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Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ18O values and in-situ Rb–Sr constraints on the initial (87Sr/86Sr)I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H2O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable 87Rb/86Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (87Sr/86Sr)I released into eclogites. Instead, unradiogenic 87Sr/86Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (87Sr/86Sr)I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data. [ABSTRACT FROM AUTHOR]

Based on the geochemical and isotopic (δ18О, δD) data, the thermal and fluid conditions during the formation of the Eldjurta granite massif were reconstructed. Analysis of rocks collected from the core of the Tyrnyauz Superdeep Well (TSW) within the depth range of 1427–3923 m revealed their homogeneous isotopic parameters: the δ18O values of bulk samples, quartz, feldspars, and biotite in 12 samples of biotite granites are 8.50 ± 0.33, 9.55 ± 0.22, 8.40 ± 0.33 and 5.45 ± 0.40‰, respectively. The δD values in the biotite vary from −103.3 to −95.6‰. The closure temperatures of the oxygen isotope system of quartz are 440–980°C. The rock cooling history was reconstructed using a new approach based on the analysis of single quartz grains. This approach can be used for detailed reconstructions of thermal history during formation of intrusive bodies. The definite samples were used to demonstrate that Dodson's equation is valid for description of the δ18O values of quartz in a granite system. The data obtained suggest that the studied part of the massif was formed in at least two almost simultaneous stages. The lower part of the massif was crystallized first, and the second injection of granite melt arrived immediately after the first portion has been crystallized, but had no yet had time to cool significantly. The Tc values in the lower part of the massif indicate the re-opening of the oxygen isotope system of quartz, with subsequent long-term isotope re-equilibration between minerals. This leads to decrease of the observed Tc values and the calculated cooling rates, which is related to increasing volume of the intrusive body and cooling within already heated rocks. Estimates of the isotopic parameters of the water component indicate the absence of exotic water fluid (meteoric or buried waters) during cooling of the massif. The variations of the δ18O values in the minerals of the Eldjurta biotite granites can be described in terms of a simple retrograde exchange at the cooling stage. [ABSTRACT FROM AUTHOR]

The complexity of petroleum phases in deep formations plays an important role in the evaluation of hydrocarbon resources. Pressure is considered to have a positive impact on the preservation of liquid oils, yet direct evidence for this phenomenon is lacking in the case of deep reservoirs due to late destruction. Here, we present fluid-inclusion assemblages from a deep reservoir in the Tazhong area of the Tarim Basin, northwestern China, which formed as a direct consequence of fluid pressure evolution. Based on thermodynamic measurements and simulations of the coexisting aqueous and petroleum inclusions in these assemblages, the history of petroleum activities was reconstructed. Our results show that all analyzed fluid-inclusion assemblages demonstrated variable pressure conditions in different charging stages, ranging from hydrostatic to overpressure (a pressure coefficient of up to 1.49). Sequential petroleum charging and partial oil cracking may have been the main contributors to overpressure. By comparing the phases of petroleum and fluid pressures in the two wells, ZS1 and ZS5, it can be inferred that overpressure inhibits oil cracking. Thus, overpressure exerts an important influence on the preservation of liquid hydrocarbon under high temperatures. Furthermore, our results reveal that the exploration potential for liquid petroleum is considerable in the deep reservoirs of the Tarim Basin. [ABSTRACT FROM AUTHOR]

Objective Calcite veins are the products of tectonic diagenesis. Information about fracture opening and fluid activity was recorded by calcite veins. Methods Multiple approaches, consisting of thin section observation, cathodoluminescence, U-Pb isotope dating, and REE, C, O and Sr isotope analyses of calcite veins from the Triassic Qinglong carbonate reservoirs in the Huangqiao area, Subei Basin, are used to analyse the origin of vein-forming fluid. Results Four stages of calcite veins were identified in the Qinglong Formation and successively formed at (115.30±0.42), (97.03±0.43), (85.29±0.25), (45.5±19.0) Ma. In the first stage, the calcite veins were derived from deep hydrothermal fluids.And in the second stage, the calcite veins were derived from mixed fluids of atmospheric fresh water and seawater. Formation water and deep shell-source hydrothermal fluid mixed in the vein-forming fluid of the third-stage calcite veins. In the fourth stage of calcite veins formation, deep mantle-source hydrothermal fluid mixed with the formation water, resulting in the precipitation of calcite veins. The formation timing of the four stages of calcite veins corresponds to multistage tectonic movements during different periods, indicating that fluid evolution in the Triassic Qinglong carbonate reservoirs was controlled by multiple stages of tectonic movements. Conclusion Importantly, three stages of deep hydrothermal fluid injectionin the reservoir were the result of deep fault opening caused by tectonic movements, which possibly indicate multistage hydrocarbon accumulation in the Triassic Qinglong Formation in the Huangqiao area.

A significant breakthrough has been made in the exploration and development of shale oil in the Subei Basin. Cores from well HY1 in the Gaoyou Sag of Subei Basin indicate the presence of well-developed natural fractures within the organic-rich shale of the second member of Paleogene Funing Formation. These fractures contain compelling evidence of shale oil, gas, and fluid activities and migration. This study specifically examines the calcite veins filling the shale fractures within the second member of Funing Formation. Through the integration of methodologies from mineralogy, elemental geochemistry, geochronology, and geological fluid analysis, the originof paleo fluids, the timing of vein formation, and fluid inclusions within the shale fracture veins are investigated. This study also sheds light on paleotemperature conditions and corresponding geological timeframes during the burial, uplift, and denudation of the shale oil-bearing layers. The primary fracture types observed in the second member of Funing Formation include bedding fractures, detachment fractures, shear fractures, shrinkage fractures, and tensile fractures, predominantly filled with fibrous calcite veins indicating a source linked to hydrothermal fluids in a reducing environment. Notably, three distinct stages of shale oil injection are evident within the organic-rich shale fracture veins of the second member of Funing Formation, marked by primary green fluorescent oil inclusions from the Middle Eocene, secondary blue fluorescent oil inclusions from the Late Eocene, and secondary green fluorescent oil inclusions from the Late Quaternary period. The primary accumulation of shale oil occurred during the period of maximum burial depth, emphasizing the significant role of tectonic movements, especially the Sanduo tectonic event, in the redistribution and migration of shale oil in the region.

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Serpentinites of the mantle wedge were identified from a composite oceanic–continental subduction zone that occurs as the Hong'an orogen in east–central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modeling as well as zircon U–Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, light rare earth elements and high field strength elements in the whole rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive ε Hf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong'an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U–Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high- to ultrahigh-pressure metamorphic rocks in the Hong'an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative ε Hf(t) values of −16.0 to −2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during the Triassic continental collision. In this context, the mantle wedge peridotites were first metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision. [ABSTRACT FROM AUTHOR]

Previous studies propose that there is a mantle upwelling that generated the Cenozoic basalts in Changbaishan. However, the dominant source and mechanism of the mantle upwelling remains highly debated. Here we apply machine learning algorithms of Random Forest and Deep Neural Network to train models using global island arc and ocean island basalts data. The trained models predict that Changbaishan basalts are highly influenced by slab-derived fluid. More importantly, the fluid effect decreases with no (87Sr/86Sr)0 and εNd(t) changes between 5 Ma and 1 Ma, then enhances with increasing εNd(t) and decreasing (87Sr/86Sr)0 after 1 Ma. We propose that a gap opened at about 5 Ma and the hot sub-slab oceanic asthenosphere rose through the gap after 1 Ma, generating the basalts enriched in fluid mobile elements and with the addition of depleted mantle component derived from the sub-slab oceanic asthenosphere.

Most mud volcanoes are located along large tectonic zones within the Alpine and Central Asian folded regions, the Pacific mobile belt and rift regions of the Atlantic and Indian oceans. One of the basic conditions for the functioning of the mobile crust of mud volcanic processes in the earth's crust is the location and creasing into folds of the complex of sedimentary formations. It is these geological conditions that determine the activity of many mud volcanoes within the Kerch-Taman mud volcanic province, including the mud volcano Shugo. Long-term studies of the phenomenon of mud volcanism, carried out under the guidance and with the direct participation of Academician Ye.F. Shnyukov created the fundamental foundations of a geological and geophysical approach to a deeper understanding of the conditions of formation and mechanisms of activity of the volcanoes of the Kerch and Taman Peninsulas. This work is devoted to the results of seismological and geoacoustic studies of the Shugo volcano, expanding our ideas about the features of the deep structure of the studied object and contributing to the further development of the ideas of the outstanding scientist, academician of the National Academy of Sciences of Ukraine Ye.F. Shnyukov.

Sichuan Basin is the only successful basin for shale gas exploration in China. In addition to the main shale in the Lower Silurian Longmaxi formation, the lower Cambrian Qiongzhusi shale is an important potential formation. However, it was once considered that shale gas is difficult to enrich because of its poor sealing conditions and hydrocarbon migration to adjacent reservoirs. With the increasing research on hydrocarbon generation and reservoir in shale of Qiongzhusi Formation in recent years, it has become an important exploration target in Sichuan Basin. The enrichment of oil and gas is closely related to fluid activities. Limited by the degree of exploration, there is little analysis of fluid activities in Qiongzhusi Formation, and there is little analysis of shale gas enrichment potential from the perspective of fluid. The hydrocarbon generated from Qiongzhusi shale in the rift could migrate laterally to the uplift area and form a reservoir in Dengying Formation. The fluid activities from source rock to reservoir are basically the same. Therefore, this paper reconstructed the history of hydrocarbon activities in Dengying reservoirs based on fluid inclusion analysis. Then the fluid activity process in Qiongzhusi shale was studied, and its enrichment conditions of shale gas was discussed. The results show that the hydrocarbon activities of Dengying Formation can be divided into three stages: 1) oil charging stage, 2) oil cracking gas generation stage and 3) gas reservoir adjustment stage. The first stage is under normal pressure, and the second and third stages developed overpressure with pressure coefficients of 1.3 and 1.2, respectively. High pressure coefficient is an important indicator of shale gas enrichment. Because the source rock of Qiongzhusi Formation has always been the main source rock of Dengying Formation, it can supply hydrocarbon to Dengying Formation only with overpressure in gas generation stage. Therefore, overpressure in the last two stages of gas generation indeed existed. As long as the sealing condition of shale itself is not particularly poor, shale gas “sweet points” would be formed. Therefore, the thick shale in Deyang-Anyue rift is the focus of shale gas exploration in Qiongzhusi Formation.

Based on the updated results of experimental petrology and phase equilibria modelling and combined with the available thermal structure models of subduction zones, this paper presents an overview on the dehydration and melting of basic, sedimentary and ultrabasic rocks that occur in the different stages during oceanic subduction processes and their influences on magmatism above subduction zones. During the subduction at the forearc depth of <90–100 km, the basic and ultrabasic rocks from most oceanic slabs can release very small amounts of water, and significant dehydration may occur in the slab superficial sediments. Strong dehydration occurs in both basic and ultrabasic rocks during subduction at the subarc depth of 90–200 km. For example, more than 90% water in basic rocks is released by the successive dehydration of chlorite, glaucophane, talc and lawsonite in the subarc depths. This is diversely in contrast to the previous results from synthetic experiments. Ultrabasic rocks may undergo strong dehydration through antigorite, chlorite and phase 10 Å at the subarc depth of 120–220 km. However, sediments can contribute minor fluids at the subarc depth, one main hydrous mineral in which is phengite (muscovite). It can stabilize to ∼300 km depth and transform into K-hollandite. After phengite breaks down, there will be no significant fluid release from oceanic slab until it is subducted to the mantle transition zone. In a few hot subduction zones, partial melting (especially flux melting) can occur in both sediments and basic rocks, generating hydrous granitic melts or supercritical fluids, and in carbonates-bearing sediments potassic carbonatite melts can be generated. In a few cold subduction zones, phase A occurs in ultrabasic rocks, which can bring water deep into the transition zone. The subducted rocks, especially the sediments, contain large quantities of incompatible minor and trace elements carried through fluids to greatly influence the geochemical compositions of the magma in subduction zones. As the geothermal gradients of subduction zones cannot cross the solidi of carbonated eclogite and peridotite during the subarc subduction stage, the carbonate minerals in them can be carried into the deep mantle. Carbonated eclogite can melt to generate alkali-rich carbonatite melts at >400 km depth, while carbonated peridotite will not melt in the mantle transition zone below a subduction zone. [ABSTRACT FROM AUTHOR]

The subduction channel is defined as a planar to wedge-like area of variable size, internal structure and composition, which forms between the upper and lower plates during slab subduction into the mantle. The materials in the channel may experience complex pressure, temperature, stress and strain evolution, as well as strong fluid and melt activity. A certain amount of these materials may subduct to and later exhume from >100 km depth, forming high to ultra-high pressure rocks on the surface as widely discovered in nature. Rock deformation in the channel is strongly assisted by metamorphic fluids activities, which change composition and mechanical properties of rocks and thus affect their subduction and exhumation histories. In this study, we investigate the detailed structure and dynamics of both oceanic and continental subduction channels, by conducting high-resolution petrological-thermomechanical numerical simulations taking into account fluid and melt activities. The numerical results demonstrate that subduction channels are composed of a tectonic rock melange formed by crustal rocks detached from the subducting slab and the hydrated mantle rocks scratched from the overriding plate. These rocks may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate, or exhume along the subduction channel to the surface near the suture zone. Based on our numerical results, we first analyze similarities and differences between oceanic and continental subduction channels. We further compare numerical models with and without fluid and melt activity and demonstrate that this activity results in strong weakening and deformation of overriding lithosphere. Finally, we show that fast convergence of orogens subjected to fluid and melt activity leads to strong deformation of the overriding lithosphere and the topography builds up mainly on the overriding plate. In contrast, slow convergence of such orogens leads to very limited deformation of the overriding lithosphere and the mountain building mainly occurs on the subducting plate. [ABSTRACT FROM AUTHOR]

The Chang 6 tight sandstone reservoir of the Upper Triassic, developed in a large lacustrine shallow-water delta depositional environment, is the most important target for oil exploration and development in the basin. This study is undertaken with the purpose of investigating the influences of lithofacies, diagenetic fluid activity and diagenetic process on quality of the Chang 6 tight sandstone reservoir situated in the southeastern Ordos Basin, based on core observation and description, petrographic investigations, measurement of electron probe microanalysis (EMPA), homogenization temperature and laser Raman spectroscopy of fluid inclusions, laser microzone sampling carbon and oxygen isotope of carbonate cement, the X-ray diffraction (XRD), back scattered (BSE) image observation. Research result shows that three lithofacies are recognized. Each lithofacies has its initial petrological components that slightly differ from other ones, and this is in turn restricted the diagenetic evolution route and imposed diagenetic process and oil-charging intensity, which led to a different reservoir quality. Three carbonate cementation of microcrystalline calcite, sparry calcite I, sparry calcite II and ankerite cements enriched near the mudstone-sandstone interface, occurred in the Chang 6 reservoir which are unfavorable for the reservoir quality. The Chang 6 reservoir experienced three phases of diagenetic fluid activities. The fluid activity of the meteoric water occurred in the eodiagenetic stage offered a carbon source for the cementation of microcrystalline calcite (δ18O PDB : 13.3‰∼-10.0‰; δ13C PDB : 0.4‰∼2.1‰) and sparry calcite I (δ18O PDB : 19.8‰∼-13.4‰; δ13C PDB : 0.4‰∼0.8‰). The latter two fluids released by the thermal decarboxylation of organic matter during the two oil-charging stages, occurred in the mesodiagenetic stage, providing a carbon source for the formation of sparry calcite II (δ18O PDB : 21.8‰∼-18.7‰; δ13C PDB : 5.1‰∼-0.4‰) and ankerite (δ18O PDB : 21.5‰∼-16.8‰; δ13C PDB : 4.6‰∼-3.9‰). The dominant impact on reservoir quality is mechanical compaction, which destroys more intergranular pore than cementation, especially in the sandstone of lithofacies III (the porosity loss by compaction and cementation is 30.9% and 7.3%, respectively), compared to that of the lithofacies I sandstone (the porosity loss by compaction and cementation is 28.7% and 6.6%%, respectively) and the lithofacies II sandstone (the porosity loss by compaction and cementation is 28.8% and 8.5%, respectively). Laumontite cement formed during the eodiagenetic stage effectively resisted the intensity of mechanical compaction. The authigenic clay minerals in the three lithofacies are dominated by chlorite and illite, and most abundant in the lithofacies III. Primary intergranular pore can be effectively protected when the chlorite cement is less than 4%. The dissolution pores with average of 3.0%, 2.4% and 2.3% in the lithofacies I, lithofacies II and lithofacies III sandstone, respectively, formed during the mesodiagenetic stage improved the reservoir quality to a certain degree. Four models of diagenesis and pore evolution of the Chang 6 reservoir are summarized. The pattern I has the best reservoir quality due its fewer ductile component and more coarser grains, the weaker mechanical compaction and weaker cementation of laumontite and carbonate, as well as a stronger dissolution attributed to the intensified oil-charging during mesodiagenesis. • The coupled relationship of lithofacies, diagenesis, and fluid activity determines the quality of the reservoir. • The initial petrological composition controlled by lithofacies has a profound effect on the diagenetic evolution process. • The acidic diagenetic environment formed in two stages of oil-charging resulted in the dissolution of feldspar and laumontite. • Four models of diagenesis and pore evolution are summarized. [ABSTRACT FROM AUTHOR]

The study on the initial activity time and mechanism of Altyn Tagh fault system since Cenozoic is one of hot topics in the northern Tibet Plateau, but is still lack of a reliable chronological constraint. Based on field investigation, the ages of associated quartz vein and gypsum from Altyn Tagh fault system were measured by electron spin resonance (ESR) dating, and the age was taken as the constraint of the latest fault activity; the structural deformation of Altyn Tagh fault system since Middle Miocene was discussed. The results show that Cenozoic large-scale structural deformation of Altyn Tagh fault system starts in Middle Miocene ((12.5 ± 1.3)-(15. 1 ± 1. 5)Ma), followed by multistage activities; time limit of large-scale strike-slip activity for Altyn Tagh main fault at least starts in (15. 1 ± 1. 5) Ma, but lime limit of compression thrust activity for the northern Altyn Tagh fault at least starts in (14. 1 ± 1. 4)Ma; there is a time difference for the activity between Altyn Tagh main fault and the northern Altyn Tagh fault, suggesting that Altyn Tagh fault system extends progressively from the center to both sides; the deep ductile shear leads to strike-slip movement of Altyn Tagh main fault and rapid uplift of neighbouring mountain, and then the deformation propagation cause the tectonic response of thrust activity of the northern Altyn Tagh fault. [ABSTRACT FROM AUTHOR]

Abstract: Natural hydrocarbon seeps in the marine environment are important sources of methane and other greenhouse gases to the ocean and the atmosphere. Accurate quantification of methane flux at hydrocarbon seeps is therefore necessary to evaluate their influence on the global methane budget and climate change. Hydrocarbon seeps on the seabed produce a near-shore gas bubble zone along the shallow western coast of Hainan Island, northern South China Sea. An in-situ and on-line gas flux measuring device was deployed over a hydrocarbon seep to quantify the gas flux by equal volume exchange venting from the seabed offshore of Ledong Town, Hainan Island, over 19 days. The physiochemical parameters and the dissolved methane concentration of the bottom water at the hydrocarbon seep were also measured. The gas flux from the hydrocarbon seep varied from 22 to 77l/day with the tidal period and was strongly negatively correlated with water depth. The flux data from the seep suggests that the variation in hydrostatic pressure induced by tidal forcing and ocean swell may control the variation of the gas flux. The bottom water dissolved methane concentration, ranging from 26 to 74nmol/L, was negatively correlated with temperature and water depth at the seabed and positively with the gas flux. The total gas volume released from the hydrocarbon seep was 30.5m3 for the 19-day period, providing an estimated gas flux of 600m3/yr. The 120 known hydrocarbon seeps along the eastern edge of the Yinggehai Basin could vent a large quantity of methane from the seafloor, which suggests that hydrocarbon seeps on the continental margin of the northern South China Sea may be an important natural source of methane to the atmosphere. [Copyright &y& Elsevier]

