HR: 1340h
AN: DI53A-1094    [Abstracts]
TI: Revelations From S660S: New Interpretations of Topography on the 660 km Discontinuity
AU: * Houser, C
EM: creif@pmc.ucsc.edu
AF: University of Califonia Santa Cruz, Earth and Planetary Sciences 1156 High Street, Santa Cruz, CA 95064, United States
AU: Williams, Q
EM: qwilliams@pmc.ucsc.edu
AF: University of Califonia Santa Cruz, Earth and Planetary Sciences 1156 High Street, Santa Cruz, CA 95064, United States
AB: Currently, the only phase capable of mapping the 660 km seismic discontinuity globally is \textsl{S660S}, the shear phase that reflects off of the 660 km discontinuity and arrives as a precursor to \textsl{SS}. The topography of the 660 km discontinuity is an essential element in understanding the dynamics of flow between the upper and lower mantle. Both experimental and theoretical mineral physics studies find that the phase transition of γ-spinel to perovskite plus magnesiowustite occurs at approximately the depth of the observed \textsl{S660S} reflections. While the positive Clapeyron slope (~3 MPa/K) of the olivine to β-spinel transition responsible for the 410 km discontinuity is well constrained, the magnitude and even the sign of the γ-spinel to perovskite plus magnesiowüstite transition are still debated. It is generally accepted that the Clapeyron slope of the γ-spinel to perovskite plus magnesiowustite transition is negative with a similar magnitude to that of the olivine transition, although some studies indicate that it may be very small and perhaps barely positive (Bina and Helffrich, 1994). Therefore, the transition zone is predicted to thicken in cold regions such as subducting slabs and to thin in warm regions such as upwelling plumes. The mapping of \textsl{SS} precursors by Houser \textsl{et al.} (2007) reveals that the 410 and 660 km discontinuities are correlated on a global scale. For instance, the data indicate that the thin transition zone under the Pacific (a region of high data quality) is produced by a large depression of the 410 km discontinuity accompanied by a smaller depression of the 660 km discontinuity. If temperature anomalies are vertically continuous across the transition zone, then this correlation implies that both transitions have positive Clapeyron slopes. At temperatures above ~1800°C in aluminum-bearing pyrolite, γ-spinel transforms to majorite within the transition zone. Subsequently, the majorite to perovskite transition has a positive Clapeyron slope and occurs at depths of approximately 660 km (Weidner and Wang, 1998; Hirose, 2002). Furthermore, the phase loop of the majorite-perovskite transition narrows substantially above 1800°C, thus enhancing the sharpness of the transition (Hirose, 2002). Accordingly, our observations of \textsl{S660S} in these regions where it is correlated with \textsl{S410S}, are most readily explained by the majorite to perovskite transition producing the seismic discontinuity near 660 km depth. Thus, the correlation between these discontinuities indicates that transition zone thinning is a consequence of anomalously hot material at depths spanning from 410 to 670 km and aluminum contents that are compatible with those of fertile peridotite.
DE: 3621 Mantle processes (1038)
DE: 7203 Body waves
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting