Fayalite Magnetite Quartz Research Articles

Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

Abstract Bimodal magmatism is characteristic of the geodynamic evolution of the Main Ethiopian Rift (MER), which is a reference area for the study of the processes leading to continental break-up before seafloor spreading. There are abundant emissions of basalts and rhyolites, which are in possible parent-daugther relationships. However, the P-T-H2O conditions of production and storage of the basaltic end member remain unclear. Crystallization experiments have been conducted on an alkali basalt from the MER to define its pre-eruptive conditions and shed light on the compositional evolution of derivative liquids and source conditions. The experiments were performed at 100-200 MPa, 975-1080°C, varying H2O/CO2 ratios, corresponding to melt water contents of 1-5 wt%, at fO2 slightly lower than the Fayalite-Magnetite-Quartz (FMQ) solid buffer. Comparison between the petrological attributes of the starting rock and the experiments shows that the basaltic magma was stored at 150-200 MPa, 1050 ± 10°C, with 1-2 wt% H2O in melt, with fO2 near FMQ prior to eruption. Geochemical modelling shows that the corresponding mantle source contained about 0.1 wt% H2O, reflecting a metasomatized source. Extensive crystallization of such basalts produces SiO2-rich liquids, which are not yet peralkaline, however. This underscores that extreme fractionation (>90 wt%) is required in order to produce peralkaline derivatives from mildly alkaline basalts. This extreme fractionation and the water-rich nature of the starting basalt readily explain the H2O-rich condition of peralkaline rhyolites that have fueled caldera forming eruptions in the Rift.

Influence of intensive parameters on the mafic paragenesis of nepheline syenites: the Monte de Trigo alkaline suite, southeastern Brazil

The Monte de Trigo Alkaline Suite is a Late Cretaceous syenite–gabbroid occurrence from the Serra do Mar Alkaline Province, southeastern Brazil. It has a zoned nepheline syenitic stock with nepheline syenite I (NSI), the most SiO 2 -undersaturated, in the centre, surrounded by nepheline syenite II (NSII) and alkali feldspar syenite with nepheline (SN). Synplutonic miaskitic and agpaitic nepheline microsyenite (MNM and ANM, respectively) dykes cut across the stock. Extensive fractionation of alkali feldspar and mafic minerals is attributed to the formation of the syenitic facies and dykes. Mineral chemical data indicate that SN crystallized at relatively high silica activity ( a SiO 2 ; 0.5–0.7), a H 2 O (0.24–0.11) and f O 2 (difference from the fayalite–magnetite–quartz buffer in log units, ΔFMQ + 0.2 to ΔFMQ − 0.2), in contrast to NSII and NSI ( a SiO 2 , ∼0.41; a H 2 O, 0.05–0.02; and f O 2 , ΔFMQ − 1.4 to ΔFMQ − 1.1). These parameters vary with magma evolution towards the phonolite eutectic and appear to control variations in paragenesis, from hastingsite + biotite ± diopside/hedenbergite in SN to hastingsite in NSII and hastingsite + hedenbergite in NSI. The MNM have high a H 2 O (0.31) and hastingsite + biotite + aegirine–augite assemblages. In contrast, the ANM are characterized by an F-rich anhydrous agpaitic assemblage.