Abstract: Rutile and zircon in eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe (EMP). Both single spot and profile analyses of large matrix rutiles (>60μm) generally demonstrate core–rim variability of Pb, Sr, Zr and U. Pb and Sr show enrichment at the rims, while Zr and U decrease in concentration from the cores to the rims. Correspondingly, temperatures estimated by Zr-in-rutile geothermometers are higher within the cores (mainly in the range of 680–740°C) than at the rims by 10–30°C. We note that estimated temperatures within the cores of matrix rutiles correlate positively with grain diameter. This suggests that the Zr content in rutiles is not only dependent on temperature and pressure, but is also affected by fluid activity and Zr diffusion. Estimated temperatures within the cores of matrix rutiles merely represent lower limits of the peak ultra-high pressure (UHP) metamorphic temperatures. Combined with Th/U and Ce/Lu ratios, rare earth element (REE) patterns and 206Pb/238U ages, Ti-in-zircon geothermometers can be used to distinguish the thermal evolution of eclogites during the UHP metamorphism. Zircons of 745±8.0Ma (2σ, n =3) show evidence of the temperatures at which they crystallized from the original magma. Zircons of 240±7.0Ma (2σ, n =11) with flat HREE patterns suggest a definite temperature range (689±18°C (2σ, n =11)) for the prograde metamorphism. Although 226Ma is generally considered the time of the peak UHP metamorphism for the Dabie–Sulu orogen, except for a few grains with a maximum temperature of 744°C, most of the 226±2.0Ma (2σ, n =12) zircons indicate lower temperatures (average=698±12°C (2σ, n =12)). These data imply that the 226Ma zircons were probably not products of the peak UHP metamorphism, but might instead be associated with the pressure decrease in the post-peak metamorphism. The youngest zircons (<220Ma) provide evidence for temperatures (682±14°C (2σ, n =11)) associated with the retrograde metamorphism. Geothermometers that incorporate both rutile and zircon indicate that there may have been experienced a locally short-lived heating event for eclogites from the CCSD-MH. [Copyright &y& Elsevier]

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values. [Display omitted] • B isotope variations indicate the geological information (metamorphism, magmatic-hydrothermal transition, devolatilization and metasomatism). • B isotopic compositions of fluids both from LHS and GHS and its effects on B isotopic diversity. • B isotopes distinguish hydration and dehydration melting in Himalaya. [ABSTRACT FROM AUTHOR]

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, time- scale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental subduction, fluid activity in the continental deep-subduction zone, partial melting during continental collision, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision orogen. Some intriguing questions and directions are also proposed for future studies. [ABSTRACT FROM AUTHOR]

Ultra-high-temperature (UHT) metamorphism occurs when the continental crust is subjected to temperatures of greater than 900 °C at depths of 20–40 km. UHT metamorphism provides evidence that major tectonic processes may operate under thermal conditions more extreme than those generally produced in numerical models of orogenesis. Evidence for UHT metamorphism is recorded in mineral assemblages formed in magnesian pelites, supported by high-temperature indicators including mesoperthitic feldspar, aluminous orthopyroxene and high Zr contents in rutile. Recent theoretical, experimental and thermodynamic data set constraints on metamorphic phase equilibria in FMAS, KFMASH and more complex chemical systems have greatly improved quantification of the P–T conditions and paths of UHT metamorphic belts. However, despite these advances key issues that remain to be addressed include improving experimental constraints on the thermodynamic properties of sapphirine, quantifying the effects of oxidation state on sapphirine, orthopyroxene and spinel stabilities and quantifying the effects of H2O–CO2 in cordierite on phase equilibria and reaction texture analysis. These areas of uncertainty mean that UHT mineral assemblages must still be examined using theoretical and semi-quantitative approaches, such as P( –T) –μ sections, and conventional thermobarometry in concert with calculated phase equilibrium methods. In the cases of UHT terranes that preserve microtextural and mineral assemblage evidence for steep or ‘near-isothermal’ decompression P–T paths, the presence of H2O and CO2 in cordierite is critical to estimates of the P–T path slopes, the pressures at which reaction textures have formed and the impact of fluid infiltration. Many UHT terranes have evolved from peak P–T conditions of 8–11 kbar and 900–1030 °C to lower pressure conditions of 8 to 6 kbar whilst still at temperature in the range of 950 to 800 °C. These decompressional P–T paths, with characteristic d P/d T gradients of ∼25 ± 10 bar °C−1, are similar in broad shape to those generated in deep-crustal channel flow models for the later stages of orogenic collapse, but lie at significantly higher temperatures for any specified pressure. This thermal gap presents a key challenge in the tectonic modelling of UHT metamorphism, with implications for the evolution of the crust, sub-crustal lithosphere and asthenospheric mantle during the development of hot orogens. [ABSTRACT FROM AUTHOR]

Quartz veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-quartz vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA-ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar δ18O values for minerals from both vein and eclogite, indicating that the vein-forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz–mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700–650 °C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ∼500 °C without corresponding metamorphism. In either case, the kyanite–quartz vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles. [ABSTRACT FROM AUTHOR]

Fluid availability during high-grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid-absent metamorphism commonly resulting in neither zircon growth/recrystallization for U-Pb dating nor Sm-Nd isotopic resetting for isochron dating. While peak ultra-high pressure (UHP) metamorphism is characterized by fluid immobility, high-pressure (HP) eclogite-facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm-Nd and Rb-Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U-Pb method for coesite-bearing domains of zircon. O isotope reequilibration was achieved between the Sm-Nd and Rb-Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite-facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U-Pb, Sm-Nd, Rb-Sr and O isotope systems in the eclogite minerals demonstrates that time-scale for the HP eclogite-facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid-facilitated process in the HP eclogite-facies regime during the exhumation of deeply subducted continental crust. [ABSTRACT FROM AUTHOR]

Differential exhumation was petrologically recognized in ultrahigh pressure metamorphic rocks from the southern and northern parts of the Sulu terrane. While a normal exhumation occurred for eclogites and gneisses in south Sulu, granulite-facies overprinting of ultrahigh pressure metamorphic rocks took place with high retrograde temperatures in north Sulu. A study of fluid inclusions reveals trapping of five type fluid inclusions in high and ultrahigh pressure eclogite minerals and vein quartz in the Sulu terrane. These are A-type N2±CO2 inclusion trapped at high and ultra-high pressure eclogite-facies metamorphic condition, B-type pure-CO2 liquid phase inclusion with higher density trapped during granulite-facies overprinting metamorphism of eclogites, C-type CO2-H2O inclusion and D-type hypersaline inclusion trapped in high pressure eclogite-facies re-crystallization stage, and E-type low salinity H2O inclusion trapped in the latest stage of ultrahigh pressure exhumation (amphibolite-facies retrogression). Identification of crowded-distributing pure-CO2 liquid inclusions with higher density trapped in garnet of eclogites provides an evidence for granulite-facies overprinting metamorphism in the north Sulu terrane. [ABSTRACT FROM AUTHOR]

