Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa

Abstract

Meteorites provide a sample of Solar System bodies and so constrain the types of objects that have collided with Earth over time. Meteorites analysed to date, however, are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time1. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth2,3. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements4. Here we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant (> 70 km diameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth.

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Figure 1: Macroscopic and microscopic images of the Morokweng meteorite.
Figure 2: Bulk chemical data for the Morokweng meteorite.
Figure 3: Mineral compositional data for the Morokweng meteorite.

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Acknowledgements

This work was logistically supported by Business Venture Investments No. 33 (Pty) Ltd. (M.A.G.A). Financial support was provided by S.-J. Barnes, Canada Research Chair in Magmatic Ore Deposits, UQAC. In addition, analytical grants were provided by the University of Pretoria (W.D.M.), and by NASA (A.S. and G.W.L.). ICP analyses at Cardiff University were supported by the Leverhulme Trust (I.McD.) and by a JIF award from the NERC. A.J.B. is supported by NERC Services and Facilities funding to the Isotope Community Support Facility at SUERC. We thank H. Gregoire for assistance in producing the EMS images, and R. Ash and R. Burgess for comments on an early version of the manuscript. Author Contributions All co-authors contributed in the form of data generation and/or discussion and critical comment.

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Correspondence to W. D. Maier.

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Supplementary information

Supplementary Table 1

Whole rock data for Morokweng meteorite. (XLS 20 kb)

Supplementary Table 2

Composition of olivines, Morokweng meteorite. (XLS 42 kb)

Supplementary Table 3

Composition of orthopyroxenes, Morokweng meteorite. (XLS 38 kb)

Supplementary Table 4

Composition of plagioclase, Morokweng meteorite (XLS 20 kb)

Supplementary Table 5

Chromite compositions: Morokweng meteorite (XLS 16 kb)

Supplementary Table 6

Composition of pyrrhotite and pentlandite in Morokweng meteorite. (XLS 17 kb)

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Maier, W., Andreoli, M., McDonald, I. et al. Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa. Nature 441, 203–206 (2006). https://doi.org/10.1038/nature04751

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