Yoshihiro HIDAKA^1,4, Naoki SHIRAI¹, Akira YAMAGUCHI^2,3, and Mitsuru EBIHARA^1,4
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13273]
¹Department of Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, Japan
²National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
³Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for AdvancedStudies), Tokyo 190-8518, Japan
⁴Present address: Department of Earth Sciences, Waseda University, Tokyo 169-8050, Japan
Published by arrangement with John Wiley & Sons

We have studied magnetic fractions of five acapulcoites, three lodranites, and two winonaites to investigate chemical compositions of their precursor materials and metallic partial melting processes occurring on their parent bodies. One winonaite metal is similar in composition to low Au, low Ni IAB iron subgroup, indicating genetic relationship between them. Magnetic fractions of chondrule‐bearing acapulcoite and winonaite have intermediate chemical compositions of metals between H chondrites and EL chondrites. This fact indicates that the precursor materials of acapulcoite–lodranites and winonaites were similar to H and/or EL chondrites in chemical compositions. Magnetic fractions in acapulcoite–lodranites have a large variety of chemical compositions. Most of them show enrichments of W, Re, Ir, Pt, Mo, and Rh, and one of them shows clear depletion in Re and Ir relative to those of chondrule‐bearing acapulcoite. Chemical compositional variations among acapulcoite–lodranite metals cannot be explained by a single Fe‐Ni‐S partial melting event, but a two‐step partial melting model can explain it.

C. A. LORENZ¹, A. A. SHIRYAEV^2,3, I. I. VLASOV⁴, and S. E. BORISOVSKY³
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13274]
¹1Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Kosygin St. 19, 119999 Moscow, Russia
²A. N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31 korp. 4, 119071, Moscow, Russia
³Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Staromonetnyi per, 35, 119017,Moscow, Russia
⁴A.M. Prokhorov General Physics Institute RAS, Vavilova St., 38, 119991, Moscow, Russia
Published by arrangement with John Wiley & Sons

Four ureilites subjected to impact metamorphism in a pressure range of ~15–100 GPa were investigated for mineralogical and petrological features and optical luminescence of their diamonds with the aim to understand how properties of ureilitic diamonds are correlated with shock and thermal histories of the host meteorite. Petrological data show that all the investigated ureilites experienced multistage metamorphic histories. Some of them were shocked at least twice or/and underwent high‐temperature thermal metamorphism and fluid metasomatism in the parent body interior. Photoluminescence spectra of individual diamond grains reveal the presence of neutral and negatively charged nitrogen‐vacancy (NV⁰ and NV⁻, respectively) and H3 (two nitrogens and a vacancy) defects, indicating relatively high nitrogen contents of the diamonds and some degree of thermal annealing of the grains. The diamond grain size and morphology, a texture of graphite‐diamond aggregates, and spectroscopic properties of the diamond phase vary widely both within an individual meteorite and between the ureilites. Shock‐driven transformation of sp2‐C into diamond provides the most natural explanation of the observed spectroscopic diversity of the diamond grains if one takes into account strong dependence of the PT parameters and efficiency of the transformation on structure of the carbonaceous precursor.

Jing Yang^a, Chi Zhang^a, Masaaki Miyahara^a, Xu Tang^a, LixinGu^a, Yangting Lin^a,c
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aKey Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
^bDepartment of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan
^cCollege of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

The ungrouped achondrite NWA 7325 is a cumulate olivine gabbro (Irving et al., 2013). It contains abundant and unique micro-inclusions of Ca-pyroxene (Bischoff et al., 2013) and spinel-like Al-Mg oxide (Goodrich et al., 2017) in plagioclase, indicating a remelting event induced either by impact (Goodrich et al., 2017) or by magma intrusion (Bischoff et al., 2013, Weber et al., 2016). In this work, a combined FIB-TEM study has been conducted on these micro-inclusions to address their petrogenesis and the related history of NWA 7325. TEM study revealed that micro-inclusions in the interiors of large plagioclase grains are Al-enriched spinel (Mg/Al atomic ratio: 0.03-0.4) with minor needle-like α-corundum, whereas those in the margins are predominantly Al-rich diopside (En_44.5-46.6Fs_1.2-1.5Wo_31.2-36.7CaTs_17.6-22.4) with minor forsterite (Fo_94.6-94.7). The Mg/Al atomic ratios of the spinel micro-inclusions are negatively correlated with the distance away from the interface of plagioclase-pyroxene. Large plagioclase grains also exhibit a decrease in the Mg/Al atomic ratio from the rims to the cores. Based on the reaction texture at the interfaces of plagioclase-pyroxene, we infer that the Mg concentration gradient in large plagioclase grains could have resulted from Mg diffusion from the remelted rims of pyroxene into plagioclase. In addition, TEM observations showed that large plagioclase grains are not single crystals, but assemblages of submicron to micron-sized crystals. The preservation of Mg concentration gradients, submicron-sized polycrystalline plagioclases, and the consistent presence of micro-inclusions within large plagioclase grains likely indicate complete remelting of plagioclase and partial remelting of pyroxene (only rims of pyroxene with plagioclase) followed by fast cooling. We propose that micro-inclusions of diopside, forsterite, Al-rich spinel and corundum crystallized from the melt, which developed a Mg concentration gradient during the remelting of NWA 7325.

The heating temperatures of pyroxene and plagioclase in the remelting event were estimated to be 1274-1327 °C and Σ1530 °C, respectively. A subsequent cooling rate of Σ500-650 °C/h at 1300 °C was found by fitting the measured Mg concentration gradient in large plagioclase grains with a Fick’s second law model that incorporated the diffusion coefficients of Mg in plagioclase-melt. These results are better explained by a shock event; a magmatic intrusion process is ruled out. To achieve the coexistence of shock-induced high temperature (Σ1274 °C) in-situmelting and only undulatory extinction of forsterite grains, an ambient temperature of 1000-1100 °C in the surrounding, parent rock of NWA 7325 was required prior to impact. This work suggests a very early shock event when NWA 7325 was hot and buried in the crust of its parental planetesimal, which is a scenario consistent with its magma crystallization age (∼4.3 Ma after CAIs, e.g., Koefoed et al., 2016). This work also implies that impacts are a potential heat source for melting hot planetesimals in the early Solar System.

^aLukáš Ackerman^a, RomanSkála^a, ŠárkaKřížová^a,b, KarelŽák^a, TomášMagna^c
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aInstitute of Geology, The Czech Academy of Sciences, Rozvojová 269, CZ-165 00 Prague 6 – Lysolaje, Czech Republic
^bInstitute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, CZ-128 43 Prague 2, Czech Republic
^cCzech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
Copyright Elsevier

Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and ¹⁸⁷Os/¹⁸⁸Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most analyzed Australasian tektites, combined with non-radiogenic ¹⁸⁷Os/¹⁸⁸Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.