Comparing chemical microstructures of some early solar system zircon from differentiated asteroids, mars and earth

Author(s): Julia Roszjar, Desmond E. Moser, Brendt C. Hyde, Chutimun Chanmuang, Kimberly Tait
Abstract: Zircon is a ubiquitous and highly refractory accessory phase in terrestrial rocks, and occurs in some extraterrestrial materials where its internal microstructure carries useful information on formation environments. We present a first step in comparing the internal zoning characteristics (i.e., chemical microstructures and degree of radiation damage) of some of the oldest known zircon grains derived from large differentiated asteroids, Mars and the early Earth spanning the time interval 4.55 Ga (eucrite) to 4.38 Ga (Earth). Electron microscopy (EPMA, SEM-CL, EBSD), Raman spectroscopy (v₃(SiO₄)), and photoluminescence (Dy³⁺) reveal a mix of ubiquitous and distinct characteristics in zircon from these different planetary sources. In the eucrite sample suite, representing a wide range of thermal and shock metamorphic conditions, igneous zoning features such as planar growth banding and sector zoning are often preserved but commonly modified by thermal metamorphism to produce rounded, sub- to anhedral grains sometimes with metamorphic rims. In contrast, the subhedral to anhedral crystal clasts in two samples of martian regolith breccia exhibit simple igneous zonation (e.g., concentric U, Th, and Yb zoning) and/ or mineral inclusions. Secondary rounding or fracturing of martian zircon can be explained by re-deposition through surface processes that differ from the billions of years of hydrous crustal metamorphic and erosion effects seen in a Jack Hills detrital zircon. Annealing history does not appear to be a clear discriminating property between different sources, as all zircon populations have Raman characteristics and reconstructed alpha doses indicating recovery of crystallinity at some point after formation. Down-shifted v₃(SiO₄) band positions may, however, distinguish martian grains and some eucrite zircon. At this point, the most promising suite of Solar System provenance discriminants includes internal zoning styles, specific Raman spectral properties, and chronological constraints.
Organisation(s): Department of Mineralogy and Crystallography
External organisation(s): Naturhistorisches Museum Wien (NHM), University of Western Ontario, Royal Ontario Museum
Pages: 113-135
No. of pages: 23
DOI: https://doi.org/10.1002/9781119227250.ch5
Publication date: 12-2017
Peer reviewed: Yes
Austrian Fields of Science 2012: 105127 Geochronology, 105116 Mineralogy
ASJC Scopus subject areas: General Earth and Planetary Sciences
Portal url: https://ucrisportal.univie.ac.at/en/publications/87a47e12-e594-4ca1-96e0-29a9f550b2fe

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