Chemical Zoning Research Articles

Gemology, Mineralogy, and Coloration Mechanism of Pinkish-Purple Cobaltoan Dolomite from the Democratic Republic of Congo

A pinkish-purple cobaltoan dolomite from the Democratic Republic of Congo (DRC) has appeared on the Chinese gemstone market recently. In this study, Raman analysis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, electron probe micro-analysis (EPMA), scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDXRF), and fiber optic spectrometry were used to explore the gemology, mineralogy, and coloration mechanism of cobaltoan dolomite. Results indicate that cobaltoan dolomite is a mineral aggregate with a granular texture. The degree of fineness and luster differs due to mineral (quartz) impurities in the texture. The cobaltoan dolomites are typically associated with carbonates, such as azurite, malachite, spherocobaltite, and the rare species kolwezite, which were documented in this study. The natural presence of magnesite-spherocobaltite solid solution with distinct chemical zoning occurs as crystals in the country rock. The pinkish-purple dolomites are mainly colored by CoO, whose concentrations ranged from 0.966 to 6.111 wt.%. Based on UV-Vis spectroscopy, cobaltoan dolomite showed broad characteristic absorption bands at 531.5 nm, which varied as the concentrations of CoO increased. The origin of the color is related to the electronic transition of Co2+ and charge transfer between the ions.

Karavannoe: Mineralogy, trace element geochemistry, and origin of Eagle Station group pallasites

ABSTRACTKaravannoe is a pallasite found in Russia in 2010. The mineralogy, chemistry, and oxygen isotopic composition indicate that Karavannoe is a member of the Eagle Station Pallasite (ESP) group. Karavannoe contains mostly olivine and subdued interstitial Fe,Ni‐metal. Zoned distribution of FeO in small, rounded grains of olivine and FeO and Al2O3 in chromite shows that the cooling rate of the melt was fast during the crystallization of the round olivine grains. Siderophile element distribution and correlations of Au‐As and Os‐Ir concentrations in Karavannoe and the other ESP metal record its magmatic origin. FeO‐rich composition of olivine, low W and Ga, and high Ni abundances in the Karavannoe metal indicate the formation of the metal from an oxidized chondrite precursor. Model calculations demonstrate that the ESPs’ metal compositions correspond to the solids of the fractional crystallization of CV‐ or CO‐chondrite‐derived metallic liquids. The Karavannoe metal composition corresponds to the solid fraction crystallized after ~40% fractional crystallization. The Mg/(Mg+Fe) atom ratio of complementary silicate liquid corresponds to Fo70, possibly indicating that the olivine is not in equilibrium with the metal and could have been a product of the late evolutionary processes in the Karavannoe parent body mantle. In any ESP genesis Karavannoe was not in equilibrium with its metal and is a product of mantle differentiation processes. Olivine of Karavannoe and ESPs is similar in composition, while the metal is different. We propose a model of ESP formation involving an impact‐induced intrusion of liquid core metal into a basal mantle layer, followed by fractional crystallization of the metal. The metal textures and chemical zoning of Karavannoe minerals point to remelting and rapid cooling due to a later impact event.

Polymetallic nanoparticles in pyrite from massive and stockwork ores of VMS deposits of the Iberian Pyrite Belt

This paper reports the first-ever study on nanoscale mineralogy in pyrite from the volcanogenic massive sulfide (VMS) deposits of the Iberian Pyrite Belt, southwestern Iberian Peninsula. It targeted colloform-textured grains formed at low temperature in the distal part of a polymetallic (Pb-Zn) massive sulfide lens hosted in felsic volcanoclastic rocks from the Masa Valverde deposit, and euhedral-textured grains (re)-deposited by higher temperature fluids in the Co-Au rich stockwork hosted in black shales of the Filón Norte orebody of the Tharsis deposit. The results acquired by a combination of techniques for mineral microanalysis and characterization (i.e., reflected light, FE-SEM, EPMA, LA-ICP-MS, HRTEM-STEM and TEM-EDS) show that trace amounts of metals (Au, Ag, As, Pb, Sb, Cu, Co) are incorporated as both lattice-bound and into nanoparticles (NPs). The mode of occurrence is strongly related with the evolutionary history of the mineralization. In the colloform pyrite collected from the massive sulfide lens, a rhythmic banding/oscillatory zonation with up to 3 wt% As, 5,000 ppm Pb, 1,070 ppm Sb and 750 ppm Cu is defined by the coexistence of several nano-sized layers (5 to 100 nm) and NPs (<100 nm) containing all these metals. The NPs include galena [PbS], tetrahedrite [(Cu,Fe)12Sb4S13)] and arsenopyrite [FeAsS] that exhibit euhedral and less frequently anhedral (i.e., droplet-like) morphologies being both randomly and preferentially oriented with respect to As-rich pyrite bands they are usually associated with. These features suggest formation of the NPs via direct deposition from the hydrothermal fluid(s) or low-temperature melts entrained in them as well as exsolution of trace elements originally dissolved in the As-rich pyrite structure. Additionally, some of these NPs are connected to late fractures disrupting the chemical zoning in colloform pyrite documenting a third genetic type of NPs related to late infiltration of fluids post-dating pyrite formation. In contrast, euhedral pyrite from the stockwork form well-developed homogeneous grains with discrete porous areas relatively depleted in Fe (45.20 wt%), and As (8,800 ppm) but enriched in Co (5,900 ppm). At the nanoscale, Co-enriched domains show patchy zoning defined by irregular distribution of Co– and As-rich bands of 200–500 nm in thickness. These nanometer Co– and As-rich bands are often disrupted by micron-to-nano-sized polycrystalline Au-Ag-Hg particles that fill voids in porous areas. Contact morphology anatomy between Co-rich pyrite and inclusions suggests that the Au-Ag-Hg particles are negative crystals occupying spaces originated in pyrite by coupled dissolution-reprecipitation reaction. Likewise, HRTEM observations along such pyrite-inclusion contacts show the existence of polycrystalline matrices in both pyrite and Au-Ag-Hg inclusions, the former consisting of nano-sized domains of arsenian pyrite and/or arsenopyrite in As-free pyrite and the Au-Ag-Hg inclusions made up of multiple crystal domains including nano-crystallites of Au0/Ag0 or electrum. Recognition of crystalline nanodomains and NPs in these polycrystalline matrices raises the possibility that Au NPs or nanomelts already present in the hydrothermal fluid catalyzed the formation of these heterogeneous crystals.

Nanoscale trace-element zoning in pyrite framboids and implications for paleoproxy applications

Abstract Pyrite framboids (spherical masses of nanoscale pyrite) are among the earliest textures of pyrite to form in sediments. It has been proposed that their trace-element (TE) contents can be used to track the TE composition of the water column in which they formed. However, it is not clear how these TEs are associated with the framboidal pyrite grains. For instance, it is important to know whether they are incorporated uniformly or are enriched in different regions of the framboid. We used high-resolution scanning transmission electron microscopy to identify chemical zoning within pyrite framboids. We found that initial, nanoscale pyrite euhedral crystals, which make up the volumetric majority of the framboids, are covered/infilled by later pyrite that templates on the earlier pyrite. Further, this later pyrite is enriched in TEs, suggesting that many TEs are incorporated in pyrite relatively late (during early diagenesis; not in the water column). This observation suggests that although chemical analyses of pyrite framboids may provide ocean-water chemistry trends through time, the details are complex. Specifically, the TEs found in pyrite may be linked to adsorption onto organic matter, detrital material, and authigenic minerals such as Fe- and Mn-oxide phases followed by desorption in the sediments or release via dissolution and incorporation into pyrite as overgrowths on the initial nanoscale euhedral crystals that make up framboids. While the use of pyrite chemistry to understand past ocean conditions remains promising, and even diagenetic additions may not preclude the utility of pyrite for reconstructing ancient ocean conditions, care must be taken in interpretations because the end concentration may be influenced by diagenesis.

Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes

Determining the ages and duration of high-grade metamorphic episodes during orogenesis and concomitant melt extraction events is pivotal in reconstructing and quantifying the chemical and petrological evolution of the Earth's continental crust over time. Major and trace elements combined with multichronometric garnet data from the Pan-African Damara belt of Namibia place absolute time constraints on high-grade regional metamorphism and melting in the lower crust that led to the production of widespread granites emplaced in the upper crust. Some of these granites contain inherited peritectic garnet with inferred igneous rims that record their genesis and evolution from formation by partially melting in the deep crust to the emplacement of their host rock at shallower levels. Particular characteristics of these garnets are textural and chemical disequilibrium features such as resorption, local development of partly euhedral crystal faces and occurrence of quartz-feldspar stringers in the rim of the garnet and trace element abundances with sharp concentration gradients. These features document a previous melting event followed by garnet crystallization in an ascending melt. With the exception of the outermost rim, which records late-stage diffusional features, the garnet crystals have constant Mg and Mn concentrations. In contrast, Ca shows pronounced chemical zoning. Trace elements (HREE, LREE, Zr, Sr, Y) are depleted in the core and enriched in the rim region indicating disequilibrium conditions during garnet growth. A pseudosection calculated with the garnet rim composition and the composition of the host granite yields P-T conditions of 0.62 GPa and 760 °C for the formation of the rim. This result is consistent with a P-T estimate of 0.68 GPa and 760 °C derived from the conventional garnet-biotite thermobarometer. Regional metamorphic conditions were above the cordierite-andalusite stability field with estimated conditions of ~550-600 °C and 0.4–0.5 GPa. This P-T regime indicates that the garnet originally crystallized at deep crustal levels. Diffusion modelling suggests that the observed concentration gradient of Sm in garnet is consistent with a maximum duration of ca. 0.5–1.5 Ma for the inferred melting interval during peak metamorphism in the lower crust. High-precision multi-point Lu-Hf garnet-whole rock data yield dates of 522.0 ± 1.0 and 522.3 ± 0.9 Ma for two samples. Because of presumed short intervals between melting in the lower crust and intrusion of the granite, these dates are interpreted as the age of the melting event during high-grade metamorphism and are temporally indistinguishable from the crystallization interval. Thus, dehydration melting in the deeper crust during peak metamorphism produced peritectic garnet that was incorporated into the granitic melt followed by subsequent new growth of igneous garnet around peritectic cores during intrusion. Previously obtained Sm-Nd garnet whole rock ages are 20–25 Ma younger and likely represent cooling ages. This study shows that granites enriched in mafic components such as garnet and biotite may intrude as mixtures of melt and the peritectic assemblage produced by incongruent melting involving dehydration-melting of biotite in the lower crust. Melting episodes represent short-lived processes relative to the absolute duration of orogenic cycles.

Skarn formation and Cu[sbnd]Ag mineralization in the McKenzie Gulch area, northern New Brunswick, Canada: Implication for the applications of mineral chemistry in exploration for porphyry copper and skarn deposits

The McKenzie Gulch (MG) copper–silver skarn occurrences are associated with the Middle Devonian intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork of veinlets, disseminations, patchy, and locally as replacement of calc-silicate skarns in argillaceous limestone. This skarn is interpreted to have developed in three stages: the earliest contact metamorphic stage resulting in hornfels developed from calcareous mudstone and very fine-grained clastic sedimentary rocks (Stage I); this was followed by metasomatic replacement resulting in prograde anhydrous skarn containing grossular-andradite (grandite), diopsidic pyroxene, and wollastonite (Stage II); increasing fluid/rock interaction results in retrograde skarn dominated by epidote, calcite, green amphibole, chlorite, sulfide minerals, titanite, andradite and hedenbergite commonly along veins and veinlets (Stage III). Subordinate sphalerite and pyrite occur in late veins cross-cutting both porphyry dykes, skarns and hornfels zone.Prograde garnets in the study area belong to the grossular-andradite solid solution that ranges from Adr16Gr82 to almost pure andradite - Adr99Gr1, whereas other garnet end-members constitute less than 5% collectively. Pyroxenes form a diopside-hedenbergite solid solution with composition between Di31Hd60 and Di93Hd7, while other pyroxenes constitute less than 10% collectively. These garnets and pyroxenes exhibit chemical zonation patterns generally characterized by a rim of Fe enrichment relative to cores of grains, which may suggest possible change in salinity (or other factor that promotes Fe transport). This means grossular and diopsidic cores are rimmed by more andradite and hedenbergitic compositions for garnet and pyroxene, respectively. In general, there is a tendency for increasing Fe component in both garnet and pyroxene from the prograde to retrograde stages.Epidote formed during the retrograde stage exhibits oscillatory zoning similar to retrograde andradite and indicates Fe enrichment increasing from core to rim. Based on the major element composition, textural and optical characteristics of garnets, we conclude that grandites (Al-rich) formed under low water/rock ratios, in equilibrium with metasomatic fluids whose composition was locally buffered by the host rocks, whereas andradite (Fe-rich) resulted from relatively high water/rock ratios that were in equilibrium with a magmatic-derived fluid. The results of this study indicate that there is a relationship between the composition of pyroxenes and garnets of the skarn alteration facies and the dominant metal of the mineralized skarns. These facies plot in the compositional field of Cu-dominated skarns. As a result, this relationship may be applied in exploration for skarn and porphyry copper mineralization.

3D zoning of barium in alkali feldspar

Abstract Interpretation of chemical zoning within igneous minerals is critical to many petrologic studies. Zoning in minerals, however, is commonly observed in thin sections or grain mounts, which are random 2D slices of a 3D system. Use of these 2D sections to infer 3D geometries requires a set of assumptions, often not directly tested, introduces several issues, and results in partial loss of zoning information. Computed X-ray microtomography (microCT) offers a way to assess 3D zoning in minerals at high resolution. To observe 3D mineral zoning using microCT, however, requires that zoning is observable as differences in X-ray attenuation. Sanidine, with its affinity for Ba in the crystal lattice, can display large, abrupt variations in Ba that are related to various magma reservoir processes. These changes in Ba also significantly change the X-ray attenuation coefficient of sanidine, allowing for discrete mineral zones to be mapped in 3D using microCT. Here we utilize microCT to show 3D chemical zoning within natural sanidines from a suite of volcanic eruptions throughout the geologic record. We also show that changes in microCT grayscale in sanidine are largely controlled by changes in Ba. Starting with 3D mineral reconstructions, we simulate thin-section making by generating random 2D slices across a mineral zone to show that slicing orientation alone can drastically change the apparent width and slope of composition transitions between different zones. Furthermore, we find that chemical zoning in sanidine can commonly occur in more complex geometries than the commonly interpreted concentric zoning patterns. Together, these findings have important implications for methodologies that rely on the interpretation of chemical zoning within minerals and align with previously published numerical models that show how chemical gradient geometries are affected by random sectioning during common sample preparation methods (e.g., thin sections and round mounts).

Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds.Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate‑carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.

Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)

Abstract Nb-Ta-oxide minerals (NTO) are commonly associated with rare-element pegmatites where they are interpreted to precipitate at magmatic to magmatic-hydrothermal conditions. Although high-temperature experiments show that phosphorus and other fluxing elements (e.g., Li, B, F) can affect the saturation of NTO in pegmatitic systems, it is still uncertain how NTO saturation occurs in natural, flux-rich pegmatitic melts and whether crystallization occurs at multiple stages during magmatic or subsolidus conditions. The lithium-cesium-tantalum (LCT) family P-rich Buranga granitic pegmatite (western Rwanda) is used as a type locality to address this question. NTO mineralization in the Buranga dike occurs in two mineralogical units: in mineralogically complex phosphatic nodules, and in albitized parts. In the phosphatic nodules, Fe-Nb-Ta-rich rutile and columbite-group minerals (CGM) are observed, while in the albitized parts, only CGM is found. Fe-Nb-Ta-rich rutile precipitates at the magmatic stage along with early primary phosphates (i.e., F-rich montebrasite, wyllieite, and fluorapatite). Conversely, CGM mineralization occurs at the magmatic-hydrothermal stage in association with replacement phosphates like bertossaite, after primary minerals (i.e., rutile, wyllieite, rosemaryite, and trolleite) are destabilized due to crystal-melt-fluid interactions. NTO textures and chemical zoning show uneven evolution from core to rim and are related to localized alteration phenomena of the surrounding minerals. This indicates that local processes and element transfers are more important than dike-scale fractionation processes for NTO mineralization in P-rich granitic pegmatites. The restricted availability of Fe and Mn in the system, which is related to the competition between phosphate and oxide minerals, is identified as the main control on the CGM mineralization in the Buranga dike. CGM precipitation is only possible during the magmatic-hydrothermal stage when Fe and Mn are leached from the primary phosphates and remobilized to the Nb-Ta-bearing residual melt by the exsolved fluid.

Decoupling of U–Pb ages and compositional zoning of garnet in a high–pressure marble from the eastern Iratsu body, Sanbagawa metamorphic terrane, Japan

Here we first report the in situ U–Pb dating of metamorphic grossular garnet (Grs) with distinction between internal zonation textures. The studied Grs occurs in an eclogite–facies marble collected from the eastern Iratsu body of the Sanbagawa metamorphic terrane, Japan. The Grs has a patchy texture, predominantly with pure Grs cores and andradite (Adr)–rich rims formed during eclogite–facies and exhumation stages, respectively. The U–Pb ages for the Grs core and Adr–rich rim were 97 ± 10 and 106 ± 16 Ma (95% confidence level), respectively. Despite the compositional zoning formed under different P–T conditions, the U–Pb ages of the core and rim were in similar values within analytical uncertainties. This decoupling of chemical zonation and U–Pb ages implies that the U–Pb chronological signatures of rims were inherited from cores owing to the redistribution of radiogenic Pb in cores during the rim formation through fluid–mediated dissolution and reprecipitation. The Grs U–Pb age (97 ± 10 Ma) thus directly corresponds to previously reported P–T conditions of the core formation during the eclogite–facies metamorphism. This advantage of Grt petrochronology as the combination of radiometric ages obtained by in situ analysis and P–T conditions deduced from paragenesis can contribute to reconstruct reliable metamorphic histories.

Post‐collisional alkaline lamprophyre magmatism in northern Iran: Implications from whole‐rock geochemistry and mineral compositions

AbstractThe Shanderman lamprophyre dykes crop out in the western part of the Alborz Mountains (Talesh). These rocks are classified as camptonites, composed of primary olivine, Ti‐rich diopside, kaersutite, biotite, plagioclase, K–feldspar, and minor Ti–rich spinels, magnetite, pentlandite–pyrrhotite/chalcopyrite, and powellite–scheelite. Secondary analcime–wairakite, serpentines, and prehnite are common minor minerals within the studied rocks. Olivine, Ti‐rich diopside, spinel, and amphibole show distinct chemical zoning. Spinels display a core‐to‐rim decrease in Cr2O3, MgO, and Al2O3 concentrations and an increase in TiO2 and FeOT (total Fe as FeO), reflecting the oxidation state increase due to hydrothermal fluid influx. Low SiO2 contents (&lt;42 wt%), high MgO (12.44 to 13.98 wt%), and Fe2O3T (12.76 to 13.43 wt%), Cr (318–537 μg/g) and Ni (231–327 μg/g) contents indicate the ultrabasic nature of the rocks. The samples show potassic character (2.1–2.8 wt% K2O), along with elevated LREE and LILE, and also exhibit minor positive Eu anomalies (Eu/Eu* = 1.09 to 1.20). Olivine–spinel geothermometry indicates a maximum crystallization temperature of 1227 °C (ave. 988 °C ± 65 °C). Exsolution of pentlandite–pyrrhotite/chalcopyrite solid solutions occurred during magma cooling and crystallization. At lower temperatures, analcime–wairakite and prehnite partially replaced plagioclases. The geochemical modeling of the rocks indicates the Shanderman lamprophyre magmas were derived from low‐grade melting (&lt;5%) of amphibole–bearing garnet lherzolite source without or with very few phlogopites. The primary magma of Shanderman lamprophyres was derived from a depth of ~135 km by partial melting of a metasomatized mantle source in a post‐collisional environment.

Dve nové lokality pyromorfitu v Slovenskej republike: Chvojnica a Hnilčík

Two new localities of pyromophite were recently discovered at the Chvojnica base metals occurrence in the Strážovské vrchy Mts. (Tatric Unit), and at the supergene zone of the Piata siderite-type hydrothermal vein near Hnilčík (Spišsko-gemerské rudohorie Mts., Gemeric Unit), Slovak Republic. At the Chvojnica occurrence pyromorphite occurs as bright green crystalline crusts and radial aggregates up to 3 mm developed in cavities and fractures in the porous quartz gangue with relicts of primary galena. It is associated with minor crystalline coatings of cerussite. The refined unit-cell parameters of pyromorphite from Chvojnica (for the hexagonal space group P63/m) are a 9.9750(2) Å, c 7.3569(2) Å and V 633.93(2) Å3. Except of dominant contents of Pb, P and Cl, only minor amounts of Ca (up to 0.10 apfu), Al (up to 0.05 apfu), Fe (up to 0.04 apfu), As (up to 0.05 apfu) and S (up to 0.02 apfu) were detected in studied pyromorphite. At the Hnilčík, pyromorphite occurs rarely in the cavities of the cavernous quartz-limonite gangue in the supergene zone of the Piata vein. It forms colourless to very light green prismatic crystals up to 2 mm in size, or crystalline aggregates associated with goethite, Mn oxides and malachite. The refined unit-cell parameters of pyromorphite from Hnilčík (for the hexagonal space group P63/m) are a 9.9484(15) Å, c 7.315(2) Å, V 627.0(2) Å3 (Ca-poor) and a 9.8983(6) Å, c 7.2589(8) Å, V 615.92(9) Å3 (Ca-rich). Pyromorphite from Hnilčík shows strong chemical zoning caused by variation of Pb and Ca contents, with outer parts of the crystals enriched in Ca. Contents of Ca are ranging between 0.09 to 0.96 apfu, with the atomic Pb/(Pb+Ca) ratio ranging from 0.98 to 0.81, which is close to the compositional boundary with phosphohedyphane. Interesting are also elevated concentrations of Cu reaching up to 0.07 apfu. Other minor elements detected in pyromorphite from Hnilčík are As (up to 0.12 apfu), Al, V and S (all up to 0.02 apfu).

Relationships between Intensity of Deformation Induced Cr-Al Chemical Zoning and Geometrical Properties of Spinel: An Approach Applying Machine Learning Analysis

Relationships between Intensity of Deformation Induced Cr-Al Chemical Zoning and Geometrical Properties of Spinel: An Approach Applying Machine Learning Analysis

Минералогия и P-T условия образования амфиболитов няртинского метаморфического комплекса и маньхобеинской свиты Приполярного Урала

The article presents the results of studying the amphibolites of the Nyartinskiy Complex and the Mankhobeyskaya Suite. Based on the petrographic and mineral features of the rocks and the chemical zoning of the garnets, two episodes of metamorphism manifestations have been established. The first episode is characterized by a higher temperature (500 amphibolite facies (B3). The pressure at this stage is estimated at 6 10 kbar. The second episode corresponds to low-temperature diaphthoresis (284 4). Similar conditions of the rocks metamorphism of the Nyartinskiy Complex and the Mankhobeyskaya Suite confirm the idea that Mankhobeyskaya Suite is not an independent stratigraphic unit, but is a complex of low-temperature diaphtorites in rocks similar to those that compose the Nyartinskiy Complex.

Garnet Chemical Zoning Based Thermobarometry: Method Evaluation and Applications in the Menderes Massif, Western Turkey

The garnet chemical zoning method (GZM) is a reliable thermodynamic approach for forward modeling pressure-temperature (P-T) paths using observed garnet and bulk rock compositions. However, intracrystalline diffusion is known to compromise the integrity of GZM modeled garnet-growth P-T paths. For this reason, extracting reliable metamorphic estimates from garnet-bearing schists in the Central Menderes Massif (CMM), western Turkey, has been difficult. To evaluate the impact of diffusion on GZM, we simulate garnet growth and diffusion for an average metapelite using the program Theria_G. Modeled garnet compositions from four simulations are used to estimate P-T conditions and paths by GZM, which are compared against Theria_G specified P-T-t trajectories. Factors influencing results are heating/cooling rate, grain size, and peak T. At a maximum T of 610 °C, both undiffused and diffused garnet compositions returned estimates comparable to prescribed conditions regardless of heating/cooling rate. Diffused profiles from simulations reaching a maximum T of 670 °C also reproduced prescribed P-T paths if tectonism occurred at high heating/cooling rates (50 °C/my). From these insights and additional Theria_G simulation-derived observations for CMM garnets, we deduce that metamorphism in the region exceeded 650 °C and achieved a maximum burial P between 8–10 kbar prior to Cenozoic exhumation.

Texture and Trace Element Composition of Rutile in Orogenic Gold Deposits

AbstractRutile from a wide range of orogenic gold deposits and districts, including representative world-class deposits, was investigated for its texture and trace element composition using scanning electron microscopy, electron probe microanalysis, and laser ablation-inductively coupled plasma-mass spectrometry. Deposits are hosted in various country rocks including felsic to ultramafic igneous rocks and sedimentary rocks, which were metamorphosed from lower greenschist to middle amphibolite facies and with ages of mineralization that range from Archean to Phanerozoic. Rutile presents a wide range of size, texture, and chemical zoning. Rutile is the dominant TiO2 polymorph in orogenic gold mineralization. Elemental plots and partial least square-discriminant analysis suggest that the composition of the country rocks exerts a strong control on concentrations of V, Nb, Ta, and Cr in rutile, whereas the metamorphic facies of the country rocks controls concentrations of V, Zr, Sc, U, rare earth elements, Y, Ca, Th, and Ba in rutile. The trace element composition of rutile in orogenic gold deposits can be distinguished from rutile in other deposit types and geologic settings. Elemental ratios Nb/V, Nb/Sb, and Sn/V differentiate the rutile trace element composition of orogenic gold deposits compared with those from other geologic settings and environments. A binary plot of Nb/V vs. W enables distinction of rutile in metamorphic-hydrothermal and hydrothermal deposits from rutile in magmatic-hydrothermal deposits and magmatic environments. The binary plot Nb/Sb vs. Sn/V distinguishes rutile in orogenic gold deposits from other geologic settings and environments. Results are used to establish geochemical criteria to constrain the source of rutile for indicator mineral surveys and potentially guide mineral exploration.

Comment on “Biogeochemical cycling of iron (hydr-)oxides and its impact on organic carbon turnover in coastal wetlands: A global synthesis and perspective” [Earth-Science Reviews (2021) 103658

Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg # ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM.A majority of kimberlitic olivines show normal (decreasing Mg #) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg # in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg #, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model.Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.

Cognate versus xenocrystic olivines in kimberlites – A review

Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg # ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM. A majority of kimberlitic olivines show normal (decreasing Mg #) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg # in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg #, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model. Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.

Origin and significance of tourmaline-bearing rocks in Boroujerd region, Sanandaj-Sirjan zone, western Iran

Tourmaline minerals are a main petrogenetic tool because of their refractory nature, chemical variability, and extensive occurrence in various geological settings. The Middle-Jurassic Boroujerd igneous rocks are one of the largest bodies that located in the middle part in the Sanandaj-Sirjan Magmatic Arc (SSMA) that introduced into regional metamorphic rocks. They are mostly composed of granitoids (granodiorite, quartz-diorite, and monzogranite) and contain apatite, zircon and tourmaline as abounadant accessory minerals. Tourmalines occur in varying types of mineral assemblages, chemical compositions and individual forms including tourmalinite, quartz-tourmaline vein, as accessory mineral in the more evolved granitoids, and schists. Tourmaline crystal sizes in these rocks varies from very fine to coarse-grained. The colour is brown to black and some crystals displaying a finescale chemical zonation. Tourmaline crystals commonly exhibit cellular features. Based on electron probe microanalysis (EPMA) analyses, schorl is the most abundant type of tourmaline in granitoids but foitite and dravite are common compositions in the pegmatite and schist, respectively. Presence of several tourmaline generations and their compositional trend may indicate time-varying relatively acidic boron-bearing fluids and reflects an anatexic melts influence in their crystallization.

Petrology and geochemistry of lunar feldspathic meteorite Northwest Africa 11111: Insights into the lithology of the lunar farside highlands

AbstractWe performed a petrological, mineralogical, and geochemical study of the lunar feldspathic meteorite Northwest Africa (NWA) 11111. This meteorite contains several types of lithic clasts, including feldspathic clasts, mafic‐rich clasts, granulites, impact melt breccias, minor basaltic clasts, and highly evolved clasts cemented in a recrystallized fine grain matrix. Both mineral chemistry and geochemical characteristics indicate a lunar origin for NWA 11111. The bulk analysis suggests that NWA 11111 is a typical feldspathic lunar meteorite, which is consistent with its large population of anorthositic clasts and plagioclase fragments. A comparison of geochemical data made by lunar orbiter missions indicates that this meteorite was likely launched from the Feldspathic Highland Terrane on the lunar farside. The chemical zoning, coupled with extensive exsolution lamellae (up to 20 μm in width) occurring in pyroxene across three sections of NWA 11111, demonstrates that this meteorite contains components derived from the surface to about 10 km of lunar crust. Magnesian anorthosite clasts are commonly present in the meteorite, indicating that magnesian anorthosite probably represents an important lithology in the lunar farside crust. Basaltic clasts in NWA 11111 range from a very low‐Ti to a low‐Ti mare basalt, possibly representing cryptomare on the lunar farside. Although a KREEPy signature for NWA 11111 is not evident, highly evolved clasts containing various silica polymorphs and/or K‐feldspar are present. They may originate from late‐stage residual liquids. Lithic clasts and mineral fragments within NWA 11111 provide new insights into the diversity of lunar crust lithology and magmatic processes on the lunar farside. This meteorite also offers rocky materials from a wide vertical section of lunar crust.