Magma oxygen fugacity of mafic-ultramafic intrusions in convergent margin settings: Insights for the role of magma oxidation states on magmatic Ni-Cu sulfide mineralization

Oxygen fugacities (fO2) of mantle-derived mafic magmas have important controls on the sulfur status and solubility of the magmas, which are key factors to the formation of magmatic Ni-Cu sulfide deposits, particularly those in convergent margin settings. To investigate the fO2 of mafic magmas related to Ni-Cu sulfide deposits in convergent margin settings, we obtained the magma fO2 of several Ni-Cu sulfide-bearing mafic-ultramafic intrusions in the Central Asian Orogenic Belt (CAOB), North China, based on the olivine-spinel oxygen barometer and the modeling of V partitioning between olivine and melt. We also calculated the mantle fO2 on the basis of V/Sc ratios of primary magmas of these intrusions. Ni-Cu sulfide-bearing mafic-ultramafic intrusions in the CAOB include arc-related Silurian-Carboniferous ones and post-collisional Permian-Triassic ones. Arc-related intrusions formed before the closure of the paleo-Asian ocean and include the Jinbulake, Heishan, Kuwei, and Erbutu intrusions. Post-collisional intrusions were emplaced in extensional settings after the closure of the paleo-Asian ocean and include the Kalatongke, Baixintan, Huangshandong, Huangshan, Poyi, Poshi, Tulaergen, and Hongqiling No. 7 intrusions. It is clear that the magma fO2 values of all these intrusions in both settings range mostly from FMQ+0.5 (FMQ means fayalite-magnetite-quartz oxygen buffer) to FMQ+3 and are generally elevated with the fractionation of magmas, much higher than that of MORBs (FMQ-1 to FMQ+0.5). However, the mantle fO2 values of these intrusions vary from ~FMQ to ~FMQ+1.0, just slightly higher than that of mid-ocean ridge basalts (MORBs) (≤FMQ). This slight difference is interpreted as the intrusions in the CAOB may have been derived from the metasomatized mantle wedges where only minor slab-derived, oxidized components were involved. Therefore, the high-magma fO2 values of most Ni-Cu sulfide-bearing mafic-ultramafic intrusions in the CAOB were attributed to the fractionation of magmas derived from the slightly oxidized metasomatized mantle. In addition, the intrusions that host economic Ni-Cu sulfide deposits in the CAOB usually have magma fO2 of >FMQ+1.0 and sulfides with mantle-like δ34S values (–1.0 to +1.1‰), indicating that the oxidized mafic magmas may be able to dissolve enough mantle-derived sulfur to form economic Ni-Cu sulfide deposits. Oxidized mafic magmas derived from metasomatized mantle sources may be an important feature of major orogenic belts.

Partitioning of V and 19 other trace elements between rutile and silicate melt as a function of oxygen fugacity and melt composition: Implications for subduction zones

Vanadium is a multivalent element that can speciate as V2+, V3+, V4+, and V5+ over a range of geologically relevant oxygen fugacities (fO2). The abundance of V in planetary materials can be exploited as a proxy for fO2 when its partitioning behavior is known. The mineral rutile (TiO2) is an important carrier of the high field strength elements Nb and Ta in the solid Earth, but it can also incorporate substantial quantities of vanadium (up to ~2000 ppm; e.g., Zack et al. 2002). However, little work has been done to systematically investigate how the partitioning of V in rutile-bearing systems changes as a function of both fO2 and composition. We measured the partitioning of V and 19 other trace elements (Sc, Cr, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu, Hf, and Ta) between rutile and three silicate melt compositions equilibrated at 1 atm pressure, 1300 °C and fO2 values from two log units below the quartz-fayalite-magnetite oxygen buffer (QFM-2) to air (QFM+6.5). Rutile/melt partition coefficients (DVrt/melt) change dynamically over an eight-log unit range of fO2 and are greatest at fO2 = QFM-2 in all compositions. Vanadium solubility in rutile declines continuously as fO2 increases from QFM-2 and approaches unity in air. Trace-element partitioning between rutile and melt is also correlated with melt composition, with the greatest values of Drt/melt measured in the most polymerized melt systems containing the least TiO2. We do not find any circumstances where V becomes incompatible in rutile. Our results indicate that rutile is a considerable sink for V at terrestrial fO2 values and will contribute to the retention of V in refractory slab residues in subduction zones. In agreement with previous work, we find that DTart/melt>DNbrt/melt under all conditions investigated, suggesting that rutile fractionation does not lead to low Nb/Ta ratios in Earth's continental crust.

Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust

Oxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/ΣFe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/ΣFe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/ΣFe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/ΣFe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.

Intra-eruptive trachyte-phonolite transition: Natural evidence and experimental constraints on the role of crystal mushes

The generation of silica undersaturated phonolite from silica saturated trachytes is uncommon, as it implies the crossing of the thermal barrier and critical plane of silica undersaturation. Nevertheless, a co-genetic suite displaying compositional transition from benmoreite-trachyte to phonolite has been observed within the Al Shaatha pyroclastic sequence in the Harrat Rahat Volcanic Field (Kingdom of Saudi Arabia). We performed crystallization experiments on benmoreite and trachyte starting compositions to simulate the pressure-temperature-volatile conditions that generated the observed liquid line of descent. The experimental conditions were 200–500 MPa, 850–1150 °C, 0–10 wt% H2O, 0.0–0.5 wt% CO2, and NNO+2 oxygen buffer. The experimental mineral assemblage consists of clinopyroxene, feldspar, and titanomagnetite, as well as glass in variable proportions. The degree of crystallinity of hydrous runs is lower than that of anhydrous ones at analogous pressure and temperature conditions. Clinopyroxene crystallizes with compositions diopside-augite and augite-hedenbergite, respectively, at 500 and 200 MPa. The saturation of feldspar is primarily controlled by temperature and volatile content, with the more potassic composition equilibrating at low temperature (850–900 °C) and anhydrous (for benmoreite) or hydrous (for trachyte) conditions. At low pressure (200 MPa), temperatures below 850 °C, and anhydrous conditions, the degree of crystallization is extremely high (>90%), and the residual glass obtained from trachyte experiments is characterized by peralkaline and sodic affinity. This finding is consistent with natural eruptive products containing interstitial phonolitic glass within an anorthoclase framework. The shift from trachyte to phonolite is therefore interpreted as the result of open system interaction between trachytic magma and intercumulus phonolitic melt, as well as of dissolution of anorthoclase from a crystal mush.

Water in parental basaltic magmasof the Menshiy Brat volcano (Iturup Island, Kurile islands)

The paper presents study of the liquidus assemblage of olivine and spinel in high-magnesian basalts (MgO up to 10 mas. %) of the Menshiy Brat volcano (Iturup Island). It was possible to reconstruct the water content and evolution of volatile components in the primary parental magmas that took part in the formation of the Medvezhya Caldera, Iturup Islands. It is shown that the initial water content in the primary melts could reach 5 mas. % with oxygen fugacity corresponding to oxygen buffer NNO + 0.4 log. units. The evolution of magmas involved continuous degassing while magma rises to the surface. The water-rich fluid, which is constantly separated by evolving magma, could play a significant role in the formation of large siliceous magma chambers, which participated in catastrophic caldera eruptions.

Supercharged ceria quantum dots with exceptionally high oxygen buffer action

Supercharged ceria nanoparticles with excellent oxygen storage capacity.

Photo, thermal and chemical degradation of riboflavin

Riboflavin (RF), also known as vitamin B2, belongs to the class of water-soluble vitamins and is widely present in a variety of food products. It is sensitive to light and high temperature, and therefore, needs a consideration of these factors for its stability in food products and pharmaceutical preparations. A number of other factors have also been identified that affect the stability of RF. These factors include radiation source, its intensity and wavelength, pH, presence of oxygen, buffer concentration and ionic strength, solvent polarity and viscosity, and use of stabilizers and complexing agents. A detailed review of the literature in this field has been made and all those factors that affect the photo, thermal and chemical degradation of RF have been discussed. RF undergoes degradation through several mechanisms and an understanding of the mode of photo- and thermal degradation of RF may help in the stabilization of the vitamin. A general scheme for the photodegradation of RF is presented.

Experimental Investigation of Aqueous Cerium Oxide Nanofluid Blend in Diesel Engine

Influence of aqueous cerium oxide nanofluid on the major physicochemical properties of diesel and the performance and exhaust emission characteristics of diesel engine were investigated. 50cc of aqueous cerium oxide nanofluid was dispersed into 1 litre of diesel fuel for preparing test fuel. The diesel with and without aqueous cerium oxide nanofluid was tested in a direct injection diesel engine at 0%, 25%, 50%, 75%, and 100% of full load condition. nanosized cerium oxide has more surface area, higher activity and can react with water at high temperature to generate hydrogen and improve fuel combustion. Also cerium oxide acts as oxygen buffer causing simultaneous oxidation of hydrocarbons as well as the reduction of oxides of nitrogen. The results on the combustion of diesel mixed with aqueous Cerium oxide nanofluid showed an increase in total heat of combustion and a decrease in concentration of HC, NOx and smoke in the exhaust emission from the diesel engine.

Study of Uranium Incorporation Into Zircon: Solubility Limits and Durability of Fixation

ABSTRACTResearch on synthetic zircons fabricated in system ZrO2-UO2-SiO2 at T = 400 - 750° C, PH20 = 100-200 MPa, and Ni-NiO oxygen buffer was made in order to estimate the limits of uranium incorporation into the phase's structure. It was shown that up to 27 wt.% of uranium may locally enter into the synthetic zircons. SEM/EDS study of the samples has showed that the element is uniformly distributed within the zircon crystals. Incorporation of U into zircon increases its unit-cell dimensions up to α = 6.70 A and c = 6.00 A relative to 6.60 and 5.98 A for a pure phase without impurities.After thermal high temperature treatment of the samples under 1400° C, in air, for 6 hours inhomogeneous distribution of uranium within the zircon grains was revealed. Very small uranium rich spots probably corresponding to a newly formed phase were observed in the zircon crystals. Contents of the element throughout the grains decreased to about 10 wt.%. It is probably caused by destruction of zircon-coffinite solid solution formed at hydrothermal conditions in the course of its re-heating.Study of interaction of natural zircons with alkaline carbonate solution at 100° C shows that U release from the phase can reach up to 6 % of total uranium in the samples even under relatively short term interaction. Loss of uranium rises with the element content (or amorphisation degree) and run duration, as well as with grain size reduction. Research on uranium distribution in natural zircons picked up from altered and weathered rocks is characterised by strong depletion of U at grain margins relative to the centre of the crystals. These results indicate that zircon may lose some uranium and, probably, the other actinides with time due to its lattice destruction under radioactive decay followed by hot water attack.