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.