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We report new experimental data from ultrabasic basanite and ultrabasic tephrite as starting material compositions in the 1350°C to 1000°C temperature range. Crystallization experiments under low- to high-pressure (0.5–2.0 GPa) were carried out under reduced conditions (≈CCO buffer), while one-atmosphere, anhydrous, experiments were performed ranging from reduced to oxidized conditions (−2 ≤ ∆QFM ≤ +2). The results highlight the ƒO2 role on the silica saturation of the alkali liquids differentiated from these primary ultrabasic magmas, on the mineral assemblage, and its composition. The liquid lines of descent (LLDs) from basanite are sodic and strongly SiO2 undersaturated, whereas from tephrite, the LLDs are sodic-potassic/potassic for both weakly SiO2-undersaturated and SiO2-saturated compositions, being more silica saturated under oxidized conditions. At the lowest temperature experiments, the percentage of liquid remaining is significantly higher in the basanite-derived products (ca. 35 wt.%) than in tephrite, indicating that the equivalent magmas are more prone to produce larger quantities of evolved melts. The best obtained Fe–Mg olivine/melt and clinopyroxene/melt exchange coefficients for these alkali compositions considering the new and available data are |${K_D}_{Fe^{2+}- Mg}^{Ol- Alkali\ melt}=0.285\pm 0.014$| and |${K_D}_{Fe^{2+}- Mg}^{Cpx- Alkali\ melt}=0.245\pm 0.008$|⁠ , slightly lower than those observed in tholeiitic melts. Clinopyroxene compositions are Ti–Al-rich and Si-poor as compared with common clinopyroxenes in subalkali systems. We suggest that Ti should be allocated in the tetrahedral sites substituting for Si and that its contents are inversely correlated with pressure. Our results allow a simple new barometer based on clinopyroxene-only compositions, as follows: $$ P\left(\pm 0.16\ GPa,1\sigma \right)=16.028\left(\pm 1.042\right) Na-6.715\left(\pm 0.727\right)\frac{Ti}{Ti+ Al(t)}+0.494\left(\pm 0.441\right) Si+0.144\left(\pm 0.079\right) $$ where Na, Ti, Al(t), and Si are molar proportions relative to 6O. This formulation accounts for the jadeite (NaAlSi2O6) component, herein computed from the Na contents, corrected for the Ti-diopside (CaMgTi2O6) component in clinopyroxene and also considers the evolutionary trend from Mg-augite to ferroan diopside. It applies to alkali ultrabasic to intermediate compositions in the examined P–T–ƒO2 range, resulting in more accurate estimates than the available calibrations. The MgO-in-melt thermometer was optimized for the studied compositions at one-atmosphere pressure and anhydrous conditions, as follows: $$ T\ \left(\pm 5{}^{\circ}C,1\sigma \right)=27.35\left(\pm 0.65\right)\ \big({MgO}^{liq}\big)+984\left(\pm 4\right) $$ which provides much more reliable liquidus temperatures for these alkali systems. Given data restriction, this formulation may be expanded to include the pressure effects for relatively low-H2O (< 3 wt.%) systems as: $$ T\ \left(\pm 20{}^{\circ}C,1\sigma \right)=27.35\left(\pm 0.65\right)\big({MgO}^{liq}\big)+80.20\left(\pm 5.50\right)P(GPa)+981\left(\pm 4\right) $$ [ABSTRACT FROM AUTHOR]

Serpentinites play an important role in the delivery of water into subduction zones. In addition, serpentinites also contain ferric Fe and can transport significant redox potential. We present high-pressure and high-temperature experiments and Mössbauer spectroscopy measurements on natural lizardite and antigorite samples equilibrated at various oxygen fugacities in order to quantify the relationship between the oxygen fugacity f (O2) and the Fe3+/Fetot ratio in these two phases. In antigorite, Fe3+ partitions into the octahedral site and is charge balanced by tetrahedral Al. In lizardite, tetrahedral Fe3+ is observed only at low temperature as well as under high f (O2), whereas Fe3+ prefers the octahedral site at temperatures exceeding 500 °C and at 3 to 5 GPa. Although metastable, lizardite remains in redox equilibrium in our experiments at conditions above the lizardite to antigorite phase transformation at 300 °C and demonstrates a similar stability to antigorite. The Al concentration of lizardite is found to be temperature dependent, and it was possible to reequilibrate the Fe3+/Fetot ratio of lizardite from 0.1 to 0.9 by using redox buffers such as Fe metal, graphite, graphite–calcite, Re–ReO2 and Ru–RuO2. Our experiments on antigorite demonstrate that antigorite does not adjust its Al concentration on experimental time scales. Since Fe3+ is charge balanced by Al, it was also not possible to manipulate the Fe3+/Fetot ratio of antigorite. The coexisting phases, however, show chemical equilibration with this antigorite composition. We have retrieved the standard Gibbs energy for Fe3+- and Al-endmembers of antigorite and lizardite and calculated the metamorphic evolution of subducting serpentinites. The lizardite to antigorite transformation does not cause a decrease in the bulk Fe3+/Fetot ratio under f (O2) buffered conditions, in contrast to observations from some natural settings, but does result in the formation of additional magnetite due to antigorite having a lower Fe3+/Fetot ratio than lizardite at equilibrium. If the f (O2) of antigorite serpentinite is buffered during subduction, such as due to the presence of graphite and carbonate, the bulk Fe3+/Fetot ratio decreases progressively. On the other hand, in a closed system where the bulk serpentinite Fe3+/Fetot ratio remains constant, the f (O2) increases during subduction. In this scenario, the f (O2) of an antigorite serpentinite with a typical Fe3+/Fetot ratio of 0.4 increases from the fayalite–magnetite–quartz to the hematite–magnetite f (O2) buffer during dehydration. These f (O2) results confirm earlier inferences that fluids produced by antigorite dehydration may not contain sufficient oxidised sulphur species to oxidise the mantle wedge. Sufficiently high levels of f (O2) to mobilise oxidised sulphur species may be reached upon antigorite dehydration, however, if closed system behaviour maintains a high bulk redox potential across the lizardite to antigorite phase transformation. Alternatively, oxidation of the mantle wedge might be achieved by oxidising agents from sources in subducted oceanic crust and sediments. [ABSTRACT FROM AUTHOR]

In this study, we present new evidence for changes in magma storage conditions that preceded the 1232 ka caldera-forming eruption of the Bandelier magmatic system in the Jemez Mountains Volcanic Field. Using high precision 40Ar/39Ar sanidine dating we determine that at least eight rhyolites erupted within 8.6 ± 3.4 kyr of the ~400 km3 eruption that formed Valles caldera. Some of those rhyolites contain fayalite with or without biotite, others contain only biotite. An eruption of fayalite-bearing rhyolite at 1240.5 ± 2.1 ka ended an eruption hiatus of at least 100 kyr. Following that first post-hiatus episode of volcanism, at least four more eruptions of fayalite-bearing rhyolite and three eruptions of biotite-bearing rhyolite occurred prior to the caldera-forming eruption. We use phase equilibrium experiments and geothermobarometry to infer the storage conditions and processes that led to these differing crystal cargos and ultimately generated ~400 km3 of predominantly fayalite rhyolite ignimbrite (Tshirege Member of the Bandelier Tuff). We find that biotite-bearing rhyolites were stored at 695–750°C, 75–160 MPa, and at an oxygen fugacity more oxidizing than the quartz-fayalite-magnetite (QFM) buffer reaction. Fayalite-bearing rhyolites were similarly stored at 695–745°C and 70–190 MPa, but at more reducing conditions (⁠|${f}_{O_2}$| ≤ QFM). We suggest that the reduced, fayalite-bearing rhyolite was most likely produced via interaction of crystal-poor rhyolitic magma with a reducing, potentially Cl-bearing, and H2O-rich supercritical fluid phase. This fluid flux event was a key component of the substantial magmatic rejuvenation that enabled the mobilization of ~400 km3 of mostly fayalite-bearing rhyolite prior to not only the Tshirege event, but the older Otowi event as well. [ABSTRACT FROM AUTHOR]

In the solar system, oldhamite (CaS) is generally considered to be formed by the condensation of solar nebula gas. Enstatite chondrites, one of the most important repositories of oldhamite, are believed to be representative of the material that formed Earth. Thus, the formation mechanism and the evolution process of oldhamite are of great significance to the deep understanding of the solar nebula, meteorites, the origin of Earth, and the C–O–S–Ca cycles of Earth. Until now, oldhamite has not been reported to occur in mantle rock. However, here we show the formation of oldhamite through the reaction between sulfide-bearing orthopyroxenite and molten CaCO3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3 GPa/1273 K. Importantly, this reaction occurs at oxygen fugacities within the range of upper-mantle conditions, six orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolysed to produce calcium hydroxide. Low oxygen fugacity of magma, extremely low oxygen content of the atmosphere, and the lack of a large amount of liquid water on the celestial body's surface are necessary for the widespread existence of oldhamite on the surface of a celestial body otherwise, anhydrite or gypsum will exist in large quantities. Oldhamites may exist in the upper mantle beneath mid-ocean ridges. Additionally, oldhamites may have been a contributing factor to the early Earth's atmospheric hypoxia environment, and the transient existence of oldhamites during the interaction between reducing sulfur-bearing magma and carbonate could have had an impact on the changes in atmospheric composition during the Permian–Triassic Boundary. [ABSTRACT FROM AUTHOR]

The liquid line of descent from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, with whole-rock compositions following a fractionation path along the 'thermal valley' in the peralkaline haplogranite system Qz-Ab-Or-Ac-Ns and terminating at a minimum on the feldspar-quartz cotectic. Although whole-rock compositions for different pantelleritic suites follow nearly identical paths in a Qz-Ab-Or projection that terminate near the experimental minimum (Qz40.5Or34.5Ab25 at 100 MPa, projected from Ac-Ns), matrix glass from samples with near-minimum compositions record extreme differentiation and form a 'cotectic delta' beyond the terminus of the 'thermal valley'. Although each glass trend shows a continuing increase in Zr to >3000 μg/g, the most evolved compositions in each suite differ in peralkalinity (mol [Na + K] / Al) and in the proportions of FeOT, Qz, Ab, Or, and other components, which are related to subtle variations in the mafic phases controlled mainly by differences in oxygen fugacity (f O2) and pressure (P). To determine the controls over mafic mineral crystallization in pantelleritic magmas and the various paths these suites take beyond the apparent (whole-rock) minimum, amphibole-phyric suites from the ~159 ka Cala dell'Altura and Cala Gadir volcanic centres and the ~8–10 ka Cuddia Mida volcanic centre on Pantelleria have been analyzed and compared with each other and with the well-characterized and amphibole-free, compositionally zoned Green Tuff, the ~46 ka caldera-forming ignimbrite of the Cinque Denti caldera. Differences between the extended fractionation trends may be ultimately attributed to variations in oxygen fugacity, depth of emplacement, and water saturation. Shallower (lower pressure) magma reservoirs such as the one for the Green Tuff are water-saturated and undergo degassing, which leads to an increase in relative oxygen fugacity. Deeper (higher pressure) magma reservoirs remain water-undersaturated and retain water in the melt, which both maintains lower relative oxygen fugacities and enables the crystallization of amphibole. Amphibole formation appears to require melt water contents >4 wt%, low oxygen fugacity (<ΔNNO-1.5), and low temperatures (<700°C), although fluorine may stabilize it at higher temperatures in some rocks. [ABSTRACT FROM AUTHOR]

Osorno volcano (41°06'S, 72°20'W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %), which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060°C to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7-km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centers built over the major regional Liquiñe-Ofqui Fault Zone. [ABSTRACT FROM AUTHOR]

The valence state of iron in mantle-derived melts, such as mid-ocean ridge basalt, is a useful proxy for oxygen fugacity (ƒ O2). On subduction, oceanic crust undergoes metamorphic reactions that alter its initial ƒ O2, generating compositional and redox heterogeneity in the supra-subduction zone and in the convecting mantle source of oceanic basalts. Kimberlite-borne eclogite xenoliths with ancient oceanic crustal protoliths represent an important archive to trace these processes in deep time. We determined, by Mössbauer spectroscopy, Fe3+/ΣFe for garnet (0.03–0.15, average 0.08; n  = 13) and clinopyroxene (0.05–0.37, average 0.23; n  = 11) in bimineralic eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), combined the results with high-quality literature data to estimate bulk Fe3+/ΣFe and ƒ O2, assessed associated uncertainties and discussed petrological implications. The incorporation of Fe3+ in, and distribution between, eclogite minerals is controlled by chemical composition (X), temperature (T), pressure (P) and ƒ O2. Therefore, Fe3+/ΣFe in garnet alone is an imperfect proxy for bulk Fe3+/ΣFe. Values for the distribution of Fe3+/ΣFe between clinopyroxene and garnet (D (Fe3+/ΣFe)cpx-grt) range from 1.4 ± 0.1 to 20 ± 13 and correlate strongly with the abundance distribution of another polyvalent element, vanadium (r2 = 0.80, n  = 28), probably because both vary as a function of X-T–P - ƒ O2. This allows to estimate Fe3+/ΣFe in clinopyroxene and bulk rocks (0.01–0.32, average 0.06, n  = 159) for the majority of literature data where only garnet Fe3+/ΣFe is known. Low Fe3+/ΣFe is retained in many high-Al2O3 eclogites with plagioclase-rich cumulate protoliths (Eu/Eu*> > 1), despite extended residence in the cratonic lithosphere. Bulk Fe3+/ΣFe increases during mantle metasomatism but is not particularly linked to enrichment in highly incompatible elements. Low grossular content in garnet ensuing from mantle metasomatism, especially at low temperature, limits uptake of Fe3+, which is accommodated in clinopyroxene instead, leading to very high D (Fe3+/ΣFe)cpx-grt. Eclogite oxybarometry requires that Fe3+/ΣFe of only garnet be known, whereby the input temperatures should be calculated with all Fe as Fe2+. For temperatures projected onto regional steady-state geothermal gradients, ƒ O2 values relative to the fayalite-magnetite-quartz buffer (∆log ƒ O2(FMQ)) range from −5.9 to −0.2 (average − 3.1, n = 174), and are too low to stabilise oxidised sulphur in all and oxidised carbon in the vast majority of samples. Thus, ancient oceanic crust was a sink rather than a source of oxygen. In particular CaO-rich eclogite xenoliths occasionally are corundum-bearing and SiO2-undersaturated, typically with silica-deficient clinopyroxene with cations per formula unit <1.97 for 6 O anions, resulting in overestimated ƒ O2. For the remaining samples, cryptic metasomatism, with enrichment in highly incompatible elements, is clearly focused in the ƒ O2 interval of FMQ−4 to FMQ−3, indicating that mantle metasomatism can have both an oxidising and a reducing effect on eclogite. Unmetasomatised bimineralic eclogites have lower ƒ O2 than coesite-bearing ones, reflecting reduction during metamorphism and silica-consuming partial melting of ancient subducted oceanic crust. [ABSTRACT FROM AUTHOR]

The caldera-forming Samalas eruption of the Samalas–Rinjani volcanic complex on Lombok, Indonesia, in 1257 CE ranks as one of the most explosive and sulphur-rich eruptions of the past thousand years. Along with other significant events (e.g. Tambora 1815, El Chichón 1982), Samalas 1257 forms a class of powerful eruptions of magmas of intermediate alkaline composition, which remain relatively understudied. This study aims to better understand the pre-eruptive magma reservoir conditions and volatile storage capacities of intermediate alkaline systems, using the 1257 Samalas eruption as a case study. Systematic partial-equilibrium experiments were run to constrain potential magma reservoir conditions (25–200 MPa and 850–1000°C) by examining changes in the chemistry and textures of experimental glass and mineral products. Natural trachydacite pumice from the 1257 Samalas eruption was used as the experimental charge for a series of water-saturated experiments run in cold-seal pressure vessels, buffered at an oxygen fugacity ƒ O2 of NNO/NNO + 1 log units. Assessing the match between natural and experimental glass compositions and mineral phases and textures reveals that pre-eruptive magma reservoir conditions were between 875°C and 930°C and ca. 100–150 MPa (4.5 ± 1km depth). Breakdown of experimental amphibole at pressures below 75 MPa, and plagioclase instability at 950°C/100 MPa, provide the strongest constraints on phase stabilities that are consistent with the magma storage region. In the observed temperature and pressure range, the natural compositions of plagioclase, amphibole and orthopyroxene are replicated in experimentally precipitated rims. Clinopyroxene and high anorthite plagioclase (An>64) in the natural mineral assemblage could not be replicated in the experiments, implying that these compositions most likely did not grow in the shallow pre-eruptive reservoir of Mt Samalas. The pre-eruptive storage depths of trachydacite magma are significantly shallower than the postulated magma storage at the currently active basaltic-andesite magmatic system at Rinjani, and thus monitoring magma storage depth may be a useful indicator of changing magma composition in the future. Our findings show that highly explosive VEI 7 eruptions of intermediate alkaline magma can be fed from a relatively limited range of storage pressures (100–150 MPa), suggesting accumulation of magma in one place, rather than the rapid extraction of magma from a vertically extensive, transcrustal magma system. [ABSTRACT FROM AUTHOR]

It is not well known which chemical differentiation pathways basaltic melts will take when they are iron metal saturated. Thermodynamically, the pathway seems predictable. So long as Fe metal is a stable liquidus phase and relative oxygen fugacity (fO2) is not subject to major fluctuations, the activity of FeO (aFeOmelt) is buffered by the iron–wüstite (IW) equilibrium 2Femetal + O2 → 2FeOmelt. Metallic Fe also stabilizes olivine through the equilibrium 2Femetal + O2 + SiO2 melt → Fe2SiO4 olivine. That equilibrium tends to suppress the enrichment in bulk SiO2 when Fe saturated basaltic melts differentiate. We document the differentiation history of tholeiitic cumulates from the Siberian craton that carry up to 30 modal % metallic Fe. Our study is complemented by differentiation experiments at two redox states, one set in Fe metal capsules at 1.6 log units below IW (IW-1.6) and a second set in graphite capsules at IW + 1.5. Iron saturated differentiation pathways do not show enrichments in FeO nor in bulk SiO2 because olivine remains stable along the entire liquid line of descent. By contrast, melts equilibrated at IW + 1.5, that is, outside metallic Fe saturation, crystallize pigeonite as first silicate and follow a normal (terrestrial) differentiation pathway involving marked SiO2 enrichment. The Fe-saturated path duplicates in detail the liquid line of descent we derive for the cumulates. Iron-saturated experiments have limited applicability to the Earth because there are so few terrestrial basalts saturated with metallic Fe; however, they might apply to the Moon. Many lunar basalts appear to have been saturated with an Fe-Ni phase during their emplacement on the lunar surface, and potentially during generation within the lunar mantle. [ABSTRACT FROM AUTHOR]

Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent SSZ magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~ + 1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of > + 2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution [ABSTRACT FROM AUTHOR]

Reports by the US Department of Energy in 2014 evaluated membrane heat pump technology as one of the most promising alternatives to conventional vapour compression methods. Vapour compression methods maintain an evaporator temperature lower than the dew point to deal with the latent heat load. In membrane heat pump systems, only the water vapour is transferred and there is no phase change. The migration is caused by the difference in vapour pressure before and after the membrane. A vacuum pump or blower is used to create the pressure difference. However, there is no methodology for predicting dehumidification performance of membranes when used as part of a cooling system. In this study, using the assumption that there is a similarity between heat transfer and moisture pervaporation, the performance indices of the membrane are derived using a well-known heat exchanger method, the ε-NTU models. Performance estimations are calculated for two representative system layouts: bypass and vacuum. Simple relations between design parameters are suggested, giving design guidelines for researchers. [ABSTRACT FROM AUTHOR]

Clinopyroxene–liquid geothermobarometry is a widely used tool for estimating the conditions under which mafic magmas are stored before they erupt. However, redox variability, sector zoning and disequilibrium crystallization present major challenges to the robust estimation of magma storage conditions. Moreover, most recent studies seeking to address these challenges have focused on clinopyroxenes from alkalic systems and are thus of limited use for understanding clinopyroxenes from the tholeiitic systems that dominate global magma budgets. Here we combine observations on natural clinopyroxenes from the 2014–2015 Holuhraun lava in Iceland with observations on experimental clinopyroxenes synthesized during high-pressure, high-temperature experiments on the same lava in order to investigate clinopyroxene–liquid equilibria in tholeiitic systems and optimize of geothermobarometric strategies. Natural clinopyroxenes from the 2014–2015 Holuhraun lava are sector zoned, with { 1 - 11 } hourglass sectors being enriched in the enstatite–ferrosillite component at the expense of all other components with respect to { hk 0 } prism sectors. In contrast with observations on clinopyroxenes from alkalic systems, sector zoning in clinopyroxenes from the 2014–2015 Holuhraun lava is characterized by differences in Ca and Na contents as well as in Ti and Al contents. The products of crystallization experiments performed at 100–600 MPa and 1140–1220 °C on a powdered starting glass at two sets of melt H2O content–oxygen fugacity conditions (∼0·1 wt % H2O and close to the graphite-oxygen redox buffer, and 0·5–1·0 wt % H2O and approximately one and half log units above the quartz–fayalite–magnetite redox buffer) demonstrate that clinopyroxene crystals from nominally equilibrium experiments can preserve strongly disequilibrium compositions. The compositional systematics of experimental clinopyroxenes are consistent with the presence of sector zoning. Furthermore, the magnitude of compositional variability increases with decreasing melt H2O content and increasing deviations of experimental temperatures below clinopyroxene liquidus temperatures (i.e. degrees of undercooling sensu lato), indicating that kinetic processes play a key role in controlling clinopyroxene compositions, even under notionally equilibrium conditions. Few published analyses of experimental clinopyroxene crystals may thus represent truly equilibrium compositions. Stoichiometric calculations on natural and experimental clinopyroxenes show that Fe3+ is a major constituent of clinopyroxenes from tholeiitic magmas under naturally relevant oxygen fugacity conditions. They also show that Fe3+ is most likely incorporated as Ca- and Al- bearing Ca–Fe-Tschermak's component rather than Na-bearing aegirine component at oxygen fugacities up to one and a half log units above the quartz–fayalite–magnetite buffer. Elevated oxygen fugacities are thus less likely to compromise clinopyroxene–liquid geothermobarometry than previously thought. Guided by our experimental results, we combined published descriptions of clinopyroxene–liquid equilibria with geothermobarometric equations to develop an internally consistent and widely applicable method for performing geothermobarometry on tholeiitic magmas that does not require equilibrium zones to be selected a priori. Applying this method to natural clinopyroxene crystals from the 2014–2015 Holuhraun lava that formed under low but variable degrees of undercooling (perhaps 25 °C or less) returns values in excellent agreement with those from independent methods (232 ± 86 MPa, 1161 ± 11 °C). Robust estimates of magma storage conditions can thus be obtained by performing clinopyroxene–liquid geothermobarometry on tholeiitic magmas when disequilibrium is suitably accounted for. [ABSTRACT FROM AUTHOR]

