EMPIRICAL GARNET THERMOBAROMETER FOR MANTLE PERIDOTITES

Great efforts made by W.L.Griffin, S.Y.O’Reilly, C.G Rayan (1996-2003) for the mapping of the mantle lithosphere of Siberian and North American cratons with garnet thermobarometry reveal general structure of the mantle. Nevertheless correlation of the Ni-Cr thermobarometry with common and precise Ga-Al-orthopyroxene barometry and two pyroxene thermometry is not high. Empirical calibration on the large volume of the associations of the xenoliths allowed to obtain the equations which give high correlations with the estimates produced by (McGregor,1974) barometer (R=0.83) and with orthopyroxene (Brey,Kohler,1990) thermometer. Dependence of ln(Kd)/P from the estimated T BreyKohlreOpx is a polynomial (R=0.783) Kd=MgO*TiO2/(CaO+Mg)^2/ FeO^2/(Al2O3) ToC=1466.5*(ln Kd/P )^3+4108.9*(ln Kd/P )^2+3670.3* ln Kd/P +1800 {1} components are in oxide wt% for more simple calculations in thermobarometric program. Analogical equation for estimates with (Nimis Taylor, 2000) thermometer is ToC=362.05*(LnKd/P)^3+1880.4*( LnKd/P)^2+2659.6 *LnKd/P+1695.5 {2} (R=0.83). Kd=Na2O*TiO2/(CaO+MgO)^2/ FeO/(Al2O3) Checking kimberlite localities in the World (Jericho, Thumb, Udachnaya etc) show that approximations to orthopyroxene temperatures produced better coincidence with estimated by other methods geotherms but for HT associations clinopyroxen- aproximation temperatures give higher values for HT metasomatites, sheared peridotites. For the pressure the dependence for Cr2O3 from the pressure was used with the corrections on the temperatures. The simple equation is P=3.725 + 4.52*Cr2O3+ 0.01285*T {3} is works better for the shallow mantle to 40 kbar. To reproduce the deep structure beneath the Udachnaya (Ashchepkov et al., 2003) and other mantle columns the more complex equation P0=((13.5+Cr2O3*4.5/(CaO+MnO+0.25*FeO) 1.25+0.01685*ToC*Cr2O3)-19/MgO/7-CaO /5- 3.75*TiO2)*5 {4} the P=0.0064*P0^2+1.3816*P0 +5.1442{5} give the R=0.9 to McGregor’s estimates. Comparison for several pipes and World data show similarity with the estimates based on clinopyroxene (Nimis, Taylor, 2000, Ashchepkov, 2002) and orthopyroxene methods (Brey, Kohler, 1990-McGregor, 1974).