HR: 12:05h
AN: T32B-08    [Abstracts]
TI: Deformation Microstructures of Garnet Peridotites From Norwegian Caledonides and Evidences for High Water Content in the Deep Upper Mantle
AU: * Katayama, I
EM: ikuo.katayama@yale.edu
AF: Department of Geology and Geophysics, Yale Univ, 210 Whitney Ave, New Haven, CT 06520 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Department of Geology and Geophysics, Yale Univ, 210 Whitney Ave, New Haven, CT 06520 United States
AU: Brandon, M
EM: mark.brandon@yale.edu
AF: Department of Geology and Geophysics, Yale Univ, 210 Whitney Ave, New Haven, CT 06520 United States
AB: Water has marked effects on both physical and chemical properties of minerals and rocks and hence the inference of distribution of water is an important subject for geological studies. Rocks carried from deep upper mantle may provide a clue as to the distribution of water in the deep mantle. However, the high diffusivity of water in minerals provides a challenge because water can be lost or added easily during their transport to the surface. Deformation microstructures have different response times for time-varying physical and chemical environment including water content. Here we attempt to read the history of deformation conditions of garnet peridotites from Norwegian Caledonides, which contain majoritic garnet, to reveal water content in the deep upper mantle. One of the peridotites from the Otroy island shows a olivine lattice-preferred orientation (LPO) with a maximum for the [001] subparallel to the stretching lineation and the (100) plane subparallel to the foliation. This pattern is similar to that found in olivine experimentally deformed at high water content ($>$1000 ppm H/Si) and relatively low stress (Jung and Karato, 2001). Spatial relationship between precipitates and subgrain boundaries in garnets indicates that the high-strain deformation occurred at high pressures ($>$5GPa). Dislocation microstructures in the olivine show well-defined glide loops on the (001) plane, suggesting the dominant slip system of [100](001). This is distinct from the observed olivine fabric, and this fabric was found at modest water content (200-1000 ppm H/Si) and low stress by our recent deformation experiments. Lattice-preferred orientation records conditions for long-term large strain deformation, whereas dislocation microstructures tend to reflect the conditions of the latest stage, short-term deformation (or annealing). Therefore, we concluded that large strain deformation with high water content ($>$1000 ppm H/Si) at deeper portion ($>$150 km) is followed by low strain deformation at lower water content in the shallower portions. These observations indicate that the deep upper mantle in this region had considerably higher water content than the upper mantle near mid-ocean ridges.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8045 Role of fluids
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting