V21F-01 INVITED
Bowen Lecture: Physical and Chemical Properties of Melts Under Deep Earth Conditions and Their Importance in Geodynamics
Physical and chemical properties of melts at high pressure are the essential factors controlling geodynamics. One of the major subjects on the melt properties is the partitioning behavior, i.e., element partitioning among silicate melts, metallic melts, and minerals, which played crucial roles in fractionation in the magma ocean and core formation stages, and determined the chemical compositions of the mantle and core. Our recent studies on element partitioning between metallic liquid and lower mantle minerals revealed that the terrestrial magma ocean was extended to the deep lower mantle [1, 2]. The density crossover between magma and crystals in the deep mantle is also an interesting phenomenon which played an essential role in solidification of the primordial magma ocean and the deep seated magma generation processes [3,4] since magmas are extremely compressible associated with their structural change compared to crystals. The density crossover between peridotite magmas and equilibrium olivine was observed at around 9.5 GPa in Martian mantle [5] and at 13 GPa [6] in the Earth's mantle. Thus, neutral buoyancy of olivine occurs in the primordial magma ocean in the early planets and effective separation of olivine could not occur in the magma oceans producing an olivine enriched upper mantle in the magma ocean stage. The deep mantle melt is also important in the present Earth both at the bottoms of the upper and lower mantles. Seismological studies revealed that there is a low velocity and low Q zone at the base of the upper mantle suggesting existence of a partial molten region at this depth [7,8]. Existence of the ultra-low velocity zone at the base of the lower mantle has also been established seismologically [9]. We determined the density of hydrous magma and carbonated magma by the sink-float method using diamond as a density marker, and determined the partial molar volumes of H2O and CO2 in magmas up to 20 GPa [10,11]. The result implies that a density crossover exists between the mantle and hydrous or carbonated magmas containing H2O or CO2 up to about 2-5 wt percent. The volatile rich magmas could be gravitationally stable at the base of the upper mantle, and can explain the low seismic velocity and low Q regions observed at this depth (e.g., [7]). The base of the lower mantle is also a possible region of accumulation of dense magmas. The origin of the ultra-low velocity zone has been interpreted as existence of dense magmas [4, 8]. We showed closure of the liquid immiscibility gaps in the FeO- Fe and FeO-FeS-Fe systems at high pressure [12]. Thus we may expect dissolution of metallic Fe component into magmas at the core-mantle boundary, producing dense magmas at the base of the lower mantle. References: [1] Kawazoe T and Ohtani E, Phys. Chem. Minerals, DOI 10.1007/s00269-006-0071-4. [2] Sakai T et al. GRL, 33, doi: 10.1029/2006GL026868. [3] Stolper EM et al., JGR, 86, 6261, 1982. [4] Ohtani E, PEPI, 33, 12-25, 1983. [5] Ohtani E et al., Proc. of Jpn Acad, 96, ser. B, 23-28, 1993. [6] Suzuki A and E. Ohtani E, Phys. Chem. Minerals., 30: 449-456 , 2003. [7] Song TR et al. Nature, 427, 530-533, 2004. [8] Bercovici G and Karato S, Nature, 425, 39-44, 2003. [9] Garnero EJ, Science, 304, 834, 2004. [10] Sakamaki T et al., Nature, Vol.439, 192-194,2006. [11] Ghosh S et al., GRL., in review 2007. [12] Tsuno Q et al., PEPI, 160, 75-85, 2006.
V21F-02 INVITED
Bowen Lecture: The origin of the Moon and the early history of the Earth revisited
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.