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The Mangodara district (southwestern of Burkina Faso, West African Craton) consists of a regional-scale Eburnean dome cored by granitoid-gneisses comprising rafts of migmatitic paragneisses and amphibolites of the Paleoproterozoic Birimian series. The occurrence of rare metal-bearing pegmatites in diffuse contact with these migmatitic and granitoid gneisses suggests that they originated from the segregation of a residual melt of these partially molten hosts. In this paper, we constrain the petrogenetic link between pegmatites and their hosts, and the mechanisms of rare metal fractionation in Lithium-Cesium-Tantalum (LCT) vs Niobium-Yttrium-Fluorine (NYF) petrogenetic signatures by the geochemistry of micas, apatite, columbite-group minerals, garnet, and zircon. Titanite-allanite pegmatites (containing titanite, allanite, epidote, zircon, and apatite as accessory minerals) and their evolved equivalent, apatite-zircon pegmatites (richer in apatite, lower K/Rb and Fe/Mn ratio in biotite but Li-depleted) are poorly mineralized metaluminous pegmatites. They display a continuous evolution trend in K/Rb and Fe/Mn in biotite and similar REE patterns in apatite, which favor an origin by segregation of residual melt within tonalitic-trondhjemitic gneiss in the core of the Mangodara dome. Garnet-columbite pegmatites containing REE-bearing phosphates and Zr-U-Th-bearing metamict minerals are mixed LCT + NYF pegmatites. Their micas, slightly enriched in Li, LREE-rich apatite, and Nb–Ta-U-rich garnet, are consistent with an origin by partial melting of a metasedimentary source, with dehydration of biotite (reservoir of Li, Rb, Nb) and dissolution of apatite-monazite (reservoir of REE). Apatite crystals in one garnet-columbite pegmatite reveal an inherited REE signature typical of apatite-zircon pegmatite, which suggests mingling of a LCT pegmatite-forming melt with the residual melt derived from crystallization of metaluminous pegmatites. Garnet-REE pegmatites, containing ilmenite-pyrophanite and euxenite-aeschynite), are NYF pegmatites that should originate from melt segregation within granodioritic gneiss associated with breakdown/entrainment of amphibole (reservoir of REE, Y) and LREE segregation by allanite and phosphates in the source. These data show that the LCT vs NYF signature of pegmatites of the Mangodara district results primarily from the chemical composition of the partially-molten source and the minerals involved in the partial melting reactions, which vary as a function of increasing depth (mica, phosphate, amphibole, garnet). The trace-element signature of anatectic peraluminous pegmatite-forming melt might then be affected by mingling with residual Nb-enriched metaluminous melt. [ABSTRACT FROM AUTHOR]

• Granitic rocks from the Yangkou region record crustal anatexis at HP to UHP conditions. • These rocks are derived from partial melting of the underlying granitic gneisses. • The pressure effect on melt composition reflects the key role of accessory minerals during anatexis. • The Nd-Hf isotope variation is due to dissolution of apatite and garnet into anatectic melt. The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb) N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K 2 O/Na 2 O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO 2 and Na 2 O contents, but lower contents of K 2 O, MgO, TiO 2 , P 2 O 5 and most trace elements (e.g., Rb, Ba, LREE) with lower K 2 O/Na 2 O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and ε Hf (t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher ε Nd (t) values than Group I ones and the granitic gneisses by 3–5 ε Nd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions. [ABSTRACT FROM AUTHOR]

The Pampean aquifer, in south east Argentina, is mainly constituted of loess-like sediments. These are clastic sediments mainly composed of quartz and aluminosilicates and calcrete concretions. Its hydrochemistry is generally studied assuming a chemical equilibrium between mineral phases and the aqueous fluids. The phases forming the matrix of this aquifer are considered to be the reactive phases responsible for the chemistry of the groundwater. In the present study, batch dissolution experiments were performed on calcrete and loess to better understand the source of the Pampean aquifer water chemistry and to measure the benefit of applying water-rock interaction models that use kinetic rate laws instead of thermodynamic equilibria. The different minerals composing the loess and calcrete samples were calculated using quantitative Rietveld refinement of X-ray powder diffraction (XRD) patterns. This analysis showed that the major phases of loess are quartz (~ 30 wt%) and feldspars (~ 70 wt%). The main components of calcrete are calcite (~ 95 wt%) and quartz (~ 5 wt%). Scanning electron microscopy with energy dispersive X-ray microanalysis (SEM/EDXS) was used to provide detailed information about the chemical composition of the powder samples, revealing the presence of traces of minerals like halite, barite and fluorapatite, which were not detected by XRD. The kinetic code KINDIS was used to carry out simulations using the minerals identified previously in their relative proportions to identify the signature of those geochemical phases on water chemistry. Experimental data from batch dissolution experiments were compared to simulated data. This investigation showed that water reached pseudo steady state concentrations due to the presence of fast dissolving phases like halite, barite, gypsum, plant phytolith. These phases appeared to be of major importance in controlling the chemical composition of the Pampean groundwater. Furthermore, this work showed that the KINDIS software can be used on all kinds of aquifers as it is very easy to modify the parameters of the simulation to adapt it to numerous situations. The modeling is a very important tool for thermodynamic and kinetic studies of groundwater chemistry; it enables the prediction of water quality and can help to understand the impact of anthropic or natural contamination on the groundwater. [ABSTRACT FROM AUTHOR]

The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950-1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800-900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900-950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528-521 Ma), peak (528-522 Ma), and peak to retrograde metamorphism (503-532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20-25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models. [ABSTRACT FROM AUTHOR]

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in KO and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm-Nd and Lu-Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust. [ABSTRACT FROM AUTHOR]

Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of >11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4-6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where HO-rich (~6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the HO-saturated liquidus at ~4 km. HO contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently 'hot' and 'cold' granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons. [ABSTRACT FROM AUTHOR]

Regionally metamorphosed pelitic rocks at Campolungo, Central Alps, contain biotite, muscovite, garnet, staurolite, kyanite, and quartz, and the minor minerals tourmaline, plagioclase, chlorite, rutile, and ilmenite. Accessory allanite, apatite, monazite, potassium feldspar, xenotime, and zircon have also been identified. The bulk-rock chemical composition is similar to that of shales, and indicates that the protolith was deposited in an active continental margin setting. Element distribution maps, electron microprobe analyses and in situ UV–laser ablation inductively coupled plasma mass spectrometry data document a pronounced zoning in garnet and tourmaline porphyroblasts. Garnet displays a typical bell-shaped MnO zoning profile, with a maximum (∼3 wt %) in the euhedral core. Cores are also rich in Y and heavy rare earth elements (HREE; e.g. 2150 ppm Y). In their broad rim, all garnet crystals display a subhedral annulus (10–15 µm wide), which is distinctly enriched in Ca, Sr, Y, and HREE, and which probably resulted from the breakdown of allanite (at ∼550°C, ∼6·4 kbar). Another characteristic feature of garnet rims is their sinusoidal chondrite-normalized REE pattern, which may represent partial equilibration with a light REE-enriched medium, probably generated through the breakdown of metamorphic allanite. Similar REE patterns are exhibited by a Ca-poor internal zone (inside the annulus), which may represent an earlier partial equilibration following the breakdown of detrital monazite. The large tourmaline crystals exhibit an optically visible three-stage zoning, which comprises: a euhedral core; a continuously zoned inner rim with a prominent euhedral Ca-rich annulus; and an outer rim, which also displays a distinct Ca-rich annulus and is separated from the inner rim by a sutured boundary. This boundary represents a marked chemical discontinuity, characterized for example by a decrease in the Zn concentration from 250 ppm (inner rim) to 20 ppm (outer rim). This change in Zn content reflects staurolite growth, which started after resorption of the inner rim of tourmaline and after a major deformation event. This chemical and textural discontinuity coincides with a marked shift in δ18O, which increases by ∼0·8‰ across the inner rim–outer rim boundary. Our thermodynamic models suggest that resorption of the inner rim of tourmaline may be associated with small amounts (5–7 vol. %) of melt formed at ∼650°C and 8·5 kbar. By using detailed textural observations, major and trace element zoning patterns and thermodynamic data, it was possible to model the metamorphic evolution of these rocks in considerable detail and, specifically, to correlate the growth and breakdown of major and accessory minerals. [ABSTRACT FROM PUBLISHER]

