Oxygen fugacity-dependent stability of magnesite along slab geotherms

In this work, a study concerning the composition of Italian papers from the seventeenth to the twentieth centuries was carried out using energy dispersive X-ray fluorescence spectrometry (EDXRF), X-ray diffraction (XRD), and scanning electron microscopy coupled to energy dispersive spectrometry (SEM-EDS). The analyzed samples consisted of papers employed for drawing, writing, printing, and absorbance. Observations carried out by SEM magnified the typical paper morphology. EDXRF in combination with XRD and SEM-EDS allowed the determination of calcite, gypsum, kaolin, talc, magnesite, and dolomite, used as fillers in the production of the papers studied herein. The inks present in the handwritten and printed papers, investigated by SEM-EDS and μ-EDXRF, were synthetic, Fe based, and iron gall inks.

Numerous important Miocene and early Pliocene epithermal Au-Ag deposits are present in the northern Great Basin. Most deposits are spatially and temporally related to two magmatic assemblages: bimodal basalt-rhyolite and western andesite. These magmatic assemblages are petrogenetic suites that reflect variations in tectonic environment of magma generation. The bimodal assemblage is a K-rich tholeiitic series formed during continental rifting. Rocks in the bimodal assemblage consist mostly of basalt to andesite and rhyolite compositions that generally contain anhydrous and reduced mineral assemblages (e.g., quartz + fayalite rhyolites). Eruptive forms include mafic lava flows, dikes, cinder and/or spatter cones, shield volcanoes, silicic flows, domes, and ash-flow calderas. Fe-Ti oxide barometry indicates oxygen fugacities between the magnetite-wustite and fayalite-magnetite-quartz oxygen buffers for this magmatic assemblage. The western andesite assemblage is a high K calc-alkaline series that formed a continental-margin arc related to subduction of oceanic crust beneath the western coast of North America. In the northern Great Basin, most of the western andesite assemblage was erupted in the Walker Lane belt, a zone of transtension and strike-slip faulting. The western andesite assemblage consists of stratovolcanoes, dome fields, and subvolcanic plutons, mostly of andesite and dacite composition. Biotite and hornblende phenocrysts are abundant in these rocks. Oxygen fugacities of the western andesite assemblage magmas were between the nickel-nickel oxide and hematite-magnetite buffers, about two to four orders of magnitude greater than magmas of the bimodal assemblage. Numerous low-sulfidation Au-Ag deposits in the bimodal assemblage include deposits in the Midas (Ken Snyder), Sleeper, DeLamar, Mule Canyon, Buckhorn, National, Hog Ranch, Ivanhoe, and Jarbidge districts; high-sulfidation gold and porphyry copper-gold deposits are absent. Both high- and low-sulfidation gold-silver and porphyry copper-gold deposits are affiliated with the western andesite assemblage and include the Comstock Lode, Tonopah, Goldfield, Aurora, Bodie, Paradise Peak, and Rawhide deposits. Low-sulfidation Au-Ag deposits in the bimodal assemblage formed under relatively low oxygen and sulfur fugacities and have generally low total base metal (Cu + Pb + Zn) contents, low Ag/Au ratios, and notably high selenide mineral contents compared to temporally equivalent low-sulfidation deposits in the western andesite assemblage. Petrologic studies suggest that these differences may reflect variations in the magmatic-tectonic settings of the associated magmatic assemblages—deposits in the western andesite assemblage formed from oxidized, water-rich, subduction-related calc-alkaline magmas, whereas deposits in the bimodal assemblage were associated with reduced, water-poor tholeiitic magmas derived from the lithospheric mantle during continental extension. The contrasting types and characteristics of epithermal deposits and their affinities with associated igneous rocks suggest that a genetic relationship is present between these Au-Ag deposits and their temporally associated magmatism, although available data do not prove this relationship for most low-sulfidation deposits.

Primary magmatic biotite in granitoid intrusions associated with Late Mesozoic Jinchanggouliang-Erdaogou lode gold deposits, North China Craton: Linkage to gold mineralization

Sedimentary rocks contain a unique record of the evolution of the Earth system. Deciphering this record requires a robust understanding of the identity, origin, composition, and post-depositional history of individual constituents. Petrographic analysis informed by Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS) mineral mapping can reveal the mineral identity, morphology and petrological context of each imaged grain, making it a valuable tool in the Earth Scientist’s analytical arsenal. Recent technological developments, including quantitative deconvolution of mixed-phase spectra (producing “mixels”), now allow rapid quantitative SEM-EDS-based analysis of a broad range of sedimentary rocks, including the previously troublesome fine-grained lithologies that comprise most of the sedimentary record. Here, we test the reliability and preferred mineral mapping work flow of a modern Field-Emission scanning electron microscope equipped with the Thermofisher Scientific Maps Mineralogy mineral mapping system, focusing on mud/siltstones and calcareous shales. We demonstrate that SEM-EDS mineral mapping that implements 1) a strict error minimization spectral matching approach and 2) spectral deconvolution to produce ‘mixels’ for mixed-phase X-ray volumes can robustly identify individual grains and produce quantitative mineralogical data sets comparable to conventional X-ray diffraction (XRD) analysis (R2 > 0.95). The correlation between SEM-EDS and XRD-derived mineralogy is influenced by mineral abundance, processing modes and mapped area characteristics. Minerals with higher abundance (>10 wt%) show better correlation, likely the result of increased uncertainty for XRD quantification of low-abundance phases. Automated spectral deconvolution to produce ‘mixels’ greatly reduces the proportion of unclassified pixels, especially in the fine-grained fraction, ultimately improving mineral identification and quantification. Mapping of larger areas benefits bulk mineralogy analysis, while customized area size and shape allows high-resolution in situ mineralogical analysis. Finally, we review SEM-EDS-based mineral mapping applications in the Earth Sciences, via case studies illustrating 1) approaches for the quantitative differentiation of various mineral components including detrital (allogenic), syndepositional (authigenic) and burial diagenetic phases, 2) the origin and significance of lamination, 3) the effectiveness and appropriateness of sequential leaching in geochemical studies, and 4) the utility of mineral maps to identify target grains within specific petrological contexts for in situ geochemical or geochronological analysis.

Comparison of corrosion resistance mechanism between ordinary Portland concrete and alkali-activated concrete subjected to biogenic sulfuric acid attack

The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure

The partitioning of molybdenum between magnetite (mt) and two rhyolitic melts has been determined at 800 degrees C, 1 kb of pressure, and at two oxygen fugacities. At an oxygen fugacity one-half a log unit above nickel-nickel oxide, D (super mt/melt) Mo = 0.21 + or - 0.08. A decrease in the oxygen fugacity to the graphite-methane buffer is accompanied by an increase in D (super mt/melt) Mo to 0.52 + or - 0.13. The differences in melt composition have no discernible effect.These data are interpreted by writing balanced chemical potential relationships involving independently variable phase components. The Nernst partition coefficients given above are then formulated as a function of these equilibria. Given the observed variation in D (super mt/melt) Mo with oxygen fugacity, and assuming that Fe 2 MoO 4 is the molybdenum-bearing phase component in magnetite, our data suggest that Mo(III) may be present in naturally occurring silicic magmas. Further, we suggest the establishment of critical bulk partition coefficients for ore-forming systems, defined in terms of critical values of the efficiency of removal function defined by Candela and Holland (1986). These preliminary results indicate that the crystallization of ferromagnesian or titanium-bearing phases may be important in reducing the amount of molybdenum available for orthomagmatic-hydrothermal ore formation. Oxygen fugacity may play a large role in the melt-crystal-vapor equilibria ofmolybdenum under magmatic conditions.

Oxidation-reduction relations in basic magma: a case for homogeneous equilibria

Oxygen fugacity dependence of Ni, Co, Mn, Cr, V, and Si partitioning between liquid metal and magnesiowüstite at 9–18 GPa and 2200°C

When coal is widely used as fuel, it also produces lower combustion efficiency and environmental pollution due to direct utilization of coal. The demineralization of coal to prepare ultra clean coal can improve utilization value and reduce pollutant emissions. Thus, Taixi anthracite is chosen to prepare ultra clean coal by microbubble flotation method. Box Behnken design (BBD), a response surface model is introduced as a factorial design of experiments setup and used to find those parameters, which affect the ash content and yield of ultra clean coal and to adjust these parameters in order to achieve the optimized effect. Especially, their interaction effect of different factors in flotation process could be explored by the mathematical model. To explore the mechanism of preparing ultra clean coal, X‐ray fluorescence (XRF), X‐ray diffraction (XRD) and scanning electron microscopy energy dispersive spectroscopy (SEM‐EDS) results are analysed. Combining XRF with SEM‐EDS analysis, the concentrate is polluted by composite particles with particle size of less than 2 μm and organic matter accounting for more than 95 %. Through XRD and SEM‐EDS analysis, it can be shown that a small amount of fine clay minerals including kaolinite and illite agglomerate heterogeneously with coal particles during flotation and pollute concentrate. Further, the existence of composite particles and slime coatings were the main reason of limiting deep demineralization in microbubble flotation process.

The cleaning of buildings, statues, and artworks composed of stone materials from metal corrosion is an important topic in the cultural heritage field. In this work the cleaning effectiveness of a PVA-PEO-borax hydrogel in removing metal corrosion products from different porosity stones has been assessed by using a multidisciplinary and non-destructive approach based on relaxation times measurement by single-sided portable Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy—Energy Dispersive Spectroscopy (SEM-EDS), and Raman Spectroscopy. To this end, samples of two lithotypes, Travertine and Carrara marble, have been soiled by triggering acidic corrosion of some copper coins in contact with the stone surface. Then, a PVA-PEO-borax hydrogel was used to clean the stone surface. NMR data were collected in untreated, soiled with corrosion products, and hydrogel-cleaned samples. Raman spectroscopy was performed on PVA-PEO-borax hydrogel before and after cleaning of metal corrosion. Furthermore, the characterization of the dirty gel was obtained by SEM-EDS. The combination of NMR, SEM-EDS and Raman results suggests that the mechanism behind the hydrogel cleaning action is to trap heavy metal corrosion products, such as Cu2+ between adjacent boron ions cross-linked with PVA. Moreover, the PVA-PEO-borax hydrogel cleaning effectiveness depends on the stone porosity, being better in Carrara marble compared to Travertine.

<p>Subduction of carbon-bearing phases throughout Earth’s history may be an important mechanism of sourcing carbon to the Earth’s lower mantle. As carbon has very low solubility in mantle silicates, it is primarily present in accessory phases such as carbonates, diamond, or metal carbides. Previous studies indicate that more than half of the carbonate contained in the oceanic crust may survive metamorphism and dehydration in the sub-arc and reach the lower mantle, even though the oxygen fugacity in the deep mantle may not favour their stability [1]. Indeed, the presence of carbonate in ultra-deep diamond inclusions provides evidence for carbonate subduction at least down to the transition zone [2].</p><p>The carbonate phases present in the lower mantle depend on their bulk composition, the oxygen fugacity, and on their stability at a given pressure and temperature. Results from high-pressure experiments show that magnesite (MgCO<sub>3</sub>) can be stable up to deep lower mantle conditions (∼80 GPa and 2500 K) [3]. Accordingly, magnesite may be considered the most probable carbonate phase present in the deep Earth. Experimental studies on magnesite decarbonation in presence of SiO<sub>2</sub> at lower mantle conditions suggest that magnesite is stable along a cold subducted slab geotherm [4, 5]. However, our understanding of magnesite’s stability in contact with bridgmanite [(Mg,Fe)SiO<sub>3</sub>],  the most abundant mineral in the lower mantle, remains incomplete.</p><p>Hence, to investigate sub-solidus reactions, melting, decarbonation, and diamond formation in the system MgCO<sub>3</sub>-(Mg,Fe)SiO<sub>3</sub>, we conducted a combination of high-pressure experiments using multi-anvil press and laser-heated diamond anvil cells (LH-DAC) at conditions ranging from 25 to 70 GPa and 1300 to 2100 K.</p><p>Multi-anvil experiments at 25 GPa and temperatures below the mantle geotherm (1700 K) show the formation of carbonate-silicate melt associated with stishovite crystallization, indicating incongruent melting of bridgmanite to stishovite, in accordance with the recent finding of Litasov and Shatskiy [4]. LH-DAC data from <em>in situ</em> X-ray diffraction show crystallization of bridgmanite and stishovite. Diamond crystallization is detected using Raman spectroscopy. A melt phase could not be detected <em>in situ</em> at high temperatures.</p><p>Our results suggest a two-step process that starts with melting at temperatures below the mantle geotherm, followed by crystallization of diamond from the melt produced.  Therefore, we propose that subducted carbonate-bearing silicate rocks will not remain stable in the lower mantle and will instead melt at upper-most lower mantle conditions, fostering diamond formation. Our study also provides additional evidence that diamond production is related to carbonated melt. Consequently, the melting of recycled crust and chemical transfer to the surrounding mantle will hinder the transport of carbon deeper into the lower mantle.</p><p>[1] Stagno et al. (2015) Contrib. Mineral. Petrol. 169(2), 16.<br>[2] Brenker et al. (2007) EPSL 260(1-2), 1-9.<br>[3] Binck, et al. (2020) Physical Review Materials, 4(5),1-9.<br>[4] Litasov & Shatskiy (2019) Geochemistry International, 57(9), 1024-1033.<br>[5] Drewitt, et al. (2019). EPSL, 511, 213-222.</p>

Gregório Lopes is one of the most famous Portuguesepainters of the 15th–16th centuries. This work is a contribution to thestudy of his painting technique, specifically addressing the methodologyused in the preparation of ground layers, which has never been carried outpreviously with this multianalytical method.For this purpose characterization of the ground layers of a selection of hispaintings was carried out by micro-Energy Dispersive X-ray Fluorescencespectroscopy (μ-EDXRF), micro-X-ray Diffraction (μ-XRD), ScanningElectron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS) andcomplemented by micro-Raman spectroscopy. This work presents the resultsobtained on two altarpieces (c.1544) produced at the same period by thisPortuguese artist. Ground layers are composed mainly of calcium sulfate —anhydrite and gypsum — and other compounds such as dolomite. Referencesamples were prepared to obtain diffraction pattern of different percentageof gypsum and anhydrite and compared with the results from historicalsamples.

Study of eight Albanian-Dalmatian axes (XIII-XII B.C.) and six celts (XI-X B.C.) found in northern Albania, by μ-XRF, OM, SEM-EDS

Estimation of natural asbestos content in rocks by fracture network modeling and petrographic characterization