Edward R.D. Scott^a, Tatiana V. Krot^a, Joseph I. Goldstein^b and Shigeru Wakita^a,c

^aHawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA
^bDept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA
^cCenter for Computational Astrophysics, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

We have studied cloudy taenite, metallographic cooling rates, and shock effects in 30 H3-6 chondrites to elucidate the thermal and early impact history of the H chondrite parent body. We focused on H chondrites with old Ar-Ar ages (>4.4 Gyr) and unshocked and mildly shocked H chondrites, as strongly shocked chondrites with such old ages are very rare. Cooling rates for most H chondrites at 500 °C are 10-50 °C/Myr and do not decrease systematically with increasing petrologic type as predicted by the onion-shell model in which types 3 to 5 are arranged in concentric layers around a type 6 core. Some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types, contrary to the onion-shell model. Cloudy taenite particle sizes, which range from 40 to 120 nm, are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating. The three H4 chondrites that were used to develop the onion-shell model, Ste. Marguerite, Beaver Creek, and Forest Vale, cooled through 500 °C at ⩾5000 °C/Myr. Our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by ²⁶Al and cooled without disturbance by impact. Published Ar-Ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism. Metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating. Impacts during metamorphism created Portales Valley and two other H6 chondrites with large metallic veins, excavated the fast-cooled H4 chondrites around 3-4 Myr after accretion, and mixed petrologic types. Metallographic data do not require catastrophic disruption by impact during cooling.

Reference
Scott ERD, Krota TV, Goldstein JI and Shigeru Wakita S (in press) Thermal and Impact History of the H Chondrite Parent Asteroid during Metamorphism: Constraints from Metallic Fe-Ni. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.038]
Copyright Elsevier

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Chi Ma¹ and Alexander N. Krot²

¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mänoa, Honolulu, Hawai’i 96822, U.S.A.

Hutcheonite (IMA 2013-029), Ca₃Ti₂(SiAl₂)O₁₂, is a new garnet mineral that occurs with monticellite, grossular, and wadalite in secondary alteration areas along some cracks between primary melilite, spinel, and Ti,Al-diopside in a Type B1 Fractionation and Unidentified Nuclear effects (FUN) Ca-Al-rich inclusion (CAI) Egg-3 from the Allende CV (Vigarano type) carbonaceous chondrite. The mean chemical composition of type hutcheonite by electron probe microanalysis is (wt%) CaO 34.6, TiO₂ 25.3, SiO₂ 20.9, Al₂O₃ 15.7, MgO 2.1, FeO 0.7, V₂O₃ 0.5, total 99.8, giving rise to an empirical formula of Ca_2.99(Ti⁴⁺_1.53Mg_0.25Al_0.17Fe²⁺_0.05V³⁺_0.03)(Si_1.68Al_1.32)O₁₂. The end-member formula is Ca₃Ti₂(SiAl₂)O₁₂. Hutcheonite has the Ia3̄d garnet structure with a = 11.843 Å, V = 1661.06 ų, and Z = 8, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.86 g/cm³. Hutcheonite is a new secondary phase in Allende, apparently formed by iron-alkali-halogen metasomatic alteration of the primary CAI phases like melilite, perovskite, and Ti,Al-diopside on the CV chondrite parent asteroid. Formation of the secondary Ti-rich minerals like hutcheonite during the metasomatic alteration of the Allende CAIs suggests some mobility of Ti during the alteration. The mineral name is in honor of Ian D. Hutcheon, a cosmochemist at Lawrence Livermore National Laboratory, California, U.S.A.

Reference
Ma C and Krot AN (2014) Hutcheonite, Ca3Ti2(SiAl2)O12, a new garnet mineral from the Allende meteorite: An alteration phase in a Ca-Al-rich inclusion. American Mineralogist 99:667.
[doi:10.2138/am.2014.4761]
Copyright: The Mineralogical Society of America

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Sabrina Ferrari^1,3, Fabrizio Nestola¹, Matteo Massironi^1,2, Alessandro Maturilli³, Jörn Helbert³, Matteo Alvaro^1,4, M. Chiara Domeneghetti⁵ and Federico Zorzi¹

¹Department of Geosciences, University of Padua, Via G. Gradenigo 6, 35131 Padova, Italy
²Astronomical Observatory of Padua, INAF, Vicolo Osservatorio 5, 35122 Padova, Italy
³Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin-Adlershof, Germany
⁴IRSPS, G. D’Annunzio University, Via Pindaro 42, 65127 Pescara, Italy
⁵Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy

This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo.
Two C2/c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively.
The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/(Mg + Fe²⁺) atomic ratio].
The analysis of the thermal behavior gives similar thermal expansion volume coefficients, α_V, for the Mg-rich and Fe-rich samples, with α_V = 2.72(8) and 2.72(7) × 10^-5 K^-1, respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples.
Our data suggest that MERTIS spectra will be able to provide indications ofC2/c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Reference
Ferrari S, Nestola F, Massironi M, Maturilli A, Helbert J, Alvaro M, Domeneghetti MC and Zorzi F (2014) In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury. American Mineralogist 99:786.
[doi:10.2138/am.2014.4698]
Copyright: The Mineralogical Society of America

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