It is well known that a great deal of fluid was released during subduction of oceanic crust, resulting in arc magmatism, quartz veining and metamorphic mineralization of syn-subduction. In contrast, the process of continental subduction is characterized by the relative lack of fluid and thus no arc magmatism has been found so far. During exhumation of deep-subducted continental crust, nevertheless, significant amounts of aqueous fluid became available from the decomposition of hydrous minerals, the decrepitation of primary fluid inclusions, and the exsolution of structural hydroxyls. This kind of metamorphic fluid has recently attracted widespread interests and thus been one of the most important targets in deciphering the geological processes concerning metamorphism, magmatism and mineralization in collisional orogens. A large number of studies involving stable isotopes, fluid inclusions and petrological phase relationships have been accomplished in past a few years with respect to the mobility and amount of metamorphic fluid in UHP metamorphic rocks from the Dabie-Sulu orogenic belt. The results demonstrate that the fluid activity during the exhumation of deep-subducted continental crust has the following effects: (1) amphibolite-facies retrogression due to pervasive fluid flow; (2) formation of HP quartz veins within eclogites due to channelized fluid flow; and (3) partial melting of overlying crustal rocks due to focused fluid flow, producing syn-exhumation magmatism within the orogenic belt. In particular, the aqueous fluid released by decompression exsolution of hydroxyl from UHP minerals is characterized by Iow salinity and is capable of resulting in pervasive and channellized flow. Therefore, the intensive study of fluid activity during exhumation of UHP metamorphic rocks can not only provide insight into geodynamic processes that occurred in continental collisional belts, but also shed light on understanding of crust-mantle recycling and relevant magmatism in subduction zone setting. [ABSTRACT FROM AUTHOR]

Mafic and felsic orthogneisses form an integral component of the rock association of the Eastern Ghats Belt (EGB). Such rocks show similar P–T-fluid evolutionary history at two different localities of the EGB from where UHT metamorphism has already been recorded. Both mafic and felsic orthogneisses show contrasting petrological characters and are classified according to structural and mineralogical characteristics. All the varieties of mafic granulite and the gneissic enderbite preserve the strong regional foliation (S2/S3). The pegmatoidal enderbite is post deformational and hybrid in nature containing xenocrysts derived from associated rocks. Textural and thermobarometric data reveal different stages of metamorphism. Peak metamorphism (M1) occurs at 8–9 kbar pressure and temperature in excess of 950 °C. Emplacement of mafic magma could be the principal cause of this metamorphism. This was followed by retrograde R1 stage when the rocks suffered a near isobaric cooling up to 700–750 °C at 7–7.5 kbar. At the terminal retrograde stage (R2) fluid–rock interaction took place at 6–6.5 kbar, 600–650 °C. The fluid composition is calculated to be poor in H2O and CO2. Textural data predict that K-rich brine could be responsible for such composition. The observed P–T-fluid characteristics show similarity with other terranes belonging to erstwhile Gondwanaland. [Copyright &y& Elsevier]

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (<14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma. [ABSTRACT FROM AUTHOR]

The Carboniferous Batamayineishan Formation of the Kelameili Gas Field is a specific weathered crust-related volcanic reservoir that has a significant production rate in the Junggar Basin, Northwestern China, attributed to debatable processes of fluid evolution. The results suggest that various types of fluids occurring in volcanic rocks lead to the filling of quartz and calcite in fractures and their associated alteration haloes. The silica that formed quartz veins was mainly derived from deep hydrothermal fluids, while the carbon dioxide that formed calcite veins originated from sources characterized by mixing and alteration of deep hydrothermal and hydrocarbon fluids. Siliceous hydrothermal fluids rich in sulphur dioxide and other volatile components were driven by a pressure gradient and buoyancy, and circulated both laterally and vertically along the fractures, forming quartz veins and tension fractures under different temperature conditions. Moreover, changes in salinity, pressure, and carbon dioxide of deep fluids, varying from acidic to weakly alkaline, resulted in earlier calcite precipitation in contraction fractures and weathered fractures. Tectonic uplift resulted in the long-term exposure of volcanic rocks, where fresh water mixed with the partially alkaline fluid escaping the basin to form calcite cements, thus retaining the characteristics of a seepage environment in the weathered fractures. Structural fractures occurred due to tectonic movements during the burial period. Filling and leakage of hydrocarbons caused pore fluids to convert from acidic to alkaline, precipitating late sparry calcite in dissolution fractures. Late hydrothermal fluid metasomatism, brought about by infiltration into the permeable zone, caused partial dissolution of local calcite along cleavage cracks. [ABSTRACT FROM AUTHOR]