Fate of carbonates within oceanic plates subducted to the lower mantle, and a possible mechanism of diamond formation

We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO₃ (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO₃ (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO₂ + perovskite via reactions with SiO₂. Magnesite is not the only host phase for subducted carbon—solid CO₂ also carries carbon in the lower mantle. Furthermore, CO₂ itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.     

Description of an aerodynamic levitation apparatus with applications in Earth sciences

Background  

In aerodynamic levitation, solids and liquids are floated in a vertical gas stream. In combination with CO₂-laser heating, containerless melting at high temperature of oxides and silicates is possible. We apply aerodynamic levitation to bulk rocks in preparation for microchemical analyses, and for evaporation and reduction experiments.     

Conditions of Formation of Iron–Carbon Melt Inclusions in Garnet and Orthopyroxene under <Emphasis Type="Italic">P-T</Emphasis> Conditions of Lithospheric Mantle

Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe⁰- or Fe₃C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300–1500°С) in the carbide–oxide–carbonate systems (Fe₃C–SiO₂–(Mg,Ca)CO₃ and Fe₃C–SiO₂–Al₂O₃–(Mg,Ca)CO₃) are reported. Conditions of formation of mantle silicates with metallic or metal–carbon melt inclusions are determined and their stability in the presence of CO₂-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO₂-fluid are formed in the carbide–oxide–carbonate system through decarbonation, with subsequent redox interaction between CO₂ and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe–C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO₂-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe–C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.     

Determining the composition of C–H–O liquids following high-pressure and high-temperature diamond-trap experiments

Here, we present the first analytical technique (the quartz tube system technique—QTS) to directly analyze H₂O and CO₂ contents in liquids following high-pressure, high-temperature experiments in capsules containing mantle minerals and a diamond layer serving as a fluid/melt trap. In this technique, the capsule is frozen prior to opening; the diamond trap is cut out of the capsule and placed inside a N₂-filled quartz tube. The diamond trap is heated up to 900 °C to release the gases to an Infrared Gas Analyzer, which determines the CO₂ and H₂O contents. Three sets of experiments containing SiO₂ and CaCO₃ powders were performed at 6 GPa and 1,000 °C in order to calibrate and validate the technique. These experiments demonstrated that when samples are prepared in a N₂ environment, CO₂ and H₂O can be directly measured with an accuracy and precision of 2–3 and 3–4 %, respectively. The QTS technique (for H₂O and CO₂ determination) together with the cryogenic technique (total dissolved solids content) can be applied to diamond-trap capsules following HP–HT experiments in order to provide direct and complete liquid compositions coexisting with mantle material. The principal advantage of the QTS technique of direct analysis of volatile content in liquids over the indirect approach of mass balance calculations is the possibility of studying carbonated and hydrous liquid compositions in equilibrium with mantle material regardless of chemistry and pressure–temperature experimental conditions.     

Study on Al2O3 content and phase stability of aluminous-CaSiO3 perovskite at high pressure and temperature

 In order to clarify Al₂O₃ content and phase stability of aluminous CaSiO₃-perovskite, high-pressure and high-temperature transformations of Ca₃Al₂Si₃O₁₂ garnet (grossular) were studied using a MA8-type high-pressure apparatus combined with synchrotron radiation. Recovered samples were examined by analytical transmission electron microscopy. At pressures of 23–25 GPa and temperatures of 1000–1600 K, grossular garnet decomposed into a mixture of aluminum-bearing Ca-perovskite and corundum, although a metastable perovskite with grossular composition was formed when the heating duration was not long enough at 1000 K. On release of pressure, this aluminum-bearing CaSiO₃-perovskite transformed to the “LiNbO₃-type phase” and/or amorphous phase depending on its Al₂O₃ content. The structure of this LiNbO₃-type phase is very similar to that of LiNbO₃ but is not identical. CaSiO₃-perovskite with 8 to 25 mol% Al₂O₃ was quenched to alternating lamellae of amorphous layer and LiNbO₃-type phase. On the other hand, a quenched product from CaSiO₃-perovskite with less than 6 mol% consisted only of amorphous phase. Most of the inconsistencies amongst previous studies could be explained by the formation of perovskite with grossular composition, amorphous phase, and the LiNbO₃-type phase. Received: 11 April 2001 / Accepted: 5 July 2002     

