^1,2Aidan J.Ross,^1,2,3,4Hilary Downes,⁵Jason S.Herrin,⁶David W.Mittlefehldt,⁷Munir Humayun,²Caroline Smith
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125539]
¹UCL/Birkbeck Centre for Planetary Sciences, University College London, Gower St, London, WC1E 6BT, UK
²Dept. of Earth Sciences, Natural History Museum, Cromwell Rd, London, SW7 5BD, UK
³Dept. of Earth and Planetary Sciences, Birkbeck University of London, Malet St., London, WC1E 7HX, UK
⁴Lunar and Planetary Institute/USRA, 3600 Bay Area Blvd, Houston, TX 77058, USA
⁵Earth Observatory of Singapore & Facility for Analysis, Characterisation, Testing, and Simulation, Nanyang Technological University, 639798, Singapore
⁶Astromaterials Research Office, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA
⁷Department of Earth, Ocean and Atmospheric, Science & National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA
Copyright Elsevier

Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni)₃Si, hapkeite (Fe₂Si) and xifengite (Fe₅Si₃). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241.

In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Si-free kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains.

On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe₂Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia.

^1,2,3Jangmi Han,⁴Benjamin Jacobsen,⁵Ming-Chang Liu,¹Adrian J.Brearley,⁴Jennifer E.Matzel,³Lindsay P.Keller
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125543]
¹Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA
²Lunar and Planetary Institute, USRA, 3600 Bay Area Boulevard, Houston, TX 77058, USA
³ARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA
⁴Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁵Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA
Copyright Elsevier

A coordinated mineralogical and oxygen isotopic study of four fine-grained calcium-, aluminum-rich inclusions (CAIs) from the ALHA77307 CO3.0 carbonaceous chondrite was conducted. Three of the inclusions studied, 05, 1-65, and 2-119, all have nodular structures that represent three major groups, melilite-rich, spinel-rich, and hibonite-rich, based on their primary core mineral assemblages. A condensation origin was inferred for these CAIs. However, the difference in their primary core mineralogy reflects unique nebular environments in which multiple gas-solid reactions occurred under disequilibrium conditions to form hibonite, spinel, and melilite with minor perovskite and Al,Ti-rich diopside. A common occurrence of a diopside rim on the CAIs records a widespread event that marks the end of their condensation as a result of isolation from a nebular gas. An exception is a rare inclusion 2-112 that contains euhedral spinel crystals embedded in melilite, suggesting this CAI had been re-melted. All of the fine-grained CAIs analyzed in ALHA77307 are uniformly ¹⁶O-rich with an average Δ¹⁷O value of ∼−22 ± 5‰ (2σ), indicating no apparent correlation between their textures and oxygen isotopic compositions. We therefore conclude that a prevalent ¹⁶O-rich gas reservoir existed in a region of the solar nebula where CO3 fine-grained CAIs formed, initially by condensation and then later, some of them were reprocessed by melting event(s).

¹A.J.King,¹K.J.H.Phillips,²S.Strekopytov,¹C.Vita-Finzi,¹S.S.Russell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.09.041]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, U.K
²Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London SW7 5BD, U.K
Copyright Elsevier

The rare CI carbonaceous chondrites are the most aqueously altered and chemically primitive meteorites but due to their porous nature and high abundance of volatile elements are susceptible to terrestrial weathering. The Ivuna meteorite, type specimen for the CI chondrites, is the largest twentieth-century CI fall and probably the CI chondrite least affected by terrestrial alteration that is available for study. The main mass of Ivuna (BM2008 M1) has been stored in a nitrogen atmosphere at least since its arrival at the Natural History Museum (NHM), London, in 2008 (70 years after its fall) and could be considered the most pristine CI chondrite stone. We report the mineralogy, petrography and bulk elemental composition of BM2008 M1 and a second Ivuna stone (BM1996 M4) stored in air within wooden cabinets. We find that both Ivuna stones are breccias consisting of multiple rounded, phyllosilicate-rich clasts that formed through aqueous alteration followed by impact processing. A polished thin section of BM2008 M1 analysed immediately after preparation was found to contain sulphate-bearing veins that formed when primary sulphides reacted with oxygen and atmospheric water. A section of BM1996 M4 lacked veins but had sulphate grains on the surface that formed in ≤6 years, ∼3 times faster than previous reports for CI chondrite sections. Differences in the extent of terrestrial alteration recorded by BM2008 M1 and BM1996 M4 probably reflect variations in the post-recovery curation history of the stones prior to entering the NHM collection, and indicate that where possible pristine samples of hydrated carbonaceous should be kept out of the terrestrial environment in a stable environment to avoid modification. The bulk elemental composition of the two Ivuna stones show some variability due to their heterogeneous nature but in general are similar to previous analyses of CI chondrites. We combine our elemental abundances with literature values to calculate a new average composition for the Ivuna meteorite, which we find is in good agreement with existing compilations of element compositions in the CI chondrites and the most recent solar photospheric abundances.