Abstract. Corundum- and spinel-bearing symplectites after muscovite were found in ultrahigh-pressure (UHP) eclogites from the Dabie terrane, China. Three types of symplectites were recognized based on their mineral assemblages: (1) symplectitic intergrowths of corundum + plagioclase + biotite after phengite (CPB), (2) symplectitic intergrowths of spinel + plagioclase + biotite after phengite (SPB), and (3) symplectitic intergrowths of spinel + plagioclase after paragonite (SP). The microtextures and mineral assemblages of the symplectites, in combination with the results of thermodynamic modeling on local regions, indicate that these symplectites formed by the breakdown of phengite and paragonite during the granulite-facies metamorphic overprint (770–850 ∘C) of the eclogite at pressures of 0.8–0.9 GPa. Dehydration partial melting reactions occurred during the breakdown of muscovite, which leads to the formation of thin plagioclase films (silicate melts) along grain (garnet, rutile, quartz) boundaries. Mass balance calculations indicate that the development of CPB and SPB symplectites after phengite requires the introduction of Al, Ca, Na, and Fe and loss of Si, Mg, and K. However, the formation of SP symplectites after paragonite requires the input of Mg, Ca, and Fe and removal of Si, Al, and Na. By summarizing the occurrence and growth mechanism of corundum- and spinel-bearing symplectites in global UHP terranes, we find that such symplectites can form by both the subsolidus replacement of an Al-rich anhydrous mineral (kyanite) and the dehydration melting of an Al-rich hydrous phase during high-temperature metamorphism. This study reveals that muscovite-bearing eclogites may experience multiple episodes of partial melting during the slab exhumation, not only at the high-pressure (HP) exhumation stage but also at the lower-pressure metamorphic overprinting stage. Kyanite is a reaction product during the HP partial melting, whereas the low-pressure (LP) melting will consume kyanite. We propose that the occurrence of corundum- and spinel-bearing symplectites after muscovite in eclogites is a potential mineralogical indicator of LP melting in exhumed slabs.
Abstract The exhumation mechanism of high‐pressure ( HP ) and ultrahigh‐pressure ( UHP ) eclogites formed by the subduction of oceanic crust (hereafter referred to as oceanic eclogites) is one of the primary uncertainties associated with the subduction factory. The phase relations and densities of eclogites with MORB compositions are modelled using thermodynamic calculations over a P–T range of 1–4 GPa and 400–800 °C, respectively, in the NCKFMASHTO ( Na 2 O – CaO – K 2 O – FeO – MgO – Al 2 O 3 – SiO 2 – H 2 O – TiO 2 – Fe 2 O 3 ) system. Our modelling suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P–T path are quite different from those of crust subducted along a warm P–T path, and that the density of oceanic eclogites is largely controlled by the stability of low‐density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P–T path with a geotherm of ~6 °C km −1 , lawsonite is always present at 1.1 to >4.0 GPa, and chlorite, glaucophane and talc can be stable at pressures of up to 2.3, 2.6 and 3.6 GPa respectively. Along such a P–T path, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110–120 km (< 3.3–3.6 GPa). However, along a warm subduction P–T path with a geotherm of ~10 °C km −1 , the P–T path is outside the stability field of lawsonite, and the hydrous minerals of chlorite, epidote and amphibole break down completely into dry dense minerals at relatively lower pressures of 1.5, 1.85 and 1.9 GPa respectively. Along such a warm subduction P–T path, the subducted oceanic crust becomes denser than the surrounding mantle at depths >60 km (>1.8 GPa). Oceanic eclogites with high H 2 O content, oxygen fugacity, bulk‐rock X Mg [ = MgO /( MgO + FeO )], X Al [ = Al 2 O 3 /( Al 2 O 3 + MgO + FeO )] and low X Ca [ = CaO /( CaO + MgO + FeO + Na 2 O )] are likely suitable for exhumation, which is consistent with the bulk‐rock compositions of the natural oceanic eclogites on the Earth's surface. On the basis of natural observations and our calculations, it is suggested that beyond depths around 110–120 km oceanic eclogites are not light enough and/or there are no blueschists to compensate the negative buoyancy of the oceanic crust, therefore explaining the lack of oceanic eclogites returned from ultradeep mantle (>120 km) to the Earth's surface. The exhumed light–cold–hydrous oceanic eclogites may have decoupled from the top part of the sinking slab at shallow depths in the forearc region and are exhumed inside the serpentinized subduction channel, whereas the dense–hot–dry eclogites may be retained in the sinking slab and recycled into deeper mantle.
Garnet orthopyroxenites of the Maowu mafic–ultramafic body occur in coesite-bearing paragneisses in the Dabieshan ultrahigh-pressure (UHP) metamorphic complex. We have distinguished six stages of metamorphism based on integrated mineralogical–petrological investigations: M1 – high-T/low-P metamorphism (˜1.4 GPa, ˜850°C); M2 – low-T/low-P metamorphism (˜1.4 GPa, ˜750°C); M3 – low-T/high-P metamorphism (2.1–2.5 GPa, 740–760°C); M4 – UHP metamorphism (5.3–6.3 GPa, ˜800°C); M5 – early retrogression (<3 GPa, >750°C); M6 – late retrogression (<2.3 GPa, <670°C). Detailed textures and mineral compositions indicate that the protolith of the Maowu garnet orthopyroxenites was a mantle refractory harzburgite or dunite that had been metasomatized by crust-derived silica-rich fluid or melt. M1–M2 defines an isobaric cooling P–T path, probably resulting from corner-flow in the mantle wedge above the subduction slab. M2–M4 defines an isothermal compressional P–T path, suggesting that the complex subsequently recycled into the deep upper mantle (up to ˜200 km depth) and underwent HP–UHP metamorphism. M4–M6 defines a retrograde P–T path, implying that the rocks ascended to crustal levels attending exhumation of the UHP terrane, and was continuously metasomatized by fluid derived from the continent country rocks.
This study complements the isocon method of Grant (1986) for describing mass transfer during metasomatic alteration by introducing a normalization solution to illustrate sequential mass transfers among multiple progressively altered geologic samples. The principle of the normalization solution involves adjusting all the isocons (defined by unaltered and altered sample pairs) to a single unified isocon without modifying the relative concentration changes of the corresponding components. When the normalization solution is applied to a sequence of progressively altered samples, an immobile component is defined or assumed, and the measured concentrations of all components in all altered samples are normalized by multiplying the corresponding normalizing factors; these factors are calculated using the concentration data for the immobile component. The normalized compositions of the altered samples are then plotted against the original measured composition of the unaltered sample in a single normalized isocon diagram that illustrates all the mass changes for a series of progressively altered samples.
This study applies the above approach to a set of progressively altered hydrothermal quartz-feldspar-porphyries at the Millenbach Cu-Zn mine (Riverin and Hodgson, 1980). The graphically displayed result indicates that the normalized isocon diagram can illustrate not only the mass changes of any component between any two altered zones, but also mass transfer trends of all components in the entire alteration zone. The normalization solution can be applied to any group of progressively altered geologic samples.
Abstract Fluid infiltration into metacarbonates is a key mechanism to induce orogenic decarbonation, which influences the global carbon cycle and long‐term climate evolution. Little is known regarding the fluid pathways during episodic infiltration events and how flow patterns control time‐integrated CO 2 outflux. We investigate the “vein‐like” polycrystalline mineral reaction zones (PMRZs) in dolomite marbles (Mogok metamorphic belt, Myanmar), which are formed by metasomatism via the infiltration of Si–Al–K–Ti–Zr‐bearing fluids. The petrographic textures and mineral U–Pb chronology reveal three episodes of fluid influx in a single PMRZ: (1) the initial episode (Stage‐I) transformed most dolomite into Mg‐rich silicates/oxides and calcite at ∼35–36 Ma indicated by baddeleyite cores; (2) baddeleyite rims gave ages of ∼23–24 Ma, representing a subsequent infiltration episode (Stage‐II) that modified Stage‐I minerals via a dissolution–precipitation mechanism; (3) the final episode (Stage‐III) is recorded by zircon replacing baddeleyite, which yielded ages of ∼17 Ma. Stage‐III fluid has a higher SiO 2 activity and [CO 2 /(CO 2 + H 2 O)] than Stage‐I/Stage‐II fluids. Thermodynamic and mass‐balance analyses indicate that Stage‐I infiltration causes >62–67% loss of CO 2 by both dolomite‐consuming reactions and calcite dissolution, whereas the latter two infiltration episodes induce <12–18% loss of CO 2 via calcite dissolution. Our results provide compelling evidence that repeated episodes of infiltration (each separated in time by 7–13 Ma) occurred along a single channel in marbles. The initial infiltration episode may create high‐permeability regions, offering favorable channels for later‐stage fluids that transfer obviously less CO 2 than the initial metasomatism. This considerably complicates a quantitative assessment of CO 2 liberation from metacarbonates during orogenesis.
Abstract Subduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high‐precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na‐Al‐Si‐rich fluids and have low δ 26 Mg values, varying from −1.55‰ to −0.92‰, whereas the green jadeitites have higher δ 26 Mg values (−0.91‰ to −0.74‰) due to metasomatic reactions between fluids and Cr spinel. The amphibole‐rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low δ 26 Mg values (−1.17‰ to −0.72‰). Therefore, the jadeite‐forming fluids have not only high concentrations of Na‐Al‐Si but also low δ 26 Mg values. The low δ 26 Mg signature of the fluids is explained by the dissolution of Ca‐rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of δ 26 Mg with CaO/TiO 2 , CaO/Al 2 O 3 , and Sr in the white jadeitites. Given the common occurrence of Ca‐rich carbonates in the subduction channel, the Mg isotope composition of low‐Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter‐scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.
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