwork and analytical laboratory studies in the 2016–2019 period. The diagnostics of ore mineral composition was carried out using 60 polished sections. The chemical composition of ores was determined for 214 samples by ICP MS and for 374 samples by atomic absorption spectrometry at the Vietnamese Centre for Geological and Experimental Analysis in the city of Hanoi. The geochemical relationships of ore components were established by the method of multivariate statistical correlations.Results. The mineral composition includes pyrrhotite, pyrite, chalcopyrite, molybdenite, magnetite, martite, hematite, cubanite, marcasite, gold, xenotime, sphalerite, uraninite, chalcocite, covellite, goethite, malachite and limonite. According to the petrographic and petrochemical composition, the geological position and the data on isotopic age, the metamorphic formations of the Kham Duc complex can be considered as Neoproterozoic-Cambrian formations having experienced metamorphism in the Ordovician.Conclusion. A scheme of the sequence of mineral formation was outlined. ICP MS results of primary ores showed that, in addition to Cu, the formations under study contain concentrations of Co, Mo, U, Au, Zn, Ni, V, Y, La, and As. A positive correlation was found between Cu, U and Au. A model of hydrothermal ore formation was proposed.

Garnet-amphibole miaskites of the Ilmenogorsky miaskite massif (Southern Urals): Mineralogy and geochemistry

Research subject. This paper presents original findings about textural-structural, mineralogical, petrological, and geochemical features of the garnet-amphibole miaskites (firstes) of the Ilmenogorsky miaskite massif.Materials and methods. The microprobe analysis of mineral composition was performed using Tescan Vega3 sbu and REMMA202M scanning microscopes equipped with microanalyzers. The content of major, trace and rareearth elements (REE) in rock samples was determined by the methods of AAS and ICP-MS.Results. The garnet-amphibole miaskites under study are characterized by a rare mineral paragenesis, i.e. garnet-amphibole-pyroxene-nepheline-plagioclase. The mafic minerals exhibit a high ferruginosity (f = 70–99), while the accessory minerals have high Al, F and low REE contents. The garnetamphibole miaskites contains high concentrations of Al, Fe3+, Ca, Na, Be, Rb, Mo, Tl and low concentrations of LILE, HFSE, REE and transit elements.Conclusions. According to the garnet composition and its ferruginosity (f = 95– 99), high contents of Al and F in accessory minerals, the prevalence of Fe3+, as well as negative Eu/Eu* and positive Ce/ Ce* anomalies, the garnet-amphibole miaskites under study are assumed to be the product of acid-alkaline metasomatism occurring under the oxidizing conditions of petrogenesis. The low ratios of Cr/V and Ni/Co indicate the immobility of transit elements during metasomatism, and their clarke of concentration corresponds to the content in metaterrigenous and metacarbonate rocks, which suggests crustal substratum for garnet–amphibole miaskites. Garnet-amphibole miaskites are the markers of the interaction of crustal material with deep fluids, which occurred during the stage of shear tectonics development (270–240 Ma) due to the broad permeability of the rocks composing the Ilmenogorsky miaskite massif.

The origin of the serpentinite enclaves in metacarbonates of the Sa Nghia area in the Kon Tum Massif, Central Vietnam

Metacarbonate rocks surrounded by schists, amphibolites and quartzites, occur abundantly in the Sa Nghia – Ho Moong area of Kham Duc Complex in the Kon Tum Massif (Central Vietnam). They contain serpentinite enclaves, which form different bodies such as: sharp-edged rock fragments, lenses, thin layers or small nests often transecting the background of host metacarbonates. The enclaves are mainly built of antigorite. Thin veins crosscutting the serpentinites are commonly filled with microcrystalline carbonates. The relics of olivine and clinopyroxene as well as flaky talc, magnetite, ilmenite, pyrite and zircon occur as accessory components. The serpentinites are slightly enriched in Cs, Th and U, and depleted in Ba, K and Ti. Most probably, such geochemistry could be caused by the influence of fluids derived from primitive tholeiitic melt, which could infiltrate the primary ultramafic rocks before they were hosted by metacarbonates. Another alternative is that the impoverishment in Al, Cr, Ni, Cu, V and Co, together with the presence of numerous grains of carbonates, zircon and pyrite might be indicative for significant secondary metasomatic alterations of pristine ultramafic rocks as the protolith of the serpentinite enclaves.

