Siderophile element characteristics of acapulcoite–lodranites and winonaites: Implications for the early differentiation processes of their parent bodies

Yoshihiro HIDAKA1,4, Naoki SHIRAI1, Akira YAMAGUCHI2,3, and Mitsuru EBIHARA1,4
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.

Metamorphism of four desert ureilites and luminescence spectroscopy of defects in ureilitic diamonds

C. A. LORENZ1, A. A. SHIRYAEV2,3, 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.

Evidence for early impact on a hot differentiated planetesimal from Al-rich micro-inclusions in ungrouped achondrite Northwest Africa 7325

Jing Yanga, Chi Zhanga, Masaaki Miyaharaa, Xu Tanga, LixinGua, Yangting Lina,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 (En44.5-46.6Fs1.2-1.5Wo31.2-36.7CaTs17.6-22.4) with minor forsterite (Fo94.6-94.7). 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.

The quest for an extraterrestrial component in Muong Nong-type and splash-form Australasian tektites from Laos using highly siderophile elements and Re-Os isotope systematics

aLukáš Ackermana, RomanSkálaa, ŠárkaKřížováa,b, KarelŽáka, TomášMagnac
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.

 

Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite

Ke Zhu (朱柯)1,2, Jia Liu1, Frédéric Moynier2,3, Liping Qin1,4, Conel M. O’D. Alexander5, and Yongsheng He4
Astrophysical Journal 873, 82 Link to Article [DOI: 10.3847/1538-4357/aafe79 ]
1CAS Key Laboratory of Crust–Mantle Materials and Environment and CAS Center for Excellence in Comparative Planetology, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, People’s Republic of China
2Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS, 1 rue Jussieu, Paris F-75005, France
3Institut Universitaire de France, Paris, France
4State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, People’s Republic of China
5Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, Washington, DC 20015, USA

Chondrules are the main components of primitive meteorites and possibly the building blocks of planetary embryos and terrestrial planets. However, their ages and modes of formation are still highly debated. Here, we present high-precision Cr isotope data of nine chondrules from one of the more primitive chondrites, the CO3 chondrite Ornans. These chondrules define an external 53Mn–53Cr isochron, with an initial 53Mn/55Mn of (7.1 ± 1.6) × 10−6, corresponding to an age of 4567.6 ± 1.3 Ma when anchored to the angrite D’Orbigny (U-corrected). This age is within error of the age of formation of calcium-aluminum-rich inclusions (CAIs). All chondrules show a wide range of ε 54Cr values (+0.20 to +1.22) and a positive correlation between ε 53Cr and ε 54Cr values, suggesting mixing of different isotopic sources in the protoplanetary disk. This could reflect that silicate materials from the CAI-forming region (with complementary compositions to CAIs, i.e., low Mn/Cr and ε 54Cr) were transported to the accretion region of the CO chondrite parent body and mixed with CI-like material (high-Mn/Cr and ε 54Cr) during chondrule formation. Such mixing must have occurred prior to the formation of chondrule precursors. Furthermore, chondrules from chondrites with more CAIs (CV and CO) exhibit greater variability in ε 54Cr than chondrules from chondrites formed later with fewer CAIs (e.g., CB and CR), suggesting that the accretion regions of the former received more material transported from the inner solar system than the latter. This dichotomy may indicate the CB and CR chondrites accreted at greater orbital distances than other chondrites.

Condensation of SiC Stardust in CO Nova Outbursts

Maitrayee Bose1,2 and Sumner Starrfield1
Astrophysical Journal 873, 14 Link to Article [DOI: 10.3847/1538-4357/aafc2f ]
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
2Center for Isotope Analysis (CIA), Arizona State University.

This study on presolar grains compares high-precision isotopic compositions of individual SiC grains with low 12C/13C ratios, low 14N/15N ratios, large 30Si excesses, and high 26Al/27Al ratios, all available in the presolar grain database, to new CO nova models with white dwarf (WD) masses from 0.6 to 1.35 M . The models were designed to match the Large Binocular Telescope high-dispersion spectra acquired for nova V5668 Sgr. These CO nova models provide elemental abundances up to calcium and include mixing of WD material into the accreted material in a binary star system under several scenarios, including one where mixing occurs only after temperatures >7 × 107 K are achieved during a thermonuclear runaway (TNR). The 0.8–1.35 M simulations where 25% of the WD core matter mixes with 75% of the accreted material (assumed solar) from its binary companion after the TNR has begun provide the best fits to the measured isotopic data in four presolar grains. One grain matches the 50% accreted 50% solar 1.35 M simulation. For these five presolar grains, less than 25% of solar system material is required to be mixed with the CO nova ejecta to account for the grains’ compositions. Thus, our study reports evidence of pure CO nova ejecta material in meteorites. Finally, we speculate that SiC grains can form in the winds of cool and dense CO novae, where the criterion C > O may not be locally imposed, and thus nova winds can be chemically inhomogeneous.

How Adsorption Affects the Gas–Ice Partitioning of Organics Erupted from Enceladus

Alexis Bouquet1,2, Christopher R. Glein1, and J. Hunter Waite Jr.1,3
Astrophysical Journal 873, 28 Link to Article [DOI: 10.3847/1538-4357/ab0100 ]
1Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX, 78238, USA
2Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
3Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA

We study the effect of adsorption of volatile organic compounds (VOCs) in Enceladus’ geysers, both onto the ice grains ejected in the plumes, and onto the ice walls of the cracks connecting Enceladus’ internal ocean to its surface. We use a model of adsorption/desorption based on the Polanyi–Wiegner equation and experimental values of binding energies (energy of desorption E des) of the adsorbed compounds to water ice. We find that under conditions expected at Enceladus, the process of adsorption tends to ensure that the VOCs with the highest binding energy are over-represented on the ice surface, even if their abundance is comparatively lower than those of other compounds. We find that VOCs with E des ≤ 0.5 eV are insignificantly affected by adsorption while compounds with E des ≥ 0.7 eV are readily retained on the surface and compete to occupy most of the adsorption sites. We also deduce that ice grains falling back onto the surface are likely to retain most of the molecules adsorbed on their surface. The implication is that remote observation or sampling of the ice in the cracks or of the surface around it would show a mixture of VOCs that would not be representative of the gas phase of the plumes, with the high E des VOCs dominating the adsorbed phase.