Precise compositional analyses at spatial resolution <1 μm, combined with structure determination using bulk (i.e. powder XRD) and individual grain (i.e. EBSD) techniques, show that both β-FexSe and ...δ-FexSe form as solids in a two-phase field above and below the apparent peritectic temperature of 457 °C. Microstructures show that β-FexSe and δ-FexSe form together via exsolution when cooled from this two-phase field; evident when annealing time and temperature are optimised. Using a facile one-pot method with elemental Fe:Se reactant ratios ranging from 0.95 to 1.14, β-FexSe occurs as the predominant phase in association with hexagonal δ-FexSe in the temperature range 330 °C < Tmax < 750 °C, where Tmax is the sintering temperature. Maximum yield of β-FexSe occurs after sintering at Tmax > 690 °C with annealing at 420 °C for ∼24 h. We define a modified phase diagram that includes this two phase field with β-FexSe and δ-FexSe for 1.02 < x < 0.90 and a second two phase field with β-FexSe and α-Fe for 1.08 < x < 1.02. This revised phase diagram for Fe:Se ∼ 1.0 suggests that the peretectic transition nominally identified at x = 1.04 is not evident.
The Herberton Mineral Field in Northeast Australia hosts world class magmatic-hydrothermal Sn–W polymetallic deposits that are enriched in In. The Baal Gammon and Isabel deposits from the Herberton ...Mineral Field contains early tin, as cassiterite, overprinted by sulfide mineralization as chalcopyrite, sphalerite, galena, pyrrhotite, and stannite. We investigated the distribution of In in the sulfide ores from these two deposits, calculated the temperature of formation via sphalerite-stannite geothermometer, and deduced the physicochemical conditions favorable for enriching In in this mineralizing environment. The Baal Gammon deposit is dominated by chalcopyrite, with In contained in chalcopyrite, sphalerite, and stannite. The average In concentrations measured by EPMA in chalcopyrite, sphalerite, and stannite are 0.10, 0.68, and 0.92 wt%, respectively. Chalcopyrite, pyrrhotite, and sphalerite textures indicate that In incorporation occurred during exsolution from an intermediate solid solution of cubanite composition. The Isabel deposit is dominated by sphalerite associated with galena and contains only minor amounts of chalcopyrite. The average concentration of In in sphalerite from the Isabel deposit is 0.11 wt%. The stannite-sphalerite geothermometer indicates mineralization temperatures of ~ 290 °C at the Baal Gammon deposit, and ~ 307 °C at the Isabel deposit. At these temperatures, the physicochemical modeling suggests that stable In chlorine complexes occur in acidic conditions (pH < 3). These results when combined with the Eh–pH phase model of the sulfide assemblage further constrain the redox conditions during mineralization.
Abstract
The Havre 2012 deep-sea rhyolite eruption went unobserved and was initially recognised from a massive pumice raft at the sea surface. Havre pumices are predominantly white or grey, however ...pink pumice is common in the raft. In subaerial explosive eruptions, pink pumice is understood to result from high-temperature atmospheric iron-oxidation. The presence of pink pumice questions the effusive eruption model for the Havre raft. Here we report results from X-ray Absorption Near Edge Structure spectroscopy, magnetic measurements, TEM imaging and glass chemistry that collectively show the colour results from increasing amounts of magnetite nanolites in the raft pumice glass oxidizing to hematite. This suggests a short-lived but powerful explosive eruption phase penetrated the water column allowing hot pyroclasts to oxidise in air. Our results therefore challenge the known depth limits for explosive eruptions in the marine realm and suggest pink pumice can be an indicator of magnetite nanolite-driven explosive eruptions.
We demonstrate synthesis of the ternary intermetallic MgNi₃B₂ using autogenous pressure from the reaction of NaBH₄ with Mg and Ni metal powder. The decomposition of NaBH₄ to H₂ and B₂H₆ commences at ...low temperatures in the presence of Mg and/or Ni and promotes formation of Ni-borides and MgNi₃B₂ with the increase in temperature. MgNi₃B₂ aggregates with Ni-boride cores are formed when the reaction temperature is >670 °C and autogenous pressure is >1.7 MPa. Morphologies and microstructures suggest that solid-gas and liquid-gas reactions are dominant mechanisms and that Ni-borides form at a lower temperature than MgNi₃B₂. Magnetic measurements of the core-shell MgNi₃B₂ aggregates are consistent with ferromagnetic behaviour in contrast to stoichiometric MgNi₃B₂ which is diamagnetic at room temperature.
The 2012 Havre eruption evacuated a crystal-poor rhyolite (∼3–7% crystals) producing a volumetrically dominant (∼1.4 km3) pumice raft, as well as seafloor giant pumice (5–8%) and lavas (12–14%) at ...the vent (∼0.1 km3), both of which have subtly higher phenocryst contents. For crystal-poor rhyolites like the Havre pumice, it can often remain ambiguous as to whether the few phenocrysts present, in this case, plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides ± quartz, are: (a) autocrysts crystallizing from the surrounding melt, (b) antecrysts being sourced from mush and the magma plumbing system, or (c) xenocrysts derived from source materials or chamber walls, or (d) possibly a combination of all of the above. In crystal-poor magmas, the few crystals present are strongly relied upon to constrain pre-eruptive conditions such as magmatic temperatures, pressures, water content and fO2. A detailed textural and compositional analysis combined with a range of equilibrium tests and rhyolite-MELTS modeling provide the basis for distinguishing autocrystic vs inherited crystal populations in the Havre eruption. An autocrystic mineral assemblage of andesine plagioclase, enstatite and Fe-Ti oxides constrains the pre-eruptive conditions of the Havre rhyolite magma: magmatic temperatures of 890 ± 27°C, crystallization pressures at 2–4 kbars, oxygen fugacity of NNO + 0.4 and water concentrations (5.6 ± 1.1 wt.%). Inherited phases not in equilibrium with the host melt composition are clinopyroxene, An-rich plagioclase (> An53) and quartz. Rhyolite-MELTs modeling indicates the clinopyroxene and quartz have most likely been sourced from cooler, silicic mush zones in the Havre magmatic system. This study demonstrates that even in crystal-poor rhyolites it cannot be assumed that all crystals are autocrystic and can be used to constrain pre-eruptive magmatic conditions.
The 12·7–10·5 Ma Cougar Point Tuff in southern Idaho, USA, consists of 10 large-volume (>102–103 km3 each), high-temperature (800–1000°C), rhyolitic ash-flow tuffs erupted from the Bruneau–Jarbidge ...volcanic center of the Yellowstone hotspot. These tuffs provide evidence for compositional and thermal zonation in pre-eruptive rhyolite magma, and suggest the presence of a long-lived reservoir that was tapped by numerous large explosive eruptions. Pyroxene compositions exhibit discrete compositional modes with respect to Fe and Mg that define a linear spectrum punctuated by conspicuous gaps. Airfall glass compositions also cluster into modes, and the presence of multiple modes indicates tapping of different magma volumes during early phases of eruption. Equilibrium assemblages of pigeonite and augite are used to reconstruct compositional and thermal gradients in the pre-eruptive reservoir. The recurrence of identical compositional modes and of mineral pairs equilibrated at high temperatures in successive eruptive units is consistent with the persistence of their respective liquids in the magma reservoir. Recurrence intervals of identical modes range from 0·3 to 0·9 Myr and suggest possible magma residence times of similar duration. Eruption ages, magma temperatures, Nd isotopes, and pyroxene and glass compositions are consistent with a long-lived, dynamically evolving magma reservoir that was chemically and thermally zoned and composed of multiple discrete magma volumes.
The Bruneau–Jarbidge eruptive center of the central Snake River Plain in southern Idaho, USA produced multiple rhyolite lava flows with volumes of <10
km
3 to 200
km
3 each from ~11.2 to 8.1
Ma, most ...of which follow its climactic phase of large-volume explosive volcanism, represented by the Cougar Point Tuff, from 12.7 to 10.5
Ma. These lavas represent the waning stages of silicic volcanism at a major eruptive center of the Yellowstone hotspot track. Here we provide pyroxene compositions and thermometry results from several lavas that demonstrate that the demise of the silicic volcanic system was characterized by sustained, high pre-eruptive magma temperatures (mostly ≥950
°C) prior to the onset of exclusively basaltic volcanism at the eruptive center. Pyroxenes display a variety of textures in single samples, including solitary euhedral crystals as well as glomerocrysts, crystal clots and annealed microgranular inclusions of pyroxene
±
magnetite
±
plagioclase. Pigeonite and augite crystals are unzoned, and there are no detectable differences in major and minor element compositions according to textural variety — mineral compositions in the microgranular inclusions and crystal clots are identical to those of phenocrysts in the host lavas. In contrast to members of the preceding Cougar Point Tuff that host polymodal glass and mineral populations, pyroxene compositions in each of the lavas are characterized by single rather than multiple discrete compositional modes. Collectively, the lavas reproduce and extend the range of Fe–Mg pyroxene compositional modes observed in the Cougar Point Tuff to more Mg-rich varieties. The compositionally homogeneous populations of pyroxene in each of the lavas, as well as the lack of core-to-rim zonation in individual crystals suggest that individual eruptions each were fed by compositionally homogeneous magma reservoirs, and similarities with the Cougar Point Tuff suggest consanguinity of such reservoirs to those that supplied the polymodal Cougar Point Tuff. Pyroxene thermometry results obtained using QUILF equilibria yield pre-eruptive magma temperatures of 905 to 980
°C, and individual modes consistently record higher Ca content and higher temperatures than pyroxenes with equivalent Fe–Mg ratios in the preceding Cougar Point Tuff. As is the case with the Cougar Point Tuff, evidence for up-temperature zonation within single crystals that would be consistent with recycling of sub- or near-solidus material from antecedent magma reservoirs by rapid reheating is extremely rare. Also, the absence of intra-crystal zonation, particularly at crystal rims, is not easily reconciled with cannibalization of caldera fill that subsided into pre-eruptive reservoirs. The textural, compositional and thermometric results rather are consistent with minor re-equilibration to higher temperatures of the unerupted crystalline residue from the explosive phase of volcanism, or perhaps with newly generated magmas from source materials very similar to those for the Cougar Point Tuff. Collectively, the data suggest that most of the pyroxene compositional diversity that is represented by the tuffs and lavas was produced early in the history of the eruptive center and that compositions across this range were preserved or duplicated through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that unerupted magmas residual to the explosive phase of volcanism may have been stored at sustained, high temperatures subsequent to the explosive phase of volcanism. If so, such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower and/or mid-crust, consistent with the tectonic setting of a continental hotspot.
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•The variolitic texture in mafic pillow lavas of the Mount Ada Basalt of the Pilbara Supergroup formed due to liquid immiscibility.•Clinopyroxene element mapping indicate that grains ...hosted in both variole and host regions preserve shared magmatic histories.•The liquid immiscibility reaction occurred due to H2O saturation in the precursor magmas.•Variole bearing lavas have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites.•The fourth volcanic cycle in the Pilbara Supergroup, the Mount Ada Basalt and the Duffer Formation, preserves distinctly different volcanic types that indicate significant changes in magma genesis during a 10 Ma timeframe.
Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation.
Industrial applications of natural zeolites depend on their mineralogical, physical and chemical characteristics. Over the last 20 years, Australian natural zeolites have been investigated for use in ...various industrial applications. However, there are few, if any, mineral characterisation studies on Australian natural zeolites since the early 1990s that use modern techniques. In this study, a detailed mineralogical analysis was conducted on zeolite specimens from Avoca and Werris Creek deposits, located in Queensland and New-South-Wales, respectively, in Australia. Their physical properties, thermal behaviour and porosity, as well as mineral compositions were determined by conventional methods, including thermogravimetry, N2 adsorption/desorption, optical microscopy, XRF, in situ XRD, SEM/EDS and EPMA/WDS. High-precision, high-accuracy measurements of the chemical compositions of fine-grained zeolites (<20 μm) were performed in situ in thin section using accepted EPMA protocols and data reduction methods. The Australian zeolites were identified as magnesium heulandite/clinoptilolite-Ca which corrects interpretations from earlier studies.
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•Combination of ex- and in-situ analysis methods were applied.•The electron probe micro analysis (EPMA) was used for in situ mineral analysis.•Accurate estimation of zeolites composition with particle size less than 20 μm.•A chemical gradient from edges into the cores of zeolite crystals was observed.•Diversity in chemical composition was reflected in physical/chemical properties.
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