Abstract: In situ analyses of hafnium isotopes in zircon combined with U–Pb zircon ages, and trace element data, provide new information on the characterization and evolution of magnetite–apatite (“Kiruna-type”) deposits and the tectonic environments in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing magnetite–apatite deposits. Hafnium isotopic compositions and rare earth element contents in individual zircon crystals were measured in situ in both the host granites and the ore bodies by laser ablation inductively coupled plasma-mass spectrometry. Hafnium isotopic compositions in the ore zircon can be divided into two groups: those that have initial εHf (t) values that are indistinguishable from those of the host granites (e.g., εHf (t) less than +7) and are typical of relatively juvenile Proterozoic crust, and those that have extremely radiogenic εHf (t) values (as high +40). Two models are proposed to explain the observed εHf (t) values in the zircon crystals: 1) early-formed ore bodies containing magnetite, apatite, and clinopyroxene were remobilized by secondary fluid alteration, releasing Zr and Hf for the crystallization of new zircon; or alternatively, 2) fluids responsible for ore formation have interacted with garnet-bearing rocks during retrograde metamorphism, scavenging rare earth elements and radiogenic Hf. Previous work done to determine the U–Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that ore bodies record a mineralizing event that is 20 to 60m.y. younger than the age of granite emplacement. This age discrepancy, plus the highly radiogenic εHf (t) values in the ore zircon crystals, suggests that the fluids responsible for this younger event could not have been derived from the granite hosts. These data argue that magnetite–apatite deposits in the LMG have multiple mineralizing events superimposed upon one another and that early-formed deposits may be reworked, modified and redeposited by fluids subsequent to magma crystallization. [Copyright &y& Elsevier]

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr, Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested. [Copyright &y& Elsevier]

Sulfide mineralization in the province of Trento (northeastern Italy) includes various mineral assemblages that are often silver-rich and have been exploited in different phases from the Middle Ages until the 20th century. This study investigates mineralized rocks from three historically important sites (Calisio mount, Erdemolo lake, and the locality of Cinque Valli), providing new analytical data (Inductively Coupled Plasma-Mass Spectrometry on bulk rocks, and Scanning Electron Microscopy on thin sections) that demonstrate that parageneses do not only include galena, chalcopyrite, and sphalerite but also accessory minerals, such as tetrahedrite, tennantite, acanthite, and sulfosalts (matildite/polybasite). This explains the high content of As (up to 278 ppm), Bi (up to 176 ppm), and Sb (up to 691 ppm) that are associated with Pb–Cu–Zn mineralization. Notably, trace-element ratios indicate that, although closely associated from a geographical point of view, the studied sites are not genetically related and have to be referred to in distinct mineralization events, possibly induced by three diverse magmatic and hydrothermal phases that occurred in the Variscan post-orogenic setting. Besides geological and petrogenetic reconstruction, the new data outline potential geochemical risks, as they reveal a high concentration of elements characterized by marked toxicity that can be transferred into the local soil and water. Therefore, future studies should be devoted to better investigating the metal distribution in the surroundings of ancient mining sites and their geochemical behavior during the weathering processes. [ABSTRACT FROM AUTHOR]