¹L. J. Hicks, ¹J. L. Macarthur, ¹J. C. Bridges, ²M. C. Price, ²J. E. Wickham-Eade, ²M. J. Burchell, ¹G. M. Hansford, ³A. L. Butterworth, ¹S. J. Gurman, ¹S. H. Baker
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12909]
¹Department of Physics & Astronomy, Space Research Centre, University of Leicester, Leicester, UK
²School of Physical Sciences, University of Kent, Canterbury, UK
³Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA
Published by arrangement with John Wiley & Sons

The mineralogy of comet 81P/Wild 2 particles, collected in aerogel by the Stardust mission, has been determined using synchrotron Fe-K X-ray absorption spectroscopy with in situ transmission XRD and X-ray fluorescence, plus complementary microRaman analyses. Our investigation focuses on the terminal grains of eight Stardust tracks: C2112,4,170,0,0; C2045,2,176,0,0; C2045,3,177,0,0; C2045,4,178,0,0; C2065,4,187,0,0; C2098,4,188,0,0; C2119,4,189,0,0; and C2119,5,190,0,0. Three terminal grains have been identified as near pure magnetite Fe3O4. The presence of magnetite shows affinities between the Wild 2 mineral assemblage and carbonaceous chondrites, and probably resulted from hydrothermal alteration of the coexisting FeNi and ferromagnesian silicates in the cometary parent body. In order to further explore this hypothesis, powdered material from a CR2 meteorite (NWA 10256) was shot into the aerogel at 6.1 km s−1, using a light-gas gun, and keystones were then prepared in the same way as the Stardust keystones. Using similar analysis techniques to the eight Stardust tracks, a CR2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel.

^1,2Valentin O. Osadchii, ³Mark V. Fedkin, ²Evgeniy G. Osadchii
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12919]
¹Department of Geochemistry, The Faculty of Geology, Moscow State University, Moscow, Russia
²Laboratory of High Temperature Electrochemistry, The Institute of ³Experimental Mineralogy, The Russian Academy of Science, Chernogolovka, Moscow Region, Russia
⁴Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennyslvania, USA
Published by arrangement with John Wiley & Sons

High-temperature solid-state electrochemistry techniques (EMF method) were used to measure the oxygen fugacity (fO2) of the ordinary chondrites Ochansk (H4), Savtschenskoje (LL4), Elenovka (L5), Vengerovo (H5), and Kharkov (L6). The fO2 results are presented in the form of the following equations:

[not displayed here due to technical reasons]

It was found that fO2 regularly increases from H chondrites to LL chondrites. Measured fO2 are ~1.5 higher than those previously calculated from mineral assemblages. Kharkov (L6) is a little more oxidized than Elenovka (L5) in agreement with the progressive oxidation model. At the same time, Ochansk (H4) is more oxidized than Vengerovo (H5) and exhibits a slightly different slope compared to other chondrites and at T > 1200 K, becomes more reduced than Kharkov (L6) or Elenovka (L5). Measured oxygen fugacity values of meteorites fall within (0.1–1.0)·log fO2 of one another. The possible explanation of discrepancies between measured and calculated values is discussed.

^1,2,3Steven B. Simon, ^1,4Stephen R. Sutton
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12908]
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
²The Field Museum of Natural History, Chicago, Illinois, USA
³Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
⁴Center for Advanced Radiation Sources (CARS), The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

The valences of Ti, V, and Cr in olivine and pyroxene, important indicators of the fO2 of the source region of their host rocks, can be readily measured nondestructively by XANES (X-ray absorption near edge structure) spectroscopy, but little such work has been done on lunar rocks, and there is some uncertainty regarding the presence of Ti3+ in lunar silicates and the redox state of the lunar mantle. This is the first study involving direct XANES measurement of valences of multivalent cations in lunar rocks. Because high alumina activity facilitates substitution of Ti cations into octahedral rather than tetrahedral sites in pyroxene and Ti3+ only enters octahedral sites, two aluminous basalts from Apollo 14, 14053 and 14072, were studied. Most pyroxene contains little or no detectable Ti3+, but in both samples relatively early, magnesian pyroxene was found that has Ti valences that are not within error of 4; in 14053, this component has an average Ti valence of 3.81 ± 0.06 (i.e., Ti3+/[Ti3+ + Ti4+ = 0.19]). This pyroxene has relatively low atomic Ti/Al ratios (<0.4) due to crystallization before plagioclase, contrary to the long-held belief that lunar pyroxene with Ti/Al > 0.5 contains Ti3+ and pyroxene with lower ratios does not. Later pyroxene, with lower Mg/Fe and higher Ti/Al ratios, has higher proportions of Ti (all Ti4+) in tetrahedral sites. All pyroxene analyzed contains divalent Cr, ranging from 15 to 30% of the Cr present, and all but one analysis spot contains divalent V, accounting for 0 to 40% (typically 20–30%) of the V present. Three analyses of olivine in 14053 do not show any Ti3+, but Ti valences in 14072 olivine range from 4 down to 3.70 ± 0.10. In 14053 olivine, ~50% of the Cr and 60% of the V are divalent. In 14072 olivine, the divalent percentages are ~20% for Cr and 20–60% for V. These results indicate significant proportions of divalent Cr and V and limited amounts of trivalent Ti in the parental melts, especially when crystal/liquid partitioning preferences are taken into account. These features are consistent with an fO2 closer to IW − 2 than to IW − 1. Apollo 15 basalt 15555, analyzed for comparison with A-14 materials, has olivine with strongly reduced Cr (Cr2+/(Cr2+ + Cr3+) ~0.9). Basalts from different sites may record redox differences between source regions.