Owen K. Neill

Nepheline structural and chemical dependence on melt composition

Abstract Nepheline crystallizes upon slow-cooling in some melts concentrated in Na2O and Al2O3, which can result in a residual glass phase of low chemical durability. Nepheline can incorporate many components often found in high-level waste radioactive borosilicate glass, including glass network ions (e.g., Si, Al, Fe), alkali metals (e.g., Cs, K, Na, and possibly Li), alkaline-earth metals (e.g., Ba, Sr,Ca, Mg), and transition metals (e.g., Mn, and possibly Cr, Zn, Ni). When crystallized from melts of different compositions, nepheline composition varies as a function of starting melt composition. Five simulated high-level nuclear waste borosilicate glasses shown to crystallize large fractions of nepheline on slow-cooling were selected for study. These starting melt compositions contained a range of Al2O3, B2O3, CaO, Na2O, K2O, Fe2O3, and SiO2 concentrations. Compositional analyses of nepheline crystals in glass by electron probe micro-analysis (EPMA) indicate that nepheline is generally rich in silica, whereas boron is unlikely to be present in any significant concentration, if at all, in nepheline. Also, several models are presented for calculating the fraction of vacancies in the nepheline structure.

Mineral chemistry and petrogenesis of a HFSE(+HREE) occurrence, peripheral to carbonatites of the Bear Lodge alkaline complex, Wyoming

Abstract Rare earth mineralization in the Bear Lodge alkaline complex (BLAC) is mainly associated with an anastomosing network of carbonatite dikes and veins, and their oxidized equivalents. Bear Lodge carbonatites are LREE-dominant, with some peripheral zones enriched in HREEs. We describe the unique chemistry and mineralogy one such peripheral zone, the Cole HFSE(+HREE) Occurrence (CHO), located ~2 km from the main carbonatite intrusions. The CHO consists of anatase, xenotime-(Y), brockite, fluorite, zircon, and K-feldspar, and contains up to 44.88% TiO2, 3.12% Nb2O5, 6.52% Y2O3, 0.80% Dy2O3, 2.63% ThO2, 6.0% P2O5, and 3.73% F. Electron microprobe analyses of xenotime-(Y) overgrowths on zircon show that oscillatory zoning is a result of variable Th and Ca content. Cheralite-type substitution, whereby Th and Ca are incorporated at the expense of REEs, is predominant over the more commonly observed thorite-type substitution in xenotime-(Y). Th/Ca-rich domains are highly beam sensitive and accompanied by high-F concentrations and low-microprobe oxide totals, suggesting cheralite-type substitution is more easily accommodated in fluorinated and hydrated/hydroxylated xenotime-(Y). Analyses of xenotime-(Y) and brockite show evidence of VO43−

Pre-eruptive magma mixing and crystal transfer revealed by phenocryst and microlite compositions in basaltic andesite from the 2008 eruption of Kasatochi Island volcano

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Focused Interest Group on Microanalytical Standards (FIGMAS): Assessing the Quality, Availability and Need for Standards in the Microanalytical Community

substitution for PO43−

Major Element and Halogen (F, Cl) Mineral–Melt–Fluid Partitioning in Hydrous Rhyodacitic Melts at Shallow Crustal Conditions

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Geochemistry and mineralogy of a saprolite developed on Columbia River Basalt: Secondary clay formation, element leaching, and mass balance during weathering

with patches of an undefined Ca-Th-Y-Ln phosphovanadate solid-solution composition within brockite clusters. Fluorite from the CHO is HREE-enriched with an average Y/Ho ratio of 33.2, while other generations of fluorite throughout the BLAC are LREE-enriched with Y/Ho ratios of 58.6–102.5. HFSE(+HREE) mineralization occurs at the interface between alkaline silicate intrusions and the first outward occurrence of calcareous Paleozoic sedimentary rocks, which may be local sources of P, Ti, V, Zr, and Y. U-Pb zircon ages determined by LA-ICP-MS reveal two definitive 207Pb/206Pb populations at 2.60–2.75 and 1.83–1.88 Ga, consistent with derivation from adjacent sandstones and Archean granite. Therefore, Zr and Hf are concentrated by a physical process independent of the Ti/Nb-enriched fluid composition responsible for anatase crystallization. The CHO exemplifies the extreme fluid compositions possible after protracted LREE-rich crystal fractionation and subsequent fluid exsolution in carbonatite-fluid systems. We suggest that the anatase+xenotime-(Y)+brockite+fluorite assemblage precipitated from highly fractionated, low-temperature (<200 °C), F-rich fluids temporally related to carbonatite emplacement, but after significant fractionation of ancylite and Ca-REE fluorocarbonates. Low-temperature aqueous conditions are supported by the presence of fine-grained anatase as the sole Ti-oxide mineral, concentrically banded botryoidal fluorite textures, and presumed hydration of phosphate minerals. Fluid interaction with Ca-rich lithologies is known to initiate fluorite crystallization which may cause destabilization of (HREE, Ti, Nb)-fluoride complexes and precipitation of REE+Th phosphates and Nb-anatase, a model valuable to the exploration for economic concentrations of HREEs, Ti, and Nb.

Amphibole-, Clinopyroxene- and Plagioclase-Melt Partitioning of Trace and Economic Metals in Halogen-Bearing Rhyodacitic Melts

Abstract The August 7-8, 2008, eruption of Kasatochi Island volcano, located in the central Aleutians Islands, Alaska, produced abundant, compositionally heterogeneous basaltic andesite (52-55 wt% SiO2) that has been interpreted to result from pre-eruptive magma mixing. The basaltic andesite contains two populations of plagioclase phenocrysts. The first, volumetrically dominant population consists of oscillatory-zoned phenocrysts with an overall normal zonation trend toward comparatively sodic rims (An55-65), interrupted by dissolution features and spikes in calcium content (up to ~An85). The second population consists of phenocrysts with highly calcic compositions (~An90). These phenocrysts contain sharp decreases in calcium content close to their rims (reaching as low as ~An60), but are otherwise texturally and compositionally homogeneous. Groundmass plagioclase microlites are generally much more calcic than rims of the first phenocryst population, with more than 50% of measured microlites containing >An80. Major, minor, and trace element concentrations of plagioclase microlites and phenocrysts indicate that oscillatory-zoned phenocrysts derived from cooler (800-950 °C), more silicic mixing magma, while unzoned, calcic phenocrysts were associated with hotter (900-1050 °C), mafic magma. The mixing of these magmas just prior to eruption, followed by decompression during the eruption itself created high effective undercoolings in the mafic end-member, and lead to the nucleation of high-An microlites. MgO and FeO concentrations of plagioclase microlites and high-An phenocryst rims (up to ~0.4 and ~1.3 wt%, respectively) provide further evidence for high mixing- and eruptioninduced effective undercoolings.

Focused Interest Group on Microanalytical Standards (FIGMAS): An Update

It has been recognized over the past years that different electron microprobe and scanning electron microprobe laboratories use different sets of standards or reference materials for quantitative analysis. Unfortunately, some of these standards either have become unavailable (e.g., some natural minerals from the Smithsonian Institution collection) or are only available to a restricted group of people (e.g., internal reference materials). Other synthetic materials are also available commercially or provided by other institutions and research centers. However, they sometimes lack either broad availability or acceptable characterization (e.g., NIST glasses, Corning glasses, Drake & Weill REE-glasses... [1,2]). Another important problem for the community is a clear assessment of standard quality (the “Good”, “Bad” and “Ugly” of Carpenter [3]): “good” homogeneous standards with accurate compositional information and without impurities or inclusions are rare, whereas “bad” standards, which lack good characterization, are more common. Individual lab managers do commonly examine their own standard collections to re-evaluate compositional homogeneity and test the accuracy of published compositions. Standards are also frequently re-analyzed at individual labs using various techniques, and therefore multiple accepted compositions for individual standards may exist.

Microanalytical Standards, Reference and Research Materials: Continuing the Effort toward Breaking the Accuracy Barrier

Twenty-four internally heated pressure vessel experiments were conducted at 810–860 C, 1 5– 4 05 kbar, and oxygen fugacities (fO2)1⁄4NNO –0 5 to NNOþ2 log units (where NNO is the nickel–nickel oxide buffer), using hydrous rhyodacitic starting glasses from Mt Usu, Japan. Aqueous solutions were added to experimental charges such that the volatile phase(s) coexisting with the crystalline phases (amphibole, plagioclase, and clinopyroxene) at run conditions buffered the F, Cl, S, and CO2 concentrations in the melt. The resultant phenocryst phases and glass chemistries were analysed by electron microprobe and Fourier transform infrared spectroscopy, and final fluid Cl contents by chloridometer. All experiments produced homogeneous glasses and large euhedral phenocrysts with minimal compositional zonation. The results of the crystal–melt partitioning data are applicable to understanding the geochemical evolution of rhyodacitic melts at fluid-saturated shallow crustal levels, the composition of fluid phases later exsolved owing to second boiling, and the effects of halogens on amphibole crystal chemistry. The residual glasses from these experiments span the dacite–rhyolite compositional join, and are hydrous (>5 wt % H2O) with F concentrations from 100 ppm to 0 63 wt % and Cl from 130 ppm to 0 72 wt %. Measured final fluid Cl contents show that Cl strongly prefers the fluid phase over the melt phase in all experiments, with DCl fluid/melt ranging from 3 5 to 22 7. Amphibole compositions are calcic, Mg-rich, and typical of those found in natural calc-alkaline arc magmas. They are particularly sensitive to changes in melt halogen chemistry, with maximum amphibole F contents of 2 59 wt % (maximum DF values of 15) and maximum Cl contents of 0 12 wt % (maximum DCl amph/melt values of 0 40). Integration of the amphibole data with other experimental data shows that Cl incorporation is a strong function of the Mg–Cl crystallographic avoidance principle, and that addition of F to the melt strongly decreases Cl partitioning at equivalent Mg# (DCl 1⁄40 106 0 02 in experiments that were not F-doped, compared with DCl 1⁄40 056 0 01 in F-doped experiments). Plagioclase compositions are relatively restricted, with anorthite contents An59–An39, and clinopyroxene is similarly calcic, containing a significant enstatite component (Wo49–Wo30). The behaviour of Fe in the glass and crystalline phases is most significantly affected by the fO2 of the experiment. Higher total Fe contents are found in amphibole and clinopyroxene from experiments with fO2<NNO þ1, but with little effect on the absolute DFeO . Plagioclase, however, shows relatively decreased DFeO plag/melt reflecting its preferential incorporation of Fe3þ, and less Fe3þ expected in melts crystallized at lower fO2 conditions. Comparison of the data with recently formulated amphibole geothermometers, barometers, and plagioclase hygrometers shows calculated results that are consistent with the actual experimental conditions but discrepancies arise owing to halogen-induced major VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2465 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 12, 2465–2492 doi: 10.1093/petrology/egy011 Advance Access Publication Date: 14 February 2018

Magmatic–hydrothermal fluids and volatile metals in the Spirit Lake pluton and Margaret Cu–Mo porphyry system, SW Washington, USA

Abstract This study presents chemical and mineralogical data on weathering trends in a saprolite that is preserved between flows of the Columbia River Basalt Group at Trinidad, Washington. Bulk chemistry, electron imaging, and X-ray mapping indicate early Fe and Mg depletion by dissolution of ferromagnesian minerals, followed by depletion of alkalis, Al, Ti, and P that corresponds to dissolution of feldspars, titanomagnetite, and apatite. Secondary coatings of nontronite clay in the deep saprolite display intricate, submicrometer-scale zoning in Fe and Mg content. Distinct aluminous zones in these clays become more prominent at shallower depths. The primary Fe-containing phase shifts from nontronite in deeper samples to hematite in shallow samples; samples at the boundary contain the assemblage kaolinite + nontronite, which may mark the transition from permeability-limited fluid flow to fully open-system behavior. This shift is observed in rocks that have lost 30–40% of the total rock mass to leaching, and coincides with the disappearance of feldspar, Fe-Ti oxides, and apatite. Rocks in the uppermost saprolite have been converted to an assemblage of Al-smectite + hematite (+kaolinite). These results suggest that the presence of nontronite in weathered samples may indicate weathering under conditions of limited permeability; however, it does not necessarily indicate weathering in a chemically closed system. These observations may be useful in interpreting the clay mineral assemblages observed on Mars and what information they contain about near-surface conditions in the planet’s ancient past.