Magma storage conditions of Lascar andesites, central volcanic zone, Chile

Abstract. Lascar volcano, located in northern Chile, is among the most active volcanoes of the Andean Central Volcanic Zone (CVZ). Its activity culminated in the last major explosive eruption in April 1993. Lascar andesites which erupted in April 1993 have a phase assemblage composed of plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides, and rhyolitic glass. To better constrain storage conditions and mechanisms of magmatic differentiation for andesitic magmas in a thick continental crust, crystallization experiments were performed in internally heated pressure vessels at 300 and 500 MPa, in the temperature (T) range of 900–1050 °C, at various water activities (aH2O) and oxygen fugacities (logfO2 between QFM+1.5 and QFM+3.3 at aH2O =1; QFM is quartz–fayalite–magnetite oxygen buffer). The comparison of experimental products with natural phase assemblages, phase compositions, and whole-rock compositions was used to estimate magma storage conditions and to reconstruct the magma plumbing system. We estimate that Lascar two-pyroxene andesitic magmas were stored at 975±25 °C, 300±50 MPa, and logfO2 of QFM+1.5±0.5, under H2O-undersaturated conditions with 2.5 wt % to 4.5 wt % H2O in the melt. The geochemical characteristics of the entire suite of Lascar volcanics indicates that a fractionating magmatic system located at a depth of 10–13 km is periodically replenished with less evolved magma. Some eruptive stages were dominated by volcanic products resulting most probably from the mixing of a mafic andesitic magma with a felsic component, whereas compositional variations in other eruptive stages are better explained by crystal fractionation processes. The relative importance of these two mechanisms (mixing vs. crystal fractionation) may be related to the amount and frequency of magma recharge in the reservoir.

Spinel chromianowy z dunitów masywu Inagli i ich termobarometria tlenowa (tarcza ałdańska, platforma syberyjska)

The Inagli dunite-peridotite-shonkinite zonal-ring intrusive with platinum-chromite mineralization is located on the Aldan shield of the Siberian Platform. Considering the structure, rock composition and ore mineralization, it is similar to the platinum-bearing zonal massifs of the "Ural-Alaskan" type, but this intrusive differs from the latter in its geological position. In order to clarify the physical and chemical conditions of formation of the Inagli massif, the mineral composition of rocks, especially сhromite-containing dunites, peridotites and shonkinites, as well as platinum-chromitite ore segregations, has been studied in detail. The rocks of the Inagli massif, from dunites to shonkinites, including peridotites, clinopyroxenites, and alkaline syenites, form a single continuous series. This is confirmed by a clear dependence of the composition of olivine, pyroxene, phlogopite and chromian spinel on the content of MgO in rocks. They were formed from the initial high-potassium picrite melt, which, during rising, underwent gradual decompression solidification and formed a cylindrical diapir-like body at the near-surface level in the Early Cretaceous. This occurred as a result of subduction processes related to the formation of the Mongol-Okhotsk orogenic belt along the southern framing of the Siberian craton. The values of oxygen volatility (lgfO2) for dunites, peridotites, shonkinites, chromitites and olivine-chromite inclusions in the isoferroplatinum of the Inagli massif, calculated using the method of the olivine-spinel oxygen thermobarometer of Ballhaus-Berry-Green (BBG), form a single trend FMQ+(2-4) in the range 620-11400C, i.e. along the band by 2-4 units of lgfO2 exceeding the fayalite-magnetite-quartz (FMQ) buffer. Such a rather narrow range of variation in the values of O2 fugacity in a wide interval of temperature indicates good comparability and reliability of the data obtained. At the same time, there is a natural decrease in temperature intervals for the formation of olivine-chromite parageneses (in 0С): with isoferroplatinum – (1140-680); in chromitite segregations – (980-710); in dunites – (930-620); peridotites – (890-770) and shonkinites – (840-710). The results obtained almost completely coincide with the field of values for dunites and chromitites from the Platinum-bearing belt of the Urals, given by other researchers. In terms of redox parameters, platinum-bearing zonal ultramafic-mafic massifs of the Ural-Alaskan and Aldanian types are close to more oxidized peridotites with a long history in the lithosphere. They differ significantly from the peridotites of the Beni-Bousera massif, as well as abyssal peridotites of the oceanic ridge systems, and others, which are formed under more reduced conditions corresponding to the range between FMQ, carbon-oxygen-CO (ССО) and iron-wustite (IW) buffers.

Iron valence systematics in clinopyroxene crystals from ocean island basalts

The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity (fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document}). However, our knowledge of how and why fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland and the Azores to explore whether they record information about magma Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} contents and magmatic fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document}, we find that up to half of the total Fe present can occur as Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}, with crystals from alkali systems typically containing more Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} than those from tholeiitic systems. Thus, Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} in these crystals is hosted within esseneite component (CaFe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}AlSiO6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{6}$$\\end{document}), though some may be hosted in aegirine component (NaFe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}Si2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document}O6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{6}$$\\end{document}) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} is related to the abundance of tetrahedrally coordinated Al (IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al), implying some steric constraints over Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} partitioning between clinopyroxene and liquid (i.e., DFe2O3cpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$D\\mathrm {^{{cpx-liq}}_{{Fe_{2}O_{3}}}}$$\\end{document} values), though this may not be an equilibrium relationship. For example, IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-rich {hk0}\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{hk0\\}$$\\end{document} prism sectors of sector-zoned crystals contain more Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} than IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-poor {1¯11}\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\{\\bar{1}11\\}$$\\end{document} hourglass sectors. Moreover, IV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {^{IV}}$$\\end{document}Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}. Apparent clinopyroxene-liquid Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document}–Mg exchange equilibria (i.e., KD,Fe2+-Mgcpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K\\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$\\end{document} values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that KD,Fe2+-Mgcpx-liq\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K\\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}$$\\end{document} values lies closer to experimentally reported values of 0.24-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-$$\\end{document}0.26 than values of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}0.28 returned from a general model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (≥\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\ge$$\\end{document}FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} degassing; our other glassy sample from Iceland was also affected by reductive SO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and fO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\ extrm{O}_{2}}$$\\end{document} conditions on clinopyroxene-liquid equilibria involving Fe2+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2+}$$\\end{document} and Fe3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document}.

Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Abstract Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

The Redox State of the Asthenospheric Mantle and the Onset of Melting Beneath Mid‐Ocean Ridges

AbstractThe redox state of the convective asthenospheric mantle governs the speciation of volatile elements such as carbon and, therefore, influences the depth at which (redox) melting can occur, with implications for seismic signals. Geophysical observations suggest the potential presence of carbonatite melts at a depth of 200–250 km. However, thermodynamic models indicate that the onset of (redox) melting would occur at 100–150 km for a mantle with 3%–4% of Fe3+/∑Fe. Here, we present a new oxybarometer that is based on the V/Sc exchange coefficient between olivine and the melt, which is insensitive to surficial alteration, volatile degassing, electron exchange reactions and fractional crystallization. By applying this method to primary mid‐ocean ridge basalts (MORBs) from Southwest Indian Ridge and East Pacific Rise, we demonstrate that the average oxygen fugacity (fo2) of MORBs corrected for the depth of formation is 0.78 ± 0.26 (1σ) log units above the fayalite‐magnetite‐quartz (FMQ) buffer, which is slightly more oxidized than previously estimated (near FMQ buffer). Our findings indicate that the convective asthenospheric mantle exhibits higher oxygen fugacity than the continental lithospheric mantle. Along an adiabat, carbonatitic melts can form from a CO2‐bearing source at a depth of 200–250 km, explaining the asthenospheric mantle's electrical conductivity and seismic velocity anomaly.

Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at <135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures.

Geochemical Evidence of Plume Sources for High-MgO Lavas in the Western Kunlun Orogenic Belt

Abstract Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian–Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6–33.8 wt %) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2 ± 0.9 wt %. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite-dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg–Fe equilibria, Al-in-olivine and olivine–spinel thermometry and oxybarometer yield a mantle potential temperature of 1522–1556 °C and high oxygen fugacity of FMQ (fayalite-magnetite-quartz) + 0.93. The H2O contents in the picrite flows are estimated as 3.67 ± 1.0 wt %, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB (oceanic island basalt)-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd–O–Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.

Redox control of the partitioning of platinum and palladium into magmatic sulfide liquids

The partitioning behavior of platinum group elements in magmas is critical for their use as tracers of planetary accretion and in understanding magmatic sulfide deposits. Here we use laboratory experiments to determine sulfide liquid–silicate melt partition coefficients for platinum and palladium at 1.5 GPa, 1400 °C, and oxygen fugacity 1.5–2 log units above the fayalite–magnetite–quartz buffer. We find that the partitioning coefficients of these elements are 2.3 × 105 to 1.1 × 106 and are independent of the platinum and palladium concentration in the system. Combined with previous data obtained at oxygen fugacity below the fayalite–magnetite–quartz buffer, this indicates redox-controlled partitioning behavior whereby at oxidizing conditions platinum- and palladium-enrichments are achieved through their dissolution in sulfide liquids, while at reducing conditions the entrapment of platinum- and palladium-rich clusters in sulfide liquids is more critical. This redox-controlled partitioning behavior should be considered when studying crust–mantle differentiation and the formation of magmatic sulfide deposits.

Experimental determination of tin partitioning between titanite, ilmenite and granitic melts using improved capsule designs

Abstract Investigating mineral/melt Sn partitioning at high temperatures and pressures is a difficult task because Sn is a redox-sensitive multivalent element and easily alloys with noble metal sample capsules. To obtain accurate Sn partition coefficients between titanite, ilmenite, and granitic melts, we developed single capsule Pt or Au and double capsule Pt95Rh5 (or Au)–Re designs to avoid significant Sn loss at a controlled oxygen fugacity (ƒO2). With these new capsule designs, we performed piston-cylinder experiments of Sn partitioning between titanite, ilmenite, and granitic melts. The experimental P–T–ƒO2 conditions were 0.5–1.0 GPa, 850–1000 °C and ~QFM+8 to ~QFM–4 (QFM: Quartz–Fayalite–Magnetite buffer), with ƒO2 controlled by the solid buffers of Ru–RuO2, Re–ReO2, Co–CoO, graphite, and Fe–FeO. The obtained mineral/melt Sn partition coefficients (DSnmin/melt) are 0.48–184.75 for titanite and 0.03–69.45 for ilmenite at the experimental conditions. The DSnmin/melt values are largely dependent on ƒO2 although the effects of temperature and melt composition are also observed. DSnTtn/melt strongly decreases with decreasing ƒO2, from ~46–185 at the most oxidizing conditions (Ru–RuO2 buffer), to ~2–16 at moderately oxidizing to moderately reducing conditions (Re–ReO2 to Co–CoO and graphite buffers), to < 1 at the most reducing conditions (Fe–FeO buffer). DSnIlm/melt exhibits a variation trend similar to DSnTtn/melt, but is always lower than DSnTtn/melt at a given ƒO2. These DSnmin/melt values can be applied to quantitatively assess the mineralization potential of granitic magmas. Using DSnTtn/melt, we estimate that Sn contents are ~150–400 ppm in the premineralization magmas of the tin-mineralized Qitianling plutons (South China).

Oxidation state of Cu in silicate melts at upper mantle conditions

Beyond its economic value, copper (Cu) serves as a valuable tracer of deep magmatic processes due to its close relationship with magmatic sulfide evolution and sensitivity to oxygen fugacity (fO2). However, determining Cu’s oxidation state (+ 1 or + 2) in silicate melts, crucial for interpreting its behavior and reconstructing fO2 in the Earth’s interior, has long been a challenge. This study utilizes X-ray Absorption Near Edge Structure spectroscopy to investigate the Cu oxidation state in hydrous mafic silicate melts equilibrated under diverse fO2 (− 1.8 to 3.1 log units relative to the Fayalite–Magnetite–Quartz buffer), temperature (1150–1300 °C), and pressure (1.0–2.5 GPa) conditions. Our results reveal that Cu predominantly exists as Cu+ across all fO2 conditions, with a minor Cu2+ component. This dominance of Cu+ persists even in relatively oxidized melts, highlighting its limited sensitivity to fO2 under upper mantle conditions. This significantly constrains the utility of Cu as an oxybarometer in hydrous silicate melts in the deep Earth. However, our findings suggest that Cu isotopes primarily reflect the interplay of sulfide segregation/accumulation during magmatic differentiation, shedding light on these fundamental processes in Earth’s interior.

Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano

Abstract. Here we present a confocal Fe K-edge μ-XANES method (where XANES stands for X-ray absorption near-edge spectroscopy) for the analysis of Fe oxidation state in heterogeneous and one-side-polished samples. The new technique allows for an analysis of small volumes with high spatial 3D resolution of <100 µm3. The probed volume is restricted to that just beneath the surface of the exposed object. This protocol avoids contamination of the signal by the host material and minimizes self-absorption effects. This technique has been tested on a set of experimental glasses with a wide range of Fe3+ / ΣFe ratios. The method was applied to the analysis of natural melt inclusions trapped in forsteritic to fayalitic olivine crystals of the Hekla volcano, Iceland. Our measurements reveal changes in Fe3+ / ΣFe from 0.17 in basaltic up to 0.45 in dacitic melts, whereas the magnetite–ilmenite equilibrium shows redox conditions with Fe3+ / ΣFe ≤0.20 (close to FMQ, fayalite–magnetite–quartz redox equilibrium) along the entire range of Hekla melt compositions. This discrepancy indicates that the oxidized nature of glasses in the melt inclusions could be related to the post-entrapment process of diffusive hydrogen loss from inclusions and associated oxidation of Fe in the melt. The Fe3+ / ΣFe ratio in silicic melts is particularly susceptible to this process due to their low FeO content, and it should be critically evaluated before petrological interpretation.

The effect of oxygen fugacity on the evaporation of boron from aluminoborosilicate melt

Abstract. We present the results of B2O3 evaporation experiments from Ca- and Mg-bearing aluminoborosilicate melts. Our experiments were conducted at 1245 to 1249 ∘C and 1350 to 1361 ∘C for different run times (60–1020 min), and at oxygen fugacities (logfO2) relative to the fayalite–magnetite–quartz (FMQ) buffer of FMQ−6 to FMQ+1.5, and in air. Our results show that with increasing fO2, evaporation of B from the melt increases by a factor of 5 compared to reducing conditions. Using Gibbs free energy minimization calculations, we suggest two possible evaporation reactions for B2O3 which constrain its speciation in the gas phase to be either 3+ or 4+ (B2O3(g) and BO2(g)). The measured B2O3 contents of the B evaporated residual glasses were used to calculate evaporation rate constants (ki) for B2O3 in oxidizing conditions (air, ki=2.09×10-4 cm min−1 at 1350 ∘C) and reducing conditions (FMQ−4, ki=4.46×10-5 cm min−1 at 1350 ∘C). The absence of diffusion profiles in the experimental glasses suggests that the evaporation rates are slower than B2O3 diffusion rates and therefore the rate-limiting process. Overall, the rate of B evaporation in air is approximately a factor of 5 higher compared to reducing conditions at FMQ−4.

An experimental study of the effect of water and chlorine on plagioclase nucleation and growth in mafic magmas: application to mafic pegmatites

