EPMA Data Research Articles

Mineralogical and geochemical studies on the Qianjiadian deposit, Songliao Basin, NE China: Insights into multiple metallogenic processes in the sandstone-type uranium deposit

Sandstone-type uranium deposits usually exhibit a range of metallogenic mechanisms. To gain insights into the nature, genetic relationships, and mineralization mechanisms of uranium and associated minerals, representative samples from the Qianjiadian uranium deposit in the Songliao Basin were investigated through detailed mineralogical, elemental geochemical, and in-situ sulfur isotopic analyses. The results provided invaluable information on the various stages of uranium mineralization. During the early diagenetic stage, uranium mineralization occurred in the local organic matter (OM)-enriched sites (TOC = 0.70 ∼ 2.05 %) in the Yaojia Formation. The paragenetic relationship revealed that uranium was firstly precipitated as pitchblende along with the biogenetic framboidal pyrite (δ34S values ranging from −44.1 ‰ to −1.8 ‰) through bacterial sulfate reduction (BSR) process. Simultaneously, organic acids generated through biodegradation of OM caused an acidic fluid condition and subsequent dissolution of feldspars, resulting in the production of kaolinite intergrowth with pitchblende. During the late diagenetic stage, the ore-forming fluid turned alkaline, leading to the conversion of pitchblende into coffinite, with elevated elements such as Si, P, Ca, Zr, and Y in the altered phase. In addition to the contribution of OM and sulfate-reducing bacteria (SRB), pyrite was confirmed as a reducing agent manifested by its replacement by uranium minerals and the Fe-oxide produced along the boundaries. The diagenetic uranium enrichment is mainly hosted in the gray sandstone that experienced reduced alteration of deep hydrocarbons, resulting in pervasive kaolinization and promotion of reduction potential. A significant amount of uranium was fixed by adsorption, as evidenced by the rare observation of independent uranium minerals, but the higher whole-rock U content (U = 133.5 ∼ 394.0 ppm) and the detection of U signal in micro-XRF analysis and EPMA data (U content in representative kaolinite ranges from bld to 0.74 wt%). Overall, the formation of sandstone-type uranium deposits is a long-term and multi-stage process. The early enriched uranium during synsedimentary-diagenetic stages is non-negligible and can even result in an appreciable amount of mineralization, further serving as an important source for the subsequent secondary enrichment.

A first record of alluvial gold in the Olănești and Cheia rivers, Southern Carpathians, Romania

We provide the first scientific record and the mineralogical characterization of alluvial gold in the Olănești area, Southern Carpathians, Romania, based on field evidence, optical microscopy, and XRD, BSE and EPMA data. Chemical data were acquired on 11 alluvial gold grains from the Olănești and Cheia rivers and revealed a generally continuous variation in gold and silver content with Ag ranging from 7.31 to 19.77 wt.% and Au ranging from 80.26 to 93.16 wt.%. The source of the primary native gold is inferred to be the Cu-Au Valea lui Stan and/or the Costești As-Au shear-zone-related ore deposits located towards the north and west, respectively, of the study area.

Accretionary and collisional processes of the Brasiliano Orogeny recorded by the Tonian La Tuna ophiolite, Dom Feliciano Belt, Uruguay

Accretionary to collisional orogenies such as the Brasiliano Orogeny of eastern South America present the opportunity to distinguish between oceanic-continental (accretionary) and continental (collisional) processes. However, the multitude of geotectonic processes involved and the reduced degree of preservation of the accretionary stage restrain the interpretation of evidence. This study focuses on a metasomatite associated with the La Tuna Ophiolite in the southern Dom Feliciano Belt of the Uruguayan Shield, as these can provide crucial insights into the accretionary stage of the Brasiliano Orogeny. The metasomatite occurs as a meter-sized massive tourmalinite body surrounded by a meter-wide chloritite lens, both enclosed by the La Tuna serpentinite. Our SEM-EDS and EPMA data revealed that the serpentines from the La Tuna ophiolite are antigorite and lizardite, while chlorite was identified as clinochlore. Abundant inclusions of sheridanite chlorite in dravite tourmaline suggests that tourmaline overgrew chlorite. Dravite crystals revealed the presence of three diffuse zones with minor compositional variations, mainly enrichments in Ca and Fe towards the rims. The U-Pb LA-ICP-MS dating of metasomatic zircon yielded ages of 743.2 ± 3.4 Ma (n = 22) for the chloritite and 743.1 ± 2.4 Ma (n = 31) for the tourmalinite. High δ11B values (+12.1‰ to + 18.1‰) of tourmaline (LA-ICP-MS) suggest that the metasomatic event was caused by seawater-derived fluids in a fluid-dominated environment. The single metasomatic U-Pb age combined with the high δ11B values suggest that the metasomatites underwent more intense metasomatic events than those associated with the Bossoroca (−8.5‰ to +1.8‰) and the Ibaré ophiolites (+3.2‰ to +5.2‰) further north in the São Gabriel Terrane of the Dom Feliciano Belt. Nonetheless, a few spots in tourmaline located within fracture zones and in contact with chlorite displayed negative (c. −5‰) and low positive δ11B values (c. 2‰). These are interpreted as overprinting by crustal fluids, either during the tectonic emplacement of the La Tuna Ophiolite or during exhumation. The calculated TDM Hf model ages for zircon in tourmalinite and chloritite range from 0.74 to 1.09 Ga and their ɛHf(T) values range from +4.9 to +11.2, indicating derivation from depleted mantle sources. The data corroborate earlier results indicating that the associated La Tuna amphibolites have composition similar to N-MORB. Furthermore, the trace element composition of zircon in the metasomatites is consistent with an oceanic environment (U/Yb < 0.1). Geotectonically, the La Tuna metasomatites formed at 743 ± 2 Ma in an oceanic setting, most likely in a juvenile Adamastor Ocean basin. During the collisional stage of the Brasiliano Orogeny, the La Tuna Ophiolite was tectonically emplaced within the <666 Ma La Micaela Schist of the Paso del Drágon Complex. As such, this study on the La Tuna Ophiolite contributes direct evidence on the oceanic evolution leading up to the accretionary stage of the Brasiliano Orogeny and provides new constraints on the transition towards the collisional stage.

Constraints on Crystallinity of Graphite Inclusions in Nephrite Jade from Xinjiang, Northwest China: Implications for Nephrite Jade Formation Temperatures

Graphite usually occurs in mineral/rock associations in the form of solid inclusions and plays an important role in tracing regional metamorphic degree, ore-forming temperature, fluid evolution, as well as the deep carbon cycle of the Earth. In this study, we investigate the placer black nephrite jade where the co-occurrence of abundant graphite inclusions and jade remains extraordinary. By employing petrographic, mineral-chemical, and Raman spectroscopic methods, we characterize the textures and crystallinity of graphite inclusions that exist in nephrite jade. EPMA and petrological data indicate that the main constituents of black jade are tremolite and graphite, with minor phases of diopside, calcite, dolomite, epidote, and apatite. Micro-Raman spectroscopic thermometry of carbonaceous material shows that most of the formation temperatures of graphite inclusions are between 378 and 556 °C, and only a few temperatures may be above 650 °C, indicating that graphite inclusions were formed at medium- to high-temperature metamorphic facies. The petrologic and spectral investigations of graphite inclusions in these nephrite jade samples show major metamorphic signatures with mixed features associated with fluid precipitation. Our results allow us to propose that primary nephrite jade was formed under multi-stage tectonic evolution conditions, and regional temperatures were predominately driven by the late continent–continent collision, while the ore-controlling temperatures of nephrite jade formation were found in a medium- to high-temperature environment.

GEOCHEMISTRY OF HIGH-PHOSPHORUS ZIRCON FROM THE UPPER RIPHEAN METASANDSTONES OF THE SOUTHERN TIMAN

A detailed mineralogical and geochemical study (using the EPMA and SIMS methods) of zircon from metasandstones of the Dzhezhim Formation in the South Timan made it possible to establish an anomalously high content of phosphorus in it (up to 10.27 wt % P2O5 according to EPMA data), which correlates with an increased content of other non-formula admixture elements, the main of which are Y, REE, Ca, Fe, Al, Ti, Sr, Ba, Th, U. Of particular note is the significant amount of volatile components in zircon (up to 0.49 wt % water and up to 0.26 wt.% fluorine, determined by the method SIMS). The total content of non-formula admixture elements can exceed 20 wt %, which is a characteristic feature of the composition of zircon exposed to fluid or formed as a result of hydrothermal-metasomatic processes. The main mechanism for the incorporation of admixture elements into the composition of zircon was xenotime-type heterovalent isomorphism, when the presence of quinquevalent phosphorus is compensated by the participation of trivalent Y and REE. Of subordinate importance was the isomorphism scheme, which ensures the entry of hydrogen (water). The areas of zircon enriched in phosphorus and other adulterants are confined to the edge of the grains, or to systems of scissures and fluid-permeable areas. Granitoids served as a possible source of zircon, and the process of transformation of its composition (recrystallization and enrichment of local areas, less often whole grains) probably occurred already in the sandstone sequence, during metamorphism and/or hydrothermal process.

Novelty on deep reduction roasting through removal of lead and zinc impurities from iron ore afore ironmaking

The novelty of deep reduction roasting is that iron (Fe) can be recovered to high degrees and a large proportion of zinc and lead impurities can be simultaneously removed. In this work, deep reduction roasting was performed at different temperatures, and phase changes were investigated during ironmaking. Haematite is the main Fe-bearing minerals in Xinjiang, China. It contains up to 91.35% iron and minor amounts of magnetite and iron silicate. The main form of zinc impurities is zinc oxide, and lead sulphide is present with low abundance. When the roasting temperature was 900 °C–1000 °C, the amount of coal reductant powder was 11%, the grade and recovery of Fe were 86% and 70% respectively. At 1050 °C–1200 °C, coal powder was added at C/O = 2.25 due to high coal powder consumption, and the magnetic separation concentrate products were obtained with an Fe grade of 86.34%–93.09% and Fe recovery rate of 75.06%–86.49%. In line with the requirement that the content of lead and zinc in iron concentrate powder products be less than 0.1%, the minimum contents of lead and zinc impurities in the magnetic separation concentrate were only 0.0084% and 0.048%, respectively. EPMA and EDS data revealed that after roasting, lead, zinc and sulphur impurities showed significant reductions and were absent from most microscopic areas. This study demonstrated that deep reduction roasting is suitable for removing lead and zinc impurities from iron ores before ironmaking.

A data screening approach to confirming a target mineral is chlorite using EPMA and LA‐ICPMS data

AbstractApplying machine learning techniques to large datasets of in situ analyses has been proven to be a powerful tool in Earth Sciences. However, problems may arise when dealing with minerals such as chlorite, that exist as a solid solution rather than a single, stoichiometric ideal. It can be difficult to determine whether the variations in major element concentrations are due to compositional difference in the mineral of interest or due to sampling of the surrounding mineral phases in addition to the mineral of interest during the analyses. If the latter, interpretations of the results would be complicated, misled or even spurious. Here we present a method to identify chlorite based on the major and minor element content, from both LA‐ICPMS and EPMA data. Further we present a dataset of 3,317 analyses of chlorite and have shown that 7.4% of these analyses include significant quantities of non‐chlorite material.

Secondary fluorescence effect quantification of EPMA analyses of olivine grains embedded in basaltic glass

Trace element analysis of olivine is challenging but vital, since trace elements in olivine can potentially record crystallization conditions and, correspondingly, conditions of magma generation, evolution, and transport dynamics. Electron microprobe analysis (EPMA) is one of the most widely used methods for trace element analysis in olivine for many reasons including but not limited to high-spatial resolution and high precision. EPMA protocols with high-beam currents and long-counting times provide accurate trace element concentrations with low-detection limits. However, these protocols neglect the effect of secondary fluorescence (SF) across phase boundaries, which may lead to fictitious analytical results when analyses are performed near other phases. In these cases, obtaining accurate EPMA data requires correction of SF effects from the adjacent or surrounding phase. In this study, the contents of olivine trace elements Ca, Ni, Mn, Ti, Cr, and Al have been measured in run products of experimental high-temperature studies on mafic magmatic systems consisting of olivine grains embedded in basaltic glass. Olivine grain fragments with known composition, were embedded in basaltic powder, and heated to 1250 °C in 1-atm vertical gas-mixing furnace for a few minutes and quenched. EPMA analyses of the olivine grains before and after the experiments were used to quantify the effect of SF, which was found to be most significant for Ca and Ti, and to a lesser extend for Al, and to depend on the distance from the interface, olivine grain geometry, accelerating voltage, and olivine and glass compositions. By combining our experimental results with systematic SF calculations performed with the Monte Carlo simulation program PENEPMA, analytical expressions that give the apparent Ca, Ti, and Al concentrations due to SF in equant olivine grains embedded in basaltic glass were derived. These equations, which allow for a rapid correction of SF effects, are used to assess the average overestimation due to SF of olivine analyses included in current experimental databases, which might be the source of major discrepancies observed in petrological tools utilizing trace element contents in olivine.

Geology of the Copper Mineralization in Proterozoic Ajabgarh Meta-Sediments of, Dokan-Dariba Belt, Sikar District Rajasthan Northwestern India

The Khetri copper belt is a well-known metallotect in northern part of Delhi fold belt in Rajasthan. On the eastern margin of the Khetri sub basin of North Delhi basin separated by a basement high, another sub basin Alwar-Ajabgarh sub basin exposes that a thick sequence of Ajabgarh group of rocks overlies a thick arenaceous sequence of Alwar group of Delhi Super Group of rocks. The Ajabgarh meta sediments here in the Neem Ka thana area are characterized by presence of Bornite dominated copper mineralization with silver association and minor presence of Pb. The mineralization has been described by various workers as strata-bound, hypogene and IOCG. But these inferences are based on part information and the inference drawn is sectorial in nature. The current study includes a holistic study based on exploration over a period of more than two decades and the data generated suggest thereof, that this syngenetic sulphide mineralization associated with the sedimentation of marl and carbonate rocks. Subsequently it has been relocated during 2nd deformation accompanied by epigenetic component of mineralization depicted in terms of vein filled coarse grained aggregates of bornite and chalcopyrite disposed across the general disposition of litho-package. The EPMA and fluid inclusion data generated from the area indicate association of typical hydrothermal environment minerals like, Perkrite, Wittchenite, Aguilarite, Molybdnite etc. The mineralizing fluids have been trapped between the temperature ranges of 130°C to 375°C with average being 250°C to 300°C. The fluid salinity also varies from near pure hot water to moderately saline fluid indicative of multi-episodic mineralization of syngenetic nature coupled with epigenetic component. The ore textures indicate 500°C temperature range; certain intergrowths of minerals like specular hematite and bornite suggest the occurrence of hypogene environment induced due to emplacement of granite/pegmatite on the eastern and southern margins of the belt. The strata bound nature suggests the euxogenic environment facilitated by carbonate facies of rocks. The parallelism of sulphide with the rock fabric was attained during first deformation and the epigenetic component coupled with the hypogene assemblage was deposited during the D2 deformation in the brittle ductile shear zones and limb shears, between the temperature ranges of 130°C to 570°C as deduced from petrochemical data. Hence a comprehensive model is suggested here on evolution of process of mineralization in the Neem Ka Thana belt.

Chemismus a klasifikace minerálů skupiny tetraedritu z ložisek v Bolívii

The quantitative study of chemical composition of ten samples of the tetrahedrite group minerals from six deposits in Bolivia (Animas - Chocaya, Pulacayo, San Vicente, Machacamarca, Cerro Rico, Oruro) provided new data enabling their detailed classification within this group. The majority of samples are usual members of tetrahedrite group: tennantite-(Zn) (San Vicente, Oruro), tetrahedrite-(Zn) (Animas - Chocaya, Pulacayo, San Vicente, Cerro Rico, Oruro) and tetrahedrite-(Fe) (Pulacayo, Machacamarca, Cerro Rico). The recently redefined members of this group - argentotetrahedrite-(Fe) and kenoargentotetrahedrite-(Fe) were found in samples from Cerro Rico and Animas - Chocaya deposits, respectively. The descriptions and quantitative EPMA data for all studied samples are given in the paper.

MagMin_PT: An Excel-based mineral classification and geothermobarometry program for magmatic rocks

ABSTRACTIgneous rock forming minerals carry valuable information from the deep earth that is not directly accessible at the surface. Each mineral represents the physico-chemical conditions at which various magmatic processes have occured over a wide range of depths from upper mantle to shallow crustal levels. These processes are cryptically inscribed in the whole-rock and mineral compositions (e.g. major elements, trace elements and isotopic ratios) and textures (equilibrium vs. disequilibrium features), together with intensive variables (e.g. pressure, P; temperature, T). Therefore, particular attention should be given to igneous minerals to understand better the processes that took place during their journey from the source through magma chambers and conduit systems to the Earth's surface.MagMin_PT is an Excel© based user-friendly program, designed to calculate mineral formulae and end-members, and to estimate pressure and temperature (e.g. geothermobarometry) from electron microprobe analytical data. The program operates using the most common igneous rock-forming minerals (olivine, pyroxene, amphibole, biotite, feldspar, magnetite, ilmenite, apatite and zircon), resulting in various classification diagrams and P–T diagrams. The program allows for whole-rock or glass composition to be entered together with the EPMA data to evalaute the equilibration status for most P–T calculation models. Fe2+ and Fe3+ estimation is routinely performed in MagMin_PT based on stoichiometric constraints, and to some extent using machine learning methods for different iron-bearing minerals. MagMin_PT is also able to carry calculations of fugacity, magmatic water content and saturation temperature. Graphical and numerical outputs produced by the program can be easily copied to other media for further processing.

Evolution of the Tepeoba porphyry-skarn Cu-Mo-Au deposit, NW Turkey: New mineralogical and geochemical findings

This study recently investigates the evolution of the Tepeoba porphyry Cu-Mo-Au deposit (Cu ∼ 3 wt%, Mo ∼ 0.16 wt%, Au ∼ 10 ppm) with associated skarn prospects in the Biga Peninsula (NW Turkey) using the geochemical and mineralogical data. Ore-related alteration types which include biotite (±muscovite)-K-feldspar at the center of the mineralization zone, actinolite-albite alteration, and outer chlorite-epidote-calcite mineral facies, are observed in the Tepeoba mine area along the southern contact between the Karakaya Complex (mainly hornfels at the mine area) and porphyry Eybek intrusion. Porphyry style alteration pattern has been detected recognized in the porphyritic microgranite host rock. On the other hand, the calcareous and carbonate as well as the mineralized hornfels refer to the skarn style alteration pattern associated with a porphyritic felsic stock. Sulfide-rich stockwork and mineralized brittle brecciated, and fracture zones are associated with these hydrothermally altered wallrocks. Ore mineralogy includes chalcopyrite, molybdenite, pyrrhotite, pyrite, subordinate cubanite and native gold. These minerals are replaced by marcasite, hematite, and goethite in the oxidized cap rocks. EPMA data of the alteration and sulfide minerals revealed that Cu-, Fe-, Mo-sulfides partially replace biotite ± muscovite-K-feldspar/actinolite-albite and chlorite-epidote-calcite alteration facies. The estimated ore formation conditions returned wide range of temperatures of 546–471 °C (biotite thermometry) to 340–264 °C (chlorite thermometry), at which sulfides had most likely precipitated while the oxygen fugacity of the magmatic-hydrothermal ore fluids was gradually lowered and developed during the stage of retrograde metasomatic reactions. Chalcopyrite and cubanite was likely to have formed via un-mixing of intermediate solid solution (iss) under oxidizing hydrothermal conditions. Under these conditions, thiosulfide complexes could have been responsible for the transport and deposition of gold. This also refers to native gold, which could have been liberated from sulfide ores (i.e., pyrite) in the oxidation zone due to the processes of supergene enrichment.

High-resolution stalagmite stratigraphy supports the Late Holocene tephrochronology of southernmost Patagonia

Volcanic ash layers are important markers for the chronostratigraphy of paleoclimate and paleoenvironmental archives at the southern tip of South America. However, this requires that tephras are well-dated. We report geochemical data from stalagmite MA1 formed in a non-karst cave near Mt. Burney volcano in southernmost Patagonia (~53°S). High-resolution LA-ICP-MS analyses, SEM imagery, EPMA data, and NanoSIMS enable to identify volcanogenic signals during the last 4.5 kyrs from sub-annual trace element variations and tephra particles in distinct laminae. Our new 230Th/U-chronology of MA1 provides precise dating of tephra from Mt. Burney (MB) and, probably, Aguilera (A) at 4,216 +93/−193 yrs BP (MB2), 2,291 ± 33 yrs BP (MB3), 853 +41/−60 yrs BP (MB4) and 2,978 +91/−104 yrs BP (A1). This unique high-resolution record holds potential to date further eruptions from Southern Andean volcanoes for the tephrochronology in this critical region, and potentially also large-volume explosive volcanism off South America.

MinPlot: A mineral formula recalculation and plotting program for electron probe microanalysis

Abstract MinPlot is a MATLAB®-based mineral formula recalculation and compositional plotting program for electron microprobe analyses (EPMA). The program offers recalculation and structural formula assignment for 15 different mineral groups: Garnet, pyroxene, olivine, amphibole, feldspar, mica, staurolite, cordierite, chlorite, chloritoid, talc, epidote, titanite, spinel, and sulfides. MinPlot is a fast and easy to use command line program and requires no prior computer programming knowledge. Percent mass fractions of oxides are loaded from datafiles and the user answers simple prompts to select mineral type, normalization scheme, and plotting options. Recalculated mineral formulas are automatically saved as output files and plots may be further manually customized by the user prior to saving. MinPlot can perform thousands of calculations in seconds and the modular nature of the program makes it simple to add new calculation routines in future releases. Combined, these features make MinPlot a powerful and useful program for the processing of EPMA data.

Identification of Fe3+ content in Epidote from Varan, Urumieh-Dokhtar Magmatic Arc, Iran: using FTIR and Raman spectroscopy

This study is aimed to determine the Fe content in natural epidote from Varan area (Urumieh-Dokhtar Magmatic Arc, Iran) by using vibrational FTIR and Raman spectroscopy and EPMA analyses. Fe3+ concentration calculated from FTIR spectroscopic data is in the range of 0.96 to 1 apfu. The results are in complete agreement with EPMA data. The comparison between obtained Raman spectra of studied epidote grains and those from the RRUFF database suggest that epidote from Varan area is rich in Fe3+. High Fe3+ content might reflect moderate to high oxygen fugacity during the crystallization of epidote. This short paper demonstrates that the quantification of the Fe content in epidote via FTIR method is as good as EPMA, whereas the utilization of a low-cost Raman spectrometer helps in quickly distinguishing between Fe-rich and Fe-poor epidote, which even could be useful in the case of field studies.spectrometer helps in quickly distinguishing between Fe-rich and Fe-poor epidote, which even could be useful in the case of field studies.Key-words: Epidote, FTIR, Raman spectroscopy, Iron content, Urumieh-Dokhtar Magmatic Arc, Iran.

Machine Learning Applied to K-Bentonite Geochemistry for Identification and Correlation: The Ordovician Hagan K-Bentonite Complex Case Study

Altered tephras (K-bentonites) are of great importance for calibration of the geologic time scale, for local, regional, and global correlations, and paleoenvironmental reconstructions. Thus, definitive identification of individual tephras is critical. Single crystal geochemistry has been used to differentiate tephra layers, and apatite is one of the phenocrysts commonly occurring in tephras that has been widely used. Here, we use existing and newly acquired analytical datasets (electron probe micro-analyzer [EPMA] data and laser ablation ICP-MS [LA-ICP-MS] data, respectively) of apatite in several Ordovician K-bentonites that were collected from localities about 1200 km apart (Minnesota/Iowa/Wisconsin and Alabama, United States) to test the use of machine-learning (ML) techniques to identify with confidence individual tephra layers. Our results show that the decision tree based on EPMA data uses the elemental concentration patterns of Mg, Mn, and Cl, consistent with previous studies that emphasizes the utility of these elements for distinguishing Ordovician K-bentonites. Differences in the experimental setups of the analyses, however, can lead to offsets in absolute elemental concentrations that can have a significant impact on the correct identification and correlation of individual K-bentonite beds. The ML model using LA-ICP-MS data was able to identify several K-bentonites in the southern Appalachians and establish links to K-bentonites samples from the Upper Mississippi Valley. Furthermore, the ML model identified individual layers of multiphase eruptions, thus illustrating very well the great potential of applying ML techniques to tephrochronology.

On the improvement of calculating biotite formula from EPMA data: Reexamination of the methods of , , and reply to the discussion of Baidya and Das

On the improvement of calculating biotite formula from EPMA data: Reexamination of the methods of , , and reply to the discussion of Baidya and Das

LA-ICPMS, TEM and Raman study of radiation damage, fluid-induced alteration and disturbance of U-Pb and Th-Pb ages in experimentally metasomatised monazite

Experimentally metasomatised monazite was studied in terms of preservation of U-Pb and Th-Pb ages during alkali-bearing fluid-induced alteration over a broad range of temperature conditions 250–750 °C. Starting materials for experiments included Burnet monazite (Concordia age 1100.5 ± 11.6 Ma, 2σ), albite, K-feldspar, biotite, muscovite, SiO2, CaF2, Na2Si2O5 and H2O. Monazite from experiments at 250–550 °C is partially replaced by secondary REE-rich fluorapatite [(Ca,LREE,Si,Na)5(PO4)3F], fluorcalciobritholite [(Ca,REE)5(SiO4,PO4)3F] and REE-rich steacyite [K1−x(Na,Ca)2(Th,U)Si8O20], and developed patchy zoning, whereas partial replacement by fluorcalciobritholite and cheralite [CaTh(PO4)2] occurred at 650 and 750 °C, with no signs of compositional alteration based on EPMA data and BSE imaging. Raman microspectroscopy results show narrowing of the ν1(PO4) stretching band in unaltered domains, which indicates advancing annealing of the monazite structure with increasing temperature, and narrow ν1(PO4) band with low FWHM values in altered domains. TEM investigations revealed that unaltered domains of monazite from experiments at 250–550 °C have mottled diffraction contrast, similar to the starting Burnet monazite, which indicates low to moderate degree of metamictization. On the contrary, the altered domains of monazite (patchy zones) show no mottled contrast, suggesting an ordered crystalline structure. TEM imaging demonstrated low degree of metamictization in monazite from the experiment at 650 °C; fluid-aided alteration along the cleavage planes resulted in the development of nanoporosity or partial replacement by fluorcalciobritholite and cheralite. Monazite from the experiment at 750 °C has crystalline structure with no signs of metamictization and shows significant development of nanoporosity and formation of secondary cheralite nanocrystals across the grain. For comparison, TEM and Raman evaluation of xenotime from similar experiments at 350 and 650 °C revealed that both starting xenotime and xenotime from experimental products are crystalline with no signs of radiation damage or fluid-induced alteration affecting internal domains on submicron scale, which could result in compositional alteration of the xenotime.The unaltered domains of monazite from runs at 250–550 °C yielded U-Pb and Th-Pb dates similar to the age of Burnet monazite, whereas altered domains yielded discordant dates due to various degree of Pb-loss (up to 99.4%). Linear regressions on the Concordia diagrams show lower intercept ages from −266 ± 160 Ma (run 350 °C, 200 MPa) to −1 ± 48 Ma (450 °C, 800 MPa), which reflect the “true age” of experimental alteration. The monazite from runs at 650 and 750 °C yielded data indicating initial disturbance of the U-Th-Pb system, ranging from 8.4% Pb-gain to 18.6% Pb-loss. Linear regressions with lower intercepts of −53 ± 420 Ma and −55 ± 610 Ma roughly correspond to the timing of the experiments. Furthermore, LA-ICPMS results demonstrate discrepancy between Th-Pb and U-Pb dates suggesting higher mobility of 208Pb than that of 207Pb and 206Pb.To summarize, TEM and Raman data indicate increasing annealing of the radiation damaged monazite with increasing temperature. Alteration processes induced by alkali-bearing fluid can result in recrystallization of monazite and various degrees of the age disturbance at temperatures 250–550 °C, whereas isotopic U-Th-Pb microanalysis provide an opportunity to constrain the age of the metasomatic processes as the lower intercept in the Concordia diagram. The particular importance of this study lies in submicron alteration of monazite at 650–750 °C induced by alkali-bearing fluid and/or melt, which remains unnoticed using common electron microscopy BSE imaging. Such alteration, however, induces substantial disturbance of U-Pb and Th-Pb ages, which can cause misinterpretations in reconstructions of geological processes.