We constrain the oxygen fugacity (⁠ f O 2 ⁠) of strongly peraluminous granites [SPGs; i.e. granites (sensu lato) generated through the partial melting of sediments] across the Archean–Proterozoic boundary, which coincides roughly with the Great Oxygenation Event (GOE), to understand whether secular changes in atmospheric O2 levels may be imprinted on the f O 2 of igneous rocks. SPGs were chosen to maximize the potential effects of sediments in their sources on the f O 2 of the magmas. We studied 28 Archean (2685–2547 Ma) and 31 Meso- to Paleoproterozoic (1885–1420 Ma) geographically distributed samples from North America, spanning two cratons (Superior and Wyoming) and both orogenic and anorogenic Proterozoic provinces (Trans-Hudson Orogen, Wopmay Orogen, and SW USA). We present an analysis of both new and previously published whole-rock major and trace element data and mineral major element chemistry from the samples. All the studied samples are peraluminous high-silica plutonic rocks (all contain >67 wt % SiO2, and 92% are true granites with >69 wt % SiO2), and biotite + muscovite ± garnet ± tourmaline ± sillimanite are the primary aluminous minerals in all samples. Whole-rock major element and trace element abundances of all samples are consistent with derivation by partial melting of aluminous sediments. To constrain the f O 2 of crystallization of the SPGs, we developed an alphaMELTS-based method that takes advantage of the sensitivity of biotite FeT/(FeT + Mg) ratios to f O 2 ⁠. This method is able to reproduce experimental and empirical data where biotite compositions and whole-rock compositions, pressures and temperatures of crystallization and f O 2 are known. For the SPGs in this study, alphaMELTS modeling indicates that 68% of Proterozoic samples crystallized at an f O 2 between NNO –1 and NNO +1·1 (where NNO is nickel–nickel oxide buffer), whereas the remaining Proterozoic samples (32%) and most of the Archean samples (75%) crystallized at ≤NNO –2. The simplest explanation of these results is that the Proterozoic SPGs were derived from metasedimentary source rocks that on average had more oxidized bulk redox states relative to their Archean counterparts. The bulk redox state of the metasedimentary source rocks of SPGs of all ages is defined by the relative abundances of oxidized (e.g. Fe3+ and S6+) and reduced (e.g. organic matter) material. The crystallization of both Archean and Proterozoic samples at f O 2 values of ≤NNO –2 is consistent with them having their f O 2 buffered by graphite (formed from organic carbon) in their source regions. However, the dominantly low f O 2 (≤NNO –2) values of the Archean SPGs plausibly reflects the presence of organic material and relatively reduced metasedimentary rocks in their source region prior to the GOE. In contrast, the elevated f O 2 values of the majority of the Proterozoic SPGs may reflect enhanced sulfate contents or increased Fe3+/FeT in sediments after the GOE, which, in terms of the bulk redox state of their metasedimentary source region, would have offset the reducing nature of organic matter present there. [ABSTRACT FROM AUTHOR]

Differentiation of hydrous primary, mantle-derived magmas is a fundamental process to generate evolved intermediate to SiO2-rich compositions forming the bulk of the continental and island arc crust. This study focuses on the results of equilibrium and fractional crystallization experiments at 1.0GPa using two different primary magmas representing deep (90 km) and shallow (35 km) mantle extraction depths. Experiments on a hydrous high-Mg basalt were conducted at graphite-saturated and more oxidized conditions (NNO to NNO+2, where NNO is nickel-nickel oxide buffer) to exploit the influence of fO2 on phase assemblages and the evolution of derivative liquids. The liquid line of descent (LLD) was simulated from liquidus to near-solidus conditions ranging from 1330°C to 720°C. H2O contents varied from about 2.0 to more than 10wt %. The LLD covers the entire compositional range from high-Mg basalt to high-silica rhyolite and evolves from metaluminous to peraluminous compositions at 56-60wt % SiO2 under oxidizing conditions. The observed crystallization sequences and the LLD reveal contrasting behavior depending on oxidation state, H2O content and equilibrium versus fractional crystallization. Equilibrium crystallization of high-Mg basalt under reducing conditions is initially dominated by olivine fractionation followed by Cr-rich spinel, clinopyroxene (cpx), and orthopyroxene (opx). Finally, between 1060 and 1000°C, amphibole formed by a peritectic reaction consuming cpx+olivine and forming amphibole+opx, resulting in 16% silica-undersaturated trachy-basaltic liquid. Equilibrium crystallization of the same composition under oxidizing conditions is characterized by strongly enhanced olivine and cpx fractionation and suppression of opx only occurring as a result of the peritectic amphibole-forming reaction at and below 1040°C. The liquid at 980°C is a peraluminous, alkali-poor, high-Al andesite representing ~15% residual liquid. Fractional crystallization of the high-Mg basalt under oxidizing conditions evolves through fractionation of early olivine joined by cpx at 1200°C, followed by opx and hercynitic spinel (1140-1080°C) and amphibole at 1050°C coexisting with cpx (and spinel) to 980°C. At 950°C both garnet and plagioclase (plag) join amphibole as liquidus phases. This paragenesis (plus ilmenite and apatite) persists to 750°C with 16% residual liquid relative to the initial basaltic composition. Liquids evolve continuously from basalt to rhyolite, crossing the metaluminous-peraluminous boundary at about 60wt % SiO2. Fractional crystallization of the basaltic andesite starting material differs at high temperature, where opx and cpx are the liquidus phases (1200-1080°C), followed by amphibole at the expense of opx. Below 950°C the phase assemblage is identical except at the lowest temperature (720°C), where quartz saturates in a high-silica rhyolitic liquid representing 20% of the initial basaltic andesite. Liquid compositions of the two starting compositions converge below 950°C, with the exception of K2O, which behaves incompatibly along the liquid line of descent. The H2O-undersaturated fractional crystallization experiments on the high-Mg basaltic composition under relatively reducing conditions evolve towards tholeiitic or alkalic compositions owing to early plagioclase (at 1140°C) and abundant opx (at 1110°C) crystallization followed by garnet-cpx-plag-ilmenite at 1080°C. Closed-system equilibrium crystallization under relatively oxidized conditions is characterized by significant expansion of the olivine stability field (>250°C, 1280-1000°C) relative to more reducing conditions, causing depletion of the liquid in ferrous iron and increase of ferric iron. The resulting fO2 of the coexisting basaltic liquids increases by more than 3 log units from about NNO+2 to NNO+5.5 over the temperature range >1200-1040°C, potentially offering an explanation for the more oxidized character of hydrous arc magmas as opposed to low-H2O tholeiitic magmas. [ABSTRACT FROM AUTHOR]

The elevated oxygen fugacity recorded by subduction-related lavas and peridotites, relative to their mid-ocean ridge counterparts, fundamentally influences the petrogenesis of arc magmas. However, the timing, process, and spatial extent of oxidizing mass transfer at subduction zones remain unknown. Forearc peridotites, which are sometimes exposed on the trench wall of the overriding plate, record chemical fingerprints of the melting and melt-rock interaction processes that occur during and following subduction initiation, and thus provide insight into the spatial and temporal evolution of this oxidized signature. In this study, we present new major element, trace element, and oxygen fugacity data for a suite of forearc peridotites recovered from the Tonga Trench, in addition to a new assessment of literature data for previously studied forearc peridotites. For Tonga samples and literature data for forearc, ridge, and subduction-zone peridotites, we calculate oxygen fugacity (fO2) using an updated method. In contrast to previous studies, we find that spinel Cr#, a proxy for extent of melt extraction, does not correlate with oxygen fugacity, such that many forearc peridotites with high spinel Cr# do not record oxygen fugacity higher than the mid-ocean ridge peridotite array. Combining these observations with trace element modeling, we conclude that forearc peridotites are less pervasively influenced by oxidation owing to subduction processes than previously reported. The oxygen fugacity recorded by Tonga forearc peridotites is heterogeneous between dredges and homogeneous within dredges. To explore these variations, we grouped the dredges into two categories. Group I peridotites have high spinel Cr#, extremely depleted trace element compositions and oxygen fugacity values consistent with the mid-ocean ridge peridotite array. We interpret these to be the residues of large degrees of fractional melting, with little influence from arc-like melts or fluids, formed during the first stages of subduction initiation. Group II peridotites have lower spinel Cr#, enriched light rare earth elements, and oxygen fugacity elevated by 1 log unit above the mid-ocean peridotite array. We interpret these peridotites to be the residues of flux melting, initiated once corner flow is established in the young subduction zone. We conclude that the forearc mantle is not pervasively oxidized relative to midocean ridge mantle, and that the asthenospheric mantle in the proto-subduction zone region is not oxidized prior to subduction initiation. As the oxidized signature in Group II peridotites accompanies geochemical evidence of interaction with subduction-related fluids and melts, this suggests that the sub-arc mantle is oxidized concurrently with addition of subduction fluids to the mantle wedge. [ABSTRACT FROM AUTHOR]

Arc magmas erupted at the Earth's surface are commonly more oxidized than those produced at mid-ocean ridges. Possible explanations for this high oxidation state are that the transfer of fluids during the subduction process results in direct oxidation of the sub-arc mantle wedge, or that oxidation is caused by the effect of later crustal processes, including protracted fractionation and degassing of volatile-rich magmas. This study sets out to investigate the effect of disequilibrium crustal processes that may involve coupled changes in H2O content and Fe oxidation state, by examining the degassing and hydration of sulphur-free rhyolites. We show that experimentally hydrated melts record strong increases in Fe3+/ΣFe with increasing H2O concentration as a result of changes in water activity. This is relevant for the passage of H2O-undersaturated melts from the deep crust towards shallow crustal storage regions, and raises the possibility that vertical variations in fO2 might develop within arc crust. Conversely, degassing experiments produce an increase in Fe3+/ΣFe with decreasing H2O concentration. In this case the oxidation is explained by loss of H2 as well as H2O into bubbles during decompression, consistent with thermodynamic modelling, and is relevant for magmas undergoing shallow degassing en route to the surface. We discuss these results in the context of the possible controls on fO2 during the generation, storage and ascent of magmas in arc settings, in particular considering the timescales of equilibration relative to observation as this affects the quality of the petrological record of magmatic fO2. [ABSTRACT FROM AUTHOR]

The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole-rock and mineral major element geochemistry, whole-rock trace element chemistry and Mössbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyroxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modified by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those represented in the Mount Morning xenolith suite has contributed to the enriched trace element (and isotope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30% melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in the harzburgites (20–30%), whereas plagioclase lherzolite mineral data suggest <15% melt depletion. The lherzolite (spinel and plagioclase) xenolith whole-rocks have compositions indicating <20% melt depletion, consistent with Proterozoic to Phanerozoic cratonization ages, and have mantle-normalized trace element patterns comparable with typical depleted mid-ocean ridge mantle. All peridotite xenoliths have undergone a number of melt–rock reaction events. Melting took place mainly in the spinel peridotite stability field, but one plagioclase peridotite group containing high-sodium clinopyroxenes is best modelled by melting in the garnet field. Median oxygen fugacity estimates based on Mössbauer spectroscopy measurements of spinel and pyroxene for spinel-facies conditions in the rifted Antarctic lithosphere are –0·6 Δlog fO2 at Mount Morning and –1·0 ± 0·1 (1σ) Δlog fO2 for all of Victoria Land, relative to the fayalite–magnetite–quartz buffer. These values are in good agreement with a calculated global median value of –0·9 ± 0·1 (1σ) Δlog fO2 for mantle spinel-facies rocks from continental rift systems. [ABSTRACT FROM AUTHOR]

Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe3+/∑Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana Trough. For single eruptions, Fe3+/∑Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation, CO2 ± H2O degassing) or reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO2 + H2O + S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H2O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe3+/∑Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean ridge basalts (∼QFM, where QFM is quartz–fayalite–magnetite buffer), ranging from QFM + 1·0 to QFM + 1·6 for Mariana arc basalts, whereas back-arc related samples record primary oxygen fugacities that range from QFM + 0·1 to QFM + 0·5. This Mariana arc sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H2O-rich component in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from the Mariana Trough through the Mariana arc, increasing by 1·5 orders of magnitude as Ba/La increases by a factor of 10 relative to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized. [ABSTRACT FROM PUBLISHER]

Hydrothermal experiments were conducted at 200 MPa and 900–1018°C to determine the solubilities, fluid(s)–melt partitioning, and mixing properties of H2O, CO2, S, Cl, and F in phonolitic–trachytic melts saturated in vapor, vapor plus saline liquid, or saline liquid. The bulk compositions and S, Cl, and F concentrations of the run-product glasses were determined by electron microprobe and the H2O and CO2 contents by Fourier-transform infrared spectroscopy (FTIR). A new parameterization was developed to calculate molar absorption coefficients for FTIR analysis of carbonate in glasses and applied to the run-product glasses. The concentrations of volatiles in the fluid(s) were determined by mass-balance calculations and checked with chloridometer analysis and gravimetry. The range in oxygen fugacity of these experiments is NNO to NNO + 2 (where NNO is nickel–nickel oxide buffer). The phonolitic–trachytic melts dissolved up to 7·5 wt % H2O, 0·94 wt % Cl, 0·73 wt % CO2, 0·75 wt % F, and 0·16 wt % S, and the integrated bulk fluid(s) contained up to 99 mol % H2O, 34 mol % Cl, 82 mol % CO2, 1·7 mol % F, and 3·7 mol % S. The mixing relationships of H2O, CO2, and Cl in melt versus fluid(s) are complex and strongly non-ideal at these pressure–temperature conditions, particularly with two fluid phases stable. The concentrations of H2O and CO2 in melt change with the addition of Cl ± S to the system, and the solubility of Cl in melt varies with S. The reductions in H2O and CO2 solubility in melt exceed those resulting from simple dilution of the coexisting fluid(s) owing to addition of other volatiles. The partitioning of H2O and CO2 between fluid(s) and melt varies as a function of fluid(s) and melt composition. The experimental data are applied to phonolitic and related magmas of Mt. Somma–Vesuvius, Italy, Mt. Erebus, Antarctica, and Cripple Creek, USA, to better interpret processes of fluid(s) exsolution in eruptive and mineralizing systems. Application of the experimental results also provides constraints on eruptive and mineralizing fluid(s) compositions. [ABSTRACT FROM AUTHOR]

To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H2O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity (). Double-capsule experiments were conducted at 2·0 and 3·0 GPa and 950–1050°C, using Co–CoO, Ni–NiO, NixPd1–x–NiO, and Fe2O3–Fe3O4 external buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing . At pyrrhotite saturation, melt S contents range from ∼30 ppm S at < FMQ – 1 to ∼500 ppm S at FMQ < ≤ FMQ + 1·1, whereas at anhydrite saturation ( ≥ FMQ + 2·5) melt S concentrations range from ∼700 ppm S to 0·3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ∼15 wt % S at 1050°C at pyrrhotite saturation ( ≤ FMQ + 1·1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that decreases markedly with increasing at pyrrhotite saturation, from several thousand at < FMQ – 1 to ∼ 200–400 at FMQ < ≤ FMQ + 1·1, owing to the increase of melt S content. At anhydrite saturation, is very low (<100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, might be in the range 200 ± 100, irrespective of . The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all . Therefore, S transfer from ocean crust to wedge mantle is not dependent and could take place over a range of conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle. [ABSTRACT FROM PUBLISHER]

The evolution of basaltic magmas depends on their redox state, hence oxygen fugacity, but there is increasing evidence that this intensive thermodynamic variable may be less well understood in basalts than commonly supposed. The redox state of terrestrial basalts has to a large extent been inferred from the Fe3+/Fe2+ ratios of their quenched glasses. However, this quantity appears to be significantly affected during late and post-eruptive processes in magmatic systems (e.g. by degassing, charge-transfer reactions of redox-variable species, and alteration), so that the degree to which the Fe3+/Fe2+ ratios preserved in basaltic glasses reflect the oxidation state of the magma at high temperature is unclear. Because olivine is the first silicate mineral to crystallize from primitive basaltic liquids on cooling following decompression, the equilibrium partitioning relations preserved in olivine phenocrysts in basalts are, in principle, less disturbed by these late and post-eruptive processes and, therefore, may better reflect the high-temperature (pre-eruptive) conditions of the magma. Here we calibrate an oxybarometer based on the strong sensitivity of the partitioning of vanadium between olivine and silicate melt to oxygen fugacity. Our empirical parameterization, calibrated over a range of redox conditions between four log10 units above and below the quartz–fayalite–magnetite (QFM) oxygen buffer, takes into account the effects of temperature, olivine composition (i.e. Mg/Fe ratios) and melt composition (namely the activities of CaO, SiO2, AlO1·5, NaO0·5 and KO0·5), and allows oxygen fugacity determinations to within ∼0·25 log10 units. We also explore the sensitivity of the exchange partitioning of Sc and Y between olivine and melt to temperature as a geothermometer. Our calibration indicates that this geothermometer allows temperature to be estimated to within 15°C, but precision is strongly dependent on the Sc and Y measurements in olivine and melt. [ABSTRACT FROM AUTHOR]

We identified molybdenite (MoS2) as an accessory magmatic phase in 13 out of 27 felsic magma systems examined worldwide. The molybdenite occurs as small (< 20 µm) triangular or hexagonal platelets included in quartz phenocrysts. Laser-ablation inductively coupled plasma mass spectrometry analyses of melt inclusions in molybdenite-saturated samples reveal 1–13 ppm Mo in the melt and geochemical signatures that imply a strong link to continental rift basalt–rhyolite associations. In contrast, arc-associated rhyolites are rarely molybdenite-saturated, despite similar Mo concentrations. This systematic dependence on tectonic setting seems to reflect the higher oxidation state of arc magmas compared with within-plate magmas. A thermodynamic model devised to investigate the effects of T, f O2 and f S2 on molybdenite solubility reliably predicts measured Mo concentrations in molybdenite-saturated samples if the magmas are assumed to have been saturated also in pyrrhotite. Whereas pyrrhotite microphenocrysts have been observed in some of these samples, they have not been observed from other molybdenite-bearing magmas. Based on the strong influence of f S2 on molybdenite solubility we calculate that also these latter magmas must have been at (or very close to) pyrrhotite saturation. In this case the Mo concentration of molybdenite-saturated melts can be used to constrain both magmatic f O2 and f S2 if temperature is known independently (e.g. by zircon saturation thermometry). Our model thus permits evaluation of magmatic f S2, which is an important variable but is difficult to estimate otherwise, particularly in slowly cooled rocks. [ABSTRACT FROM AUTHOR]

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na2O + K2O) < Al2O3] and peralkaline [(Na2O + K2O) > Al2O3] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa2O, µCaO, µK2O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks. [ABSTRACT FROM AUTHOR]

The formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation. The wide range in mineral assemblages, mineral compositions, and fluid compositions in peralkaline melts are, to a large extent, a function of the sodium/potassium content of these mantle-metasomatizing fluids, which also governs redox conditions. We propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene. In contrast, potassic fluids would oxidize the mantle assemblage by extracting Al2O3 from the garnet to make phlogopite. We suggest that during differentiation of peralkaline melts, once Fe–Ti oxides have been depleted from the assemblage, simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as Na2O + 2 FeO + 4 SiO2+1/2O2=2 NaFeSi2O6. Hence, the more persodic a melt is, the more aegirine will be crystallized and the more reduced this melt will be. This is because without a mineral donor for ferric iron, the crystal chemical forcing of the coupled crystallization of Na with Fe3+ drives oxidation of the dominant Fe2+ in the melt and thereby reduction of the C–H–S–O-bearing fluid and/or melt phase. This is in convincing agreement with observations from a variety of silica-undersaturated peralkaline complexes such as Ilimaussaq in Greenland, Mt. St. Hilaire in Canada, Lovozero and Khibina in Russia, and Tamazeght in Morocco, and explains the occurrence of magmatic methane in the most sodic of these complexes. In silica-oversaturated peralkaline melts such as comendites and pantellerites, however, this equilibrium appears to be less influential and these melts are accordingly less reduced. In perpotassic melts, in contrast, no K–Fe3+ pyroxene is stable and K can only be incorporated into dominantly Fe2+-bearing phases such as biotite or K-amphibole, if no K-feldspar is stable. Here, redox conditions in the coexisting fluid and/or melt are either unaffected or even driven to more oxidized values by the crystallization reactions, which is in accordance with observations on the ultrapotassic rocks of the Roman Province (Italy) or the kalsilite-bearing Murun complex in Siberia. We conclude that peralkalinity in magmatic rocks is tightly connected to oxygen fugacity, that both the generation and the evolution and crystallization of peralkaline melts are governed by redox equilibria, and that these depend strongly on the Na/K ratio as well as on the (Na + K)/Al ratio of the melt. The specific features of peralkaline rocks, such as the difference between agpaitic and miaskitic rocks, the unusual enrichment of high field strength elements and the occurrence of magmatic hydrocarbons in some of them, can all be explained by the oxygen fugacity-controlling model reactions discussed in this study. [ABSTRACT FROM PUBLISHER]

Vanadium exists in multiple valence states in silicate and oxide systems, namely V2+, V3+, V4+ and V5+. This special characteristic has been exploited in several ways to estimate the redox conditions of high-temperature planetary processes, such as partial melting and core formation. However, the use of V as a universal redox indicator (i.e. suitable for the entire range of oxygen fugacities found in the inner Solar System) requires precise knowledge of the partitioning of all of its several valence states. Here we report the results of a series of 1 atm (1300°C) and high-pressure (1–3 GPa, 1315–1450°C) experiments carried out over a range of redox conditions sufficiently large (from QFM−13·3 to QFM+11·4, where QFM is the quartz–fayalite–magnetite oxygen buffer) to constrain the full panoply of V chemical behaviour. Partition coefficients between the major upper-mantle minerals (olivine, clinopyroxene, orthopyroxene, spinel and garnet) and silicate melt were precisely measured with laser ablation inductively coupled plasma mass spectrometry for V and other selected heterovalent and homovalent elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb). The particularly large range of redox conditions investigated here enabled concentrations of V2+ and V5+ to be constrained along with V3+ and V4+, allowing modelling of the change in bulk V partition coefficient with oxygen fugacity to be performed in a robust thermodynamic fashion. As in previous studies, we found a trend of increasing incompatibility from V3+ to V4+ to V5+ for all phases. Partition coefficients for V2+ can be either higher or lower than for V3+, depending on the phase. Additionally, we found evidence for changes in the oxidation state of Cr (Cr2+ ↔ Cr3+), Fe (Fe2+ ↔ Fe3+) and Ti (Ti3+ ↔ Ti4+). There is also indication that P may behave as a heterovalent element, occurring as a trivalent cation at very reducing conditions (P3+ ↔ P5+). For all other trace elements, the oxidation state remained constant. The data presented here can be used to implement methods of estimating the redox state of mantle and mantle-derived planetary samples from bulk-rock concentration and crystal/melt partitioning with only minor extrapolation and bias, allowing better precision and accuracy than previously possible. The method of estimating the redox conditions of basalt suites from bulk-rock V concentrations relative to homovalent elements with similar compatibility (e.g. V/Sc and V/Ga) was tested using databases for mid-ocean ridge, ocean island and island arc basalts. Within the many assumptions involved in forward trace-element modelling (e.g. degree and style of melting, source composition, constant partition coefficients), we show that the redox states of the source regions of these different types of terrestrial basalts are indistinguishable from each other, having relative oxygen fugacities at ∼QFM ± 1. The fact that arc magmas have higher Fe3+/Fe2+ ratios than other types of basalts, making them appear to be more oxidized, may be due to late-stage processes rather than derivation from a more oxidized part of the asthenosphere. [ABSTRACT FROM PUBLISHER]

An overview is presented of the construction and use of algebraic partition functions to represent the equilibrium statistical mechanics of multimolecular complexes and their action within a larger regulatory network. Unlike many applications of equilibrium statistical mechanics, multimolecular complexes may operate with various subsets of their components present and connected to the others, the rest remaining in solution. Thus they are variable-structure systems. This aspect of their behavior may be accounted for by the use of ‘fugacity’ variables as a representation within the partition functions. Four principles are proposed by which the combinatorics of molecular complex construction can be reflected in the construction of their partition functions. The corresponding algebraic operations on partition functions are multiplication, addition, function composition and a less commonly used operation called contraction. Each has a natural interpretation in terms of probability distributions on multimolecular structures. Possible generalizations to nonequilibrium statistical mechanics are briefly discussed. [ABSTRACT FROM AUTHOR]

We report the occurrence of unusual, high-magnesium (Fo96) olivine phenocrysts in a basaltic lava and an ejected lithic block from the Upper Vancori period (∼13 ka) and the recent activity (2002–2003) of Stromboli volcano, Italy. The samples that contain this distinctive mineral chemistry are a shoshonitic basalt and a basaltic andesite with anomalous bulk-rock chemical characteristics in which the iron is highly oxidized (6–8 wt % Fe2O3 and <1 wt % FeO). In other respects these samples are similar to the majority of Stromboli basalts, characterized by the coexistence of olivine, clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts, and clinopyroxene, plagioclase and Fe–Ti oxides in the groundmass. In the high-magnesium olivine samples, Fe–Ti oxides (pseudobrookite) typically occur as symplectitic intergrowths with the olivine phenocrysts, indicating simultaneous growth of the two phases. We propose, as a paragenetic model, that the Fo96 olivine phenocrysts crystallized from a highly oxidized basaltic magma in which most of the iron was in the ferric state; hence, only magnesium was available to form olivine. The highly oxidized state of the magma reflects sudden degassing of volatile phases associated with instantaneous, irreversible, transient degassing of the magma chamber; this is postulated to occur during periods of sudden decompression induced by fracturing of the volcanic edifice associated with paroxysmic activity and edifice collapse. [ABSTRACT FROM AUTHOR]

V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluid-mobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO2 during melting. However, for the most part, the average V/Sc-inferred fO2s of arc basalts, MORB and peridotites are remarkably similar (−1·25 to +0·5 log units from the FMQ buffer) and disagree with the observation that the barometric fO2s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth's mantle may be strongly buffered in terms of fO2. The higher barometric fO2s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Crystallization temperatures (T) and oxygen fugacities (fO2) of kimberlite magma are estimated from oxides included in olivine phenocrysts from the Leslie, Aaron, Grizzly and Torrie kimberlite pipes in the central Slave Province, Canada. Crystallization temperatures recorded by olivine–chromite pairs at an assumed pressure of 1·0 GPa are 1030–1170°C ± 50°C, with a mean of ∼1080°C. At these temperatures, the fO2 of coexisting olivine and chromite is 2–3 log units less oxidized than the nickel–nickel oxide (NNO) buffer at a silica activity limited by the presence of monticellite. Mass balance of olivine, bulk-rock and liquid compositions in equilibrium with olivine phenocryst rims suggests that these kimberlites represent crystallization from a magma with 11–28 mol % of liquid, 10 mol % of earlier precipitated olivine phenocrysts and 62–79 mol % of mantle xenocryst olivine. The calculated T–fO2 values indicate that diamonds entrained in the Lac de Gras kimberlites were probably transported to the surface within the stability field of graphite but close to the graphite–CO2 boundary. [ABSTRACT FROM AUTHOR]

Studies of iron-bearing silicate melt (ferrobasalt) + iron metallic phase + graphite + hydrogen equilibria show that carbon and hydrogen solubilities in melts are important for the evolution of the upper mantle. In a series of experiments conducted at 3·7 GPa and 1520–1600°C, we have characterized the nature (oxidized vs reduced) and quantified the abundances of C- and H-compounds dissolved in iron-bearing silicate melts. Experiments were carried out in an anvil-with-hole apparatus permitting the achievement of equal chemical potentials of H2 in the inner Pt capsule and outer furnace assembly. The fO2 for silicate melt–iron equilibrium was 2·32 ± 0·04 log units below iron–wüstite (IW). The ferrobasalt used as starting material experienced a reduction of its iron oxides and silicate network. The counterpart was a liberation of oxygen reacting with the hydrogen entering the capsule. The amount of H2O dissolved in the glasses was measured by ion microprobe and by step-heating and was found to be between 1 and 2 wt %. The dissolved carbon content was found to be 1600 ppm C by step-heating. The speciation of C and H components was determined by IR and Raman spectroscopy. It was established that the main part of the liberated oxygen was used to form OH− and to a much lesser extent H2O, and only traces of H2, CO2 and CO32−. Dissolved carbon is mainly present as atomic carbon or amorphous carbon. It was possible to measure an isotopic fractionation of 0·8‰ between graphite and dissolved or amorphous carbon at the temperatures of experiments. The Raman spectra also suggest that the network units might contain Si–C bonds. Comparison of our results with the literature demonstrates that the amount of dissolved species decreases as fO2 decreases. In the light of these experimental data, it appears that large-scale melting of the proto-Earth could be associated with melts containing an oxidized form of hydrogen. The early Earth, however, was likely to have been a very reducing environment, in which most of the carbon remained stable in the form of graphite. As the Earth became more and more oxidized, melts formed at depth would have dissolved larger amounts of water, and also carbon in the form of CO2, which would have made the degassing of the upper mantle more and more efficient. [ABSTRACT FROM AUTHOR]

The redox state of sillimanite zone (650–700°C, 5–6 kbar) metasediments of the Barrovian type area, Scotland, was investigated using estimates of metamorphic oxygen fugacity (fO2), sulfur fugacity (fS2), and fluid chemistry based on new determinations of mineral and rock compositions from 33 samples. A total of 94% of the samples lack graphite, contain both ilmenite–hematite solid solutions (RHOMOX) and magnetite, and had metamorphic fO2 about 2 log10 units above the quartz–fayalite–magnetite (QFM) buffer. The regional variation in metamorphic fO2 for these rocks was minimal, about ±0·3 log10 units, reflecting either a protolith that was homogeneous with respect to redox state, or an initially variable protolith whose redox state was homogenized by metamorphic fluid–rock interaction. RHOMOX inclusions in garnet porphyroblasts that become richer in ilmenite from the interior to the edge of the host porphyroblast suggest that at least some syn-metamorphic reduction of rock occurred. Significant variations in bulk-rock oxidation ratio (OR) that are probably inherited from sedimentary protoliths are found from one layer to the next; OR ranges mostly between ∼20 and ∼50 [OR = molecular 2Fe2O3 × 100/(2Fe2O3 + FeO)]. These OR variations are uncorrelated with fO2 and do not indicate that large, order-of-magnitude gradients in fO2 and redox state existed or were preserved between layers during metamorphism. The other 6% of the samples contain ilmenite, lack magnetite, and had low fO2 0–1 order of magnitude below QFM in the stability field of graphite. They are characterized by combinations of the following: large fluid HF/H2O; metasomatic, tourmaline-bearing veins; absence or rarity of primary organic matter; and crosscutting late metamorphic shear zones rich in carbonaceous material. Such observations suggest that locally low fO2 conditions may have been related to the influx of reducing fluids from elsewhere in the area. [ABSTRACT FROM PUBLISHER]

This comment addresses the interpretation of oxygen fugacity data for spinel peridotite xenoliths from five Mexican volcanic fields presented by Luhr & Aranda-Gomez (Journal of Petrology, 38, 1075–1112, 1997). The postulated east–west increase of the ΔFMQ (‘relative oxygen fugacity’, where FMQ is fayalite–magnetite–quartz) values is inherent to the method and therefore of questionable geological significance. Increases in ΔFMQ do not necessarily mirror oxidation processes in the mantle controlled by subduction-related fluids. [ABSTRACT FROM PUBLISHER]

The Jamon granite is representative of the Lower Proterozoic (1·88 Ga) oxidized A-type granites of the eastern part of the Amazonian craton. The dominant facies is a metaluminous to slightly peraluminous hornblende biotite monzogranite with K2O/Na2O between 0·8 and 1·5 and FeOt/(FeOt + MgO) between 0·8 and 0·9. In contrast to many other A-type granites, the Jamon granite is characterized by early hornblende crystallization and the presence of magnetite. Crystallization experiments were performed on glass at 300 MPa between 700 and 900°C for various melt H2O contents and for both oxidizing and reducing fO2 [NNO (nickel-nickel oxide) + 2·5 and NNO - 1·5 on average]. For NNO + 2·5 and under H2O-rich conditions, ilmenite, clinopyroxene, magnetite and hornblende are near-liquidus phases, followed by plagioclase. The orthopyroxene stability field is restricted to high temperatures and H2O contents in the melt <5 wt %. In contrast, for NNO - 1·5, magnetite and titanite are absent and orthopyroxene (never observed in the granite), clinopyroxene and ilmenite are the liquidus phases. Conditions of crystallization of amphibole, magnetite and plagioclase constrain the initial melt H2O content to between 4·5 and 6·5 wt %. Plagioclase cores crystallized from 870 to 720°C. Clinopyroxene, amphibole and biotite Fe/(Fe + Mg) values suggest fO2 around NNO + 0·5 during crystallization of the granite. The demonstration of relatively hydrous conditions and oxidizing fO2 for the Jamon granite stresses the diversity of A-type magmas in terms of H2O contents and redox states. The Jamon granite was most probably generated from oxidized Archaean igneous rocks of mafic-intermediate composition. [ABSTRACT FROM PUBLISHER]

Because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour. Comparison of experimental data on an iron-rich basaltic composition with predictions of the MELTS free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well, the saturation of Fe–Ti oxides is not We have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations, mineral–melt partitioning of major elements, and mass balance constraints. The model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen. Liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar. In a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids, whereas systems closed to oxygen crystallize less abundant magnetite, leading to a less pronounced iron depletion in the liquid. Predicted bulk solid compositions and variations of fo2, with falling temperature agree well with those observed or inferred from the cumulates of the Skaergaard intrusion, but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations. Compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the Basistoppen sill (which closely resemble those of Skaergaard) may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen. A comparison of the predicted T-fo2, paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as Iceland and Hawaii evolve under conditions open to oxygen, whereas evidence from plutonic environments (e.g. Skaergaard and Kiglapait layered intrusions) suggests that they evolved under conditions more closed to oxygen. The compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different, although the results of this study suggest that magnetite saturation will limit Fe enrichment in all environments to <20wt% FeO*, consistent with enrichments reported for volcanic glasses. [ABSTRACT FROM AUTHOR]

Standard Electrical Olivine 3 (SEO3) is a new model of electrical conductivity for olivine as a function of temperature and oxygen fugacity, and is based on a previous study of point defect behaviour in a dunite. That study produced quantitative estimates of mobility and concentration for small polarons and magnesium vacancies as a function of temperature and oxygen fucacity. Unlike earlier SO1 and SO2 models of olivine conductivity, the dunite sample was buffered by pyroxene, and avoided alteration at high temperature. The low-temperature extrapolation produced by SEO3 is in good agreement with SO2, while the high-temperature extrapolation is significantly more conductive, which is in general agreement with many field studies that show the Earth's upper mantle to be more conductive than predicted by SO2 for reasonable temperatures. Plausible variations in mantle oxygen fugacity correspond to about half an order of magnitude variation in electrical conductivity, equivalent to about 100 degrees in temperature. [ABSTRACT FROM AUTHOR]