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• Uranium solubility was determined in two simplified glass systems over a wide redox range. • The effect of aluminum content is attributed to the proportion of [5]Al. • Uranium volatilization in each MAS system glass was observed under oxidizing atmospheres. Uranium solubility was measured in melts belonging to the CaO-Al 2 O 3 -SiO 2 (CAS) and MgO-Al 2 O 3 -SiO 2 (MAS) systems using the Pt wire loop technique, enabling independent control of the temperature (1400 °C), glass composition, and oxygen fugacity (-16.1

The internal zone of the Pan-African Lufilian orogenic belt (Zambia) hosts a dozen uranium occurrences mostly located within kyanite micaschists in a shear zone marking the contact between metasedimentary rocks attributed to the Katanga Neoproterozoic sedimentary sequence and migmatites coring domes developed dominantly at the expense of the pre-Neoproterozoic basement. The P–T–t–d paths reconstructed for these rocks combining field observations, microstructural analysis, metamorphic petrology and thermobarometry and geochronology indicate that they have recorded burial and exhumation during the Pan-African orogeny. Both units of the Katanga metasedimentary sequence and pre-Katanga migmatitic basement have underwent minimum peak P–T conditions of ~ 9–11 kbar and ~ 640–660 °C, dated at ca. 530 Ma by garnet-whole rock Lu–Hf isochrons. This suggests that this entire continental segment has been buried up to a depth of 40–50 km with geothermal gradients of 15–20 °C.km − 1 during the Pan-African orogeny and the formation of the West Gondwana supercontinent. Syn-orogenic exhumation of the partially molten root of the Lufilian belt is attested by isothermal decompression under P–T conditions of ~ 6–8 kbar at ca. 530–500 Ma, witnessing an increase of the geothermal gradients to 25–30 °C·km − 1 . Uranium mineralizations that consist of uraninite and brannerite took place at temperatures ranging from ~ 600 to 700 °C, and have been dated at ca. 540–530 Ma by U–Pb ages on uraninite. The main uranium deposition thus occurred at the transition from the syn-orogenic burial to the syn-orogenic exhumation stages and has been then partially transposed and locally remobilized during the post-orogenic exhumation accommodated by activation of low-angle extensional detachment. [ABSTRACT FROM AUTHOR]

A dozen uranium occurrences are known in the Western Zambian Copperbelt (Domes region, Zambia). The host-rocks, Neoproterozoic kyanite ± talc micaschists, have been interpreted as meta-evaporites that formed part of an epicontinental sequence affected by upper amphibolite metamorphism during the Pan-African orogeny. Ca–Na metasomatism, expressed by the presence of apatite, epidote and scapolite, is regionally extensive in the Katanga and the pre-Katanga rocks in the Domes region. The present study focuses on the Solwezi dome in the Western Zambian Copperbelt. Two distinct uranium mineralization events that took place here mark the transition from crustal-thickening by tectonic accretion to exhumation of the metamorphic host rocks. U–Pb dating of uraninite and Th–Pb dating of monazite show that the first U-mineralization event took place between ~ 550 Ma and ~ 530 Ma. The event is therefore contemporaneous with crustal thickening and, on the basis of fluid inclusion analyses, can be related to circulation of hypersaline Ca-rich metamorphic fluids trapped at minimum P – T conditions of 460 MPa and 500 °C. On a regional scale, the event is related to an epidote–apatite metasomatic assemblage. The second mineralization event, dated at between ~ 510 and ~ 500 Ma by Th–Pb dating of monazite, is related to high-salinity Ca–Na-rich metamorphic fluids and occurred during exhumation of the high-grade metamorphic rocks. These fluids were trapped at the ductile/brittle transition at 150 to 260 MPa and 200 to 300 °C. A regional-scale scapolitization is attributed to these late fluid circulations. These fluid–rock interactions, described at the regional scale, are also characterized at the mineral scale. Alteration reactions, assisted by CaCl 2 -rich fluids, indicate that monazite and allanite hosted by the pre-Katanga rocks potentially acted as U sources for these Pan-African synmetamorphic uranium mineralizations. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge basalts (MORBs) are the most abundant magmas produced on Earth. They are widely studied to infer mantle compositions and melting processes. However, MORB liquids are also the complex end-product of a variety of intra-crustal processes such as partial or fractional crystallization, melt-rock interaction, and contamination. Deciphering the relative contribution of these different processes is of first-order importance. Contamination at ocean crustal levels is likely, and may occur at magma chamber margins where fresh magmas can interact with previously hydrothermally altered rocks. Characterizing the composition of this crustal contaminant component is critical if we are to understand the relative importance of each component in the resulting MORB liquid. Here we present the results of experiments designed to reproduce the processes occurring at oceanic magma chamber roofs, where crustal contamination should be most extensive, by melting a representative sample of the sheeted dike complex. Anatectic melts thus produced are likely to represent the principal crustal contaminant in MORB. These melts were characterized for major and trace elements, showing B, Zr, Hf, and U enrichment, and Sr, Ti, and V depletion relative to original MORB liquids. In comparison to the starting material, relative element fractionations are observed in the anatectic melts, with enrichments of: U relative to Ba, Nb, and Th; LREE and MREE relative to Sr; and Zr-Hf relative to LREE. Bulk partition coefficients for element partitioning during magma chamber roof anatexis are derived and proposed as valuable tools for tracking MORB contamination. Comparison with natural samples from the East Pacific Rise and the Oman ophiolite shows that anatectic melts can crystallize in situ to form oceanic plagiogranite intrusions, and that residual assemblages associated with the hydrous partial melting stage are represented by hornfelsic dikes and enclaves (also named granoblastic basalts). We now recognize these as commonplace at the root of the sheeted dike complex both at present-day and fossil oceanic spreading centers. [ABSTRACT FROM AUTHOR]

The intensive variables of dacitic–rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850–900°C, fluid (H2O + CO2)-saturation, XH2O [= molar H2O/(H2O + CO2)] between 0·6 and 1 (melt H2O contents of 2–10 wt %), and redox conditions of FMQ (fayalite–magnetite–quartz buffer) or NNO + 1 (where NNO is Ni–NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850–900°C and P ≥ 2 kbar, under moderately reduced conditions (ΔNNO = −0·9 to −0·1), and were poor in fluorine (∼500–800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5–6 wt % and 2500–3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H2O, but most probably saturated with respect to H2O + Cl ± CO2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system. [ABSTRACT FROM PUBLISHER]

Abstract: The Zabili granitic pluton (SW Chad) exposed in the Mayo Kebbi massif is dominated by a coarse-grained hornblende biotite granite grading into a fine-grained biotite granite along its southern margin. Petrologic (micrographic intergrowth of quartz and alkali feldspars, granophyric microstructures, the presence of fluorite and bastnaesite as accessory minerals) and geochemical data (high silica, alkalis and Fe/Mg, depletions in CaO, MgO, TiO2; high Ga, Nb, Zr, Ga/Al, REE, depletions in Ba, Sr, Eu and compatible elements) indicate that this pluton consists of A-type granites crystallized from hot (apatite and zircon saturation temperatures ranging from 744°C to 923°C), extremely differentiated magmas. U–Pb zircon geochronology indicates that the magmas crystallized at 567±10Ma and reveals the presence of older Neoproterozoic xenocrystic zircons at 668±5Ma in both facies. Within the fine-grained biotite granite, discordant zircons with U–Pb and Pb–Pb ages ranging from Neoproterozoic to Archaean are also reported. The 668±5Ma old zircons are considered to derive from country-rocks while discordant zircons, characterized by angular shapes, internal fractures and inherited cores, are likely to represent multi-sources detrital crystals that have recorded at least one metamorphic event. Old pre-Neoproterozoic zircons are reported for the first time for rocks of the Mayo Kebbi massif and they attest to the contribution of an old basement (likely to be the Eastern Nigeria basement and/or the Congo craton) involved in a collisional event with a juvenile Neoproterozoic crust prior to the emplacement of the Zabili granitic pluton. Initial εNd values calculated for the Zabili pluton range from +2.6 to +7.0, the highest value recorded by one sample from the coarse-grained hornblende-biotite granite being close to the one of the depleted mantle at 570Ma (εNd=+7.4). Combining geochronology, Nd isotopes composition and geochemical modeling, leads us to suggest the following model for the origin of the Zabili granitic pluton: (i) contribution of juvenile magmas or partial melting of a juvenile basaltic protolith characterized by a short crustal residence time; (ii) interaction of granitic magmas with older continental materials as suggested by the presence of pre-Neoproterozoic zircons and lower initial εNd values of the fine-grained biotite granite; and (iii) fractional crystallization of feldspars and ferromagnesians to produce the observed geochemical features of sample GAB-B, from which an initial εNd value of +7.0 has been calculated. [Copyright &y& Elsevier]

Abstract: Mylonites display petrographical and geochemical characteristics that can be related to syn-deformation fluid circulation. The South Armorican Shear Zone, a major structural feature of the Armorican Massif (France), is outlined by the presence of mylonitic rocks cropping out mostly in open quarries. These mylonites were essentially formed at the expense of peraluminous granitic bodies. Deformation occurred from ductile conditions in the biotite stability field (>400°C) down to lower greenschist cataclasis and brecciation, where carbonation developed. U–Pb analyses on zircon and monazite define a minimum duration of 15Ma for the deformation and hydrothermal history, between 315Ma and 300Ma. Fluid circulations are well documented, by way of petrographic observation (chlorite and carbonate crystallization), mineralogical composition analysis (muscovite chemistry), erratic mobility behavior of some elements (As, Sn, U for instance), and stable isotope composition analysis of the infiltrated rocks. High temperature deformation is not accompanied by alteration of the O isotope system, which implies either low fluid/rock ratio and/or the involvement of δ18O crustal fluids with a composition similar to that of the rocks. On the other hand, some low temperature mylonites show a drastic decrease in the δ18O values, which has to be related to the influx of surface derived waters. The heat source necessary for this crustal scale downward infiltration of fluids followed by upward motion was likely provided by the exhumation of lower crustal units in the South Armorican domain. [Copyright &y& Elsevier]

Abstract: To better understand glass alteration mechanisms, especially alteration layers formation, leaching experiments of a borosilicate glass (SON68) doped with a different rare earth element (La, Ce, or Nd) with solutions rich in 29Si and 18O were carried out. The coupled analyses of glass, alteration products, and solution led to a complete elemental and isotopic (29Si and 18O) budget. They revealed different behaviours of elements that depend not only on their structural role in the glass, but also on their affinity for alteration products (gel, phyllosilicates, phosphates). However, analyses of both glass and solution are not sufficient to describe the real exchanges occurring between glass and solution. The use of 29Si and 18O tracers gives new insights on the formation of alteration layers. During alteration the phyllosilicates records the isotopic variations of the leaching solution. Their isotopic signatures highlight a mechanism of dissolution/precipitation, which implies equilibrium between the secondary phases and the solution. On the other hand the gel isotopic signature, that is intermediate between the glass and the solution, substantiates the hypothesis that the gel is formed by hydrolysis/condensation reactions. This mechanism can thus explain the influence of the gel formation conditions (alteration conditions, solution saturation) on the structure (reorganisation) and texture (porosity) of this layer and on its protective effect. These hydrolysis/condensation reactions are also certainly involved in the aluminosilicate glass alteration and in the formation of palagonite. [Copyright &y& Elsevier]

• Fresh Southwestern Indian Ridge basalt glasses are found to have high water contents and H 2 O/Ce ratios (>600). • High water contents and H 2 O/Ce ratios are not caused by assimilation or diffusion. • The water enrichment is associated with the recycling of arc-mantle residue. Trace amounts of water in the sub-oceanic mantle play crucial role in the vigor of mantle convection and the production of oceanic crust, and other many geodynamic processes. Consequently, the cycling of H 2 O between the mantle and the exosphere in the mantle is one of the critical processes governing Earth's geodynamical and geochemical evolution. While the deep cycling of altered oceanic lithosphere was considered as the main way to replenish the water in oceanic upper mantle, the significance of the arc mantle wedge after the genesis of arc magmatism dragged down by the subducting slab concomitantly has been not well constrained. Here, we report that fresh depleted basaltic glasses from the ultraslow-spreading Southwestern Indian Ridge (SWIR), located far from any recent subduction zones, show unusually high H 2 O/Ce ratios (>600), water contents and heavy hydrogen isotopic compositions. These results could be best explained by recycling of water through melting of a residual hydrous mantle wedge after early melt extraction. Considering that such mantle wedges residue dragged down to the deeper mantle could occupy a volume one order of magnitude larger than that of the subducted lithosphere in the earth history, we suggest that the potential role of such shallow recycling should be considered in studies of global water recycling and the origin of water in the upper asthenosphere. [ABSTRACT FROM AUTHOR]

Abstract: The Sidi Driss and Douahria sulfide ore deposits and showings are located in the Nefza mining district, northern Tunisia. The ores are hosted within Upper Miocene (Messinian) basins, within carbonate lenses composed of Fe–Mn-enriched dedolomite partially or totally replaced by early barite and celestite. The ore mainly consists of galena and spherulitic/colloform sphalerite (partially replaced by later Fe-sulfides). It is present as disseminated or banded ores and has been deposited within the host-rock through dissolution/replacement process or through void infilling. The remaining voids are filled by late calcite/barite locally associated with a late oxidation event. In situ ion microprobe sulfur isotope analyses show significant variability for sphalerite (from −43.9 to 1.2‰), galena (from −30.3 to −2‰) and marcasite δ 34S (from −35.9 to 25.8‰). These values are interpreted as the result of (i) the mixing of two sulfur end-members in the mineralizing fluids, corresponding to different reduction processes (bacterially-mediated sulfate reduction and thermochemical sulfate reduction, with early celestite and barite and/or contemporaneous Messinian seawater, together with Triassic sulfate, as sulfur sources, and (ii) Rayleigh fractionation process in a closed system. Several arguments (Fe–Mn enrichment in early diagenetic carbonates, involvement of Messinian seawater as a sulfur source, low temperature of deposition and soft-style sedimentary figures within sulfides) suggest that the Sidi Driss and Douahria Pb–Zn ore deposits are syn-diagenetic. Moreover, the properties of the ore deposits, with Fe–Mn carbonates having a wider distribution than that of the base metal sulfides, and the characteristics of the mineralization, in which diagenetic sulfates are a sulfur source and Cd enrichment in sphalerite, support the classification of the Sidi Driss and Douahria Pb–Zn deposits as SHMS-Sedex type. Late Miocene extension, comprising rifting dynamics and basalt emplacement, favored a thermally-driven fluid circulation origin for the Sidi Driss and Douahria deposits, enhancing inception of small-scale shallow convection cells. Such sulfide emplacement is significantly different from that of southernmost MVT ore deposits of Tunisia, which are associated with diapirs and are interpreted as the result of a Serravalian–Tortonian gravity-driven fluid circulation event related to late Alpine convergence. [Copyright &y& Elsevier]

Abstract: Zircons from the polyphase Tamlalt–Menhouhou gold deposit were examined in order to define the age of the sodic metasomatism (albitization), which is assumed to be related to the primary gold mineralization. Magmatic and hydrothermal zircons were distinguished in albitized banded rhyolitic metatuffites (BRM) and in albitite on the basis of petrographical associations, mineralogical character and REE signature. Direct crystallization from zircon-saturated fluids is proposed for the formation of hydrothermal zircons in albitized BRM. In albitite, some zircons present dissolution–reprecipitation textures, probably related to a high fluid/rock ratio during alteration. Petrographical observations and geochemical data are inconsistent with a Zr contribution from hydrothermal fluids, but reflect the release of Zr stock from the pristine BRM (Zr>300 ppm) during sodic metasomatism. Secondary ion mass spectrometry (SIMS) U–Pb analysis provides Ediacaran (569±8 Ma) and Late Ordovician ages (449±8 Ma) for magmatic and hydrothermal zircons respectively. Additionally, the hydrothermal zircon REE signatures, in particular the Ce and Eu anomalies, suggest that hydrothermal zircons grew in the presence of a hot, reduced hydrothermal fluid, and during mixing between this fluid and a cold, oxidized hydrothermal fluid. The Late Ordovician age is confirmed by 40Ar/39Ar dating of two muscovite monograins, and is additional to the late Variscan hydrothermal event (293±7 Ma) previously recognized for the secondary gold mineralization. This study underlines the potential of zircons to record strong hydrothermal events in polyphase deposits, and the value of these phases for metallogeny. [Copyright &y& Elsevier]

Abstract: TIMS-ID and SIMS U–Pb dating on zircons from metaplutonic rocks involved in the Pan-African nappe of southern Cameroon allow definition of three groups of subduction-related intrusions: group-I intrusions represented by the Masins metagabbro in the Lomie region yielded 666±26Ma; group-II intrusions represented by the Mamb metasyenogabbro and the Yaoundé pyriclasite yielded ca. 620Ma and are broadly coeval with the deposition of the Yaoundé metasediments; group-III intrusions represented by the Elon augen metagranite and the Ngaa Mbappe metamonzodiorite yielded ca. 600Ma. The onset of the nappe tectonics occurred under high-grade conditions in the range 616 to 610Ma and continued around 600Ma with the emplacement of the shallowest nappes. Finally, the construction of southern Cameroon proceeded by a multi-stage evolution characterized by a long-lived development of magmatic arcs associated with rapid opening and closure of sedimentary marginal basins in relation to a northward subduction. [Copyright &y& Elsevier]

• Uranium solubility has been determined in soda-lime aluminosilicate glasses • HR-XANES can be used to determine uranium oxidation states in glass • Uranium solubility in glass melt decreases in the order UVI > UV > UIV • The hexavalent uranium content is a key factor for uranium solubility in glass Uranium solubility in aluminosilicate melts of the Na 2 O-CaO-Al 2 O 3 -SiO 2 system with two different Na/Ca ratios was studied at temperatures of 1250–1400°C and under various redox conditions. A closed thermochemical reactor was used to control the alkali metal activity (sodium oxide content) and the oxygen fugacity imposing the reducing environment on the glass melt (10−5 atm < fO 2 < 10−15 atm). The compositions of the quenched glasses were analyzed by scanning electron microscopy and electron probe microanalysis. It appeared that uranium solubility decreased with decreasing oxygen fugacity, elucidating the roles of the different valences of uranium. To account for the respective effects of theses valences, we proposed a method to determine the proportion of each uranium oxidation state in the glass sample. The coexisting UVI, UV, and UIV species have been characterized for the first time in glass samples using U M 4 edge high energy resolution X-ray absorption near-edge structure. Results showed that the lowest solubility values, of approximately 1 mol% UO 2 , were obtained under strongly reducing conditions, and thus with UIV as the main valence. Under higher oxygen fugacity, uranium solubility was controlled and drastically enhanced by the UVI concentration in the melt. [ABSTRACT FROM AUTHOR]

The French Massif Central (FMC) is part of the West European Variscan belt. The economic and sub-economic ore deposits from the Limousin area are the consequence of intensive fluid-rock interactions related to a lithospheric delamination process that occurred during the Neo-Variscan Period (320–290 Ma). This study focuses on the fingerprints of fluid-rock interactions in the Limousin ophiolite-derived rocks and granitic massifs, in order to trace the source of trace elements transported by fluids involved in the formation of the Variscan ore deposits (including Sn, W, Au and U deposits). A series of compatible and incompatible metallic trace elements (Ag, As, Au, Bi, Li, Co, Cr, Cu, Ga, Ge, Ni, Pb, Sb, Sc, Sn, Ti, V, W) were analysed in a series of basic (metagabbros, basic dykes), ultrabasic (serpentinites) and felsic (granites and gneisses) rocks, as well as in the mineral assemblages, in order to determine their relationships with the Limousin ore deposits. Serpentinites and their mineral assemblages (serpentine, olivine, amphibole, chlorite and spinel) show high concentrations in compatible transition metals (Sc, V, Cr, Co, Ni, Zn, Cu) and low abundances of incompatible metals (Ge, Ga, Mo, Sn, Sb, W). Associated amphibolites (metagabbros and metamorphosed dykes) show high contents of compatible metals, though lower than ultrabasic rocks. Amphibole and, to a lesser extent, titanite, are the main hosts for metallic trace elements. Abundances in incompatible metals are generally low. Amphibole has low Sn and Sb contents, while higher Sn contents were measured in titanite. In a UHP zoisite-eclogite cropping out close to one of the ophiolite-derived massifs, zoisite, rutile, omphacite and retrograde amphibole show high concentrations of incompatible elements (Sn, Sb, W, As, Ga, Ge, Mo and Bi). However, it has been shown that high-temperature and low-temperature hydrothermal alteration that have affected the basic and ultrabasic protoliths is c. 100 Ma older than those that were responsible for the formation of ore deposits. For this reason, and as well because of their low W and Sn abundances, the ultrabasic and basic rocks cannot be considered as source for the Limousin W Sn ore deposits. In granite and gneisses, the metallic trace elements are mainly hosted by biotite and muscovite. Quartz and feldspars only show minor amounts of Sc, Ga, Ge, Sn and Pb. The micas contain low amounts of Ge, As, Sb, Bi and variable amounts of Sc, Cu and Ga. Besides, micas display significant Sn and W contents. Variations in Sn and W in micas allow tracing the pre-mineralisation concentrations in the granitic massifs: there is a clear geographical correlation between the Sn and W composition of micas and the importance of the W Sn ore deposits, arguing for a local source of pre-concentrations. Granites from the Vaulry W Sn ore deposit display the highest W and Sn concentrations, while granites hosting the Au deposits in the southern part of the Limousin (St-Yrieix, Cheni, Lauriéras) show the smallest concentrations. • Trace elements were analysed in the Limousin ophiolite-related rocks and granites. • Ultrabasic and basic rocks are unlikely sources for the Limousin W Sn ore deposits. • Biotite and muscovite are major hosts for trace elements in granites. • Sn and W variations in micas allow tracing pre-mineralisation concentrations. • Trace element data argue for a local source of Sn and W pre-concentrations. [ABSTRACT FROM AUTHOR]

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth's reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism., (© 2022. The Author(s).)