Decomposition Of Serpentine Research Articles

Impact of Solid Hydrocarbon on the Composition of Fluid Phase at the Subduction (Experimental Simulation)

Experiments conducted in the olivine–serpentine–anthracene–metal (FeNi) system have shown that the recrystallization of olivines occurs under substantially reduced conditions with active participation of hydrocarbons, especially paraffins; moreover, their amount increases with increasing pressure and temperature. During the decomposition of serpentine, a large amount of water is released; therefore, the fluid at relatively low P-T parameters (2 GPa, 1100 °C) has mainly water–hydrocarbon composition. With an increase in pressure up to 3–4.5 GPa and temperature up to 1300–1400 °C, the composition of the fluid changes greatly towards an increase in the relative amount of hydrocarbons, while the main share is occupied by light (C1–C4) aliphatic hydrocarbons. Therefore, a biogenic material with a carbon–hydrogen composition can make a certain contribution to the carbon budget in subduction processes when falling into the subduction zones and may affect the oxygen fugitivity in the subducted slab.

Spectral and mineralogical effects of heating on CM chondrite and related asteroids

Several carbonaceous chondrites (CCs) display evidence of aqueous and thermal alteration. However, the process of thermal alteration is not fully understood. To investigate the spectral variations caused by thermal alteration, we heated powders of CM2 CCs Murchison and Jbilet Winselwan, as well as a simulant Murchison mixture (WMM) and its end members. Heating was conducted up to 1200 °C, in 100 °C increments under a purified nitrogen environment. We also compared the findings of our study with results of previous heating experiments conducted on CCs to better understand the effect differing conditions have on the spectral properties observed. Formation of Fe3+ oxyhydroxides and the decomposition of serpentine due to heating are confirmed by both reflectance and X-ray diffraction (XRD) data. Fe3+ oxyhydroxides features such as a steep slope in between 350 to ∼700 nm, and an ∼850 nm feature can be seen starting at ∼300 and 400 °C, respectively. The serpentine-associated features start to decompose at ∼700 °C and disappear by ∼900 °C. Spectra >1000 °C are generally dark and featureless and above this temperature, mafic silicate absorption bands begin to appear. Our results show that heating-induced spectral variations are evident, and the nature of these changes depends on various parameters including temperature, experimental conditions, duration of heating, sample grain size, as well as mineralogical changes accompanying heating, and heterogeneity between CCs.

Mineralogy of the 2019 Aguas Zarcas (CM2) carbonaceous chondrite meteorite fall

AbstractThe 2019 Aguas Zarcas CM2 meteorite is the most significant carbonaceous chondrite CM2 fall since Murchison in 1969. Samples collected immediately following the fall and studied here provide the rare opportunity to analyze the bulk mineralogy of a CM2 largely free of terrestrial contamination. Bulk samples were analyzed by powder X-ray diffraction (XRD), thermal gravimetric (TG) analysis, evolved gas analysis (EGA), and scanning electron microscopy (SEM) with an electron-probe micro-analyzer (EPMA). Water-extracted salts were analyzed by XRD. In hand specimen, the stones are brecciated and dominated by chondrule-rich and chondrule-poor lithologies, and locally, a matrix-rich lithology. Powder XRD patterns from multiple stones are dominated by reflections from serpentine group minerals, on which are superimposed reflections for ferrotochilinite, 1:1 regularly interstratified ferrotochilinite/cronstedtite, anhydrous silicates, calcite, pentlandite, pyrrhotite, and minor phases. Reflections for magnetite are present only from a metal-rich breccia clast. The serpentine XRD reflections from the chondrule-rich and chondrule-poor lithologies match those from 1T cronstedtite, whereas those from the matrix-rich lithology match the 1M polytype. Patterns with the 1M polytype also show a distinct low-angle scattering to the serpentine basal reflection centered near 8.6 Å, the origin of which is obscure. Further matching of the known serpentines to the Aguas Zarcas data shows that cronstedtite accounts for a subordinate amount of the clays, and at least three other chemically and structurally distinct serpentines are likely present. A typical fragment of Aguas Zarcas yielded 0.6 wt% water-extractable salts. The powder XRD pattern of the dried water extract shows reflections for halite = NaCl; chlorartinite = Mg2(CO3)(OH)Cl·2H2O; thenardite = Na2SO4; and sodium chlorate = NaClO4. The TG mass losses of 11.4 to 14.7 wt% are consistent with other CM2 chondrites. The gases detected by EGA are dominated by H2O and CO2, largely derived from the dehydroxylation and decomposition of serpentine and calcite, respectively. Also detected are gases with masses matching SO2/S2 and H2S, which are primarily released below 480 °C, and a mass of 30, which matches the molecular weight of formaldehyde and ethane, shows a maximum at 376 °C. These organic gases likely derive from the pyrolysis of indigenous organic matter. Taken together, the millimeter-scale mineralogical study of Aguas Zarcas reveals a complex breccia dominated by CM2-like clasts. The detailed study of this meteorite, together with similar studies from a range of carbonaceous chondrites, provides the foundations for studying and interpreting the samples returned from the NASA OSIRIS-REx and JAXA Hayabusa2 missions.

Carbonation of heat-activated serpentine driven by Microcystis aeruginosa PCC7806

The capture and storage of CO2 has been a critical strategy towards coping with global warming in recent years. Mineral carbonation is gaining much attention as a novel method of CO2 storage owing to its stability and safety. In this study, the mechanism and effect of carbonation of heat-activated serpentine [Mg3Si2O5(OH)4] driven by cyanobacteria were explored, taking Microcystis aeruginosa PCC7806 as an example. The serpentine was heated at different temperatures, and thermal decomposition of serpentine was characterized by simultaneous thermogravimetry and differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and scanning electron microscope (SEM). The pH, ion concentrations, mineral composition and surface morphology in the process of interactions between the algae cells and heat-activated serpentine were analyzed. The results show that the serpentine can be transformed to forsterite after heat treatment at over 614 °C, and the medium for the addition of heat-activated serpentine at over 614 °C did not allow algae growth. The algae grew well in BG11 liquid medium with 400 °C heat-activated serpentine, and the pH of the culture liquid reached 10.2–10.3, which is in favor of the carbonation of serpentine. The analysis results show the carbonation of trace serpentine. Moreover, the carbonation effect of 400 °C heat-activated serpentine driven by the algae in a solid medium was the most significant, and the carbonate content of serpentine increased by 75.21% as detected by chemical analysis. XRD, SEM, and energy dispersive spectrometer (EDS) results indicate that amorphous CaMg(CO3)2 and MgCO3 were formed on the surface of serpentine driven by the algae. This study advances our understanding of the biological mechanism of serpentine carbonation, and provides a new way to fully exploit serpentine resources in carbon sequestration.

Kinetics of Magnesium Removal from Garnierite by Carbothermal Reduction in Vacuum

The removal of magnesium from garnierite in Yuanjiang area of Yunnan was performed by carbothermal reduction in vacuum. The effects of reduction temperature and reduction time on the removal rate of magnesium were investigated. The kinetics of the removal of magnesium by carbothermal reduction in vacuum was studied. The thermodynamic calculation results show that it is feasible to remove magnesium from garnierite by carbothermal reduction in vacuum. The experimental results show that the removal rate of magnesium in garnierite can reach 93.23% under the conditions of 1823K for 120min. The reduction process conforms to the chemical reaction kinetics model, which indicated that the reduction process is controlled by chemical reaction and whose expression is 1-(1-α)1/3=(-22850.1/T+2.6296) t, the apparent activation energy (Ea) and the pre-exponential factor (A) are 189.97 kJ/mol and 13.87 s-1, respectively. The results of XRD and SEM analysis show that the condensate obtained by carbothermal reduction in vacuum of the garnierite is magnesium, which is mainly obtained by the reduction reaction between magnesium silicate produced by the decomposition of serpentine in minerals and coal. At the same time, it is proved that it is feasible to directly extract magnesium metal from the garnierite.

Characterization and evaluation of magnesite ore mining by-products of Gerakini mines (Chalkidiki, N. Greece)

The qualitative and quantitative characterization of several mining by-product samples, were collected from the magnesite mine of “Grecian Magnesite SA” company (Gerakini, Chalkidiki, North Greece), was aiming to evaluate the possibility of upgrading their refractory properties by applying thermal treatment. The concentration range of main components for the selected best qualified samples was 38.7–43 wt% for MgO, 37.5–44.1 wt% for SiO2 and 6.5–7.3 wt% for FeO. The mineralogical characterization revealed the presence of olivine, pyroxenes and serpentine, with the concentration of the latter positively correlated to LOI. Microprobe analyses clarified the presence of olivine [(Mg1.79Fe0.19Ni0.01)SiO4], consisting mainly of 90 wt% of forsterite (Mg2SiO4) and 10 wt% of fayalite (Fe2SiO4), as well as of pyroxene group minerals [(Mg0.87Fe0.08Ca0.01Cr0.01)(Si0.98Al0.04)O3], consisting mainly of 91 wt% enstatite (MgSiO3) and 9 wt% of ferrosilite (FeSiO3), respectively. The thermal treatment of the qualified samples demonstrated that at the temperature of 650–680 °C serpentine is almost totally decomposed and at the temperature of 850 °C it has been totally recrystallized to olivine and pyroxenes. At higher temperature treatment (1300 °C), it seems that there is a recrystallization process that favors the deformation of olivine and the further formation of pyroxenes, due to the excess of Si available from the initial decomposition of serpentine, while the presence of magnesite resulted to the restriction of olivine deformation through the partial capture of available Si. For increasing the olivine percentage and, subsequently, the improvement of refractory properties of this material, at temperature > 1300°C, the ideal theoretical addition dose of wt% MgO for optimizing the formation of olivine was calculated, ranging from 7.4 to 17.5 wt%. The latter calculations are reported for the first time in the literature regarding this kind of materials.

Peculiarities of Serpentine Decomposition in the Presence of Alkaline Chloride: Raman Study at High Pressure and Temperature

In situ Raman spectroscopy combined with a resistively heated diamond anvil cell was used to study the behavior of serpentine (chrysotile) Mg3Si2O5(OH)4 in the presence of NaCl at 100–400°С and 10 kbar. Low H2O activity leads to a decrease in the dehydration temperature by about 200°C compared to a salt-free system, as well as the formation of only anhydrous products.

Experimental crystallization of a subcalcic Cr-rich pyrope in the presence of REE-bearing carbonatite

This paper focuses on formation of subcalcic Cr-rich garnet (up to 14.25 wt% Cr2O3) in the model ultramafic system corresponding to natural harzburgite with the presence of REE-bearing fluid phase. The experiments were carried out using a “split-sphere” type multi-anvil high-pressure apparatus (BARS) at a pressure of 5 GPa and a temperature of 1300 °C. Natural serpentine, chromite, corundum and REE-carbonatite were used as starting components. Crystallization of garnet occurred in subsolidus conditions by the reaction of orthopyroxene and spinel in the presence of fluid phase. Composition of fluid was controlled by interaction of water released by decomposition of serpentine with carbonate. By using different amounts of carbonatite (0.5 and 1.5 wt%) as a source of calcium and REE, subcalcic Cr-rich garnets with up to 3.5 wt% CaO were crystallized, which are typical for inclusions of harzburgitic paragenesis in natural diamonds. The experiments demonstrated that the rare earth elements (REE) released from the initial carbonatite were transported by the fluid and were incorporated into the newly formed garnet. The distribution of REE in garnet revealed a vivid enrichment toward the heavy REE (HREE), showing the pattern with a very steep slope. These results confirmed high partitioning of HREE into garnet. The present study indicates that the mantle carbonatites, which contain very high proportions of light REE (LREE) to HREE, can play an important role as source material in formation of REE-rich fluids to crystallize garnets with typical REE patterns in mantle peridotites.

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Formation of A-type granites in the Lower Yangtze River Belt: A perspective from apatite geochemistry

Apatite is ubiquitous in A-type granites, and can be used to elucidate the volatile contents of the silicate melt, which reflect its source characteristics. A-type granites have been recognized as a distinct group of granites. A1- and A2-type subgroups are produced under different extensional settings. However, the details of the mechanisms behind the distinctive geochemical characteristics of A1- and A2-type granites remain obscure. Belts of Cretaceous A1- and A2-type granites occur along the Lower Yangtze River Belt in eastern China. Here we investigated the major and trace element compositions of apatites from contemporary A1- and A2-type granites at different localities along the Lower Yangtze River Belt, in order to decipher their discrepant source processes. Apatites from A1- and A2-type granites show similar major and trace elements, but differ in their F and Cl concentrations. Apatites from A1-type granites in the eastern part of the Lower Yangtze River Belt have much lower F and higher Cl concentrations compared to A2-type granites in the western part. Moreover, from the east to the west, the F concentrations of apatites from A1-type granites increase, while the Cl concentrations decline. In a subducted plate, F is retained by amphibole, chlorite, serpentine and mica minerals through the amphibolite stage, and finally by phengite and lawsonite during the eclogite stage, whereas, Cl is controlled by amphibole, chlorite and serpentine. The high and varied Cl concentrations in A1 subgroup apatites, therefore, may be attributed to the breakdown of amphibole, chlorite and/or serpentine decomposition during partial melting of subducted oceanic crust releasing a large amount of Cl at shallower depth. In contrast, F is transported to deeper depths in the subducted oceanic crust, and released through breakdown of phengite and lawsonite, making an important contribution to the formation of A2-type granites. Apatites from A1- and A2-type granite samples show regular changes in LREE/HREE, LREE/MREE and MREE/HREE ratios with increasing distance from the location of the subduction zone, probably as the result of nonsynchronous dehydration of the serpentine and phengite at different stages during subduction. We propose that A1- and A2-type granites on the Lower Yangtze River Belt were derived from sources metasomatised by fluids originating from the breakdown of amphibole, chlorite and/or serpentine with higher Cl, lower F, and from phengite and/or lawsonite with relatively higher F but lower Cl, respectively.

Pulse-laser irradiation experiments of Murchison CM2 chondrite for reproducing space weathering on C-type asteroids

We performed pulse-laser irradiation experiments of a primitive meteorite to simulate space weathering by micrometeorite bombardments on C-type asteroids. Pellets of powdered Murchison CM2 chondrite were set in vacuum and exposed to pulse laser with a diameter of 0.5mm and delivered energies of 5, 10, and 15mJ. We measured reflectance spectra of unirradiated and irradiated surfaces of the pellets. During analysis the pellet was heated to approximately 100°C and purged in N2 gas in order to reduce absorption of ambient water. The spectra become darker and bluer with increasing laser energies. Their UV reflectance increases and 0.7- and 3-μm band depths decrease from 0 to 15mJ. The spectral bluing observed in our experiments reproduces the bluing occurred during space weathering of C-type asteroids. High-resolution observation by a transmission electron microscope showed that the laser heating causes preferential melting and evaporation in FeS-rich fine-grained portions, which results in dispersion and deposition of numerous FeS-rich amorphous silicate particles 20–1000nm in size on the surface of the pellet. In addition, at the laser-irradiated but unmelted areas, heat-induced amorphization and decomposition of serpentine occur. These mineralogical changes make the reflectance spectra of the Murchison CM chondrite darker and bluer.

The mechanism of decomposition of serpentines from peridotites on heating

Serpentines from peridotites of San Jose and New Idria (California, USA) were previously heat treated ranging from 400 to 1,260 °C, and then leached by using an original approach for acid processing of dehydrated serpentines. This approach is capable of releasing from the dehydrated serpentines ortho-[SiO4]4−, di-[Si2O7]6−, and other silicate anions, and moving them into solution in the form of soluble silicic acids. The discovery of these anions has led to the idea of the existence of differences in electronic configuration of Si–O bonds in siloxane bridges and different amount and allocation of ortho-[SiO4]4− and meta- [(SiO3)2−]n silicate anions in serpentines silicate layers thus providing a new deeper insight into the mechanism of the temperature-induced decomposition of the serpentine silicate structure. It should be emphasized that in spite of a great number of studies devoted to the temperature-induced dehydroxylation and recrystallization processes of serpentine minerals, the mechanism of serpentine decomposition is still poorly understood because no one has studied particularities of the structural organization of silicate layers in serpentines. The effect of thermal treatment at different temperatures ranging from 400 to 1,260 °C for 2 h on the above mentioned serpentinite samples was characterized by thermal analysis, X-ray diffraction, and chemical analysis. The observed evidence made possible a comprehensive modeling of the relevant dehydroxylation, high-temperature crystallization, and recrystallization reactions. Complemented by chemical analysis data, the results obtained allows understanding the basic principles of the serpentine decomposition and crystallization processes, which govern the formation of stable high-temperature products like forsterite, enstatite, and protoenstatite, as well as the formation of different amount of silicic acids in solution via the new approach. These studies are of great interest and value to the pure and applied material sciences connected with serpentinites.

The Study on the Mineral Phase Transformations and Thermodynamics of Laterite Ores Calcination

In this paper, mineralogical phase transformations of Indonesia nickel laterite ores are studied during calcinations, the thermodynamics of process are also investigated. Different factors that affect the reduction of the laterite ores are tested and analyzed by Differential Scanning Calorimeter (DSC), X-ray Diffraction (XRD), Brunauer Emmett Teller (BET) and Scanning Electron Microscope (SEM). The results show that during calcination process, goethite is dehydrolyzed by removing structural water at about 300°C, which then transforms to hematite, and serpentine decomposes at around 600~700°C, leading to formulate olivine if agglomerating at higher temperature. By thermodynamics calculation, the goethite dehydration temperature is under 100°C, and about 200°C for serpentine decomposition. It might be the inclusions in ore which make laterite become hard to decompose.

Thermal activation of serpentine prior to acid leaching

The overall process for producing pure magnesium compounds from serpentinite usually starts with acid leaching. However, if serpentinite is calcined prior to leaching, not only faster magnesium dissolution is achieved, but use of thermally activated serpentine also significantly reduces the problems relating to corrosion of the leaching equipment, because less aggressive leaching agents can be used and/or lower leaching temperatures and pressures can be applied. This paper compares how calcination influences the dissolution behaviour of magnesium in solutions of hydrochloric acid, acetic acid and ammonium chloride. Fine-grained serpentinite, characterized by the extent of serpentine decomposition between 85 and 95%, displayed the highest reactivity. The initial magnesium dissolution rate of calcined serpentinite was up to 30-, 125- and 165-times higher as compared to that of uncalcined serpentinite in solutions of hydrochloric acid, acetic acid and ammonium chloride, respectively, under identical reaction conditions.

The conservation of an aqueous fluid in inclusions in minerals and their interstices at high pressures and temperatures during the decomposition of antigorite

An experimental study of serpentine decomposition at high pressure (4.5 GPa) was carried out to elucidate if water can be preserved in the system in the form other than structural admixtures in minerals. This problem is of interest because it is water that plays a leading role in the melting in a subducted slab and a mantle wedge. To estimate the possible content of an aqueous fluid in deep-seated rocks, a BARS pressless split-sphere apparatus was used in complex with thermobarogeochemistry and gas chromatography. It has been established that the serpentine decomposition is accompanied by the release of water, which concentrates in inclusions in the produced minerals (olivine and orthopyroxene) and their interstices. Chromatographic analysis with a stepwise heating of samples showed that most of the released water is localized in the interstices, and the rest is conserved in fluid inclusions in the minerals. The produced solid phases conserve 0.13 to 2.43 wt.% fluids as inclusions, with water amounting to 0.1–2.06 wt.%. The content of inclusions determined by microscopic examination falls in this region. Since the mobility of the fluid conserved as inclusions in the olivine and orthopyroxene is significantly lower than that in the interstices, this fluid might be better preserved in olivine-containing rocks subsided to depth.

Effect of Oxalic Acid on Heat Pretreatment for Serpentine Carbonation

A novel pretreatment method for the mineral serpentine was proposed to develop an effective carbonation process for CO2 sequestration. Basically, the method involved preheating a mixture of oxalic acid and serpentine and the subsequent aqueous carbonation. The addition of oxalic acid was found to stimulate the decomposition of serpentine during heat treatment. X-ray diffraction analysis revealed that serpentine is transformed into magnesium oxalate and magnesium oxide by heat treatments at 200°C and 500°C, respectively. The addition of oxalic acid was found to enhance the overall carbonation reaction owing to the formation of magnesium oxide during heat treatment. Thermogravimetric analysis showed that approximately 70% of the Mg2+ was transformed into magnesium oxide. Using this method, a carbonation rate as high as 72% could be obtained by aqueous carbonation at 100°C under 4 MPa. This novel method can potentially reduce the high energy cost and unavoidable use of expensive chemicals for pH control.

Thermal decomposition of serpentine during coseismic faulting: Nanostructures and mineral reactions

This paper reports a detailed characterization of an antigorite-bearing serpentinite, deformed at seismic slip-rate (1.1 m/s) in a high-velocity friction apparatus. Micro/nanostructural investigation of the slip zone (200 μm thick) revealed a zonal arrangement, with a close juxtaposition of horizons with significantly different strength, respectively consisting of amorphous to poorly-crystalline phases (with bulk anhydrous composition close to starting antigorite) and of highly-crystalline assemblages of forsterite and disordered enstatite (200 nm in size and in polygonal-like nanotextures). The slip zone also hosts micro/nanometre sized Cr-magnetite grains, aligned at low angle with respect to the slipping surface and inherited from the host serpentinite. Overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820–1200 °C (in agreement with flash temperature theory), responsible for serpentine complete dehydration and amorphization, followed by crystallization of forsterite and enstatite (under post-deformation, static conditions). The results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite-hosted faults through the seismic cycle.

