HR: 09:30h
AN: V21F-02 INVITED    [Abstracts]
TI: Bowen Lecture: The origin of the Moon and the early history of the Earth revisited
AU: * O'Neill, H S
EM: hugh.oneill@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
AB: The last decade has seen a remarkable increase in our knowledge of the isotopic characteristics of solar system materials, including the planetary isotopic characteristics of the Moon, which can potentially place constraints on its origins. Several of the dominant paradigms of lunar geology have also been considerably revised, e.g., the volume of the crust. Our understanding of the metal-silicate partitioning relationships of the siderophile elements from experimental petrology has also improved, as has that of the oxidation states of the Earth's deep mantle. These advances make it timely to revisit the question of the compositional relationship between the Earth and the Moon and its implications for lunar origins. The currently widely adopted paradigm is that the Moon was formed by a giant impact in the latter stages of the planet-building epoch of the inner solar system, some tens of millions of years after the origin of the solar system, with most of the material forming the Moon originating in the impactor (`Theia'). The moon-forming event was accompanied by selective loss of volatile elements, and it is probable that the Moon has a small secondary metallic core, which, if present, must have depleted its silicate portion in the more siderophile elements (like Ni, Co, Cu and Mo, but not V, Cr, Mn, or, more arguably, W). It is likely that a `late veneer' was added subsequently to the Earth and presumably also to the Moon. Taking these modifications into account, it is remarkable how similar the chemistry of the Earth and Moon are. As for isotopes, not only does the Moon have exactly the same oxygen isotopic composition as the Earth [1], but also similar Si [2]. These similarities cannot be explained within current models of terrestrial planet formation by the proto-Earth and `Theia' both being derived at c. 1 AU, because such models predict that the latter stages see the Earth being built from material sourced over large heliocentric distances. The Hf-W systematics are similar [3]. Both the isotopic make- up of Cr and its depletion are similar, and distinctive, as are the depletions of V. The Earth and Moon share the same depletions of the slightly volatile elements Li and Mn, which depletions are considerably larger than in any other solar system planetary body. Unless such similarities are dismissed as a collection of unexplained coincidences, then the Moon and the Earth derive from a similar but not identical mixture of proto-Earth and impactor; the different proportions being reflected by their different FeO contents [4]. A possible mechanism is the turbulent mixing of material sourced from proto-Earth and impactor in the time between the giant impact and lunar accretion [5]. Several features of the Moon-Earth chemistry imply that `Theia' was an oxidized body [4]. Refs.: [1] Wiechert et al., Science 294, 345-348 (2001) [2] Georg et al., Nature 447, 1102-1106 (2007) [3] Touboul et al., Abstract 2385, Lunar Planet. Sci. XXXVIII (2007) [4] O'Neill, Geochim. Cosmochim. Acta 55, 1135-1157 [5] Pahlevan and Stevenson, Earth Planet. Sci. Lett., in press.
DE: 1025 Composition of the mantle
DE: 1026 Composition of the moon
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting