HR: 17:00h
AN: T44B-01 INVITED     [Abstracts]
TI: Birch Lecture: Where on Earth is the Ocean?
AU: * Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06520 United States
AB: Hydrogen (water) is a unique element (substance). Although it is a minor element in solid Earth, it has marked influence on melting relationship and rheological properties of Earth materials and therefore has important influence on the dynamics and evolution of this planet. Since hydrogen (water) can change these properties so much it is often said that hydrogen (water) is a Maxwell's demon that can do whatever you wish it to do. It is critical, therefore, to find out how water is distributed in real Earth's mantle. Recent progress in high-pressure mineral physics has shown that a large amount of water (hydrogen), equivalent to more than 10 times of the ocean, CAN be present in Earth's mantle. However, ACTUAL amount of water or its distribution has been poorly constrained: the estimates of water content in the mantle transition zone, for example, range from nearly saturated ($\sim$2-3 wt$%$) to nearly water-free (less than 0.01 wt$%$). Here I report the latest results on mapping the radial and lateral distribution of water in Earth's upper mantle and transition zone through the combination of mineral physics studies with geophysical observations. Key mineral physics observations include: (1) the enhancement of electrical conductivity by hydrogen, (2) the modification of lattice-preferred orientation by hydrogen and (3) likely enhancement of anelasticity by hydrogen through enhanced dislocation motion and/or grain-boundary sliding. These mineral physics observations provide a {\it dictionary} to {\it read} geophysical and geological observations in terms of water content. Application of this approach shows that the distribution of hydrogen in Earth is heterogeneous both radially and laterally. The transition zone of the Earth's mantle ($\sim$410-660 km depth) has more than 10 times water concentration than that in the upper mantle, and the mantle in the subduction zone has larger but highly heterogeneous water content than the typical mantle beneath the ocean down to at least $\sim$400 km. Such water distribution indicates that (i) water is transported to deep upper mantle or deeper portions (transition zone or deeper), and that (ii) the upwelling of water-rich transition zone materials across the 410 km discontinuity results in dehydration melting that controls the distribution of trace elements including hydrogen (water) itself.
DE: 3904 Defects
DE: 1655 Water cycles (1836)
DE: 1025 Composition of the mantle
DE: 1212 Earth's interior--composition and state (8105)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting