GEOTHERMOBAROMETRY IN FOUR-PHASE LHERZOLITES I. EXPERIMENTAL RESULTS FROM 10 TO 60 KB

Abstract

Experiments were carried out on primitive natural lherzolitic compositions from 10 to 60 kb and 900 to 1400-degrees-C. Mineral compositions were reversed by using mixes of finely ground (< 20-mu-m) minerals from natural rock samples with different initial compositions. The minerals were mixed in proportions to give a primitive upper mantle composition. Further starting materials were a synthetic mineral mix and a sintered oxide mix. Equilibrium was thus approached from different chemical directions, and equilibrium mineral compositions were inferred from the overlap of microprobe analyses. Fe-loss problems were avoided by using single-crystal olivines as sample containers. Samples were placed into holes drilled into cylindrically shaped olivine crystals, which in turn were welded into Pt capsules. A further advantage of this method is that oxygen fugacities seem to be buffered by the Ni precipitation curve. This curve lies very close to the iron-wustite (IW) buffer curve for mantle olivines. Oxygen fugacities calculated from compositions of experimentally produced spinels give values slightly above the IW buffer curve. Therefore, practically no Fe3+ should be present in our experimental charges. Ortho- and clinopyroxene compositions were strictly reversed with respect to Al, Cr, and Na (only cpx) contents and their mutual amounts of enstatite and diopside components. The data show the fundamental influence of Na in cpx on two-pyroxene thermometry. Garnet compositions were reversed with respect to Ca, Al, Cr, Ti, and Mn. Other than in CMAS, Ca in garnet is mostly dependent on temperature in the natural system and not so much on pressure. This behaviour of Ca will msot strongly influence barometers based on the Al content or orthopyroxene coexisting with garnet. The mg-numbers of coexisting phases were not strictly reversed. The homogeneity of olivine compositions and internal consistency of the data set give us confidence that equilibrium values of mg-number have been reached in almost all cases. The experimental results presented in this paper offer the unique possibility of simultaneously testing many different thermobarometers which are based on different exchange reactions and which were calibrated in different laboratories.