HR: 14:10h
AN: V32E-03    [PDF]
TI: Seismic mantle discontinuities and mineral physics
AU: Woodhouse, J H
EM: john.woodhouse@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AU: * Deuss, A
EM: arwen.deuss@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AB: Seismologists have been successful in measuring Clapeyron slopes for the transition zone discontinuities at 410 and 660 km depths; these are in approximate agreement with phase transitions in olivine. Clapeyron slopes are key to explaining the seismic discontinuities as either manifestations of phase transitions or of other mineral physical processes. Such observations place constraints on the thermal, compositional and physical state of the Earth's mantle and provide critical information able to distinguish between competing hypotheses concerning the upper mantle and transition zone reflectors. Here we extend the measurement of Clapeyron slopes to upper mantle discontinuities, in particular the Lehmann discontinuity at approximately 220 km depth, and seek to interpret the results in terms of mineral physics. We find that the Lehmann discontinuity is characterized by a regionally varying negative seismological Clapeyron slope. Reflections from greater depths (250-350 km) are required to possess a large negative seismological Clapeyron slope. In seeking mineralogical explanations of the Lehmann and X-discontinuities, we can reject hypotheses in disagreement with these observations. Of hypotheses proposed in the literature to account for the Lehmann discontinuity, only a transition in deformation mechanism from dislocation to diffusion creep has the required behavior. In the case of the X-discontinuity we are not aware of mechanisms having the observed characteristics. In addition, we show detailed seismic observations of the weak transition zone discontinuity at 520 km depth. In many regions we have confirmed the existence of a discontinuity at 520 km depth, but there are a number of regions in which we found two discontinuities at about 500 and 560 km depth, an effect which can be interpreted as `splitting' of the discontinuity. These observations cannot be explained as the single effect of the olivine transition from the $\beta$-phase to the $\gamma$-phase, and the non-olivine transition from garnet to Ca-rich perovskite needs to be considered as well. Local variations in minor phases containing Fe, Ca and H$_2$0 will probably also be needed to explain the regional variability in the seismic observations. This shows that mantle transition zone is less homogeneous than is generally assumed.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting