Origin and age of the earliest Martian crust from meteorite NWA 7533
| Autor: | Roger H. Hewins, Christa Göpel, Sylvain Pont, Brigitte Zanda, Marion Grange, Munir Humayun, Damien Deldicque, Alexander A. Nemchin, Allen K. Kennedy, Jean-Pierre Lorand, C. Fieni |
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| Rok vydání: | 2013 |
| Předmět: | |
| Zdroj: | Nature. 503:513-516 |
| ISSN: | 1476-4687 0028-0836 |
| Popis: | Chemical analysis of the meteorite NWA 7533 indicates that it may be a Martian regolith breccia and, if so, that the crust of Mars may have formed in the first 100 million years of the planet’s history. This paper identifies the NWA 7533 meteorite from Northwest Africa as the first sample of Martian highlands rock in the meteorite collection. Munir Humayun and co-authors show that NWA 7533 has a composition indicative of a highlands breccia. It also contains zircons more than 4.4 billion years old, implying that early crustal differentiation on Mars occurred in the first 100 million years of its history, coeval with earliest crust formation on the Moon and the Earth. The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation1,2, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 70343) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory4, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon5 and the Earth6. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically2,7. |
| Databáze: | OpenAIRE |
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