APPLICATION OF THE OLIVINE-ORTHOPYROXENE-SPINEL OXYBAROMETER TO CHROME AND PLATINUM ORES OF THE BUSHVELD AND STILLWATER ULTRAMAFIC COMPLEXES

     The common ultramafic assemblage olivine-orthopyroxene-spinel may be used to calculate the oxygen fugacity of the magma chamber with which the assemblage equilibrated.  The basis for this thermobarometer is the reaction: 

 

6 Fe₂SiO₄ + O₂ = 6 FeSiO₃ + 2 Fe₃O₄

 

The Fe₂SiO₄ content of olivine and the FeSiO₃ of orthopyroxene are measured readily on the electron microprobe. Determination of Fe₃O₄ in the spinel is made difficult by complex solid solutions.  The electron microprobe analyses for total iron and calculation of Fe³⁺ must be done based on assumed R₃O₄ stoichiometry of the spinel.  Activity-composition relations in spinel are further complicated by cation ordering between the octahedral and tetrahedral sites of spinel.  Experimental calibrations of cation disorder and reversal experiments have refined the thermobaromter into a practical tool for measuring fO₂.  A version of the thermobarometer is given in a simplified parameterized form

 

log(f_O2 - QFM) =  220/T + 0.35 – 0.0369P/T – 12 log X^ol_Fe – 2620/T X^ol_Mg²

+ 3 log(X^M1_Fe * X^M2_Fe)^opx + 2 log a^sp_Fe3O4

 

where P is pressure in bars and T is absolute temperature (Wood et al., 1990; Sack and Ghiorso, 1991b).

     Samples of Pt-Pd ore from the Merensky reef and chromite ore from the Jackless mine, South Africa were analyzed by EMPA.  Olivine compositions range from Fa_0.11-0.18, pyroxene from Fs_0.10-0.16 and spinel from Fe_0.58-0.68Mg_0.35-0.43Fe³⁺_0.08-0.20Al_0.57-0.74Cr_1.00-1.33Ti_0.01-0.03O₄.  Oxygen fugacity values range from 0 to +1 log fO₂ relative to QFM, somewhat more oxidizing than MORB glasses and typical mantle lherzolite xenoliths.  Temperatures obtained from olivine-spinel geothermometry extend from 600 °C to 1110 °C, likely due to subsolidus reequilibration and accompanying hydrothermal alteration.  Multiple injections of magma into the body of the intrusion are directly related to the formation of the ore horizon at the Merensky Reef, and the current data suggest that this occurs under relatively reducing conditions despite the substitution of 12-25% Fe₃O₄ in chromite.