Raman spectroscopic study of PbCO3 at high pressures and temperatures

Cerussite (PbCO₃) has been investigated by high-pressure and high-temperature Raman spectroscopy up to pressures of 17.2 GPa and temperatures of 723 K. Two pressure induced phase transitions were observed at about 8.0(2) and 16.0(2) GPa, respectively. The post-aragonite transition (PbCO₃-II) at 8.0(2) GPa is accompanied by softening of the v ₂-out-of-plane mode of the CO ₃ ^2? group and disappearance of the B_1g (v ₄-in-plane band of the CO ₃ ^2? group) mode. Stronger shifts of the carbonate group modes after the phase transition suggest that the new structure is more compressible. The formation of a second high-pressure polymorph begins at about 10 GPa. It is accompanied by the occurrence of three new bands at different pressures and splitting of the v ₁-symmetric C–O stretching mode of the CO ₃ ^2? group. The transitions are reversible on pressure release. A semi-quantitative phase diagram for PbCO₃ as a function of pressure and temperature is proposed.     

An ultraviolet laser microprobe for the in situ analysis of multisulfur isotopes and its use in measuring Archean sulfur isotope mass-independent anomalies

A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO₂ laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO₂ laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ³⁴S and δ³³S compositions of reference materials measured with a CO₂ laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency [Ding et al 2001] and [Taylor]. The precision of replicate analyses of powdered sulfide minerals with the CO₂ laser is typically ±0.2‰ (1σ) for δ³⁴S.The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO₂ laser. In situ laser craters and drill hole powders give the same δ³⁴S_V-CDT and δ³³S_V-CDT values within 0.2‰. The δ³⁴S_V-CDT and δ³³S_V-CDT values of both powders and in situ analyses are independent of F₂ gas pressure over a range of 15 to 65 torr. No dependence of δ³⁴S_V-CDT and δ³³S_V-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios.A range of Δ³³S anomalies of from - 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.     

CO2, 13C/12C and H2O variability in natural basaltic glasses: a study comparing stepped heating and ftir spectroscopic techniques

A comparison of two independent techniques was used to assess the homogeneity of CO₂ and H₂O concentrations in a number of natural basaltic glasses. Variations in carbon concentration and isotopic ratio were determined by comparison of stepped heating data obtained in two different laboratories. Dissolved volatile concentrations were also obtained by stepped heating and Fourier Transform Infrared (FTIR) spectroscopy. Replicate stepped heating analyses of a mid-ocean ridge basaltic glass show that the concentration and ¹³C/¹²C of bulk magmatic and dissolved CO₂ vary by less than ±10% and ±0.5‰, respectively. A similar degree of correlation is observed for replicate stepped heating analyses of Mariana Trough glasses conducted in two different laboratories. Dissolved CO₂ concentrations determined by stepped heating also correlate well with concentrations measured by FTIR spectroscopy. The correspondence of results obtained in these experiments provide an upper limit to the degree of natural variation in concentrations and isotopic ratios of these volatiles in basaltic glasses and suggest that intrinsic, magmatic carbon has a relatively homogeneous distribution in these glasses. Water concentrations determined through extraction by heating and FTIR also show excellent agreement.     

Fluid-assisted granulite metamorphism: A continental journey

Lower crustal granulites, which constitute the base of all continents, belong to two series: high-pressure granulites generated by crustal thickening (subduction) and (ultra)high-temperature granulites associated with crustal extension. Fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low-water activity fluids (high-density CO₂ and brines), which have invaded the lower crust at peak metamorphic conditions (fluid-assisted granulite metamorphism). High-pressure and (ultra)high-temperature granulites commonly occur along elongated paired belts. They were formed, from the early Proterozoic onwards, during a small number of active periods lasting a few hundreds of m.y. These periods were separated from each other by longer periods of stability. Each period ended with the formation of a supercontinent whose amalgamation coincided with low- to medium pressure (ultra)high-temperature granulite metamorphism, immediately before continental break-up. It is proposed that large quantities of mantle-derived CO₂ stored in the lower crust at the final stage of supercontinent amalgamation, are released into the hydro- and atmosphere during breakup of the supercontinent. Fluid-assisted granulite metamorphism, therefore, appears to be an important mechanism for transferring deep mantle fluids towards the Earth's surface. Possible consequences were, for example, the sudden end of Proterozoic glaciations, as well as the post-Cambrian explosion of life.     

