HR: 0830h
AN: S21E-0361    [PDF]
TI: The Effect of Iron Content on the High-Pressure Elasticity of Olivine: Implications for Chemical Heterogeneities in the Upper Mantle
AU: * Speziale, S
EM: speziale@Princeton.EDU
AF: Princeton University, Department of Geosciences Princeton University, Princeton, NJ 08544 United States
AU: Duffy, T S
EM: duffy@Princeton.EDU
AF: Princeton University, Department of Geosciences Princeton University, Princeton, NJ 08544 United States
AU: Angel, R J
EM: rangel@vt.edu
AF: Virginia Tech, Department of Geological Sciences, Blacksburg, VA 24061 United States
AB: Olivine is a major component in mineralogical models for the Earth's upper mantle (Ringwood, 1975; Duffy and Anderson, 1989). The thermo-elastic properties of olivines with compositions between Mg$_{2}$SiO$_{4}$ and (Mg$_{0.8}$Fe$_{0.2}$)$_{2}$SiO$_{4}$ have been the subject of extensive study, but this compositional range is too limited to reliably constrain the effect of Fe - Mg substitution on the elastic properties. The combined effect of Fe - Mg substitution in olivine and garnets is probably the most important factor to connect detectable seismic anomalies to chemical heterogeneities in the upper mantle. In the tectosphere model of Jordan (1978), Fe-depletion as a result of basaltic melt extraction from peridotite plays an important role in stabilizing the continental lithosphere. However, the effect of Fe - Mg substitution on seismic wave velocities is not well constrained at high pressures (e.g., Karato and Karki, 2001). In order to better understand how pressure influences compositional effects, we have determined the single-crystal elastic constants of natural Fe-rich olivine (Fe$_{0.94}$Mn$_{0.06}$)$_{2}$SiO$_{4}$ by Brillouin scattering to 12.1 GPa at ambient temperature. The aggregate bulk modulus, shear modulus and their pressure derivatives are: $K_{0S}$ = 136.3 (2) GPa, $G_{0}$ = 51.2 (2) GPa, $\partial$$K_{0S}/\partial$$P$ = 4.9 (1), $\partial$$G_{0}/\partial$$P$ = 1.8 (1), $\partial^{2}G_{0}/\partial$$P^{2}$ = -0.11 (1) GPa$^{-1}$. These results greatly improve our knowledge of the high-pressure elastic properties of fayalite, which was previously based on limited ultrasonic data and on static compression experiments often performed under non-hydrostatic conditions. Our results also show a strong compositional effect on the pressure dependence of the bulk modulus of olivine, which increases 17% from forsterite ($\partial$$K_{0S}/\partial$$P$ = 4.2) to fayalite ($\partial$$K_{0S}/\partial$$P$ = 4.9). Changes in Fe content can have large effects on seismic velocities. For example, an increment in Fe content from 10 mol% to 20 mol% would produce a 2.4% decrease of compressional velocity and 3.8% decrease of shear velocity in olivine at a pressure of 5 GPa along the 1673 K adiabat. The scaling coefficient for shear velocity as a function of Fe content, $\partial$$lnV_{S}/\partial\chi_{Fe}$ is found to be -0.39 at ambient conditions and to decrease in magnitude by 10% at 8 GPa. These results differ by as much as 20% from the earlier estimate of Karato and Karki (2001).
DE: 1025 Composition of the mantle
DE: 3655 Major element composition
DE: 3924 High-pressure behavior
DE: 7218 Lithosphere and upper mantle
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting