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
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.