^1,2He, Yuyang^3,4Zhou, You,⁵Wen, Tao,⁶Zhang, Shuang,⁷Huang, Fang,⁸Zou, Xinyu,⁹Ma, Xiaogang,¹⁰Zhu, Yueqin
Applied Geochemistry 140, 105273 Link to Article [DOI 10.1016/j.apgeochem.2022.105273]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
²State Key Laboratory of High Temperature Gas Dynamics, Institute of Mechanics, Chinese Academy of Sciences, Beijing, 100190, China
³International Research Center for Planetary Science, College of Earth Sciences, Chengdu University of Technology, Chengdu, 61005, China
⁴CAS Center for Excellence in Comparative Planetology, Hefei, 230026, China
⁵Department of Earth and Environmental Sciences, Syracuse University, Syracuse, 13244, NY, United States
⁶Department of Oceanography, Texas A&M University, College Station, 77843, TX, United States
⁷CSIRO Mineral Resources, Kensington, 6151, WA, Australia
⁸Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
⁹Computer Science Department, University of Idaho, 875 Perimeter Drive, MS 1010, Moscow, 83844-1010, ID, United States
¹⁰National Institute of Natural Hazards, Ministry of Emergency Management of the People’s Republic of China, Beijing, 100085, China

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¹M.M.Hirschmann
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.04.005]
¹Dept. of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455 USA
Copyright Elsevier

The mantles of both Earth and Mars are more oxidized than would be expected based on low pressure equilibration of molten silicate and alloy during their magma ocean stages. High pressure silicate-alloy equilibration in a magma ocean can produce appreciable ferric iron in the silicate, leading to comparatively oxidized near surface conditions and overlying atmospheres. Upon crystallization, this may feasibly be sufficient to account for oxygen fugacities prevailing in basalt source regions of Earth and Mars. Experiments and first principles studies affirm that Fe3+ is stabilized at high pressure, but to date there has been no model that accounts accurately for the combined effects of melt composition, temperature, pressure, and oxygen fugacity on magma ocean Fe3+/FeT. We calibrate a new model for Fe3+/FeT as a function of temperature, pressure, melt composition, and fO2 which reproduces Fe3+/FeT for experimental peridotite liquids and which incorporates differences in FeO and Fe2O3 liquid heat capacities into a potentially realistic temperature function. For the effects of pressure, two versions of the model are implemented based on recent equations of state (EOS), though only the EOS of Deng et al. (2020) is applicable to pressures relevant to metal-silicate equilibration in a deep terrestrial magma ocean. For Earth, metal-silicate equilibration at 28-53 GPa, 2300-4100 K, and fO2 set by plausible mantle and core compositions produces Fe3+/FeT between 0.034 and 0.10, with variation mostly owing to differences in assumed temperatures. For Mars, different proposed mantle compositions produce Fe3+/FeT ratios that range from 0.026 for FeO* of 13.5 wt.% up to 0.038 for FeO* of 18.1 wt.%.

Although significant Fe3+ may be present in magma oceans owing to high pressure equilibration with alloy, the budget of Fe2O3 in crystallized mantles is expected to be modified from that in the molten state. An important additional factor is the influence of Cr, which is Cr2+ in molten silicate equilibrated with alloy and Cr3+ in terrestrial upper mantles. Production of Cr3+ and Fe2+ by reaction with Cr2+ and Fe3+ during crystallization can destroy much of the Fe2O3 present during the magma ocean stage. Considering the stability of Cr2+ in olivine and the temperature-dependent partitioning of Cr3+ between mantle silicates, we construct an empirical model for the fraction of Cr that is Cr2O3 in solid spinel peridotite as a function of temperature and fO2. For Earth, at least 0.35 wt.% Fe2O3 is destroyed by oxidation of magma ocean CrO and for Mars, more than 0.55 wt.% Fe2O3 should be destroyed. Consequently, either the terrestrial and martian magma oceans were significantly more enriched in Fe2O3 than their present-day upper mantles or other processes contributed to oxidation of the latter. Over-enrichment of Fe2O3 in the magma oceans is plausible only if terrestrial metal-silicate equilibration occurred above 3300 K and if the martian mantle contains >17 wt.% FeO*. Subsolidus disproportionation of ferrous iron may have contributed to the present-day redox state of the Earth’s mantle, and late accretion of chondrite-like material and hydrogen degassing also likely affected the solidified mantles of both Earth and Mars.

¹Ray D.,¹Panda D.K.,¹Arora G.,^bGhosh S.,³Murty S.V.S.,⁴Chakraborty S.
Journal of the Geological Society of India 98, 323-328 Link to Article [DOI 10.1007/s12594-022-1983-4]
¹Physical Research Laboratory, Ahmedabad, 380 009, India
²54/3 M.B. Road, Kolkata, 700 072, India
³Lad Society Road, Ahmedabad, 380 015, India
⁴Department of Chemistry, University of California, Urey Hall 5112, San Diego, La Jolla, 92093-0356, United States

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¹S.E.Suarez,¹T.J.Lapen¹M.Righter,²B.L.Beard,³A.J.Irving
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.04.020]
¹Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204–5007, USA
²Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706–1692, USA
³Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195–1310, USA
Copyright: Elsevier

The Tissint strewn field has produced over 16 kg of material that has minimal terrestrial weathering and/or contamination. Tissint, along with 16 other incompatible trace element (ITE)-depleted shergottite specimens with igneous crystallization ages spanning 327 to 2403 Ma, were ejected together from Mars 1.1 m.y. ago. Despite the geochemical similarities of fragments from the Tissint strewn field, there are reported discrepancies in age determinations from different fragments that raise the possibility that the strewn field may be heterogenous. There are also questions about whether the shock ejection event incorporated martian soil components into impact glass, and the sources of radiogenic Sr and Pb that have been measured from leachate fractions in previous studies. An impact melt-rich fragment of Tissint was analyzed by LA-ICPMS for rare-earth element (REE) and highly siderophile element (HSE) concentrations and Pb isotopic compositions. Leachate and residues from 8 specimens representing separate individual fragments collected from the strewn field were analyzed for Rb-Sr. Unleached fractions of the 8 specimens were also analyzed for Sm-Nd and Lu-Hf.

The measured REE and HSE concentrations of impact melt glass and associated sulfide measured by LA-ICPMS are consistent with bulk rock compositions of Tissint and show no evidence for incorporation of more ITE-enriched martian surface components. Measured Pb isotopic compositions confirm that the impact melt glass and associated sulfide contain no evidence for incorporation of more radiogenic materials than the Pb compositions inherited from the primary magma. In situ Pb isotopic data from sulfide likely represents the most robust method for constraining initial Pb isotopic compositions of shergottites whereas approaches that rely on leaching and digestion may not remove all mineral and/or crack surface contaminants.

Rubidium-strontium analyses of the 8 Tissint specimens indicate that labile components hosting HCl-soluble Rb and Sr are not in isotopic equilibrium with the igneous assemblage and that the washed residues are in isotopic equilibrium with the igneous assemblage. The Sr isotopic compositions of the leachate are within the range of ‘more ITE-enriched’ depleted shergottites, perhaps indicating sources from the igneous pile on Mars. The radiogenic Sr component could represent crack and mineral surface coatings of volatilized materials derived from nearby depleted shergottite rock units during the impact ejection process but are not radiogenic enough to represent ITE-enriched crust or mantle components.

The Lu-Hf isotopic data from the specimens indicate no evidence of contamination or element mobility, whereas the Rb-Sr and Sm-Nd isotopic systems show evidence for element mobility and potential mixing with an isotopic component not in equilibrium with the igneous phases. The calculated ages using data compiled from Brennecka et al. (2014), and Grosshans (2013) for Lu-Hf, Rb-Sr, and Sm-Nd are 571 ± 84 Ma, 590 ± 49 Ma and 559 ± 39 Ma, respectively. These data indicate that the specimens analyzed here are cogenetic and the Tissint strewn field appears to be homogeneous.

^1,2Jangmi Han,³Ichiro Ohnishi,³Akira Yasuhara,²Lindsay P. Keller
American Mineralogist 107 873–884 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P0873.pdf]
¹Lunar and Planetary Institute, USRA, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
³JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo, 196-8558, Japan
Copyright: The Mineralogical Society of America

Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive
meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects
in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c*
in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and
energy-dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar
features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing,
and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm
(002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured
(“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block
such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is
RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite
Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution
EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on
tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R
blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show
Al-rich compositions compared to stoichiometric MgAl2O4 spinel. Therefore, Mg likely played the
major role in the formation and metastability of planar defects in hibonite. Electron energy loss spec-
troscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti
as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85%
of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled
substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite.
The formation of planar defects in hibonite occurred during high-temperature nebular condensa-
tion or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende
CAI deviates from the mineral formation sequence predicted from equilibrium condensation models.
Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal
growth during the high-temperature formation of refractory solids in the early solar nebula.