Melting of clinopyroxene + magnesite in iron-bearing planetary mantles and implications for the Earth and Mars

The assemblage clinopyroxene + magnesite was observed in Earth’s high-pressure metamorphic samples, and its stability in subducting slabs was confirmed by experiments. Recent studies also suggested that the fO₂ variations observed in SNC meteorites can be explained by polybaric graphite-CO-CO₂ equilibria in the Martian mantle. Although there is no direct evidence for the stability of the cpx + mc assemblage in Mars mantle, its high-pressure–high-temperature decomposition to cpx + fo + CO₂ makes it a good analogue for the source of carbon metasomatism, in particular, to study nakhlites formation. Iron, which is present in the Earth’s and Martian mantles, may, however, influence the speciation of carbon. We performed experiments on a clinopyroxene + magnesite assemblage at 1.8 and 3.0 GPa and temperatures corresponding to the Earth’s and Martian mantles. The role of iron and of fO₂ was investigated by (1) replacing all or part of the magnesite by siderite FeCO₃, (2) adding Fe⁰ and (3) using graphite C capsules. A carbonate-silicate melt forms at both Earth and Mars conditions. Clinopyroxene and olivine are the main solid phases in the iron-free experiments. Fe²⁺ and Fe⁰ decrease their melting temperatures and increase the silicate fraction in the melt. The produced carbonate-silicate melts may be involved in the formation of some carbon-rich lavas on Earth (e.g., carbonatites, ultramafic lamprophyres, or kamafugites). Our results may also be used to interpret ophiolite samples or inclusions. In particular, we show that wüstite may form in equilibrium with carbonate-silicate melt in opx-(and silica-) poor regions of the mantle below 3 GPa. Our results also confirm the hypothesis of carbon metasomatism in the Martian nakhlites source. Immiscibility or reduction could explain the absence or rarity of C in Martian lavas.     

Physical and numerical simulations of the presence of a forsterite transitional zone at high temperature and pressure: insight for scCO2 geological storage

Geological sequestration is one of the most effective ways to reduce greenhouse gases, such as carbon dioxide (CO₂). The deep oceanic crust dominated by ultrabasic rock could store CO₂ permanently. However, the storage mechanism has not been thoroughly understood because of the limited amount of research and experiments conducted. This study explored the reactive mechanisms of water–rock–gas in an ultrabasic system under different conditions. Forsterite, the most dominant mineral found in ultrabasic reservoirs, was used to conduct laboratory physical simulation experiments. Two experimental systems were designed including an scCO₂–forsterite–water system and an N₂–forsterite–water system. All experiments were performed for 1000 h at an experimental temperature of 150°C and a pressure of 150 bar, respectively, to mimic the geological conditions. The liquid products from the experiments were analysed by inductively coupled plasma-optical emission spectrometry, whereas the solid samples were analysed by scanning electron microscopy with energy disperse spectroscopy. Results showed that: (1) in the early stage during scCO₂/N₂–forsterite–water interaction, forsterite was dissolved with a reactive transitional zone forming on the surface, which caused H⁺ to enter into the silicate framework and accelerated the reaction; (2) in the N₂ system, the dissolution of forsterite was inhibited by the Mg²⁺ concentration after reaching its saturation in the late stage; and (3) in the scCO₂ system, magnesite was precipitated as a secondary mineral during the late stage, which promoted the dissolution of forsterite. As a result, the degree of dissolution of forsterite in the scCO₂ system was far higher than in the N₂ system. The experimental results are consistent with the numerical simulation using TOUGHREACT, a geochemical simulation procedure, which showed that CO₂ promotes the dissolution of forsterite greatly at high temperature and pressure.     

Determination of the second critical end point in silicate-H2O systems using high-pressure and high-temperature X-ray radiography

To determine the second critical end point in silicate-H₂O systems, a new method for the direct observations of immiscible fluids has been developed using a synchrotron X-ray radiography technique. High-pressure and high-temperature experiments were carried out with a Kawai-type, double-stage, multi-anvil high-pressure apparatus (SPEED-1500) installed at BL04B1, SPring-8, Japan. The Sr-plagioclase (SrAl₂Si₂O₈)-H₂O system was used as an illustrative example. A new sample container composed of a metal (Pt) tube with a pair of lids, made of single crystal diamonds, was used under pressures between 3.0 and 4.3 GPa, and temperatures up to ∼1600°C. The sample in the container could be directly observed through the diamond lids with X-ray radiography. At around 980 to 1060°C and pressures between 3.0 and 4.0 GPa, light gray spherical bubbles moving upward through the dark gray matrix were observed. The light gray spheres that absorb less X-rays represent an aqueous fluid, whereas the dark gray matrix represents a silicate melt. These two immiscible phases (aqueous fluid and silicate melt) were observed up to 4.0 GPa. At 4.3 GPa, no bubbles were observed. These observations suggest that the second critical end point in the Sr-plagioclase-H₂O system occurs at around 4.2 ± 0.2 GPa and 1020 ± 50°C. Our new technique can be applied to the direct observations of various systems with two coexisting fluids under deep mantle conditions.     

一种高精度台式旋转超声岩石取心装置

将野外采集或实验室合成的岩石样品制备成小尺寸柱状岩心样品,是进行地质学岩石高温高压物性实验的重要环节.岩石样本的脆硬性和成分不均匀性严重影响常规岩石磨削取心加工过程的稳定性和取心质量.将取样工具的旋转超声振动与岩石磨削加工过程相结合,并通过气动系统实现岩石样本在竖直方向上的柔性进给,所研制的台式旋转超声岩石取心装置可实现实验室小尺寸柱状岩心样品的高精度自动磨削取心加工过程.多种不同地质材料的取心测试结果表明,该装置能进行不同硬度岩石样品的高效率和高质量小直径取心加工,满足地质学高温高压实验的高标准制样需求.      

Experimental investigation of single carbon compounds under hydrothermal conditions

The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present-day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO₂ + H₂ = CO + H₂O). Aqueous fluids containing added CO₂, CO, HCOOH, NaHCO₃, NaHCOO, and H₂ were heated at 150, 200, and 300 °C and 350 bar in flexible-cell hydrothermal apparatus, and the abundances of carbon compounds was monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO₂ to CH₃OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH₂O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH₃OH to CH₄ allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable thermodynamic equilibrium. Reaction of CO₂ to CO involves a two-step process in which CO₂ initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 × 10⁶ s⁻¹ and an activation energy of 102 kJ/mol were retrieved from the experimental results for the oxidation of CO to CO₂. Reaction rates amongst single carbon compounds during the experiments suggest that ΣCO₂ (CO₂ + HCO₃⁻ + CO₃²⁻), CO, ΣHCOOH (HCOOH + HCOO⁻), and CH₃OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH₃OH, and the consumption of H₂. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.     

C�CO�CH�CS fluids and granitic magma: how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200?MPa

Hydrothermal volatile-solubility and partitioning experiments were conducted with fluid-saturated haplogranitic melt, H₂O, CO₂, and S in an internally heated pressure vessel at 900°C and 200?MPa; three additional experiments were conducted with iron-bearing melt. The run-product glasses were analyzed by electron microprobe, FTIR, and SIMS; and they contain ??0.12 wt% S, ??0.097 wt% CO₂, and ??6.4 wt% H₂O. Apparent values of log f _O2 for the experiments at run conditions were computed from the [(S⁶⁺)/(S⁶⁺+S^2?)] ratio of the glasses, and they range from NNO ?0.4 to NNO?+?1.4. The C?CO?CH?CS fluid compositions at run conditions were computed by mass balance, and they contained 22?C99?mol% H₂O, 0?C78?mol% CO₂, 0?C12?mol% S, and <3 wt% alkalis. Eight S-free experiments were conducted to determine the H₂O and CO₂ concentrations of melt and fluid compositions and to compare them with prior experimental results for C?CO?CH fluid-saturated rhyolite melt, and the agreement is excellent. Sulfur partitions very strongly in favor of fluid in all experiments, and the presence of S modifies the fluid compositions, and hence, the CO₂ solubilities in coexisting felsic melt. The square of the mole fraction of H₂O in melt increases in a linear fashion, from 0.05 to 0.25, with the H₂O concentration of the fluid. The mole fraction of CO₂ in melt increases linearly, from 0.0003 to 0.0045, with the CO₂ concentration of C?CO?CH?CS fluids. Interestingly, the CO₂ concentration in melts, involving relatively reduced runs (log f _O2????NNO?+?0.3) that contain 2.5?C7?mol% S in the fluid, decreases significantly with increasing S in the system. This response to the changing fluid composition causes the H₂O and CO₂ solubility curve for C?CO?CH?CS fluid-saturated haplogranitic melts at 200?MPa to shift to values near that modeled for C?CO?CH fluid-saturated, S-free rhyolite melt at 150?MPa. The concentration of S in haplogranitic melt increases in a linear fashion with increasing S in C?CO?CH?CS fluids, but these data show significant dispersion that likely reflects the strong influence of f _O2 on S speciation in melt and fluid. Importantly, the partitioning of S between fluid and melt does not vary with the (H₂O/H₂O?+?CO₂) ratio of the fluid. The fluid-melt partition coefficients for H₂O, CO₂, and S and the atomic (C/S) ratios of the run-product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the H, S, and C contents of pre-eruptive magmatic fluids and volcanic gases for subduction-related magmatic systems thus confirming our experiments are relevant to natural eruptive systems.     

Stability of dense hydrous magnesium silicate phases in the systems Mg2SiO4-H2O and MgSiO3-H2O at pressures up to 27 GPa

We conducted high-pressure phase equilibrium experiments in the systems MgSiO₃ with 15 wt% H₂O and Mg₂SiO₄ with 5 wt% and 11 wt% H₂O at 20 ∼ 27 GPa. Based on the phase relations in these systems, together with the previous works on the related systems, we have clarified the stability relations of dense hydrous magnesium silicates in the system MgO-SiO₂-H₂O in the pressure range from 10 to 27 GPa. The results show that the stability field of phase G, which is identical to phase D and phase F, expands with increasing water contents. Water stored in serpentine in the descending cold slabs is transported into depths greater than 200 km, where serpentine decomposes to a mixture of phase A, enstatite, and fluid. Reaction sequences of the hydrous phases which appear at higher pressures vary with water content. In the slabs with a water content less than about 2 wt%, phase A carries water to a depth of 450 km. Hydrous wadsleyite, hydrous ringwoodite, and ilmenite are the main water reservoirs in the transition zone from 450 to 660 km. Superhydrous phase B is the water reservoir in the uppermost part of the lower mantle from 670 to 800 km, whereas phase G appears in the lower mantle only at depths greater than 800 km. In cold slabs with local water enrichment greater than 2 wt%, the following hydrous phases appear with increasing depths; phase A to 450 km, phase A and phase G from 450 km to 550 km, brucite, superhydrous phase B, and phase G from 550 km to 800 km, and phase G at depths greater than 800 km. Received: 4 August 1999 / Accepted: 1 March 2000     

Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite (α-Al(OH3)): CO2 abundance and δC values

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO₂ and H₂O. “Plateau” F (CO₂/H₂O ratios) and δ¹³C values of the CO₂ derived from gibbsite were attained over the dehydration interval, X_v(H₂) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ¹³C value of evolved CO₂ was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of ¹³C-depleted CO₂ (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ¹³C for CO₂ evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO₂ occluded in gibbsite may preserve information on δ¹³C values of CO₂ in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO₂ occluded in gibbsite and CO₂ in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems.     

Analysis of carbon and oxygen stable isotopes in carbonate rocks by the laser micro-sampling technique

The analysis of stable isotopes of carbon and oxygen in different carbonate rocks by the phosphoric acid method is not easier than that by the laser sampling method developed in recent years, which optically focuses laser beams with sufficient energy on a micro area of a thin section in a vacuum sample box via microscope. CO 2 produced by heating decomposition of carbonate was purified by the vacuum system, and the stable isotopic values of carbon and oxygen were calculated and analyzed on a mass spectrometer. This paper adopted the laser micro-sampling technique to analyze the stable isotopes of carbon and oxygen in dolomite, carbonate cement, stromatolite and different forms of dawsonite (donbassite). Results indicated that the laser micro-sampling method is effective in analyzing carbonate composition and could be a convincing proof for justification on carbonate composition analysis.     

Fluid characteristics of retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture zone in southern India

Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet–clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P–T conditions of 19 kbar and 1,010°C. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite–high pressure granulite suite. The results suggest that the early fluids were a mixture of CO₂, CH₄ and N₂ probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO₂ composition with moderate to high densities (up to 1.154 g/cm³). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968–1.154 g/cm³) and Kanji Malai (1.092–1.116 g/cm³). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO₂-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO₂-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite–high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO₂-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction–collision process is in favor of a model involving the derivation of CO₂ from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas.