European Journal of Mineralogy最新文献

{"title":"Partial melting of amphibole–clinozoisite eclogite at the pressure maximum (eclogite type locality, Eastern Alps, Austria)","authors":"S. Schorn, Anna Rogowitz, C. Hauzenberger","doi":"10.5194/ejm-35-715-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-715-2023","url":null,"abstract":"Abstract. Pristine amphibole–clinozoisite eclogite from within the eclogite type locality (Hohl, Koralpe) of the Eastern Alps in Austria preserves centimetre-thick, concordant, laterally continuous leucocratic segregations of coarse-grained (up to ∼ 1 cm grain diameter) euhedral amphibole–clinozoisite–quartz and disseminated garnet–omphacite–rutile. The segregations locally show selvedges dominated by coarse-grained amphibole at the interface with their host eclogite. Retrogression is limited to thin films of texturally late plagioclase ± amphibole and minor symplectites of diopside–plagioclase partially replacing omphacite. Mineral compositions are largely homogeneous except for clinozoisite, which is significantly enriched in Fe3+, rare-earth and high-field-strength elements in the rock matrix compared to that in segregations. Petrography, mineral chemical data and phase diagram modelling are interpreted in terms of limited melting under high-aH2O conditions, at or close to the well-established pressure maximum (21 ± 3 kbar and 680–740 ∘C), followed by melt crystallization near these conditions. Exsolution of melt-dissolved H2O led to the formation of the amphibole-rich selvedges at the leucosome–eclogite interface. Plagioclase ± amphibole/clinopyroxene films formed at lower pressure from final melt vestiges adhering to grain boundaries or from secondary fluid–rock interaction. Natural variability in rock composition and the bulk oxidation state leads to variable mineral modes and calculated high-pressure solidus temperatures for compositional end-members sampled at Hohl. Modelling suggests that oxidized conditions (XFe3+<0.5) favour hydrated but refractory amphibole–clinozoisite-rich assemblages with a fluid-present solidus temperature of ∼ 740 ∘C at 20 kbar, whereas more reduced conditions (XFe3+∼0.2) yield “true” eclogites (> 80 vol % garnet + omphacite) that commence melting at ∼ 720 ∘C at the same pressure. The interlayering of such eclogites potentially constitutes a fluid source–sink couple under appropriate pressure–temperature conditions, favouring fluid transfer from neighbouring dehydrating layers to melt-bearing ones down gradients in the chemical potential of H2O (μH2O). Phase diagram calculations show that for moderate degrees of fluid-fluxed melting (≤ 10 vol % melt) near the pressure maximum, the observed equilibrium assemblage is preserved, provided the melt is subsequently removed from the source rock. The resulting hydrous melts may be, in part, parents to similar eclogite-hosted pegmatitic segregations described in the eclogite type locality. We suggest that eclogites with a comparable composition and metamorphic history are however unlikely to produce voluminous melts.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46925796","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Batoniite, [Al8(OH)14(H2O)18](SO4)5  ⋅  5H2O, a new mineral with the [Al8(OH)14(H2O)18]10+ polyoxocation from the Cetine di Cotorniano Mine, Tuscany, Italy","authors":"D. Mauro, C. Biagioni, J. Sejkora, Z. Dolníček, R. Škoda","doi":"10.5194/ejm-35-703-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-703-2023","url":null,"abstract":"Abstract. The new mineral batoniite,\u0000[Al8(OH)14(H2O)18](SO4)5 ⋅ 5H2O,\u0000was discovered in the Cetine di Cotorniano Mine, Chiusdino, Siena,\u0000Tuscany, Italy. It occurs as hemispherical aggregates composed of brittle\u0000tabular crystals, up to 1 mm in size, white to colorless in color, with a\u0000white streak and a vitreous to greasy luster. Batoniite is biaxial negative,\u0000with α= 1.4833(6), β= 1.4948(6), γ= 1.5019(5)\u0000(589 nm), and 2V(meas.)= 71(1)∘. Electron microprobe\u0000analysis, affected by the dehydration of batoniite under the chamber vacuum,\u0000gave (in wt %) the following: Al2O3 33.48, Fe2O3 0.05, SO3\u000033.00, and H2Ocalc 44.41, total 110.94. It corresponds to the chemical\u0000formula (Al7.98Fe0.013+)Σ7.99(SO4)5.01(OH)13.95(H2O)18 ⋅ 5H2O. Batoniite is triclinic, belonging to space group P1‾, with a= 9.1757(6), b= 12.0886(9), c= 20.9218(15) Å, α= 82.901(3),\u0000β= 87.334(3), γ= 86.999(2)∘, V= 2297.8(3) Å3, and Z= 2. The crystal structure was refined to R1= 0.0916\u0000for 8118 unique reflections with Fo>4σ(Fo) and\u0000811 refined parameters and 60 restraints. Batoniite is characterized by\u0000isolated [Al8(OH)14(H2O)18]10+ polyoxocations,\u0000H-bonded to five interstitial (SO4)2− and five H2O groups. In\u0000type material, it is associated with gypsum and a poorly crystallized Al–Fe\u0000sulfate. The crystallization of batoniite is probably due to the action of\u0000H2SO4 on Al-bearing rocks of Paleozoic age cropping out in the\u0000Garibaldi Tunnel, the lowest mining level of the Cetine di Cotorniano Mine.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46518585","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle","authors":"J. Reynes, Jörg Hermann, P. Lanari, T. Bovay","doi":"10.5194/ejm-35-679-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-679-2023","url":null,"abstract":"Abstract. The incorporation mechanisms of OH groups in garnet were\u0000investigated in a suite of high-pressure rocks from the Zermatt–Saas area\u0000(Switzerland) using a combination of Fourier transform infrared spectroscopy\u0000(FTIR) and electron probe micro-analysis (EPMA). Investigated garnet\u0000specimens include grossular–andradite–uvarovite solid solutions in\u0000serpentinite and rodingite and almandine–grossular–pyrope–spessartine\u0000solid solutions in eclogite, mafic fels and meta-sediment. All rocks\u0000experienced the same peak metamorphic conditions corresponding to a burial\u0000depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa),\u0000allowing determination of the OH content in garnet as a function of rock\u0000type. The capacity for OH incorporation into garnet strongly depends on its\u0000composition. Andradite-rich (400–5000 µg g−1 H2O) and\u0000grossular-rich garnet (200–1800 µg g−1 H2O) contain at\u0000least 1 order of magnitude more H2O than almandine-rich garnet\u0000(< 120 µg g−1 H2O). Microscale analyses using FTIR\u0000and EPMA profiles and maps reveal the preservation of OH zoning throughout\u0000the metamorphic history of the samples. The OH content correlates strongly\u0000with Mn, Ca and Ti zoning and produces distinct absorption bands that are\u0000characteristic of multiple nano-scale OH environments. The use of 2D\u0000diffusion modelling suggests that H diffusion rates in these rocks is as low\u0000as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were\u0000collected for the main garnet-bearing rock types of the Zermatt–Saas area\u0000allowing a mass balance model of H2O to be calculated. The result shows\u0000that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1\u0000could be transported by garnet in the subducting slab beyond 80 km depth and\u0000contributed to the deep-Earth water cycle during the Eocene subduction of\u0000the Piemonte–Liguria Ocean.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42469585","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"First measurements of the Fe oxidation state of spinel inclusions in olivine single crystals from Vulture (Italy) with the in situ synchrotron micro-Mössbauer technique","authors":"Giulia Marras, G. Carnevale, A. Caracausi, S. Rotolo, V. Stagno","doi":"10.5194/ejm-35-665-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-665-2023","url":null,"abstract":"Abstract. The redox state of the Earth's upper mantle (i.e., oxygen\u0000fugacity, fO2) is a key variable that influences numerous processes\u0000occurring at depth like the mobility of volatile species, partial melting,\u0000and metasomatism. It is linked to the oxidation state of peridotite rocks,\u0000which is normally determined through the available oxythermobarometers\u0000after measuring the chemical composition of equilibrated rock-forming\u0000minerals and the Fe3+ in redox-sensitive minerals like spinel or\u0000garnet. To date, accurate measurements of Fe3+ / ∑Fe in peridotites\u0000have been limited to those peridotites (e.g., harzburgites and lherzolites) for\u0000which an oxythermobarometer exists and where spinel (or garnet) crystals\u0000can be easily separated and measured by conventional 57Fe Mössbauer\u0000spectroscopy. Wehrlitic rocks have been generally formed by the interaction of a\u0000lherzolite with carbonatitic melts and, therefore, have recorded the passage of\u0000(metasomatic) fluids at mantle conditions. However, no oxythermobarometer\u0000exists to determine their equilibrium fO2. The aim of this study was to retrieve the fO2 of the mantle beneath\u0000Mt. Vulture volcano (Italy) through the study of a wehrlitic lapillus emitted\u0000during the last eruption (∼ 140 kyr ago) that contain olivines\u0000with multiple tiny spinel inclusions with sizes < 40 µm. To\u0000our knowledge, the Fe oxidation state of these inclusions has been never\u0000determined with the Mössbauer technique due to their small sizes. Here, we present measurements of the Fe3+ / ∑Fe using in situ\u0000synchrotron Mössbauer spectroscopy coupled with chemical and\u0000spectroscopic analysis of both host olivine and spinel inclusions. The results show Fe3+ / ∑Fe ratios of 0.03–0.05 for olivine and\u00000.40–0.45 for the included spinels, the latter of which appear higher than\u0000those reported in literature for mantle spinel harzburgites and lherzolites.\u0000Given the evidence of the mantle origin of the trapped spinels, we\u0000propose that the high fO2 (between 0.81 and 1.00 log above the\u0000fayalite–magnetite–quartz buffer; FMQ) likely results from the interaction\u0000between the pristine spinel lherzolite and a CO2-rich metasomatic\u0000agent prior to the spinel entrapment in olivines at mantle depths.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46524999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) – Newsletter 74","authors":"Ferdinando Bosi, Frédéric Hatert, Marco Pasero, Stuart J. Mills","doi":"10.5194/ejm-35-659-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-659-2023","url":null,"abstract":"","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":"125 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136019792","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy","authors":"J. Gilotti, W. McClelland, S. Schorn, R. Compagnoni, M. Coble","doi":"10.5194/ejm-35-645-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-645-2023","url":null,"abstract":"Abstract. An eclogite-facies orthogneiss and host paragneiss from a quarry\u0000near Tavagnasco in the Lower Aosta Valley were studied in order to refine\u0000the protolith, provenance and metamorphic ages of the Eclogitic Micaschist\u0000Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz + phengite + K-feldspar ± garnet + rutile + zircon, whereas the\u0000paragneiss hosts garnet + jadeite + phengite ± glaucophane + epidote + rutile + quartz. Phase diagram modeling of two representative\u0000samples yields minimum equilibration conditions of 550 ± 50 ∘C and 18 ± 2 kbar. Cathodoluminescence images of zircon from the\u0000orthogneiss show oscillatory-zoned cores that are embayed and overgrown by\u0000complex, oscillatory-zoned rims. Four concordant secondary ion mass\u0000spectrometry analyses from the cores give a weighted mean\u0000206Pb / 238U age of 457 ± 5 Ma. The cores have Th/U = 0.1\u0000and negative Eu anomalies indicative of an igneous protolith, which we\u0000interpret to have crystallized in the Ordovician at 780 ∘C, based\u0000on Ti-in-zircon measurements. Zircon rims yield a range of\u0000206Pb / 238U dates from 74 to 86 Ma, and four concordant analyses\u0000define a weighted mean 206Pb / 238U age of 78 ± 2 Ma. The rims\u0000are interpreted to have grown in the eclogite facies based on their lower\u0000Th/U (0.01), less negative Eu anomalies and steeper heavy rare\u0000earth element (HREE) patterns at <600 ∘C. The paragneiss yielded a detrital zircon population with\u0000major peaks at 575–600, 655 and 765 Ma; minor older components; and a\u0000maximum depositional age of approximately 570 Ma. The prominent\u0000Neoproterozoic zircon population and Ediacaran depositional age suggest\u0000derivation from the Gondwana margin. The metamorphic zircon is consistent\u0000with the oldest eclogite-facies event in the Sesia Zone; it does not show\u0000evidence of multiple periods of rim growth or any pre-Alpine (e.g.,\u0000Variscan) metamorphism.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43364377","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hochleitnerite, [K(H2O)]Mn2(Ti2Fe)(PO4)4O2(H2O)10  ⋅  4H2O, a new paulkerrite-group mineral, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria, Germany","authors":"I. Grey, E. Keck, A. R. Kampf, C. MacRae, R. Gable, W. G. Mumme, N. Wilson, A. Glenn, C. Davidson","doi":"10.5194/ejm-35-635-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-635-2023","url":null,"abstract":"Abstract. Hochleitnerite,\u0000[K(H2O)]Mn2(Ti2Fe)(PO4)4O2(H2O)10 ⋅ 4H2O, is a new paulkerrite-group mineral from the Hagendorf-Süd\u0000pegmatite, Oberpfalz, Bavaria, Germany. It was found in specimens of altered\u0000zwieselite, in association with fluorapatite, rockbridgeite, columbite and\u0000sub-micrometre rods of uranophane. Hochleitnerite occurs as isolated and\u0000intergrown pale-yellow, diamond-shaped tablets with thicknesses reaching 50 µm and lengths of 120 µm. The crystals are flattened on\u0000{010}, slightly elongated on [001], and bounded\u0000by the {111} and {010} forms. The calculated density is\u00002.40 g cm−3. Optically, hochleitnerite crystals are biaxial\u0000(+), with α= 1.615(2), β= 1.621(2) and γ= 1.645(2) (measured in white light). The calculated 2V is 53.8∘.\u0000The empirical formula is\u0000[K(H2O)](Mn1.512+Fe0.492+)Σ2.00(Ti1.624+Fe0.193+Al0.15)Σ2.96(PO4)4.00[O1.50F0.23(OH)0.27]Σ2.00(H2O)10 ⋅ 4H2O. Hochleitnerite has space\u0000group Pbca and unit-cell parameters a= 10.5513(3) Å, b= 20.6855(17) Å,\u0000c= 12.4575(4) Å, V= 2718.96(15) Å3 and Z= 4. The crystal\u0000structure was refined using single-crystal data to wRobs= 0.082 for\u00002242 reflections with I > 3σ(I). The crystal structure\u0000contains corner-connected linear trimers of Ti-centred octahedra that share\u0000corners with PO4 tetrahedra to form 10-member rings parallel to (010).\u0000K+ cations and water molecules are located within the rings. Additional\u0000corner sharing of the PO4 tetrahedra with MnO2(H2O)4\u0000octahedra occurs along [010] to complete the 3D framework structure.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46729512","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrological study of an eclogite-facies metagranite from the Champtoceaux Complex (La Picherais, Armorican Massif, France)","authors":"Thomas Gyomlai, P. Yamato, G. Godard","doi":"10.5194/ejm-35-589-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-589-2023","url":null,"abstract":"Abstract. The high-pressure metagranite of La Picherais\u0000belongs to the Cellier Unit (part of the lower allochthon of the\u0000Champtoceaux Complex; Armorican Massif, western France), where it crops out\u0000as an undeformed body embedded within the orthogneisses of the Cellier Unit\u0000and is closely associated with numerous mafic eclogite lenses and seldom\u0000metahornfels. The petrographic observations of this metagranite reveal the\u0000presence of well-developed reaction textures: (1) pseudomorph after\u0000plagioclase, (2) garnet and phengite coronae at biotite–plagioclase\u0000interfaces, (3) garnet and phengite coronae at biotite–K-feldspar interfaces,\u0000and (4) garnet and rutile coronae at ilmenite–plagioclase interfaces,\u0000attesting that it underwent high-pressure and low-temperature conditions\u0000after the granite intrusion and its cooling. The analysis of the coronae and\u0000of a xenolith inclusion found in this granite points to pressure (P) and\u0000temperature (T) estimates of P>1.7 GPa and T=600–650 ∘C for the peak of metamorphism. P–T estimates performed on\u0000the mafic eclogite collected in the vicinity of the metagranite give values\u0000of 2.0–2.2 GPa and 640–680 ∘C, in good agreement with previous\u0000estimates made in other places within the Cellier Unit. The La Picherais metagranite is a key example of undeformed high-pressure metagranite allowing the study of the reactivity and degree of transformation of\u0000quartzofeldspathic rocks during subduction and constitutes a Variscan\u0000equivalent of the Alpine Monte Mucrone or Brossasco–Isasca metagranitoids.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42484621","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"H2O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach","authors":"Patricia Louisa Marks, A. Allabar, M. Nowak","doi":"10.5194/ejm-35-613-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-613-2023","url":null,"abstract":"Abstract. The injection of mafic magma into a hydrous felsic magma chamber is a\u0000potential trigger mechanism for bimodal explosive volcanism. As H2O is\u0000the most abundant volatile component in magmas, the interaction and the\u0000degassing behavior of mildly peralkaline hydrous rhyolitic melt in contact\u0000with hydrous basaltic melt were investigated by decompression experiments.\u0000Preparatory hydration experiments and bimodal magma decompression\u0000experiments, as well as reference experiments, were carried out in an\u0000internally heated argon pressure vessel. Pre-hydrated rhyolite and\u0000basalt cylinders were perfectly contacted together in a precious-metal\u0000capsule, heated to 1348 K at 210 MPa, and thermally equilibrated for 10 min.\u0000The initial sample properties were determined by a bimodal reference\u0000experiment, quenched immediately after equilibration. To simulate the magma\u0000ascent, three bimodal samples and a decompression experiment with two\u0000contacted rhyolite cylinders for testing the experimental setup were\u0000decompressed with 0.17 or 1.7 MPa s−1 to the final pressure of 100 MPa and then quenched. All decompression experiments resulted in vesiculated samples. The H2O\u0000vesicles observed in the decompressed sample of the monomodal\u0000rhyolite–rhyolite reference experiment are homogeneously distributed\u0000throughout the sample. The former interface between the contacted glass\u0000cylinders is invisible after decompression and quench. This reference\u0000experiment proves that the two-cylinder design does not influence the\u0000degassing behavior of the hydrous melt, e.g., an increased formation of\u0000vesicles at possible nucleation sites at the contact plane of the cylinders. The undecompressed bimodal rhyolite–basalt sample shows crystal-free\u0000rhyolitic glass, whereas 3 µm sized idiomorphic magnetite crystals\u0000coexist with glass in the basaltic part of the sample. Within the 10 min run\u0000time, a ∼ 300 µm wide hybrid composition zone developed\u0000between the hydrous rhyolitic and basaltic endmembers, caused by\u0000diffusion-induced mixing processes. Decompression and quenching of the bimodal melts resulted in vesiculated\u0000glass samples. A ∼ 100 µm wide zone of alkali-depleted\u0000rhyolitic glass as part of the ∼ 300–560 µm wide\u0000hybrid zone is covered with an enhanced number of H2O vesicles\u0000compared to the pristine rhyolitic and basaltic glass volumes. We suggest\u0000that this enhanced vesiculated zone forms by a rapid diffusional loss of\u0000alkalis from the mildly peralkaline rhyolitic melt into the basaltic melt of\u0000the sample. The reduced alkali concentration significantly reduces the\u0000H2O solubility of the rhyolitic melt. This process enhances the\u0000H2O supersaturation necessary for vesicle formation during\u0000decompression. In summary, the new findings imply that convective magma ascent driven by\u0000the injection of hot basaltic magma into a hydrous peralkaline rhyolitic\u0000melt reservoir leads to enhanced H2O vesicle formation near the melt\u0000interface and thus to efficient degassing. This in turn can accelerate\u0000buoyancy-","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45979429","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Corundum-bearing and spinel-bearing symplectites in ultrahigh-pressure eclogites record high-temperature overprint and partial melting during slab exhumation","authors":"Pan Tang, Shun Guo","doi":"10.5194/ejm-35-569-2023","DOIUrl":"https://doi.org/10.5194/ejm-35-569-2023","url":null,"abstract":"Abstract. Corundum- and spinel-bearing symplectites after muscovite were found in\u0000ultrahigh-pressure (UHP) eclogites from the Dabie terrane, China. Three\u0000types of symplectites were recognized based on their mineral assemblages:\u0000(1) symplectitic intergrowths of corundum + plagioclase + biotite after\u0000phengite (CPB), (2) symplectitic intergrowths of spinel + plagioclase +\u0000biotite after phengite (SPB), and (3) symplectitic intergrowths of spinel\u0000+ plagioclase after paragonite (SP). The microtextures and mineral\u0000assemblages of the symplectites, in combination with the results of\u0000thermodynamic modeling on local regions, indicate that these symplectites\u0000formed by the breakdown of phengite and paragonite during the granulite-facies metamorphic overprint (770–850 ∘C) of the eclogite at pressures\u0000of 0.8–0.9 GPa. Dehydration partial melting reactions occurred during the\u0000breakdown of muscovite, which leads to the formation of thin plagioclase\u0000films (silicate melts) along grain (garnet, rutile, quartz) boundaries. Mass\u0000balance calculations indicate that the development of CPB and SPB\u0000symplectites after phengite requires the introduction of Al, Ca, Na, and Fe\u0000and loss of Si, Mg, and K. However, the formation of SP symplectites after\u0000paragonite requires the input of Mg, Ca, and Fe and removal of Si, Al, and\u0000Na. By summarizing the occurrence and growth mechanism of\u0000corundum- and spinel-bearing symplectites in global UHP terranes, we find that\u0000such symplectites can form by both the subsolidus replacement of an Al-rich\u0000anhydrous mineral (kyanite) and the dehydration melting of an Al-rich\u0000hydrous phase during high-temperature metamorphism. This study reveals that\u0000muscovite-bearing eclogites may experience multiple episodes of partial\u0000melting during the slab exhumation, not only at the high-pressure (HP)\u0000exhumation stage but also at the lower-pressure metamorphic overprinting stage.\u0000Kyanite is a reaction product during the HP partial melting, whereas the\u0000low-pressure (LP) melting will consume kyanite. We propose that the\u0000occurrence of corundum- and spinel-bearing symplectites after muscovite in\u0000eclogites is a potential mineralogical indicator of LP melting in exhumed\u0000slabs.\u0000","PeriodicalId":11971,"journal":{"name":"European Journal of Mineralogy","volume":" ","pages":""},"PeriodicalIF":2.1,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48391849","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}