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Fluids in subduction zones significantly influence element mobility, isotope fractionation, and mass transfer. However, unraveling the source, composition, and redox state of fluids in continental subduction zones poses a significant challenge. This study focuses on a granitic melt‐eclogite contact interface, along with adjacent granite and eclogite from the Sulu ultrahigh‐pressure metamorphic belt in East China. The interface exhibits complex mineral assemblages, enriched rare earth elements (REEs), and high field strength elements (HFSEs). Zircon grains from the interface show an age of ∼217 ± 9 Ma, slightly later than peak metamorphism, along with the presence of coesite inclusions. These findings suggest that the interfacial fluid likely formed from the mixing of granitic anatectic melt and aqueous fluid from the eclogite during the initial exhumation of the Sulu terrane. The interaction resulted in the eclogite acquiring substantial REEs and HFSEs, suggesting the interfacial fluid's effective element‐transporting capability and potential supercritical fluid properties. Zircon Ce anomaly and Fe3+/Fe2+ oxybarometer data indicate a highly oxidizing interfacial fluid, analogous to arc magmas in oxygen fugacity. This led to the preferential loss of isotopically heavier Cr from the eclogite during fluid‐eclogite interaction, evidenced by heavier Cr isotopic compositions in the interface (δ53Cr = −0.04 to −0.05‰) compared to adjacent eclogite (δ53Cr as low as −0.11‰). In summary, our results highlight the presence of strong oxidizing and element‐mobilizing fluids in continental subduction zones, offering insights into supercritical fluid recognition and the genesis of oxidizing arc magmas in subduction zones. Plain Language Summary: Fluids are important agents for material migrations in subduction zones, yet the characteristics of fluids in continental subduction zones remain poorly understood, including their redox state, element‐carrying capacity, and isotopic features. In our study, we uncovered a unique interfacial fluid process occurring during the interaction of granitic melt and eclogite in continental subduction zones. This interfacial fluid transported significant amounts of insoluble trace elements into the eclogite, resulting in superchondritic Nb/Ta ratios. These characteristics indicate the interfacial fluid's exceptional element‐transporting capability distinct from silicate melts or aqueous fluids. Additionally, the interfacial fluid demonstrates a strong oxidizing property, comparable to or even exceeding that of arc magmas. This distinctive feature leads to the preferential loss of isotopically heavier Cr in the eclogite during fluid‐rock interaction. In summary, our study provides a compelling demonstration of the presence of strong oxidizing and element‐mobilizing fluids, carrying significant understanding for fluid properties in subduction zones. Key Points: Significant migrations of rare earth elements and high field strength elements are observed in granitic anatectic melt‐eclogite interfaceThe interfacial fluid is highly oxidizing, with oxygen fugacity equivalent to or even surpassing that of arc magmasThe strong oxidizing nature of the interfacial fluid resulted in the preferential loss of isotopically heavier Cr of the eclogite [ABSTRACT FROM AUTHOR]

Element mobility and chemical mass transfer are evaluated in the formation of Cu—Fe exoskarn deposits and endoskarn and minor porphyry‐style alteration in the Tonglushan quartz monzodiorite (QMD) system, eastern China. Endoskarn formation involved the migration of Ca into the QMD from the exoskarnification of carbonates (now marble) xenoliths and wall rocks, addition of Fe and Mn by magmatic‐hydrothermal fluids emanating from the interior of the QMD, and removal of alkali elements due to the replacement of feldspars and mica by prograde skarn minerals. Zirconium, Hf, U, and rare earth elements (REE) were added by hydrothermal fluids which were able to carry these often poorly mobile high field strength elements (HFSE) due to elevated F activity. Additions of Al were likely from Na‐rich fluids that also caused sodic alteration. Several factors favored mineralization within the exoskarns rather than endoskarns and QMD. The endoskarns were relatively oxidizing, as evidenced by a significant addition of Fe3+, which caused Cu to remain in magmatic‐hydrothermal fluids until they entered and precipitated sulphides in the more reducing environment of the exoskarns. Fluid migration from the QMD through the endoskarns and into the exoskarns was favored due to decarbonation of wall rock carbonates and related upwards migration of CO2 to produce a self‐sustaining chimney effect, which drew further fluids towards the carbonates to form, alter and mineralize the exoskarns. The higher porosity and permeability of the endoskarns compared with the QMD further promoted the lateral flow of Cu‐bearing fluids towards the exoskarns and limited porphyry‐style alteration and mineralization within the QMD. This proposed mechanism is only likely to be relevant for porphyry‐type systems developed predominantly within carbonate host rocks. Its significance for exploration models is that relatively poorly mineralized porphyry stocks in this setting may be associated with more substantive exoskarn deposits on their margins. [ABSTRACT FROM AUTHOR]

The Ruwai deposit is Indonesia's largest Zn‐Pb‐Ag skarn deposit and is located in Lamandau district, Central Borneo, within the Central Borneo metallogenic belt. This skarn deposit consists of four main zones, namely Gojo, Karim, Ruwai, and Southwest Gossan Zones. The skarn orebodies are mostly hosted by limestone of the Jurassic Ketapang Complex where quartz diorite of the Cretaceous Sukadana Granitoid is the ore‐causative intrusion. Despite the several mineralogical studies carried out in this deposit, there is still a lack of knowledge of its geochemical characteristics. This study evaluates the element mobility during skarn formation on the basis of skarn and ore mineralogy combined with lithogeochemical data of the intrusions, sedimentary wall rocks, and skarn bodies. The skarn mineralogy of the Ruwai skarn complex can be divided into prograde, retrograde and supergene stages. The prograde stage is characterized by the formation of an anhydrous assemblage of garnet‐pyroxene, while the retrograde stage features the replacement of prograde minerals by predominant epidote‐chlorite‐actinolite. The mineralization was first introduced during the late prograde stage, while the formation of massive orebodies attributed to the retrograde stage. The skarn samples show a wide range of major element contents, but both the mineralized skarn and massive orebodies show similar trace and rare‐earth elements patterns in global Phanerozoic limestone‐ and upper crust sedimentary rocks‐normalized spider diagrams. The skarn and orebodies, as well as the metalimestone in this study area, are depleted in REE, although HREE are higher than LREE. Most metals (e.g., Zn, Pb, Ag, Cu, Fe) in skarn and associated orebodies, interpreted to be predominantly magmatic‐sourced, show co‐occurring enrichment or depletion relative to the metalimestone and intrusive rocks. The isocon analysis shows that there was significant mass loss as a consequence of significant volatile loss, such as CO2, during skarn formation. Major oxides and large ion lithophile elements mostly behaved as mobile elements during skarn formation, whereas rare‐earth and high field strength elements tended to be immobile. However, the occurrence of several HFSE‐ and REE‐bearing minerals in Ruwai deposit (i.e., zircon, thorite, cerite, cerianite, monazite, allanite), suggesting minor or local mobility of these elements. Such unexpected behavior can be justified by the occurrence of fluorine‐rich hydrothermal fluid, which could have been responsible for the increasing mobility of these elements. [ABSTRACT FROM AUTHOR]

Assimilation serves as a significant part during magma's evolution. Because of rapid cooling of mafic–ultramafic magma, assimilation between mantle‐derived magma and crustal rocks (wall rocks) is often neglected. The Beidaihe diabase dyke in North China Craton suggests that assimilation of the rapidly cooled mantle‐derived magma cannot be ignored. The diabase dyke is very thin (~1 m in width) with chilled margin. The dyke centre is porphyritic texture while the chilled margin is aphanitic texture, suggesting a rapid crystallization process. From the centre to margin of the dyke, contents of SiO2 (from 48.12–49.94 wt.% to 38.29–46.63 wt.%), MgO (from 7.46–9.17 wt.% to 3.39–4.71 wt.%), Ni (from 91.5–118 ppm to 6.5–45.8 ppm), and Cr (from 243–306 ppm to 1.7–80.5 ppm) are obviously decreased, but contents of Fe2O3T (from 8.71–9.39 wt.% to 9.21–12.30 wt.%), CaO (from 5.67–7.08 wt.% to 6.91–10.26 wt.%), and CO2 (from 0.25–1.92 wt.% to 4.52–6.58 wt.%) are distinctly increased. The dyke intruded discordantly into Ordovician limestone. Limestone far away from the dyke mainly consists of micro calcites with low MgO (0.55–0.77 wt.%) and FeO (0.07–0.09 wt.%) but high CaO (53.82–55.51 wt.%) and CO2 (44.46–45.02 wt.%) contents. Limestone close to the dyke metamorphosed into marble, which mainly comprises of recrystallized calcite/dolomite and diopside, with high MgO (1.85–2.35 wt.%) and FeO (0.17–1.36 wt.%) but relatively low CaO (52.30–54.15 wt.%) and CO2 (38.77–41.04 wt.%). All these suggest an assimilation process of the mantle‐derived magma with wall rocks limestone. Based on the chemical balance at the interface between the diabase and limestone, CO2 exchange model is used to estimate the degree of assimilation. The calculated assimilation degree is about 8–12% at the dyke margin. There is obvious elements exchange between magma and wall rocks, and the transfer ability of Mg is greater than that of Ca. Meanwhile, Fe oxides quickly precipitated at the margins of the dyke for the increase of fO2. Therefore, even for mantle‐derived magma which chilled rapidly, it may also be obviously influenced by assimilation, especially when wall rocks are carbonate sediments. Simultaneously, this study provides a possible explanation for some mantle‐derived rocks in carbonate areas with high LOI. [ABSTRACT FROM AUTHOR]

Despite recent advances, diagenetic processes in fine‐grained sediments are still relatively poorly understood. Key questions still to be resolved include the types of diagenetic minerals present in mudstones and the extent of element mobility in these low permeability systems. This study utilizes data from the Haynesville‐Bossier Shale, USA, to analyze lithologies, discriminate authigenic phases and identify mobile elements during diagenesis. It has implications for understanding how authigenic minerals develop and the sources of those authigenic minerals in fine‐grained sediments. On the basis of grain‐size and mineralogy five lithologies are designated: (i) silica‐rich argillaceous mudstones; (ii) argillaceous siliceous mudstones; (iii) mixed siliceous mudstones; (iv) mixed mudstones; and (v) authigenically‐dominated mudstones. The diagenetic development of the Haynesville‐Bossier Shale can be divided into early and late diagenesis. Ferroan and non‐ferroan dolomite, framboidal pyrite and bioclast pore‐filling kaolinite and calcite grain replacements and cements all formed during early diagenesis. Late diagenetic mineral phases include illite formed by the illitization of smectite, replacive and displacive chlorite, calcite‐replacive albite, quartz‐replacive calcite and replacive and/or displacive quartz. The presence of extensive late diagenetic mineral precipitates indicates that there was a degree of element mobility on at least the local scale. Aluminium present in albite is most likely to have resulted from the illitization of smectite. Quartz overgrowths probably resulted from illitization and the pressure dissolution at quartz silt grain boundaries. Externally, hydrothermal fluids resulting from regional‐scale igneous activity appear to have played a role in the formation of chlorite and possibly albite. The work indicates that extensive mineral development and element mobility occurred during late diagenesis. [ABSTRACT FROM AUTHOR]

A petrological study was performed on an eclogite‐vein system from the Dabie ultrahigh‐pressure terrane by focusing on high‐pressure (HP) fluid‐rock interaction and mass transfer during exhumation. The veins comprise mainly garnet and quartz and resulted from a HP leaching process along fractures in the eclogite during exhumation. Relict peak garnet, phengite, and apatite in this rock system were altered by HP fluids forming characteristic compositional zonings. Zoned garnet in the eclogite records an isothermal exhumation from 3.3 GPa and 665–680 °C. Partial dissolution of garnet and a back incorporation of Mn‐heavy rare earth element (REE)‐Y increased the content of these elements in the altered garnet zone in the veins. Altered phengite is characterized by a Ba‐rich rim. Fluid‐aided alteration reduced the light REE (LREE) contents and produced oriented monazite, calcite, and pyrrhotite inclusions within apatite, reflecting mobilization and redistribution of LREEs, CO32−, Fe, and S between these minerals. The active HP fluid is proposed to have been a Si‐Al‐Na‐Ba‐rich aqueous fluid, as indicated by sodic plagioclase rims around various eclogitic minerals and by the altered phengite composition. Mass balance calculations reveal that LREEs, middle REEs, and Zr‐Hf‐Th‐U were added to and Li, large ion lithophile elements, and Ti‐Nb were removed from the quartz‐garnet veins, requiring a high fluid flux. These findings shed new light on the nature and behavior of HP fluids and highlight their key role in resetting mineral chemistry and mass transfer in subduction zone metamorphic environments. Key Points: Garnet, phengite, and apatite record HP fluid‐aided alteration in an eclogite‐vein system from the Dabie UHP terraneThe active HP fluid is a Si‐Al‐Na‐Ba‐rich aqueous fluid formed closed to the wet solidusA high fluid flux caused considerable element mobility during the vein formation [ABSTRACT FROM AUTHOR]

Metamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction-zone P-T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh-pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite-bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K-feldspar ± plagioclase ± K-bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute-rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U-Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ18O and δD values, suggesting that the vein-forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water-soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2-5.7 times) and Nb-Ta (1.8-2.0 times) than those in the eclogite, indicating that these normally water-insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein-forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction-zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein-forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust. [ABSTRACT FROM AUTHOR]

Abstract Mineralogical and geochemical studies on the fault rocks from the Nojima–Hirabayashi borehole, south-west Japan, are performed to clarify the alteration and mass transfer in the Nojima Fault Zone at shallow depths. A complete sequence from the hornblende–biotite granodiorite protolith to the fault core can be observed without serious disorganization by surface weathering. The parts deeper than 426.2 m are in the fault zone where rocks have suffered fault-related deformation and alteration. Characteristic alteration minerals in the fault zone are smectite, zeolites (laumontite, stilbite), and carbonate minerals (calcite and siderite). It is inferred that laumontite veins formed at temperatures higher than approximately 100°C during the fault activity. A reverse component in the movement of the Nojima Fault influences the distribution of zeolites. Zeolite is the main sealing mineral in relatively deep parts, whereas carbonate is the main sealing mineral at shallower depths. Several shear zones are recognized in the fault zone. Intense alteration is localized in the gouge zones. Rock chemistry changes in a different manner between different shear zones in the fault zone. The main shear zone (MSZ), which corresponds to the core of the Nojima Fault, shows increased concentration of most elements except Si, Al, Na, and K. However, a lower shear zone (LSZ-2), which is characterized by intense alteration rather than cataclastic deformation, shows a decreased concentration of most elements including Ti and Zr. A simple volume change analysis based on Ti and Zr immobility, commonly used to examine the changes in fault rock chemistry, cannot account fully for the different behaviors of Ti and Zr among the two gouge zones. [ABSTRACT FROM AUTHOR]