Orthopyroxene, an important phase in mantle-derived rocks, has become a powerful tool to unravel mantle nature and magma processes. However, the applications have been hindered by the lag in the development of analytical techniques, such as shortage of reference materials. Orthopyroxene grains derived from an ultramafic intrusion at the Mogok metamorphic belt (Myanmar) were evaluated for the potential use of orthopyroxene as a reference material for in-situ microanalysis. Approximately 20 g of 0.5–3 mm pure orthopyroxene grains were separated under binocular microscope and analyzed using EPMA, LA-ICPMS, and bulk analytical methods (XRD, XRF, and solution-ICPMS) for major and trace elements at four institutions. Eleven core-to-rim profiles carried out using EPMA and twelve core-to-rim profiles determined using LA-ICPMS suggest that MK-1 orthopyroxene grains are sufficiently homogeneous, with RSD < ±2% (1σ) for major elements (Mg, Si, and Fe) and RSD < ±10% (1σ) for trace elements (Na, Al, Ca, Ti, Cr, Co, Zn, Ni, Mn, Sc, and V). In addition, the composition of MK-1 orthopyroxene was also measured by XRF and solution-ICPMS measurements in two different laboratories, to compare with the results measured using EPMA and LA-ICPMS. The results indicated a good agreement with RSE < ±2% (1σ) for major elements and RSE < ±5% (1σ) for most trace elements, except for Na (±9.73%) and Ti (±6.80%). In an overall assessment of these data, MK-1 orthopyroxene can be considered as a reference material for in-situ microanalysis, which would provide solid trace elements data for a better understanding of mantle source and magmatic evolution.
Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high XMg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (˜1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.
AbstractThe multi-stage metamorphic evolution and high-pressure (HP) to ultra-high-pressure (UHP) fluid events of vein-bearing eclogites from the Ganghe in the Dabie UHP terrane have been established based on a detailed study of compositional zonations in garnet, omphacite, and epidote. Comparison of mineral assemblages and zoned garnet compositions with the predictions of phase equilibrium modelling results in six stages of metamorphism in eclogites: (i) prograde low-temperature (LT)-HP eclogite-facies metamorphism (2.0–2.3 GPa, <540°C); (ii) peak UHP lawsonite eclogite-facies metamorphism (~3.1 GPa, ~650°C); (iii) decompressional breakdown of UHP lawsonite during initial exhumation (2.8–3.0 GPa, 650–680°C), as we have previously reported; (iv) middle-temperature (MT) and HP eclogite-facies metamorphism (~2.6 GPa, ~720°C); (v) retrograde eclogite-facies metamorphism (1.5–1.7 GPa, 610–670°C); and (vi) amphibolite-facies metamorphism (<1.1 GPa, <600°C). A heating event with decompression is confirmed from metamorphic stages (iii) to (iv), indicating that the Ganghe eclogites underwent a relatively sluggish exhumation during the stages from the UHP to HP conditions. During prograde metamorphic stages (i) and (ii), fluid activities in the eclogites are limited because lawsonite growth consumed large amounts of early-stage hydrous minerals and potential free water. Dehydration reactions during metamorphic stage (iii) triggered a significant liberation of UHP fluid. Intense fluid-eclogite interaction and dissolution-precipitation processes during metamorphic stages (iii) to (iv) led to the occurrence of patchy zones in epidote cores and the systematic compositional change of epidote and omphacite cores near the vein. A late-stage infiltration of HP fluid into eclogites occurred in metamorphic stage (v), leading to the final-stage growth (mineral rims) of garnet, omphacite, and epidote. This study presents detailed mineral zonation patterns and mineral compositions of the vein-hosting eclogites to link the complicated metamorphic P-T evolution with episodic multi-stage HP-UHP fluid events in deeply subducted continental crust.Keywords: high-pressure veineclogitemetamorphic evolutionmulti-stage fluid eventscompositional zonationsDabie UHP terrane AcknowledgementsWe sincerely thank Drs Qian Mao and Yuguang Ma for their assistance in the EMP analyses and Drs Lingmin Zhang, Yueheng Yang, Zhichao Liu, Qin Zhou, and Jing Sun for their assistance in the LA-ICP-MS analyses. Two anonymous reviewers are highly appreciated for their helpful comments and suggestions. Editorial handling by editors Robert J. Stern and Weidong Sun is gratefully acknowledged.Additional informationFundingThis study was supported by The National Science Foundation of China [grant numbers 41090371, 41102034, and 41372079], The National Basic Research Programme of China (973 Programme 2009CB825001), and a China Postdoctoral Science Foundation -funded project (2011M500384).
Abstract Mantle wedge hybridization by crust‐derived melt is a crucial mechanism responsible for arc lavas. However, how the melt‐rock reactions proceed in the mantle wedge and affect melt compositions is poorly understood. Garnet peridotites from Jiangzhuang in the Sulu orogen (eastern China) host garnetite and pyroxenite veins formed by slab‐mantle interactions at different melt/rock ratios. The Jiangzhuang peridotites consist mainly of garnet lherzolites and minor harzburgites and represent a fragment of the mantle wedge influenced by ultrahigh‐pressure metamorphism (5.2–6.1 GPa) in the subduction channel. Petrography, major and trace element geochemistry, and in situ clinopyroxene Sr isotope values of the garnetite and pyroxenite veins reveal their derivation from interactions between mantle wedge peridotites and deeply subducted crust‐derived melts. The two veins share a common metamorphic and metasomatic history and have similar mineral assemblages and compositions, enriched isotope signatures, and formation P ‐ T conditions, indicating the same source for their reacting melts. The different mineral proportions and microtextures between the garnetite and pyroxenite veins are ascribed to different melt/rock ratios. The garnetite vein formed at relatively high melt/rock ratios (>1:1), which would likely produce hybrid slab melts with Mg‐rich, high‐silica adakitic signatures. In contrast, the pyroxenite vein formed at low melt/rock ratios (<1:1), and the expected hybrid slab melts would evolve into high‐Mg andesites. Moreover, recycled heterogeneous garnetite and pyroxenite could contribute to the mantle sources of intraplate magmas. Therefore, slab‐mantle interactions at different melt/rock ratios could be an important crustal input to lithological and geochemical heterogeneities in the mantle.
The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity (fo2) conditions, approximately 2.5–4.5 logfo2 units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replacement reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb (<10 ppm) and Ta (<2 ppm) and Nb/Ta ratios (<10) and higher contents of Cr (>340 ppm) and V (>1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks.
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