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| Titel: |
Lunar impact breccias: petrology, crater setting, and bombardment history of the Moon |
| Auteur: |
Norman, M. D. |
| Verschenen in: |
Australian journal of earth sciences |
| Paginering: | Jaargang 52 (2005) nr. 4-5 pagina's 711-723 |
| Jaar: |
2005 |
| Inhoud: |
Two general classes of lunar impact breccias have been recognised: fragmental breccias and melt breccias. Fragmental breccias are composed of clastic-rock debris in a finely comminuted grain-supported matrix of mineral and lithic fragments. Impact melt breccias have crystalline to glassy matrices that formed by cooling of a silicate melt. Most lunar impact breccias in our collection probably sample ejecta from large complex craters or multi-ring basins, although linking individual breccias to specific impact events has proven surprisingly difficult. A long-standing problem in lunar science has been distinguishing clast-poor impact melt breccias from igneous rocks produced by melting of the lunar interior. Concentrations and relative abundances of highly siderophile elements derived from the meteoritic impactor provide a useful discriminant, especially when combined with petrologic and geochemical evidence for mechanical mixing. Most lunar impact melt breccias have crystallisation ages of 4.0 - 3.8 Ga, corresponding to an episode of intensive crustal metamorphism recorded by whole-rock U - Pb isotopic compositions of lunar anorthosites. This may reflect a short-lived spike in the cratering rate, although other explanations are possible. The question of whether or not a cataclysmic bombardment struck the Earth and Moon at ca 3.9 Ga remains open and the subject of continuing investigations. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
Details van artikel 12 van 23 gevonden artikelen
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