HR: 0800h
AN: T21C-0519    [Abstracts]
TI: Effect Of Phase Transitions On Seismic Properties Of A Metapelite
AU: * Zappone, A S
EM: alba@sed.ethz.ch
AF: Schweiz. Erdbebendienst (SED), Schafmattstr. 30, ETH H”nggerberg, Zuerich, 8093 Switzerland
AU: * Zappone, A S
EM: alba@sed.ethz.ch
AF: CNR-IDPA, Via Botticelli, 23, Milano, 20133 Italy
AU: Burlini, L
EM: burlini@erdw.ethz.ch
AF: Geological Institute, ETH, Sonneggestrasse, 5, Zuerich, 8092 Switzerland
AU: Connolly, J A
EM: connolly@erdw.ethz.ch
AF: Geological Institute, ETH, Sonneggestrasse, 5, Zuerich, 8092 Switzerland
AB: Seismic properties of an upper amphibolite facies metapelite sample from the Orobic basement (Southern Alps, N-Italy) were measured on an internally heated gas apparatus (Paterson rig) under hydrostatic conditions at pressure up to 500 MPa, and temperature up to 1020 K. The rock is composed of quartz, plagioclase, biotite and muscovite; minor chlorite is found in the pressure shadows of porphyroclasts or as crack-filling. Traces of sillimanite are also present. Vp and Vs were measured at room temperature parallel and normal to lineation and foliation, Average Vp at 400 MPa is 6.4 km/s and Vp anisotropy is 12%. Average Vs1 is 3.55 km/s (8.5% anisotropy), and Vs2 is 3.45 km/s (2.9% anisotropy); shear waves splitting is 0.2 km/s. These are typical values for upper amphibolite facies metapelitic rocks. To observe the effects of the alpha-beta quartz transition under perfectly hydrostatic conditions and, possibly, of muscovite dehydration on seismic velocity, velocities were monitored as the sample was heated to 1020K. At 400 MPa, Vp decreases monotonically with temperature to 970 K, at higher temperatures Vp increases, very rapidly at first, and then slowly. Since this is a perfectly reversible effect, we interpret it to be a consequence of the alpha-beta transition in quartz, in agreement with previous studies. The continuous increase of Vp at elevated temperatures (to 1020K) may reflect the dehydration of muscovite + quartz to K-feldspar + sillimanite. Indeed, after recovering the sample, we observed the presence of water in the assembly and newly formed K-feldspar and sillimanite. The alpha-beta transition in quartz is almost instantaneous in both directions (increasing and decreasing temperature), whilst the dehydration reaction is kinetically hindered since quartz and muscovite remain present in the recovered sample. During the phase transition the velocity increases almost 10 %.
DE: 5102 Acoustic properties
DE: 8020 Mechanics, theory, and modeling
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005