U-Pb dating and composition of columbite from Vishteritsa: Implication for timing of granite magmatism and rare-element granitic pegmatites in the Western Rhodopes, Bulgaria

The economic significance of pegmatites as a source of strategic rare metals for high-tech products and green energy motivated the present study on Ta–Nb oxides from Vishteritsa rare-element beryl–columbite LCT pegmatites of the Rila–West Rhodopes Batholith in the Western Rhodopes, Bulgaria. Here, we present the first U/Pb age data from columbite with application of the LA–ICP–MS U–Pb technique and a new X36 columbite standard reference material. The obtained Concordia age of 47.57 ± 0.32 Ma with a small spread of the individual 206Pb/238U ages between 45 and 51.3 Ma argues for Early Eocene magmatism and pegmatite formation. The host granite of the rare-element pegmatites is dated 51.94 ± 0.61 Ma with LA–ICP–MS U–Pb technique on zircon and suggests a fertile Early Eocene magmatic period in the Western Rhodopes. EPMA data for the composition of the columbite is used to refine the formula of the mineral (Mn0.554Fe0.427U0.006)0.987(Nb1.826Ta0.085Ti0.116)2.03O6 and define it as columbite-(Mn). Application of the in-situ LA–ICP–MS data technique establishes a series of typical trace elements (Ti, U, Zr, Hf, Y, W, and Zn) that are usually found in content above 500 ppm. The studied columbite is enriched in heavy rare earth elements (HREE sum: 306–697 ppm) and depleted in light REE and Eu. These geochemical characteristics are collectively interpreted as evidence for crystallization from highly fractionated fluid-rich magma. High UO2 content reaching 0.89 wt. % is characteristic for the Vishteritsa columbite. The decrease of U proximal to cracks and in outer crystal zones documents U-mobility during overprinting hydrothermal processes.

Pseudomorphic 9-line silician ferrihydrite and Fe-rich serpentine-group minerals in FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, U.S.A.

AbstractLow-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the replacement of an igneous sulfide such as pyrrhotite. Electron diffraction shows the ferrihydrite to be a 9-line variety. Si-rich cronstedtite formed thin rims around the ferrihydrite, and talc grew patchily around the cronstedtite. Greenalite formed in ~10 μm cracks through all the above minerals and olivine, and hisingerite microveinlets partially replaced olivine. Igneous minerals remaining include olivine Fa46, magnetite, ilmenite, hornblende, biotite, and trace clinopyroxene. Correlations among the constituents of ferrihydrite determined by electron microprobe, including anhydrous totals, indicate progress during the growth of two charge-balanced exchanges involving silica enrichment: an inverse cronstedtite substitution (MgFe2+,Si) (Fe3+Mn3+)–2 and an inverse hydrogarnet substitution SiH–4. The cronstedtite exchange requires charge and size balance across nearest-neighbor T and O crystal sites, suggesting crystal-interior rather than crystal-surface control. Ferrihydrite’s composition reflects time- and space-related variations in the chemical potentials of components in the hydrous fluid at the site of alteration. An upper limit for SiO2 of 14–15 wt%, or ≈1.0 Si per 5-cation formula unit, would seem to correspond to the limit of availability in ferrihydrite of tetrahedral sites open to the entry of Si. Our EPMA data, projected to zero SiO2, indicate an anhydrous total of ≈83 wt% for end-member ferrihydrite, a number that matches the formula: Fe10O15·9H2O. The geochemical properties of Laramie ferrihydrite are shared by some samples of altered chondritic and Martian meteorites. Ferrihydrite on Earth commonly occurs as a surface deposit; unlike the Laramie occurrence, these lack the microspatial coherence of replacements/pseudomorphs to show systematic, structure-related element variations. The superior crystal quality of the Laramie ferrihydrite likely contributed to its unique compositional variability.