¹Aya Iwai, ²Yoichi Itoh, ³Tsuyoshi Terai, ⁴Ranjan Gupta, ⁵Asoke Sen, ²Jun Takahash
Research in Astronomy and Astrophysics 17, 17 Link to Article [http://dx.doi.org/10.1088/1674%E2%80%934527%2F17%2F2%2F17]
¹National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8563, Japan
²Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, Hyogo 679-5313, Japan
³National Astronomical Observatory of Japan, Hilo, Hawaii 96720, USA
United States
⁴Inter-University Centre for Astronomy and Astrophysics, Ganeshkhind, Pune 411 007, India
⁵Department of Physics, Assam University, Silchar 788 001, India

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¹Roberts Blukis, ²Rudolf Rüffer, ²Aleksandr I. Chumakov, ¹Richard J. Harrison
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12841]
¹Department of Earth Sciences, University of Cambridge, Cambridge, UK
²European Synchrotron Radiation Facility, Grenoble, France
Published by Arrangement with John Wiley & Sons

Metallic phases in the Tazewell IIICD iron and Esquel pallasite meteorites were examined using 57Fe synchrotron Mössbauer spectroscopy. Spatial resolution of ~10–20 μm was achieved, together with high throughput, enabling individual spectra to be recorded in less than 1 h. Spectra were recorded every 5–10 μm, allowing phase fractions and hyperfine parameters to be traced along transects of key microstructural features. The main focus of the study was the transitional region between kamacite and plessite, known as the “cloudy zone.” Results confirm the presence of tetrataenite and antitaenite in the cloudy zone as its only components. However, both phases were also found in plessite, indicating that antitaenite is not restricted exclusively to the cloudy zone, as previously thought. The confirmation of paramagnetic antitaenite as the matrix phase of the cloudy zone contrasts with recent observations of a ferromagnetic matrix phase using X-ray photoemission electron spectroscopy. Possible explanations for the different results seen using these techniques are proposed.

¹C.J. Renggli, ¹P.L. King, ¹R.W. Henley, ¹M.D. Norman
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.012]
¹Research School of Earth Sciences, Australian National University, ACT 2601, Australia
Copyright Elsevier

Transport of metals in volcanic gases on the Moon differs greatly from their transport on the Earth because metal speciation depends largely on gas composition, temperature, pressure and oxidation state. We present a new thermochemical model for the major and trace element composition of lunar volcanic gas during pyroclastic eruptions of picritic magmas calculated at 200-1500 °C and over 10-9-103 bar. Using published volatile component concentrations in picritic lunar glasses, we have calculated the speciation of major elements (H, O, C, Cl, S and F) in the coexisting volcanic gas as the eruption proceeds. The most abundant gases are CO, H2, H2S, COS and S2, with a transition from predominantly triatomic gases to diatomic gases with increasing temperatures and decreasing pressures. Hydrogen occurs as H2, H2S, H2S2, HCl, and HF, with H2 making up 0.5 to 0.8 mole fractions of the total H. Water (H2O) concentrations are at trace levels, which implies that H-species other than H2O need to be considered in lunar melts and estimates of the bulk lunar composition. The Cl and S contents of the gas control metal chloride gas species, and sulfide gas and precipitated solid species. We calculate the speciation of trace metals (Zn, Ga, Cu, Pb, Ni, Fe) in the gas phase, and also the pressure and temperature conditions at which solids form from the gas. During initial stages of the eruption, elemental gases are the dominant metal species. As the gas loses heat, chloride and sulfide species become more abundant. Our chemical speciation model is applied to a lunar pyroclastic eruption model with isentropic gas decompression. The relative abundances of the deposited metal-bearing solids with distance from the vent are predicted for slow cooling rates (< 5 °C/s). Close to a volcanic vent we predict native metals are deposited, whereas metal sulfides dominate with increasing distance from the vent. Finally, the lunar gas speciation model is compared with the speciation of a H2O-, CO2- and Cl-rich volcanic gas from Erta Ale volcano (Ethiopia) as an analogy for more oxidized planetary eruptions. In the terrestrial Cl-rich gas the metals are predominantly transported as chlorides, as opposed to metallic vapours and sulphides in the lunar gas. Due to the presence of Cl-species, metal transport is more efficient in the volcanic gas from Erta Ale compared to the Moon.