Genesis of baddeleyite and high δ18O zircon in impure marble from the Tongbai orogen, Central China: insights from petrochronology and Hf–O isotope compositions

Coexistence of baddeleyite and zircon is rarely documented in metacarbonate rocks and poorly understood. In this study, metamorphic baddeleyite and zircon were recognized in marbles from the Tongbai orogen, central China. We carried out high spatial resolution analysis of trace element geochemistry and U‒Pb‒Lu‒Hf‒O isotope systematics for the zircon and baddeleyite. SIMS (Secondary Ion Mass Spectroscopy) U‒Pb analysis yields weighted average 207Pb/206Pb ages of 1848.0 ± 3.3 Ma (MSDW = 1.2, n = 12) for baddeleyite and 1840 ± 13 Ma (MSWD = 1.7, n = 13) for zircon. The baddeleyite grains have relatively low REE (Rare Earth Element) contents (ΣREE = 1.30–47.3 ppm) with flat chondrite-normalized patterns and moderate contents of HFSE (High Field Strength Element), which are ascribed to the nature of its host metacarbonate rock and mineral assemblage. The zircon grains have low Ti contents (1.63–3.56 ppm), positive Ce anomalies (Ce/Ce* = 1.91–6.96), and mineral inclusions of hematite, calcite, dolomite, lizardite and H2O. These characteristics indicate zircon formation in a relatively low-temperature setting under oxidizing and water-rich conditions. The baddeleyite and zircon have similar ranges of eHf(t) values of – 7.7 to – 10.5 and – 7.8 to – 11.6, respectively, which suggest their derivation from a relatively uniform Hf source. The high δ18O values of these zircon grains (20.0–21.0‰) preclude any percolation of external meteoric fluids during their crystallization. Therefore, a “detrital-zircon-dissolution” reaction is preferred to provide the Zr for the metamorphic zircon and baddeleyite in this study. The O isotope disequilibrium between whole-rock (δ18O = 16.2–16.3‰) and metamorphic zircon is most likely caused by metamorphic devolatilization or interaction with an H2O-dominant fluid during later metamorphism. On the other hand, the baddeleyite grains yield δ18O values ranging from 4.5 to 8.7‰, which is in disequilibrium with metamorphic zircon at the temperature of “detrital-zircon-dissolution” reaction (ca. 660–710 °C; Zircon + Dolomite → Baddeleyite + Forsterite + Calcite + CO2). The genesis of metamorphic zircon can be interpreted by a baddeleyite-to-zircon reaction of ZrO2 + SiO2 → ZrSiO4 at a retrograde stage, consistent with the low Ti-in-Zircon temperatures. The results highlight the importance of integrating trace element and multiple isotopic compositions in studying the genesis of metamorphic baddeleyite and zircon. Additionally, the U–Pb geochronology data also indicate that the protoliths of the marble were deposited in the early Paleoproterozoic and experienced a metamorphism at 1.85 Ga. The ca. 1.85 Ga high-grade metamorphic event might have been induced by the coeval magmatism in an extensional setting.

Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones

Subduction of serpentinite-hosted carbonate rocks formed during the oceanic stage in the oceanic lithosphere (ophicalcite) or by metasomatism at the slab mantle-wedge interface (hybrid carbonate–talc rocks) constitute important fluxes of carbon in subduction zones. If hosted in the hydrated mantle lithosphere of the slab, these carbonate-rocks may be preserved from the infiltration of fluids produced by prograde metamorphism of the overlying meta-sediments and the hydrated oceanic crust to subarc depths, where they are fluxed by fluids derived from dehydration of their host serpentinite. Because the dissolution of carbonate in aqueous fluids is enhanced at high P and T, fluid-mediated carbon release at subarc depth is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. This contribution presents thermodynamic modelling —using the implementation of the DEW aqueous database in Perple_X— of prograde devolatilization reactions (solid = fluid + solid) and infiltration-driven devolatilization reactions (external fluid + solid = fluid + solid) of serpentinite-hosted carbonate rocks. In line with previous studies of prograde devolatilization of carbon-bearing oceanic crust, our models show that, in warm and cold subductions zones, the carbon released into fluids produced by prograde devolatilization reactions of meta-ophicalcite and carbonate-talc rocks is limited even if electrolytic complexing in the fluid phase is taken into account. Therefore, serpentinite-hosted meta-carbonate rocks are likely to be preserved to subarc depths, where they undergo infiltration-driven devolatilization by fluids derived from Atg-serpentinite dehydration. At the P–T conditions of Atg-breakdown in cold to warm subduction zones, our models of infiltration-driven devolatilization of meta-ophicalcite and carbonate-talc rocks indicate that carbon release is accompanied by significant Ca loss, particularly at high pressures (P>3.5 GPa), due to the high concentration of CaHCO+3 in the fluid. The carbon solubility in fluids equilibrated with meta-ophicalcite and carbonate–talc rocks is markedly higher than for pure aragonite. Mass balance considerations combined with the parameterization of the P–T conditions of Atg-serpentinite dehydration as a function of the thermal parameter of subduction zones allow us to investigate the carbon loss caused by infiltration of serpentinite dehydration fluids. Unlike the dissolution of pure CaCO3, carbonate dissolution in serpentinite-hosted meta-carbonate rocks is highest at the slab surface of warm subduction zones and lowest at Moho depths of cold subduction zones, where Atg-serpentinite dehydration occurs at greater depths. Carbon in subducted meta-ophicalcite —with thicknesses of up to 70 m typically found in oceanic ophicalcite— will be preserved beyond the subarc depths of cold and hot subduction zones, and recycled into the deep mantle as carbonate-garnet-clinopyroxene-olivine rocks. At the subarc depths of most subduction zone regimes, infiltration-driven devolatilization of subducted carbonate-talc rocks is a highly efficient decarbonation mechanism transforming these rocks into carbon-free orthopyroxenite.

Pre-UHP titanite archives pro- and retrograde episodes of fluid-marble-interaction (Dabie Shan UHP unit, China)

Deeply subducted crustal carbonate rocks are major transporters of carbon into the asthenosphere. However, mechanism and extent of the interaction of carbonate rocks at mantle depth with external fluids remain unclear. Carbonate interaction with external aqueous fluids derived from silicate rocks is of particular importance, because the associated decarbonation and dissolution reactions (i) influence the input into the subduction carbon cycle, (ii) influence rheology, and (iii) control isotope fingerprints of the interacting rocks, including the mantle.We investigate meta-carbonate rocks from the Dabie UHP unit, where marbles within a several km2 exposure show 87Sr/86Sr isotope signatures as low as 0.7037, which are unusual for crustal carbonates, but typical for mantle rocks. We focus on a sample with a large pre-UHP titanite, crystallized in a vein, with crustal 87Sr/86Sr signatures (87Sr/86Sri: 0.7071–0.7075) and its fluid-mediated replacement. The titanite survived subduction to mantle depth and was altered only at its margins during subduction and exhumation. This fluid-mediated replacement is characterized by (i) very low, mantle-like 87Sr/86Sr signatures (as low as 0.7045), both in the titanite reaction rim and the marble matrix, and (ii) significantly higher 87Sr/86Sr signatures (ca. 0.7085) related to allanite replacing titanite along discrete veins during retrogression.Combining mineral-scale Sr-isotope analyses with microfabric and mineral-chemical data of titanite, white mica, and calcite, allows for establishing five episodes (A to E) of fluid-mineral interaction linked with the pro- and retrograde reaction history: (A) prograde episode of interaction with aqueous fluid sourced from dehydration reactions in the impure marble or from subducting crustal rocks; titanite crystallized from fluid in a vein and acted as a sink for Ti, REE and Sr from the protolith, (B) fluid-interaction resulted in the replacement of titanite by rutile in the rim of the prograde titanite, ductile deformation lead to shape-preferred orientation of mineral relicts of the HP-UHP stage, (C) infiltration of a mantle-derived fluid is recorded by the back – reaction to titanite, whose 87Sr/86Sr signatures reflect mixing between the fluid from dissolved prograde titanite and the incoming fluid, (D) the continued interaction of the mantle-derived fluid with the marble resulted in pervasive replacement and mantle-like 87Sr/86Sr signatures of the minerals in the matrix, and (E) infiltration of a fluid sourced from crustal rocks or from metasomatized mantle fragments along grain boundaries and fractures at the retrograde episode E related to emplacement in the middle crust. We show that dissolution-precipitation reactions efficiently supported material transport and isotope chemical exchange between titanite and its meta-carbonate host rock, depicting Sr isotope mixing associated with dissolution and precipitation. Deformation was not a controlling factor. The fluid-mineral interaction episodes are related to map-scale metasomatism and the change of the isotope fingerprint of the marbles.

Metasomatic flow of metacarbonate-derived fluids carrying isotopically heavy boron in continental subduction zones: Insights from tourmaline-bearing ultra-high pressure eclogites and veins (Dabie terrane, eastern China)

Abstract Tourmaline, one of the most important hosts of boron (B) in crustal rocks, has rarely been found in natural high-pressure (HP) and ultra-HP (UHP) eclogites. Here, we report the first finding of tourmaline-bearing UHP eclogites and veins in the Dabie terrane, China. These distinctive samples occur exclusively in impure UHP marbles. Investigations on the eclogite-vein-marble system provide important insights into the origin, flow, and metasomatic effects of eclogite-facies, 11B-rich fluids in continental subduction zones. Petrologic and geochemical evidence indicates that tourmaline in the eclogites (Tur-E) formed by metasomatism due to the infiltration of a B-rich fluid under conditions of ca. 2.2–2.6 GPa and 610–660 °C. The HP veins, containing euhedral tourmaline (Tur-V) and occurring at the eclogite-marble contacts or in the interiors of eclogite lenses, represent the crystallized products of the infiltrating fluid. Systematic compositional variations along the profile from the interiors of the eclogite lenses to their margins and mass-balance calculations indicate large inputs of B, carbon, large ion lithophile elements, and light rare earth elements to the eclogites during fluid infiltration. All types of tourmaline, including the Tur-E and Tur-V as well as minor amounts of tourmaline in the marbles (Tur-M), have similar compositions (dravitic) with XMg values [= Mg/(Mg + Fe)] of 0.7–0.8. In situ analyses using laser ablation multicollector inductively coupled plasma mass spectrometry show that all of tourmaline has high δ11B values (ranging from +6 to +15‰), which suggest a 11B-rich isotope signature for the infiltrating fluid. The tourmaline B isotope data, together with the detailed field observations and whole-rock Sr-Nd isotopes, reveal that the infiltrating fluid was derived from the impure marbles. Influx of such fluid was highly channelized and was mainly achieved along lithologic boundaries between UHP marbles and eclogites or along fractures in the eclogites. This study indicates that impure marbles are an important reservoir of isotopically heavy B in deeply subducted continental slabs. This point may be of particular importance because most B reservoirs in subducted continental crust typically have light B isotopic compositions. Our results highlight that subducted metacarbonate rocks could liberate eclogite-facies, B-rich, high-δ11B fluids and thus might exert important effects on the cycle of B and its isotopes in subduction zones.

Metacarbonatite rocks from Amesmessa area (In Ouzzal Terrane), Hoggar shield, Algeria

The Hoggar Shield (South Algeria) belongs to the 3,000 km-long Pan-African Trans-Saharan Belt which formed in the Neoproterozoic and Cambrian, between 750 and 545 Ma. At the western part of this shield, carbonate rocks crop out to the north and south of Amesmessa gold deposit. They occur as lensoid bodies and dikes cross-cutting Archean granulitic rocks, and are consistently associated with mafic rocks and syenite. The petrogenesis of the Amesmessa metacarbonate rocks, which have previously been interpreted as marbles of Archean age, are reconsidered in the present study. Within the Amesmessa area, a small volume of carbonate rocks consisting of calcite, biotite, clinopyroxene, hornblende, olivine and apatite, and are spatially associated with ultramafic rocks, are surrounded by granosyenite, pyroxenite and rhyolite. The cross-cutting relationship and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Archean rocks. The average total REE concentration in the metacarbonate is up to 5,700 ppm. However the associated rocks; syenite and pyroxenite contain only 60 and 12 ppm, respectively. Sr concentration is also high (>1,390 ppm) which is normally high in carbonatitic rocks and low in limestones. On the basis of available chemical analyses and outcrop evidence, it becomes clear that the Amesmessa carbonate rocks consist of metamorphosed carbonatite.

Carbonate-related metallic and non-metallic mineralization within and proximal to granites (Fichtelgebirge Pluton, Germany): “Mantle-crust marker mineralization”

Metacarbonate rocks (marble, calcsilicate rock, skarn) hosting strata- and structurebound Fe-, As-Sb, Bi-, Ni-Co, Cu-Pb-Zn, U-, W- ore minerals as well as talc, clay minerals, barite and fluorite are widespread in the country rocks south of the collisional calc-alkaline felsic to intermediate intrusive rocks of the Fichtelgebirge Pluton, Germany. At its western rim, thrustbound and vein-type mineral assemblages with Au-As-Sb and F-Ba minerals associated with carbonate gangue minerals developed. Only recently, a calcite-hosted Sb mineralization was encountered in a deep-seated lineamentary fault zone cutting through metaultrabasic rocks along the northern edge of the granites. The same structure zone forms the loci of calcite-bearing U episyenite situated within this pluton. A composite geological, mineralogical and chemical (major and trace elements, REE, C- and O isotopes) study has been conducted to distinguish the heat and element source in subcrustal or deep-seated crustal areas and in the largely exposed granite complex. Only Sn-W skarn deposits are genetically related to the highly fractionated granitic members of the Variscan pluton. Pegmatite skarn has a strong subcrustal component as to the heat source and the provenance of rare elements and a moderate crustal one as far as the silicates are concerned. Deep-seated fault zones were active over a rather long period of time and acted as conduits venting magmas and hydrothermal fluids from the waning stages of the Variscan deformation through the Neogene. Calcsilicate and carbonate mineral assemblages are an efficacious tool to constrain the physico-chemical regime in this mineral province, covering the temperature range from 745 °C down to 53 °C in a medium to low pressure regime at strongly varying redox conditions. The major and trace elements, the REE variation, the Ce and Eu anomalies as well as carbon and oxygen isotopes of the various mineral assemblages enable us to identify the fluid sources and depict the element concentration processes, e.g., mixing of fluids, connate fluid and meteoric fluid interaction. From the economic point of view, the mineralizing system is most prospective for rare element deposits, talc, kaolinite-group minerals, iron, and uranium. Accumulations of fluorite and barite are subeconomic in mineral assemblages inside as well as outside the granites, while base metals and precious metals are only of mineralogical interest.

Zircon U-Pb age constraints for Tonian-early Cryogenian deposition of metasedimentary rocks from the Schirmacher Oasis, East Antarctica: Implications for correlations across the Mozambique Ocean

Schirmacher Oasis, central Dronning Maud Land, East Antarctica, a dispersed fragment of the 660-610 Ma East African Orogen (EAO), exposes platformal-facies high-grade metasedimentary rocks. Here we constrain the depositional age of these sedimentary rocks to test models relating to timing of the closure of the Mozambique Ocean, the southern segment of the EAO, since Tonian time. Zircon, a proxy for detritus, was dated by LA-MC-ICPMS (206Pb/238U ages) that includes both magmatic and (older) metamorphic populations, in addition to those formed during the granulite-facies metamorphic events (660-600 Ma and 580-555 Ma) having an age range between 1240 and 500 Ma. In conjunction with field evidence, their depositional time is constrained to the Neoproterozoic (Tonian to early Cryogenian, i.e. 1000 >Tdep >800 Ma and 930 >Tdep>800 Ma) with their provenance likely dominated by metamorphic (orogenic) over magmatic rocks.These metacarbonate and psammitic rocks are correlated with similar rocks from the Montepuetz Complex, Northeastern Mozambique, metacarbonate rocks from the Sør Rondane Mountains and with the Linta Group, Vohibory Domain, Southeastern Madagascar. The coeval deposition of platformal sedimentary protoliths, interpreted as occurrence of multiple Mozambique Oceanic strands, were incorporated into the continental arc crust during collision between the Congo and Azania cratonic blocks forming the EAO.

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

AbstractThe Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (>780–810°C) and [=CO2/(CO2+H2O) >0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (<620°C) and (<0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.

Infiltration-driven metamorphism, New England, USA: Regional CO2 fluxes and implications for Devonian climate and extinctions

We undertake thermodynamic pseudosection modeling of metacarbonate rocks in the Wepawaug Schist, Connecticut, USA, and examine the implications for CO2 outgassing from collisional orogenic belts. Two broad types of pseudosections are calculated: (1) a fully closed-system model with no fluid infiltration and (2) a fluid-buffered model including an H2O–CO2 fluid of a fixed composition. This fluid-buffered model is used to approximate a system open to infiltration by a water-bearing fluid. In all cases the fully closed-system model fails to reproduce the observed major mineral zones, mineral compositions, reaction temperatures, and fluid compositions. The fluid-infiltrated models, on the other hand, successfully reproduce these observations when the XCO2 of the fluid is in the range ∼0.05 to ∼0.15. Fluid-infiltrated models predict significant progressive CO2 loss, peaking at ∼50% decarbonation at amphibolite facies. The closed-system models dramatically underestimate the degree of decarbonation, predicting only ∼15% CO2 loss at peak conditions, and, remarkably, <1% CO2 loss below ∼600 °C. We propagate the results of fluid-infiltrated pseudosections to determine an areal CO2 flux for the Wepawaug Schist. This yields ∼1012 mol CO2 km−2 Myr−1, consistent with multiple independent estimates of the metamorphic CO2 flux, and comparable in magnitude to fluxes from mid-ocean ridges and volcanic arcs. Extrapolating to the area of the Acadian orogenic belt, we suggest that metamorphic CO2 degassing is a plausible driver of global warming, sea level rise, and, perhaps, extinction in the mid- to late-Devonian.

Element mobility during regional metamorphism in crustal and subduction zone environments with a focus on the rare earth elements (REE)

This paper explores bulk-rock geochemical data for a wide array of metamorphosed mafic, quartzo-feldspathic, pelitic, and metacarbonate rocks using a quantitative mass-balance approach to assess fluid-driven element mobility—particularly of the rare earth elements (REE)—in regional metamorphic and some high-pressure subduction zone environments (40 examples; over 240 individual analyses). Most examples are from focused fluid flow settings, such as veins and lithologic contacts, where fluxes are large and metasomatic signals are thus strong. A variety of REE behaviors are observed, including little or no REE mobility (roughly a third of the data set); light REE (LREE), mid-REE (MREE), and/or heavy (HREE) mobility; europium “anomalies”; overall REE losses; and local REE redistribution. The REE are typically fractionated by mass transfer, with the exception of several examples that underwent fairly uniform overall losses of REE. The fractionation reflects strong mineralogical controls on REE uptake/loss by a comparatively small number of phases. Examples include: HREE mass changes associated with garnet, xenotime, and sphene; LREE and MREE changes associated with apatite, monazite, and allanite; and Eu changes associated with plagioclase and lawsonite. As mineralogy is a dominant control, the nature of the metasomatism is not strongly correlated with metamorphic grade, other than obvious mineralogical differences between settings (e.g., plagioclase in Barrovian metamorphism, lawsonite in subduction zones). Extensive mobilization of non-REE major and trace elements can happen without significant open-system transport of the REE. If REE mobility occurs, it is always accompanied by mobilization of other non-REE. When mobile, neighboring REE (e.g., Sm and Nd) typically have strongly correlated mass changes indicating that both were mobilized to about the same degree. Although individual examples of metasomatism can show correlations between patterns of mass transfer for the REE and the non-REE, little such correlation is evident across the entire data set, with the exception of P. Once again, this highlights the importance of individual minerals in controlling REE systematics. Broad correlations of REE and P mobility suggest REE transport by P complexes, or REE and P transport together by some other complexing agent. Mass changes for REE and Y are more strongly coupled, reflecting the geochemical similarity of these elements and perhaps indicating a role for Y complexing as well. The REE are the most mobile of the high field strength elements (HFSE). On a percentage mass basis, the amount of HFSE mobility decreases roughly in the order: REE > U > Nb > Ti > Th ~ Zr. Th mobility is rare but when present is positively correlated with U and REE mobility. The mobility of the more refractory HFSE is low in aqueous fluids, but is a larger concern in more extreme environments such as magmatic/magmatic-hydrothermal systems, charnockite metamorphism, and supercritical fluids in high-pressure/ultrahigh-pressure settings. The mobility of certain HFSE (e.g., Nb) can be large enough to affect rock plotting positions on petrotectonic discrimination diagrams. Potassium and related (Rb, Ba, Cs) large-ion lithophile elements (LILE) are typically lost in association with mica breakdown. As the data set focuses on high fluid flow environments, LILE changes will likely be smaller for rocks that undergo less fluid-rock interaction. Gains of K and related LILE are coupled to volatile gain and Na loss, consistent with fluid flow down temperature gradients, or infiltration from K-rich sources (e.g., pegmatites). In Barrovian settings, two other mass transfer trends are evident in addition to K mass transfer. First, silica loss is coupled to volatile loss, illustrating the dependence of silica mass transfer on devolatilization. Most of the silica loss reflects local mass transfer into adjacent vein fluid flow conduits were additional silica is precipitated; indeed, wallrock inclusions in veins can be highly silicified. Second, Na and Ca mass gains and losses are well correlated in the Barrovian examples, reflecting control of plagioclase growth or destruction. Strontium and Pb behaviors also appear to be largely related to plagioclase behavior (in subduction settings, this role can be played by phases such as lawsonite or epidote group minerals). Carbon dioxide mass transfer by conventional devolatilization as well as near-stoichiometric CaCO 3 dissolution or precipitation is represented in the data set. Determining the relative roles of these processes as functions of metamorphic grade, intensity of fluid-rock interaction, and tectonic setting will be an important challenge for future research.

Compositional Appraisal and Quality Implication of Emiworo Metacarbonate Deposit in Central Nigeria

ABSTRACT Metacarbonate deposit occurring in Emiworo area, northcentral Nigeria are low grade marble associated with Proterozoic basement complex rocks. Integrated inorganic geochemical, physical and thermal characteristics of the samples were investigated with the aim of revealing their economic viability. The phase characteristics of the metacarbonate show that dolomite is the dominant phase with calcite and phlogopite as minor phases, while silicates, graphites and pyrites constitute the accessories. Reactivity of the quicklime measured after calcination at different temperatures revealed that only the quicklime produced in 1 hour at 950°C and 1000°C was found to be reactive. Physical properties and comparative data show that the Emiworo marble displays lower water absorption and higher compressive strength, loss on ignition and bulk specific gravity, compared to other metacarbonate rocks occurring in other parts of Nigeria and elsewhere. These attributes makes the deposit suitable for many conventional industrial applications.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures.

Biological activity is a major factor in Earth's chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth's evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

Is the Lomagundi Event present on the Rae craton? A case study from the Murmac Bay Group

Shortly after the initial rise of atmospheric oxygen in the Paleoproterozoic Era, a major perturbation occurred in the global carbon cycle, which is manifested as a long-lived positive carbon isotope excursion recorded in ∼2.22–2.06 Ga carbonate rocks, known as the Lomagundi Event. Beyond its significance for evolving seawater composition, this geochemical event can be used as an indirect age marker in Paleoproterozoic sedimentary successions. Documenting further occurrences of this event in other Paleoproterozoic carbonate rocks confirms that the event was global and reflects ambient seawater composition. This event, however, has only been documented in two successions in Canada, despite the ubiquity of Paleoproterozoic-aged rocks on the Canadian Shield. Our study focuses on metacarbonate rocks from the Paleoproterozoic-aged Murmac Bay Group on the southwestern margin of the Rae craton in northern Saskatchewan. Measured δ13C values (up to 7.8‰) fall within the range of the Lomagundi Event, but most values are relatively low, suggesting CO2 loss (decarbonation) altered the δ13C values. Stable carbon isotope data coupled with major element geochemical data allowed us to account for the degree to which the values have changed due to metamorphic CO2 loss. We also compare δ13C values from micro-drilled (dolomite and calcite phases) with whole-rock δ13C values from the same samples to characterize the δ13C composition of pre-metamorphic carbonate minerals. Both measured and corrected δ13C values place the Murmac Bay Group metacarbonates within the range of values that characterize the Lomagundi Event and indicate, for the first time, the presence of the Lomagundi Event on the Rae craton.

Pb-Sr-O-C isotope compositions of metacarbonate rocks of the Derbina Formation (East Sayan): Chemostratigraphic and geochronological significance

The Pb-Sr-O-C isotope compositions of calcite marbles of the Derbina Formation, exposed in the northwestern part of the Derbina block of the East Sayan, were studied. Rocks of the Derbina Formation were metamorphosed under high-temperature amphibolite facies conditions. The carbonate constituent of marbles contains (ppm) 15–130 Mn, 130–160 Fe, 0.008–0.039 Rb, 645–2190 Sr, 0.565–0.894 U, and 0.288–1.42 Pb. These concentrations are similar to those in modern carbonate sediments. The values of δ13C in marbles of the Derbina Formation range from–0.6 to +1.4‰ PDB; the values of δ18O range from 21.5 to 28.6‰ SMOW. The 87Sr/86Sr ratio values in the two least altered rocks, which meet geochemical criteria of the Rb-Sr system preservation in high-grade carbonate rocks, are 0.70804 and 0.70829. The protolith ages of marbles determined using Sr and C chemostratigraphy lie within the interval of 560–530 Ma, which is regarded as the period of carbonate sedimentation. The slope of the straight line on the 206Pb/204Pb–207Pb/204Pb diagram (n = 9, MSWD = 19) constructed on the basis of the data points of bulk carbonate constituents of all samples studied and those representing leachate steps of one of them in 0.5N HBr yields Late Vendian age (556 ± 31 (2σ) Ma. Taking into account the data on Sr and C isotope systematics of Derbina marbles, this age is regarded as the age of early diagenesis of carbonate sediments close to the age of sedimentation. Thus, metacarbonate rocks of the Derbina Formation preserved the pre-metamorphic chemostratigraphic and isotope-geochronological information. The age obtained testifies that formation of the carbonate cover of the Derbina block occurred in the Late Vendian. At the end of the Cambrian, carbonate deposits were metamorphosed during the Early Caledonian tectonic event in the southeastern part of the Central Asian Fold Belt.

Sr and Nd Isotopic evidence in metacarbonate rocks for an extinct Island arc–ocean system in East Antarctica

Sr and Nd Isotopic evidence in metacarbonate rocks for an extinct Island arc–ocean system in East Antarctica

Mayenite-supergroup minerals from burned dump of the Chelyabinsk Coal Basin

Three minerals of the mayenite supergroup have been found in fluorellestadite-bearing metacarbonate rock (former fragment of petrified wood of ankeritic composition) from the dump at the Baturinskaya-Vostochnaya-1-2 mine. These are eltyubyuite Ca12Fe1°Si4O32Cl6, its fluorine analog Ca12Fe103+Si4O30F10, and chlormayenite-wadalite Ca12(Al,Fe)14O32Cl2-Ca12(Al,Fe)10Si4O32Cl2. The first two phases occur in the reaction mantle around hematite, magnesioferrite, and Ca-ferrite aggregates (“calciohexaferrite” CaFe12O19, “grandiferrite” CaFe4O7, and “dorrite phase” Ca2(Fe53+Mn00.5Mg0.5)(Si0.5Fe5.53+)O20) and, rarely, as individuals in grained aggregates of fluorellestadite-cuspidine (± lar- nite±rusinovite Ca10(Si2O7)3Cl2). Assemblages of zoned chlormayenite-wadalite crystals are found in grained aggregates of fluorellestadite- cuspidine, which lack Ca-ferrite. Also, harmunite CaFe2O4, chlorellestadite, fluorapatite, anhydrite, rondorfite CasMg(SiO4)4Cl2, fluorine analog of rondorfite CasMg(SiO4)4F2, “Mg-cuspidine” Ca3.5(Mg,Fe)0.5(Si2O7)F2, fluorite, barioferrite BaFe12O19, zhangpeishanite BaFCl, and other rare phases are identified in this rock. Data on the chemical composition and Raman spectroscopy of the mayenite-supergroup minerals are given. The genesis of metacarbonate rock is considered in detail: “oxidizing calcination” of Ca-Fe-carbonates with the formation of hematite and lime; reaction between hematite and lime with the formation of different Ca-ferrites; formation of larnite as a result of reaction between SiO2 and lime or CaCO3; and reactionary impact of hot Cl-F-S-bearing gases on early assemblages. Eltyubyuite and its fluorine analog crystallized at the stages of gas impact. It is presumed that the maximum temperature during the formation of rock reached 1200–1230°C. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.