HR: 0800h
AN: V41E-1508    [Abstracts]
TI: Mantle temperatures, and tests of experimentally calibrated olivine-melt equilibria
AU: * Putirka, K D
EM: kputirka@csufresno.edu
AF: California State University, Fresno, 2576 E. San Ramon Ave., MS/ST24, Fresno, CA 93710 United States
AB: Because the ratio Mg^ol/Mg^liq (Kd(Mg)) is sensitive to T, olivine-liquid Kd's have long been used as geothermometers, and more recently, maximum Fo contents from volcanic rocks have been used to estimate mantle potential temperatures. Such estimates by Putirka (2005, G³) indicate higher mantle equilibration temperatures at Hawaii, compared to temperatures derived from earlier calibrations. Several published models were thus tested for their ability to reproduce T for 862 experimental data. The Putirka (2005) models did not include P corrections, which are added here: lnKd(Mg)=-1.88 + 30.85P(GPa)/T(C) - 0.04[H₂O]^liq + 0.068[Na₂O+K₂O]^liq + 3629.7/T(C) + 0.0087[SiO₂]^liq - 0.015[CaO]^liq lnKd(Fe)= -2.92 - 0.05[H₂O]^liq + 0.0264[Na₂O+K₂O]^liq + 2976.13/T(C) + 0.01847[SiO₂]^liq + 0.0171[Al₂O₃]^liq - 0.039[CaO]^liq + 33.17P(GPa)/T(C) In these expressions, Kd(Mg) and Kd(Fe) are the partition coefficients for Mg and Fe between olivine and liquid, expressed as cation fractions; compositional corrections are in weight percent. The models are calibrated from 785 experimental data (P = 0.0001-15.5 GPa; 1213-2353 K). In the tests, the expressions of Beattie (1993) performed exceptionally well for dry systems with MgO^liq < 17 wt. %, with a standard error of estimate of 35 K, compared to an SEE of 59 K for Ford et al. (1983) and an SEE of 51 K for the inversion of the Kd(Mg) model above. Recalibration of the Beattie (1993) model over this composition range thus appears unnecessary. But Beattie (1993), Ford et al. (1983), and other models overestimate T for hydrous systems, and for compositions with MgO^liq > 17 wt. %; new models are therefore needed. Over the greater compositional range, model 1 above can be inverted to yield T with a SEE of 56 K, and an average mean (systematic) error of +3 K for 856 experimental data; this compares to a systematic error of -26 K for Beattie (1993) and -36 K for Ford et al. (1983). For use in equation (1) of Putirka (2005), the models above are also more precise at both low and high MgO, and hydrous and non-hydrous systems compared to Beattie (1993) and Ford et al. (1983). Herzberg (pers. comm.) has modeled olivine-melt pairs for Hawaii and MOR's, which are in accord with Putirka (2005); these pairs are used to test for the effects of systematic model error on estimates of mantle temperatures. The Beattie (1993) and Ford (1983) models appear to predict too low a value for T (by 100-150 K), given comparisons of lnKd(Mg) v. 1/T(K) for experimental data, and the very low values for Kd(Mg) observed at Hawaii. The new calibrations indicate mantle equilibration temperatures of 1855 K at Hawaii and 1608 K at MOR's, in agreement with calculations by Putirka (2005). Excess temperatures at Hawaii thus likely exceed 200 K, as suggested by dynamic model predictions (Sleep, 1990; Schilling, 1991) for a thermal plume origin for the Hawaiian Islands and other hot spots.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005