日本列島直下で現在進行中の広域変成作用

Recent advances in seismic studies have revealed that earthquakes have a close link with chemical processes, i.e. metamorphic dehydration. From this point of view, we provide a new scheme for observation of on-going regional metamorphism in a subduction zone.Combining the phase diagrams of MORB + water and peridotite + water with the thermal structure of the descending Pacific oceanic plate in NE Japan and in the Philippine Sea (PHS) plate in SW Japan, we can draw the distribution of metamorphic facies of regional metamorphism. However, the most uncertain parameter is thermal structure, even though it has been calculated numerically, because of the difficulty of evaluating frictional heating, heat transportation by dehydrated fluids and mantle convection in the hanging wall. To overcome this problem, we have carried out different approach from seismic observations in estimating the thermal structure of a subduction zone, by applying the dehydration-induced earthquake hypothesis. This hypothesis involves the assumptions as follows : 1) any dehydration in the subducted slab induces earthquakes, 2) peridotite of the subducting plate is more or less hydrated, as well as the oceanic crust, and 3) the dehydration reactions proceed in near equilibrium condition.Direct seismic determination of the depths of the blueschist or epidote-amphibolite facies to eclogite transformation, decomposition of serpentine (antigorite), and the stability limit of clinochlore enable us to establish fixed points for the slab temperature. The seismogenic zone (150°C to 350°C), the depth limit of non-volcanic tremor seismicity, and the slab melting in SW Japan (800-900°C) were also used to fix temperatures at given depths. Three profiles in NE-Japan and two profiles in SW-Japan were examined, and their P-T paths along the Wadati-Benioff zone were estimated to be anti-clockwise in all cases. The P-T paths are consistent with those of metamorphic facies series from well-studied on-land regional metamorphic belts. The P-T path of the subducting slab in NE-Japan is colder than that of the eastern-Shikoku section in SW-Japan, except for the Kii-peninsula section which has an almost similar P-T path to that in NE-Japan.Comparison between the on-going metamorphism beneath the Japanese islands and the on-land regional metamorphic belts in the Sanbagawa and Kokchetav shows that the P-T conditions of these two metamorphic belts are located between that of NE-Japan and of the eastern-Shikoku profile.A numerical model for wedge-mantle convection shows that the direction of the small corner flow of the wedge mantle causes a back current along the subducting slab. The area of the corner flow is wider in a shallow subduction zone corresponding to that in eastern Shikoku, and is narrower in a steeper subduction zone represented by NE-Japan. Since the exhumed metamorphic belts have intermediate P-T conditions between those in NE Japan and eastern Shikoku, We suggest that a change in the mode of wedge-corner flow from steep to shallow subduction plays some role in the exhumation of a metamorphic belt. Such a change in Cretaceous time from 120 Ma to 80 Ma may have promoted the exhumation of the Sanbagawa belt.

Rayleigh–Taylor instabilities from hydration and melting propel ‘cold plumes’ at subduction zones

It is commonly thought that hot diapiric flows prevail in the mantle wedge above the subducting slab. However, hydration and partial melting along the slab can create a situation in which a Rayleigh–Taylor instability can develop at the top of a cold subducting slab. We have numerically modeled this parodoxically interesting geological phenomenon, in which rising diapiric structures, colder than the asthenosphere by 300–400°C, are driven upward by compositional buoyancy, with a high-resolution two-dimensional regional model. These ‘cold plumes’ with a compositional, hydrous origin, launched from a depth of greater than 100 km, are lubricated by viscous heating, have an upward velocity in excess of 10 cm/yr, penetrate the relatively hot asthenosphere in the mantle wedge within a couple of million years and thus can cool the surroundings. These ‘cold plumes’ are fueled by partial melting of the hydrated mantle and subducted oceanic crust due to fluid release from dehydration reactions wihin the slab, including the decomposition of serpentine. There may be a spatial correlation between seismicity and the particular depth of cold-plume initiation.

Chemical constraints on the weathering of serpentinites containing nickel-iron sulphides

A review of mineralogical observations and results of Eh-pH measurements made on weathered profiles over serpentinites containing Ni-Fe sulphides in several areas of Western Australia permits some conclusions to be drawn relating to the chemical conditions prevailing during, and as the result of, weathering. At depth, conditions of relatively low Eh and high pH prevail. Eh increases and pH decreases upward in response chiefly to a series of anodic oxidative changes in the sulphides. Sulphides decompose at the water table, resulting in a sharp drop in pH which, in turn, promotes the decomposition of serpentine and carbonates. Above the water table, where the mineralogy is dominated by goethite, the pH increases again, promoting the local deposition of carbonates. Near-surface silicification is attributed to deposition of silica from silicic acid generated by the decomposition of serpentine during an earlier phase of weathering.