HR: 14:40h
AN: V32E-05    [PDF]
TI: What do we Really Know About the Elastic Properties of Garnets at Upper Mantle Pressures?
AU: Jiang, F
EM: fumingj@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AU: Speziale, S
EM: speziale@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AU: * Duffy, T S
EM: duffy@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AB: Garnets are a major constituent of the Earth's upper mantle. Detailed understanding of their elastic properties is essential for interpretation of seismic velocity profiles of the mantle. Garnets exhibit a wide compositional range and the elasticity of garnets have been studied far more extensively than any other mineral group. At ambient pressure, there is good agreement in both individual and aggregate elastic properties among various studies using different experimental techniques. At high pressures, however, there are drastic disagreements (up to 50% or more) in reported pressure derivatives of the bulk and shear modulus for a given composition. The magnitude of this effect completely precludes any assessment of high-pressure compositional effects using the current data set, and introduces considerable uncertainty in interpretation of features such as the transition zone seismic velocity gradient. To address these problems, we have carried out detailed single-crystal elasticity measurements on andradite (Ca$_3$Fe$_2$Si$_3$O$_{12}$), grossular (Ca$_3$Al$_2$Si$_3$O$_{12}$), and almandine-pyrope ((Fe,Mg)$_3$Al$_2$Si$_3$O$_{12}$) garnets to pressures in excess of 11 GPa by Brillouin spectroscopy. In these studies we have paid careful attention to sources of systematic error (e.g., vignetting), compositional heterogeneity, and have maintained strictly hydrostatic conditions in the diamond anvil cell. For the andradite-rich garnet (An98Gr2) we obtain K = 155 GPa, K' = 4.7, G = 90 GPa, and G' = 1.3. For the grossular-rich garnet (Gr87An9Py2Alm2), we obtain K = 165 GPa, K' = 3.8, G = 104 GPa, G' = 1.1. For the almandine-pyrope garnet (Alm72Py20Sp3Gr3An2), we obtain K = 175 GPa, K' = 4.7, G = 96 GPa, G' = 1.4. Compression curves derived from our results are generally consistent with static data under hydrostatic conditions, and the effects of non-hydrostaticity on previous static compression data can be identified. Our results allow us to assess the effects at high pressure of Fe-Al substitution on the octahedral site in andradite-grossular and in combination with earlier Brillouin scattering data for pyrope (Sinogeiken and Bass, 2000), the effect of Fe-Mg substitution on the dodecahedral site in pyrope-almandine. This new data set allows us to place improved constraints on the effect of chemical variation on seismic wave velocities at upper mantle pressures.
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 7207 Core and mantle
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting