The Salmas region in northwestern Iran contains serpentinized peridotites that form part of a Mesozoic ophiolite mélange complex. Based on their mineral assemblages, these peridotites are classified as clinopyroxene-bearing harzburgites. Whole-rock compositions, with oxide contents of MgO 32.59–37.67 wt
Many studies have shown that geological processes, such as metamorphism and metasomatism, consist of multiple superimposed events developing over various timescales, with each stage overprinting the previous one and thus complicating reconstruction of the complete history. Here, we report the use of a natural calcium aluminum borosilicate mineral axinite-(Fe) [(Ca2Fe2+Al2BSi4O15(OH)], which occurs in skarn deposits, as a starting material in a series of experiments representing various metasomatic processes. The fluids used include NaF+H2O, NaCl+H2O, KCl+H2O, CaCl2+H2O, 2 M NaOH, 2 M KOH, Ca(OH)2+H2O, and Na2Si2O5+H2O, which are representative of common crustal fluids. The experiments were conducted at 600 °C; 200 MPa and at 800 °C; 500 MPa. The dominant experimental product was grandite garnet and, to a minor extent, wollastonite, titanite, and fluorite. Grandite garnet compositions vary significantly with respect to the fluid composition and the P-T conditions used, showing a wide range of grossular to andradite solid solutions. Under both P-T conditions, andradite-grossular zoning develops in experiments with metal-hydroxide–bearing fluids, whereas andradite-rich grandite garnet crystallizes in experiments utilizing Na2Si2O5+H2O. Despite varying physicochemical conditions, no differences are observed in the REE-chondrite normalized patterns from the product grandite garnet in each of the experiments. Wollastonite contains bustamite and ferrobustamite components whose composition is strongly controlled by the coexisting grandite garnet. These experiments provide important insights into understanding how first order skarn mineral assemblages may be altered during subsequent fluid-rock interaction under a variety of P-T conditions, highlighting how variations in the grandite garnet composition respond to variations in fluid composition.
The rapakivi microstructure has been widely studied in granites, but few investigations have focused on volcanic rocks, especially Paleoarchean types. The Contendas Rhyolite (CR), approximately 3.3 billion years old, is a rare example of a Paleoarchean volcanic rock with preserved rapakivi microstructure. This work provides a microstructural description, results of energy-dispersive X-ray spectrometry (EDS) analyses, and electron-backscatter diffraction (EBSD) studies of the rapakivi feldspars in the CR. Petrographic analysis shows that the microstructure features a single, thick rim of euhedral to subhedral albite surrounding a core of variable-shaped orthoclase. Unlike rapakivi in other volcanic rocks, which often have dendritic or cellular plagioclase rims, the CR displays well-developed plagioclase rims, indicating that they formed before reaching the surface, during rapid cooling. EBSD confirms that the albite rim shares crystallographic orientation with the orthoclase core, with parallel planes such as (001)Ab || (001)Or, (010)Ab || (010)Or, and (110)Ab || (110)Or, as well as continuity in twinning planes, evidencing epitaxial nucleation. Microstructural and chemical data suggest that sub-isothermal decompression is the primary mechanism of formation for rapakivi feldspars. This study is among the first to describe rapakivi microstructure in a Paleoarchean volcanic rock and to confirm EBSD evidence of crystallographic continuity between rim and core feldspars.
This thermobarometry study employs calcite-dolomite solvus thermometry and thermodynamic pseudosections applied to marbles in the Făgăraș Unit, the uppermost unit of the Variscan metamorphic basement of the Alpine Supragetic units, Central Southern Carpathians. The marbles exhibit extensive relict granoblastic dolomitic bands and quartz inclusions, indicating formation through the metasomatic replacement of dolomite by calcite, aided by a hydrothermal F-rich fluid. The data reveal a thermal history ranging from amphibolite facies (572°–617°C) to greenschist and sub-greenschist facies ( 435°C) during polyphasic metamorphism, followed by late-stage hydrothermal activity (105°–195°C). Mineral assemblages in the marbles suggest equilibration at 435 ± 40°C and 0.42 ± 0.12 GPa, with the CO2/H2O ratio influencing reactions. Talc formation indicates fluid interactions, while tectonic processes controlled hydrothermal fluid flow and pathways. This study quantitatively confirms and refines earlier P–T (pressure–temperature) estimates regarding the metamorphic and hydrothermal evolution of the Făgăraș Unit. The results also reveal a distinct P–T history compared to other metamorphic units within the Alpine nappe stacking in the Făgăraș Mountains and support the interpretation of a complex nappe-stacking system composed of pre-Alpine units juxtaposed during the Variscan orogeny. These units were subsequently thrust during the Alpine orogeny over the Lerești-Călușu Unit, a low-grade metamorphic unit with no marbles and no evidence of polymetamorphism. The observed mineralogical composition, microtextures, and distinct P–T signatures of the dolomitic and calcitic marbles highlight their potential to preserve diagnostic geochemical and isotopic signals, offering valuable insights into the geologic evolution of the host petrologic assemblage.
In the northwestern part of the Afyon Zone, along the northern margin of the Anatolide–Tauride Block, which is generally regarded as a fragment of northern Gondwana during the Late Paleozoic, Triassic metavolcanic rocks are extensively exposed. These rocks comprise felsic and basic types distinguished by mineralogical and geochemical variations. Zircon U-Pb dating of the felsic metavolcanics yields an age of 233.4 ± 2.1 Ma, indicating a Triassic magmatic episode. Metavolcanics have high potassium-shoshonitic and calc-alkaline characteristics. They are enriched in large-ion lithophile elements (LILE), and depleted in high-field-strength elements (HFSE). They also show enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE). The relative enrichment of low-field-strength elements (LFSE) over HFSE, along with average (La/Lu)N ratios of 3–25, reflects typical calc-alkaline volcanism. The geochemical trends suggest that assimilation and/or crustal contamination may have occurred during magma evolution. Furthermore, they are considered to be related to the island arc and resemble Tholeiitic-type mid-ocean ridge basalts. Isotope data supports an enriched mantle source. The initial ⁸⁷Sr/⁸⁶Sr isotope ratios are 0.711429–0.716276 in metadacite and 0.709275–0.714190 in metabasic. The 143Nd/144Nd isotope ratios vary from 0.512349 to 0.512369 in metadacite, and from 0.512463 to 0.512509 in metabasic. The geochemical and isotopic data suggest that the parental magmas were derived from an enriched mantle source metasomatized by subduction-related components beneath the Afyon Zone. The northward subduction of the Paleozoic Tethyan oceanic lithosphere under the southern margin of Laurasia during the Triassic is interpreted to have triggered a regionally rift-related regime along the northern margin of Gondwana. The Triassic metavolcanic rocks are interpreted to represent the earliest magmatic products associated with the Paleotethys Ocean crust, formed along the northern margin of Gondwana.
MgO (periclase, magnesia) is a key component of refractory materials used for the lining of high-temperature aggregates in the metallic and non-metallic sectors alike. The primary source in production is magnesite (MgCO₃), which is thermally decomposed to magnesium oxide. However, the excellent refractory properties of magnesia are only achieved at specific crystallite sizes, which requires thermal post-treatment of MgO using sintering at temperatures of 1500 to 2200 °C in rotary or a shaft kiln or by fusion well above the melting point in an electric arc furnace, respectively. The present contribution is part of an exploratory study that investigates the possibility of obtaining periclase crystals with dimensions of at least 500 μm using alternative synthesis routes that demand lower temperatures. Therefore, the flux method using two alkali borate-based systems were successfully tested. The investigations included the determination of MgO solubilities in a pure NaBO2 melt and a melt of eutectic composition in the system NaBO2-KBO2. It is worthy to note that synthesis runs employing pure potassium metaborate (KBO₂) as a solvent proved less successful and resulted in the concomitant formation of Mg-containing borates such as Mg3(BO3)2. Temperatures and annealing times were systematically varied to identify the optimal growth conditions through slow evaporation of the solvents. The samples were characterized employing the following methods: optical microscopy, SEM, and PXRD. High-quality octahedra of sufficient size and free from flux inclusions were obtained after 96 hours and 120 hours at 1000 °C using the eutectic mixture. The investigation was conceived as a proof-of-concept study. Some issues related to a potential upscaling of the process are also discussed.
The tectonic architecture of a mountain belt is associated to a specific metamorphic architecture, revealing how deep-crustal processes interact during continental subduction and mountain building. Specifically, the Pinerolo Unit in the Dora-Maira Massif (Western Alps) crops out as a tectonic window below the overlying polycyclic ultra-high-pressure units and represents the structurally deepest unit in the Alpine nappe stack. The largest section of the Pinerolo Unit, which is essentially made of graphite-bearing metaconglomerates, metasandstones, and a few metapelites, is exposed in the northern part of the Dora-Maira Massif. Metasandstones and metapelites preserve early, garnet-bearing assemblages that record peak pressure and temperature conditions. Thermodynamic modelling indicates that garnet growth occurs in the quartz stability field, in a narrow temperature range, first during isobaric heating culminating at ⁓1.9-2.0 GPa ⁓500 °C, then at the beginning of decompression. The obtained values are similar to those previously reported for the Pinerolo Unit in the southern Dora-Maira Massif, as well as for the Money Unit in the Gran Paradiso Massif. These results confirm that the internal zones of the Western Alps display an apparent ‘inverted’ pressure gradient, with ultra-high-pressure units overthrusting high-pressure units, resulting from the detachment of subducted crustal slices from the downgoing slab and their stacking during the ensuing continental collision.
Four crystal structures of the uranyl carbonate mineral andersonite were refined from single-crystal X-ray diffraction data. One sample is a natural andersonite from the Monte Cristo mine, Utah, USA. Structure refinement at 297 K yielded the composition Na2Ca[(UO2)(CO3)(3)]& centerdot;5.28H(2)O, space group R(-)3m, a = 17.9012(6) angstrom, c = 23.7978(12) angstrom, Z = 18, and R1 = 0.0139 for 2283 F-o > 4 sigma(F-o). Three synthetic andersonite samples refined at T = 297, 200, and 100 K in space group R(-)3m (R1 = 0.0122-0.0144) have similar compositions with 5.27-5.29 structure-based H2O per formula unit. Unlike the natural sample, they exhibit an additional electron-density peak (X) with rho peak=1.8-2.8 e(-)/angstrom & sup3;, attributed to an as-yet unidentified lattice defect. The crystal structure of andersonite consists of a framework with composition Na2Ca[(UO2)(CO3)(3)]& centerdot;5H(2)O, constructed from uranyl tricarbonate groups and CaO6(H2O) polyhedra organized into {Ca(H2O)[(UO2)(CO3)(3)]}(2-) layers linked by NaO4(H2O)(2) and NaO2(H2O)(4) octahedra. The framework contains a distinctive closed cavity bounded by twelve Na-bonded H2O groups forming a hexagonal prism and six carbonate oxygen atoms. Each cavity accommodates two symmetry-equivalent interstitial H2O molecules, corresponding to 1/3 H2O per formula unit at full occupancy. The result is a unique water-in-water capsule. Site splitting affects approximately & sup1;& frasl;(6) of the twelve H2O groups surrounding the interstitial H2O in all structures and is attributed to hydrogen-bond donation to the cavity water molecules. After providing deeper insight into the complexity of the andersonite structure and its close relationships to l & iacute;nekite and markeyite-type minerals, a comparison with existing crystal structures of andersonite is presented. It is shown that the idealized chemical formula of andersonite should be written as Na2Ca[(UO2)(CO3)(3)]& centerdot;51/3H(2)O and that there is no space to accommodate additional H2O.
Spinel is a key mineral in magmatic systems, preserving signatures of their formation tectonic environments. However, processes like fluid-rock interactions and regional metamorphism can significantly alter their original chemical composition and microtexture. Chloritization is a process where primary spinel can react with surrounding silicates in the presence of H2O and SiO2-rich fluids in a reducing environment, leading to the formation of Cr- and Fe2+-rich ferrous chromite and pores filled with chlorite. While chloritization under reducing conditions is well known, its formation in oxidizing environments is undocumented. In the Archean Somnavandlapalli Ultramafic-Mafic (UM-M) Complex, disseminated Cr-spinels in ultramafic rocks are completely transformed with their primary composition erased, recording their chemical evolution in the progressive oxidizing environment. The Cr-spinel exhibit two distinct textures: clean (inclusion-free) and chloritized porous spinel with chlorite-filled cavities. Both display systematic zoning with core-rim texture, from a modified altered core with ferrian chromite (RIM-I) and Cr-magnetite (RIM-II) formed during progressive hydrothermal alteration under oxidizing conditions. Oxidation-induced chloritization and forming Fe3+-rich and Cr-depleted ferrian chromite (RIM-I) without ferrous chromite, followed by chlorite dissolution and magnetite precipitation, leads to the development of Cr-magnetite (Rim-II). Subsequent re-equilibration and homogenization resulted in the distinct zoning patterns in spinel grains. Final martitization converted magnetite to hematite, marking the terminal stage of the spinel alteration in a progressively oxidizing environment.
Detailed research on a small, isolated granite body, the so-called the Staré mesto granite (SMG), has yielded different information than are known from the main body of the Bratislava massif (Malé Karpaty Mts. in Western Carpathians, Slovakia). The SMG with a known gold mineralization is exposed as a small, elongated granite body hosted within the biotite gneisses of the Pezinok Group that are missing in the main Bratislava granite massif, because of a deeper erosion level. The high-potassium content, peraluminous and magnesium character of studied SMG granites are analogous to the evolved felsic members of the main granitic body. However, new dating results, i.e., a zircon U–Th–Pb isotopic age of 363.5 ± 2.1 Ma and a monazite chemical Th–U–Pb age of 360.7 ± 2.8 Ma are different from the magmatic age of ca. 355–353 Ma determined for the main granite body of the Bratislava massif. It is suggested that melting of the SMG granite source occurred during the initial stage of the Variscan continental collision before ca. 360 Ma, while the main granitic body of the Bratislava Massif was formed in the course of the late-collision granite flare-up before ca. 355–353 Ma.
The Tighza District, located in the NE part of the Moroccan Hercynian Massif, is renowned for its multiple mineralizations (W-Au, Pb-Zn-Ag). This Hercynian chain segment exhibits vigorous magmatic activity, characterized by granitic plutons, dykes, and lenses that extend over several kilometers. These rocks belong to a calc-alkaline series with high potassium content, formed in a geological setting that evolved from syn- to post-collisional conditions between the Upper Carboniferous and Permian. These magmatic associations can be divided into three events: The first event, marked by granitic intrusions, represents the peak of a subsurface plutonic body, forming a contact metamorphic aureole and syn-granite dykes. These dykes, composed of microdiorites, microgranites, and microgranodiorites, exhibit medium- to microgranular textures with biotite, plagioclase, K-feldspar, and quartz. The end of this event is marked by hydrothermal activity, leading to the formation of a quartz vein associated with W-Au mineralization. The second event corresponds to the crystallization of intermediate to felsic dykes (microtonalite, microdiorite, microgranite) belonging to the calc-alkaline suite, indicating a resurgence of magmatism associated with the underlying batholith. The last event is represented by (i) a dense network of dykes in southern Tighza, forming an E-W hypovolcanic complex of rhyolites and microgranodiorites, suggesting emplacement in a sinistral shear context. Disseminated sulfides in these dykes indicate they formed contemporaneously with the hydrothermal event linked to Pb-Zn-Ag mineralization. Furthermore, (ii) the microleucogranites, emplaced in echelon tension slots under sinistral shear controlled by the Tighza fault. They correlate with two-mica granitoids of the Moroccan Central Massif, marking the transition from the Hercynian orogeny to the Permo-Triassic extension.
Tetravalent Si, typically the predominant cation occupying the tetrahedral (T) site of the tourmaline structure, can be partially replaced by B3+ in Al-rich tourmalines. It is relatively common for Al-rich and Li-containing tourmalines (without significant amounts of Mg) to also contain some [4]B when the Al2O3 content is > 38 wt
H2O-undersaturated experiments and phase-equilibrium modelling demonstrate that orthopyroxene is the main ferromagnesian mineral formed during high-temperature crystallisation of most S-type and many I-type granitic (s.l.) magmas. In most examples, at any pressure relevant to crustal magmatism, orthopyroxene will be stable to T > 935 °C, in typical calc-alkaline metaluminous or peraluminous granitic magmas. Once such magmas cool to between 800 and 750 °C, almost all the ferromagnesian component will have crystalised as pyroxene. On further cooling, orthopyroxene will react to biotite within a narrow temperature band ( 30 °C). Textures in granitic rocks attest to this, although the significance of the textures produced by this reaction can be overlooked. Pristine orthopyroxene is rarely found in fully crystallised granitic rocks because such preservation would require solidification at P < 1 kbar. During crystallisation at higher P, biotite-quartz aggregates replace orthopyroxene, resulting in a variety of diagnostic textures. Due to the normal, high-temperature, perisolidus, textural modifications in granitic rocks, the biotite-quartz pseudomorphs after early-formed orthopyroxene may be disregarded or simply described as ‘mafic clots’. As the presence of orthopyroxene has important implications for the temperatures and H2O contents of granitic magmas, we present an atlas of textures to assist in recognising these pseudomorphs. An important implication of the presence of orthopyroxene near the liquidi of many granitic magmas is that they formed at T > 800 °C, through fluid-absent or H2O-undersaturated melting of biotite-bearing or biotite + hornblende-bearing assemblages. Complete supersolidus reaction of orthopyroxene to biotite would be prevented by significant melt loss from the system. Thus, the rarity of orthopyroxene in granitic rocks indicates that fractional crystallization of granitic magmas at T > 750 °C is uncommon and that the loss of a silicic volcanic liquid fraction is also generally unlikely.
In the present study, the stability of zircon and xenotime-(Y) are experimentally tested by using a mixture of solids including xenotime-(Y), zircon, albite, K-feldspar, muscovite, biotite, magnetite, SiO2 and different fluids, i.e. 2 M KOH, 2 M NaOH, Ca(OH)2 solution, and CaCl2 solution, to replicate a granitic alkali- and Fe-enriched environment in four Au-capsules. The experiments were conducted at 200 MPa, 650 °C and over 7 d. The zircon and xenotime-(Y) were altered at various degrees. The experiment with 2 M KOH resulted in the lowest degree of zircon and xenotime-(Y) alteration, whereas experiments with 2 M NaOH and Ca(OH)2–solution resulted in a moderate degree of alteration with well-developed porosity and partial dissolution of zircon, xenotime-(Y), and magnetite. The U-Pb ages of zircon are significantly disturbed resulting in a reverse discordant trend of data that plot along the concordia curve towards older ages. Xenotime-(Y) U-Pb ages remained mostly undisturbed. The experiment with CaCl2–solution was the most effective considering alteration of the entire mineral assemblage. Xenotime-(Y) and zircon are strongly altered with significantly disturbed U-Pb ages. Furthermore, xenotime-(Y) is partially replaced by secondary britholite-(Y). The experimental results provide new insights to our understanding of xenotime-(Y) and zircon interactions with alkali- and alkaline-rich fluids. The particular importance is that of the observed reverse discordant U-Pb ages of altered zircon and, to a lesser degree, xenotime-(Y) towards older ages along the concordia curve, which may be related to coupled U6+ and Fe2+ mobilization by Fe-bearing reactive fluids due to dissolution of magnetite.
Four crystal structures of the uranyl carbonate mineral andersonite were refined from single–crystal X–ray diffraction data. One sample is a natural andersonite from the Monte Cristo mine, Utah, USA. Structure refinement at 297 K yielded the composition Na2Ca[(UO2)(CO3)3]·5.28H2O, space group R3m , a = 17.9012(6) Å, c = 23.7978(12) Å, Z = 18, and R1 = 0.0139 for 2283 Fo > 4σ(Fo). Three synthetic andersonite samples refined at T = 297, 200, and 100 K in space group R3m (R1 = 0.0122–0.0144) have similar compositions with 5.27–5.29 structure–based H2O per formula unit. Unlike the natural sample, they exhibit an additional electron-density peak (X) with ρₚₑₐₖ=1.8–2.8 e⁻/ų, attributed to an as–yet unidentified lattice defect. The crystal structure of andersonite consists of a framework with composition Na2Ca[(UO2)(CO3)3]·5H2O, constructed from uranyl tricarbonate groups and CaO6(H2O) polyhedra organized into Ca(H2O)[(UO2)(CO3)3]2− layers linked by NaO4(H2O)2 and NaO2(H2O)4 octahedra. The framework contains a distinctive closed cavity bounded by twelve Na–bonded H2O groups forming a hexagonal prism and six carbonate oxygen atoms. Each cavity accommodates two symmetry–equivalent interstitial H2O molecules, corresponding to ⅓ H2O per formula unit at full occupancy. The result is a unique water-in-water capsule. Site splitting affects approximately ¹⁄₆ of the twelve H2O groups surrounding the interstitial H2O in all structures and is attributed to hydrogen–bond donation to the cavity water molecules. After providing deeper insight into the complexity of the andersonite structure and its close relationships to línekite and markeyite-type minerals, a comparison with existing crystal structures of andersonite is presented. It is shown that the idealized chemical formula of andersonite should be written as Na2Ca[(UO2)(CO3)3]·5⅓H2O and that there is no space to accommodate additional H2O.
This study presents new U–Pb–Hf isotopic data and zircon ages from the Ediacaran to Ordovician Ötztal Complex of the Eastern Alps in Austria to provide new constraints on the evolution of the northern Gondwana active margin in the “proto-Alpine” realm. The multistage tectonic evolution of the complex started with siliciclastic deposition presumably in an accretionary wedge that may have lasted from ca. 600 Ma to ca. 517 Ma. The age spectra are dominated by Neoproterozoic zircon grains indicating that the complex was most likely sourced from the Arabian–Nubian shield, with a contribution of older Proterozoic and Archean grains from the more westerly Saharan metacraton. The deposition was partly overlapping in time with Cambrian to Early Ordovician mafic magmatism that formed either as mafic underplate below the accretionary wedge or outboard, being later accreted as part of the lower plate. The wedge was then intruded by compositionally diverse granitoids from ca. 500 Ma until ca. 440 Ma. By comparing the Ediacaran and Early Paleozoic evolution of the Ötztal Complex with originally more westerly Cadomian-basement terranes (e.g., those now found in the Bohemian Massif), we concluded that the Cenerian orogeny was generally younger in the proto-Alps than elsewhere in the former Cadomian belt. This was possibly due to a significantly curved geometry of the northern Gondwana margin and/or due to an eastward ridge–trench–transform triple point migration. Arrival of a warmer part of the oceanic plate then may have caused mantle melting, mafic underplating, and voluminous granitic plutonism in the forearc, perhaps finally terminated by ridge–trench interaction and slab break-off.
The thermal decomposition of four iron carbonates (siderites) and their decomposition products in a vacuum ( 2·10− 5 Pa) was investigated for the first time using in-situ transmission 57Fe Mössbauer spectroscopy. These measurements were supported by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, magnetic measurements, and transmission electron microscopy. Mössbauer spectra were collected from room temperature (RT) to 750 °C, and the sample was then cooled to RT. The initial samples contained siderite as the only Fe-mineral, and its content in the analyzed samples was greater than 80
The Chumathang granitoid of eastern Ladakh Trans-Himalaya records a magmatic evolution from subduction-related arc magmatism to syn-collisional crustal melting during the Himalayan orogeny. Field and petrographic relations define two contrasting phases an early hornblende-bearing metaluminous granodiorite and a later muscovite-bearing peraluminous leucogranite. The granodiorite displays calc-alkaline, magnesian and oxidised signatures (SiO2= 63.7–68.7 wt
