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Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores. Fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution-reprecipitation mechanism. Many trace elements, including Na, Cl, S, Si, Mg, Sr, U, Th, and (REEs+Y), were variably leached from the fluorapatite grains during this process and the Sr and O isotopic signatures of the grains were also modified. The altered fluorapatite grains/domains have in situ 87Sr/86Sr ratios (0.70829-0.70971) slightly higher than those of the fresh fluorapatite (0.70777-0.70868), and δ18O values (-3.0 to +3.4 ppm) variably lower than the primary domains (+5.3 to +7.5 ppm). The Sr and O isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore-hosting intrusion, implying that the early-stage, ore-forming fluids were magmatic in origin but underwent weak interaction with the country rocks. U-Pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of ∼131 Ma, which are the same as the age of the ore-hosting intrusion. In combination with fluid inclusion data, we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore-forming event. The shifts to higher 87Sr/86Sr ratios and lower δ18O values in the altered fluorapatite indicate that the alteration was induced by fluids with more radioactive Sr and lighter O isotope signatures. The metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks. Our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore-forming events. Thus, instead of traditional bulk-rock analysis, in situ microanalysis is important to provide accurate constraints on the magmatic and/or hydrothermal evolution of complex ore-forming systems.

Small bodies of mantle-derived peridotites and other ultramafic rocks are commonly found in exhumed lower crustal units of collisional orogens. They provide a direct record of the complex evolution of the upper mantle before and during an orogeny, and are therefore key markers of the geodynamic evolution of an orogen. We report here the discovery of such mantle-derived peridotites, which occur as fragmented enclaves in migmatites of the high-grade Variscan lower crust exposed in the Pelvoux Massif (external Western Alps). A wide petrographic diversity has been observed, from very fertile, garnet-bearing lherzolites, to more depleted spinel/chromite-bearing harzburgites. Thermobarometric calculations on a garnet lherzolite indicate an initial stage at 3.0-4.0 GPa and 970-1140°C, followed by exhumation to 0.8-1.3 GPa and 800-850°C, while the harzburgites do not show any evidence of equilibration in the garnet field. Petrological observations, whole-rock geochemistry and in situ mineral compositions suggest the peridotites have undergone a complex history prior to their incorporation in the lower crust during the Variscan Orogeny. They derive from a refractory mantle, which has experienced variable degrees of melt depletion, and has then been extensively refertilized. Cryptic metasomatism is observed in all samples. It is characterized by an enrichment in large-ion lithophile elements (LILE, in particular Cs, Rb, U and Pb) relative to high field strength elements (HFSE), in particular Nb and Ta. This cryptic metasomatism is presumably related to percolation of subduction-related fluids or melts in the mantle. In addition, modal metasomatism occurred in some samples, where crystallization of phlogopite, pargasite, chromite and apatite has been observed. This modal metasomatism resulted in significant enrichment in K2O, P2O5 and Cr2O3 of the bulk rock, together with a strong enrichment in incompatible LREE relative to HREE. These geochemical characteristics are strikingly similar to that of syn-collisional, Mg-Cr-LILE rich mantle-derived (ultra)-potassic magmas such as durbachites and vaugnerites, which are ubiquitous in the Variscan metamorphic allochthons of Massif Central, external Alps, Vosges and Bohemian Massif. We therefore suggest that this metasomatism results from dynamic percolation of the peridotites by K2O-P2O5-Cr2O3-rich melts from which the durbachites and vaugnerites are primarily derived. These geochemical characteristics are in line with whole-rock Nd isotopic compositions, which indicate enrichment of the mantle by a continental crust component, presumably related to Variscan subductions. This evolution is consistent with that of other Variscan peridotites in the Eastern Alps (Ulten) and the Bohemian Massif, where multiple metasomatic episodes related to melts or fluids released in Variscan subduction zones have been documented.

The redox state of the Earth's upper mantle (i.e., oxygen fugacity, fO2) is a key variable that influences numerous processes occurring at depth like the mobility of volatile species, partial melting, and metasomatism. It is linked to the oxidation state of peridotite rocks, which is normally determined through the available oxythermobarometers after measuring the chemical composition of equilibrated rock-forming minerals and the Fe.sup.3+ in redox-sensitive minerals like spinel or garnet. To date, accurate measurements of Fe.sup.3+ / âFe in peridotites have been limited to those peridotites (e.g., harzburgites and lherzolites) for which an oxythermobarometer exists and where spinel (or garnet) crystals can be easily separated and measured by conventional .sup.57 Fe Mössbauer spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a lherzolite with carbonatitic melts and, therefore, have recorded the passage of (metasomatic) fluids at mantle conditions. However, no oxythermobarometer exists to determine their equilibrium fO2.

Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass‐transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high‐pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk‐rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg‐metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg‐rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si‐metasomatism of serpentinite in subduction zones is limited by pressure‐dependent increase in the silica activity buffered by the serpentine‐talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P‐T conditions. Alternatively, a different set of mineral assemblages, such as serpentine‐ or chlorite‐rich rocks, may be controlling the coupling‐decoupling transition of the plate interface. Key Points Fluid‐mediated mass transfer between mafic and ultramafic rocks can occur from prograde to peak, to retrograde conditions The formation of chlorite‐rich assemblages through Mg metasomatism of mafic rocks is prevalent at high P‐T conditions Talc formation via Si‐metasomatism of serpentinite is more limited in subduction zones than in oceanic plates

Whether arc magmatism occurs above oceanic subduction zones is the forefront of studies on convergent plate margins. The most important petrologic issue related to the evolution of arc systems is the origin of arc magmatism, among which arc basalts are the most important one because they provide insights into mantle enrichment mechanism and crust-mantle interaction at oceanic subduction zones. Fluids or melts released either by dehydration or by melting of subducting oceanic slab infiltrate and metasomatize the overlying mantle wedge at varying depth, leading to the formation of source regions of arc basalts. Such processes make most of arc basalts commonly enriched in large ion lithosphile elements and light rare earth elements, but depleted in high-field strength elements and heavy rare earth elements. Small amounts of arc basalts are characterized by relatively high Nb contents or by Nb enrichment. Rare basalts with compositions similar to ocean island basalts or mid-ocean ridge basalt also occur in arc systems. For these peculiar rocks, it remains debated whether their source is affected by subduction-related components. During their ascent and before their eruption, arc basaltic magmas are subjected to crystal fractionation, mixing and crustal contamination. In addition to the contribution of subducting slab components to the mantle source of arc basalts, the materials above the subducting slab at forearc depths would have been transported either by drag or by subduction erosion into the subarc mantle and into the source of arc magmas. Heats and materials brought by corner flows also play important roles in the generation of arc basalts. Despite the important progresses made in recent studies of arc basalts, further efforts are needed to investigate subarc mantle metasomatism, material recycling, the formation of arc magma sources, geodynamic mechanism in generating arc basalts, and their implicationd s for the initiation of plate tectonics on Earth.

Mantle xenoliths recovered from the modern backarc region of the northern Altiplano Plateau record metasomatism by slab‐derived silicic melts, and a suite of Quaternary volcanics suggest that melting of accreted crustal material has persisted since shallow subduction in the Oligocene. Xenoliths recovered from a suite of high‐K andesitic lava flows include phlogopite‐ and calcite‐rich orthopyroxenites and harzburgites, a wehrlite, and a phlogopite schist. Xenolith hosted calcite yields δ13C and δ18O values of −2.49 to +0.77‰ VPDB and +15.8 to +16.4‰ VSMOW, respectively, reflecting inputs of subducted marine carbonates in the metasomatizing melt. Arc‐like trace element patterns and 87Sr/86Sr ratios further support a subduction influence. Major and trace element characteristics of Quaternary potassic basalts and intermediate alkaline lavas, with the presence of mantle xenoliths, contradict magmatic differentiation or mixing models to yield intermediate composition melts. Instead, we suggest that intermediate composition lavas are derived from the melting of sediments accreted to the base of the continental lithosphere during shallow subduction in the late Eocene‐Oligocene. Melting of accreted material produces silicic alkaline melts, which react with peridotite to produce alkaline basaltic melts and residual phlogopite‐orthopyroxenites. These processes explain the observed xenolith suite and local high‐K basaltic volcanism, and the intermediate lavas may represent sediment melts that ascended to the surface minimally altered by exploiting the Cusco‐Vilconata fault system. These observations inform mass transfers during shallow subduction and melting and metasomatism in the lithospheric mantle, with implications for the generation of alkaline magmatism and rheologic weakening in cordilleran regions globally.

Carbon plays important roles in the evolution of the atmosphere and biosphere, and geochemical differentiation in Earth's interior. Most subducted C is recycled to the deep mantle and returns to the surface by degassing from erupted basalts and associated fluids. The Mg–Ca isotopic systems have been widely used in tracing recycled C. However, the idea that these geochemical proxies truly reflect the deep C cycle has been challenged. Here we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen, China. These alkaline basalts are associated with carbonatites and are characterized by low SiO2 (39.0–43.2 wt.%) and Al2O3 (6.98–9.15 wt.%) contents, high CaO/Al2O3 ratios (1.4–1.8), and positive Nb–Ta and negative Pb–Zr–Hf–Ti anomalies, suggesting they were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle. The studied samples have δ26Mg values of −0.24‰ to −0.44‰, ranging from mantle‐like values (δ26Mg = −0.25‰ ± 0.07‰) to lower values. This implies that carbonatite metasomatism does not always produce low‐δ26Mg anomalies. The samples have δ44/40Ca values (0.59‰–0.77‰; relative to the standard SRM915a) that are lower than Bulk Silicate Earth (0.94‰ ± 0.05‰), which are attributed to the involvement of low δ44/40Ca recycled carbonate in the mantle source. We suggest that the shift in mantle δ26Mg values during carbonatite metasomatism is controlled by the type and amount of carbonatite involved, while Ca isotope variations depend largely on the δ44/40Ca values of subducted carbonates. Mg or Ca isotopes alone, however, may not be sufficient to track the deep carbon cycle. Plain Language Summary Most subducted carbon is retained in the descending slab and recycles to the deep mantle. Mg‐Ca isotopes have been used to trace recycled carbon, as sedimentary carbonates have distinctive Mg‐Ca isotopic compositions compared to the mantle. In this study, we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen. Results indicate that contribution of recycled carbonates does not always produce low‐δ26Mg anomalies as previously suggested. Combined with evidence from whole‐rock major and trace elements, light δ44/40Ca values can be attributed to the involvement of recycled carbonate. However, Ca isotope data alone may not provide sufficient evidence to track recycled carbonate. Key Points Western Qinling alkaline basalts were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle The shift in mantle δ26Mg values during carbonatite metasomatism depends on the type and amount of carbonatite involved The δ44/40Ca offset during carbonatite metasomatism depended on the Ca isotope composition of subducted carbonate

The texture and mineral compositions of mantle xenoliths from the West Eifel volcanic field (WEVF) reveal three distinct mantle events. The first, identified in LREE-depleted and isotopically depleted anhydrous (type Ib) xenoliths, resulted from partial melting of fertile mantle around 2 Ga. The second event, marked by the formation of Ti-poor, LREE-rich clinopyroxene, amphibole, and phlogopite in hydrous type Ia xenoliths, is attributed to mantle metasomatism. The third event, evidenced by LREE-depleted and MREE-enriched clinopyroxene in olivine-clinopyroxenite veins and in more evolved hornblendite and phlogopite-clinopyroxenite veins that occur in type Ia xenoliths, is associated with the passage of primitive alkaline mafic magma related to the Quaternary host magmatism.