CARBON-DIOXIDE IN STRONGLY SILICA UNDERSATURATED MELTS AND ORIGIN OF KIMBERLITE MAGMAS

Determination of CO2 solubility in melts of compositions lying on the olivine melilitite - [CaCO3-MgCO3](CaCO3:MgCO3 wt ratio of 58:42) join have been performed between 5 and 30 kbars. The accuracy of gas chrom atographic analyses and homogeneity of quenched run products from abov e liquidus were tested by electron microprobe measurements which also included quantitative carbon determination in case of a CO2-bearing ol ivine melilitite glass. The CO2 content of gas-saturated melts in almo st linearly with Ca and Mg from 9 to 37 wt% at 30 kbar. At 10 kbar it increases non-linearly with Ca and Mg, and more rapidly so on the carb onate-rich side of the join. Liquidus temperatures exhibit minima whic h are shifted to the silicate-rich side with decreasing pressure. A si mple thermodynamic model for CO2 solubility in such melts is proposed which takes into account higher stability of CaCO3 as compared with (M g,Fe)CO3 complexes in the melt. It describes satisfactorily the report ed experimental data and also previously published results for Ca- and Mg-rich compositions. The CO2 solubility for the compositions similar to type IA kimberlites changes sharply in the pressure range correspo nding to the stability field of diamond. The vigorous degassing result ing in hydraulic fracturing of wall rocks may explain the diamondifero us character of these MgO-rich melts.