Abstract. In this study, the effects of H2O and Cl on the grain size and nucleation delay of plagioclase in basaltic magma were investigated using dynamic and equilibrium experiments at 1150 ∘C, 300 MPa, and oxygen fugacity between FMQ − 1.65 and FMQ + 0.05 (fayalite–magnetite–quartz). Each experiment consisted of five samples of basaltic composition (from the Hamn intrusion in Northern Norway) containing varying amounts of H2O (up to 2 wt %) and Cl (up to 1 wt %). The equilibrium experiments were used as a reference frame for the phase assemblage, geochemical composition, and liquidus temperatures and were compared to thermodynamic models using MELTS software. Experimental phase abundances and plagioclase compositions are in good agreement with the predictions of MELTS. The dynamic experiments were initially heated above the liquidus temperature to destroy crystal nuclei and then kept at 1150 ∘C for 100, 250, or 1800 min. These experiments show that as the concentration of H2O in the melt increases, plagioclase nucleation is delayed, plagioclase abundance decreases, but its size increases. Therefore, the addition of H2O seems to favor plagioclase growth at the expense of nucleation. Thermodynamic and kinetic calculations corroborate an increase in the nucleation delay of plagioclase with increasing H2O content dissolved in the melt, suggesting that H2O decreases the undercooling of the silicate melt. The addition of Cl also seems to delay plagioclase nucleation, although this is not supported by kinetic calculations. Increasing the Cl content decreases plagioclase abundance but does not significantly affect its size. The homogeneous pegmatitic pockets of the mafic–ultramafic Hamn intrusion exhibit several petrological and geochemical features, suggesting that H2O and Cl enrichment in the silicate melt was the origin of the pegmatitic texture. The experimental results presented here indicate that H2O, rather than Cl, may have played an important role in the formation of the pegmatitic texture.

The role of high oxygen fugacity on genesis of the late Cenozoic Abaga basalts in Xilin Gol League, Inner Mongolia, China

ABSTRACT The Abaga basalts, which erupted within an intraplate background in the late Cenozoic, are situated in eastern central Inner Mongolia. Previous studies have explored the genesis of these basalts, which were influenced by the subduction of slab. Although trace element ratios demonstrate that altered oceanic crust (AOC) and sediments have contributed to the genesis of the basalts, limited information about the detailed percentages of mixing among sediments, AOC, and depleted mantle (DM) based on Sr‒Nd‒Pb isotopic values has been reported. In this study, we modelled the possible mixing percentages among AOC, DM, and sediments. The calculated results illustrate that a proportion of 80% DM, 16% AOC, and 4% sediments is appropriate for interpreting the Sr‒Nd‒Pb isotopic variation of the basalts. In addition, the associated major and trace elements of olivine and the partition coefficient of V between olivine and melt are used to estimate the crystallization temperatures of the olivine phenocrysts and oxygen fugacity of the basalts. The calculated results display that the olivine mainly crystallized between 1248 and 1293°C (standard error estimate: 51°C), and the fO2 values of the Abaga basalts range from FMQ + 0.14 to FMQ + 1.46 (FMQ, fayalite–magnetite–quartz buffer). The estimated oxygen fugacity values plot in the range of island arc basalts (from FMQ to FMQ + 2) but are higher than those of both mid-ocean ridge basalt (MORB) (from FMQ-0.4 to FMQ + 0.5) and Abaga xenoliths (from FMQ-1.55 to FMQ + 0.53), which indicates that the basaltic melts were more oxidized than those of MORB and the Abaga lithospheric mantle. Furthermore, the primary magma and the corresponding mantle source were more oxidized than MORB. The Ba/Th and Ce/Pb ratios of the basalt present positive correlations with fO2 values, demonstrating that the addition of sediments and oceanic crust into the mantle source may have contributed to increasing the fO2 values of the basalts.

Reduced post-collisional magmas for Tengchong volcanoes, southeastern Tibetan Plateau

Rapidly cooled and well-preserved magnetite and ilmenite in young volcanic rocks provide good samples for constraining the magmatic redox conditions. We report chemical compositions of magnetite and ilmenite from the youngest Tengchong volcanoes at the southeastern margin of the Tibetan Plateau, to constrain the oxygen fugacity and temperatures for the Tengchong magmas. Oxygen fugacity values (△FMQ) relative to the fayalite-magnetite-quartz (FMQ) buffer are −1.8 ∼ −0.7 (averaging −1.1 ± 0.4) for Dayingshan; −1.3 ∼ −0.7（averaging −1.1 ± 0.3） for Maanshan; and −1.3 for Heikongshan. The temperatures for magnetite-ilmenite pairs of Tengchong lavas range from 789 to 1090 °C. The relatively reducing oxygen fugacity values (△FMQ = −1.8 ∼ −0.7) of the Tengchong magmas are significantly lower than those oxidizing arc magmas (△FMQ = +0.5 ∼ +3.5). The reduced magmas at Tengchong may exemplify other Quaternary post-collisional lavas of the Tibetan Plateau. The crystallization temperatures from mineral-liquid equilibrium are 1160–1192 °C for olivine, 1049–1110 °C for plagioclase, 978–1122 °C for clinopyroxene, 789–1090 °C for Fe-Ti oxides, 743–792 °C for zircon. The high Fe-Ti oxide temperature and Ti-in-zircon temperature indicate warm magmatic conditions (>750 °C) prior to eruptions of Tengchong magmas.

The oxidation state of titanium in silicate melts

Titanium occurs as Ti3+, in addition to the more usual Ti4+, in extraterrestrial materials such as Lunar basalts and chondritic meteorites. The proportion of Ti as Ti3+ was investigated by Ti K-edge X-ray absorption near edge structure (XANES) spectroscopy for five silicate glass compositions quenched from melts equilibrated at 1400 °C, atmospheric pressure, and oxygen fugacities (fO2) in log units relative to the fayalite-magnetite-quartz (FMQ) buffer from FMQ+3.3 to FMQ-10.2 (+6.6 to −6.9 log units relative to the iron-wüstite, IW, buffer). All spectra could be well fit using a linear combination of the spectra recorded from the most oxidised and reduced samples of the same composition, indicating that the samples only contain two Ti species. Ti3+/ΣTi (where ΣTi = Ti3+ + Ti4+) = 0 for the most oxidised samples but is unknown for the most reduced. Thus, the linear combination fit results were used in a regression model in which Ti3+/ΣTi of the reduced end-member was varied to give Ti3+/ΣTi values of the other samples that best fit the thermodynamically expected dependence of Ti3+/ΣTi on fO2. The most reduced samples were found to have Ti3+/ΣTi ∼ 0.6. The resulting modified equilibrium constants of the Ti oxidation reaction, logK', are linearly correlated with the optical basicity (Λ) parameterisation of melt composition, such that as Λ increases, Ti3+/ΣTi decreases, at constant fO2. This correlation allows Ti3+/ΣTi to be predicted for other compositions and, assuming that the temperature dependence of Ti3+/Ti4+ is parallel to FMQ, a general equation relating Ti3+/Ti4+ to fO2 was obtained: log(Ti3+/Ti4+) = −0.25ΔFMQ − 0.32(19) − 3.44(32)Λ. This equation was used to predict Ti3+/ΣTi as a function of fO2 for high-Ti Mare basalt, chondrule (CV and CM), and calcium aluminium inclusion (CAI; Type A and B) compositions. For melts of these compositions Ti3+ = Ti4+ at ∼ FMQ-10.8, −9.5, −9.3, −10.6, and −10.2 (∼IW-7.5, −6.2, −6.0, −7.3, and −6.9), respectively, independent of temperature.

An Extended Calibration of the Olivine–Spinel Aluminum Exchange Thermometer: Application to the Melting Conditions and Mantle Lithologies of Large Igneous Provinces

Abstract The application of the olivine–spinel aluminum exchange thermometer to natural samples is limited by the restricted experimental data set on which it was calibrated. Here, we present a new data set of 46 high-temperature crystallization experiments and 21 reanalyzed published experiments, which we used to extend the calibration to higher and lower temperatures. The final calibration data set spans a range of conditions relevant to crustal and upper mantle processes: 1174–1606°C, 0.1–1350 MPa, QFM − 2.5 to QFM + 7.2 (oxygen fugacity, fO2, reported in log units relative to the quartz–fayalite–magnetite buffer, QFM), and 0–7.4 wt % H2Omelt. We propose three new models. The first is thermodynamically self-consistent, based on spinel Fe, Mg, Al, and Cr compositions and Al exchange between olivine and spinel. The second and third are empirical models that consider fewer elemental exchanges: the second uses only Al exchange and spinel compositions, whereas the third considers olivine–spinel Al and Cr exchange. All models include the modest effect of pressure on olivine-spinel equilibrium chemistry, whereas fO2 and water content have negligible effects. In general, as fewer elements are considered in the olivine–spinel exchange, the fit to experimental data worsens. Conversely, the associated decrease in model complexity improves their robustness against systematic errors when applied to natural crystal pairs: the thermodynamic model may underestimate crystallization temperatures in natural samples due to spinel subsolidus re-equilibration, whereas the empirical models (independent of Fe and Mg in spinel) are less sensitive to re-equilibration but yield temperatures with larger uncertainties. We applied a statistical test to select the most appropriate model for application to natural samples. When applied to lavas from mid-ocean ridges, Iceland, Skye, Emeishan, Etendeka, and Tortugal, our new temperature estimates are 30–100°C lower than previously proposed. The lower temperature estimates cause a lower mantle melting temperature and significant impacts on the mantle lithology constraints.

Petrogenesis and implications for tin mineralization of the Beidashan granitic pluton, southern Great Xing'an Range, NE China: Constraints from whole-rock and accessory mineral geochemistry

The Beidashan pluton, consisting of quartz monzonite in the northeast and biotite granite in the southwest, is a representative granitic intrusion in the southern Great Xing'an Range. To investigate its petrogenesis and relationship with tin mineralization, whole-rock and accessory minerals (zircon and apatite) geochemical analyses were carried out. The quartz monzonite mainly belongs to high-K calc-alkaline strongly peraluminous granite, and the biotite granite is high-K calc-alkaline weakly peraluminous granite. Zircon U-Pb dating indicates a younger age for the biotite granite (138 Ma) than the quartz monzonite (140 Ma). The zircon Hf and apatite Nd isotopic compositions of the quartz monzonite (εHf(t) = 5.2 to 9.1 and εNd(t) = −1.1 to 1.3) and those of the biotite granite (εHf(t) = 5.6 to 8.0 and εNd(t) = −0.9 to 1.0) are indistinguishable, implying that they were derived from the same sources. Modeling results using apatite Nd isotope data show that they were from a mixing source of the Xilinhot Group and mantle-derived basaltic melt. Whole-rock and apatite trace elements reveal that the granitic magma mainly experienced fractional crystallization of amphibole, biotite, plagioclase, K-feldspar, apatite, and monazite from the quartz monzonite to the biotite granite. Mineralogical, whole-rock geochemical, and apatite trace element data indicate that the Beidashan pluton has an I-type granite affinity. A crystal mush model was proposed accounting for the composition gap between these two granites (e.g., SiO2, Al2O3, and Ba), in which the biotite granite represents the highly fractionated interstitial melt extracted from the crystal mush system whereas the quartz monzonite is the residual crystal mush consisting of the cumulate crystals and trapped interstitial melt. The melt was extracted when the crystallinity of the crystal mush reached about 33–37%, and the residual crystal mush trapped about 38–46% interstitial melt. Both Ce anomalies in zircon and Eu anomalies in apatite imply reducing conditions. The calculated values of oxygen fugacity for the Beidashan pluton range from −19.3 to −15.7, and most of them are lower than the fayalite-magnetite-quartz (FMQ) oxygen buffer. Apatite in the Beidashan pluton has high F (2.35–3.61%) and low Cl (0–0.41%) concentrations, which are similar to those of Sn-W deposits worldwide. The Sn contents of apatite increase from the quartz monzonite (mean 0.30 ppm) to the biotite granite (mean 1.09 ppm), suggesting that magma evolution is important for the enrichment of Sn in the melt.