THE OXIDATION STATE OF LAMPROITIC MAGMAS

Abstract

Olivine leucitites from the Gaussberg volcano, Antarctica are primitive members of the lamproite group of ultrapotassic rocks. They are glass rich, have an Mg number of around 70, and carry spinel lherzolite xenoliths. Liquidus phase fields and compositions were studied experimentally at atmospheric pressure with controlled oxygen fugacities. Chrome-spinels occur as inclusions in olivines in the natural rock, but it was necessary to add Cr2O3 to the experiments to stabilize spinels at the liquidus, indicating that some fractionation of spinel has almost certainly occurred. Experimental results show that fO2 conditions of crystallization can be characterized by ferric value (100 Fe3+/(Fe3++Fe2+)) of spinel, ferric iron content of leucite, and Mg-number (100 Mg/(Mg+Fe2+)) of olivine. The results demonstrate that the liquidus phases of the Gaussberg rocks crystallized at fO2 slightly below that of the NNO buffer. Application of the results to other lamproites indicates that they began to crystallize at oxygen fugacities varying from well above NNO (Leucite Hills) to around MW (West Kimberley and the Spanish fortunites). The Gaussberg olivine leucitite contains leucite cores poor in ferric iron with rims richer in ferric iron, indicating oxidation during emplacement. The ferric value of spinel is very sensitive to changes in oxygen fugacity and recognizing that some lamproitic magmas are known to contain diamonds, it may prove to be useful as a 'diamond survival indicator'. The preservation of diamonds in lamproitic rocks will depend critically on fO2: diamonds are not likely to be preserved in rocks which reach the surface as liquids at fO2 near NNO or above.