HR: 0830h
AN: S21E-0364    [PDF]
TI: Comparison of gold and MgO pressure scales at 22-56 GPa and 300-1150 K and its implications for mantle models
AU: * Shim, S
EM: sangshim@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Frank, M R
EM: mfrank@niu.edu
AF: Northern Illinois University, 312 Normal Rd, DeKalb, IL 60115 United States
AU: Fei, Y
EM: y.fei@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd, Washington, DC 20015 United States
AU: Jeanloz, R
EM: jeanloz@mit.edu
AF: University of California, McCone Hall, Berkeley, CA 94720 United States
AB: Recent developments in mineral physics enable in-situ measurements of the phase boundaries and the equations of state of the mantle minerals. Nevertheless, existing experiments for the post-spinel transition in Mg$_2$SiO$_4$ show a discrepancy (2 GPa, 60 km) with the seismologically inferred depth of the boundary. Inconsistencies in the pressure scales at high temperature may be responsible for much of the discrepancy. We investigated the consistency of the gold (Jamieson et al. 1982, Shim et al. 2002, Heinz et al. 1984, and O.L. Anderson et al. 1989) and MgO (Speziale et al. 2001) scales through in-situ measurements using resistant heated diamond cells at 22-56 GPa and 300-1150 K. Angle-dispersive X-ray diffraction was performed for gold+MgO mixtures in a neon medium at the GSECARS sector of APS. Above 900 K and below 27 GPa, both Jamieson's and Shim's gold scales are consistent with the MgO scale within 0.5 GPa, whereas O.L. Anderson's gold scale underestimates pressure significantly (1 GPa) compared with the MgO scale. A greater magnitude of discrepancy is found above 30 GPa: 300 K isotherms for gold are discrepant by 1 GPa with the isotherm for MgO. In addition, thermal pressure calculated from the gold scales is systematically smaller than that from the MgO scale by 3 GPa. Possible reasons for the discrepancies at high temperature are: (1) electronic and anharmonic contributions, (2) different equations of state used, and (3) differential stress in the samples. Because of their consistency and use of higher quality data over a wide P-T range, it is legitimate to use Speziale's MgO and Shim's gold scales at 20-30 GPa and above 1000 K. When these scales are used, the earlier in-situ study for the post-spinel boundary becomes consistent within 1 GPa with the 660-km discontinuity. One of the most important studies (e.g., Funamori et al. 1998) for thermal equation of state of (Mg,Fe)SiO$_3$ perovskite is calibrated with Anderson's gold scale. The use of Shim's gold and Speziale's MgO scales tends to increase the Gruneisen parameter of the Mg-perovskite in the lower mantle.
DE: 3630 Experimental mineralogy and petrology
DE: 3919 Equations of state
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting