THE COMPOSITIONS OF PRIMARY CARBONATE MELTS AND THEIR EVOLUTION THROUGH WALLROCK REACTION IN THE MANTLE

We have experimentally determined the compositions of near-solidus mel ts from depleted natural lherzolite at pressures greater than 25 kbar. The melts are carbonatitic with low alkali contents and Ca/(Ca + Mg) ratios of 0.72-0.74. Primary carbonate melts from fertile mantle are m ore sodic with Ca/(Ca + Mg + Fe + Na) of 0.52 and Na/(Na + Ca + Mg + F e) up to 0.15. The melt compositions are similar to many natural magne sio-carbonatites, but differ substantially from the more abundant calc io-carbonatites. Experimentally we find that calcio-carbonatites are p roduced by wallrock reaction of primary melts with harzburgite at pres sures of < 25 kbar. At 15 kbar we have obtained a Ca/(Ca + Mg + Fe + N a) ratio of up to 0.87 and very low Na contents generated by this proc ess. Values of Ca/(Ca + Mg + Fe + Na) up to 0.95 are possible at lower pressures. Low pressure wallrock reaction of primary carbonate melt w ith fertile lherzolite produces melts richer in Na2CO3, corresponding to possible parental magmas of natrocarbonatite. Wallrock reaction at low pressures transforms the bulk peridotite composition from that of a harzburgite or lherzolite to wehrlite. Examples of such carbonatite metasomatism are now widely documented. Our experiments show that the calcium content of olivine and the jadeite content of clinopyroxene ma y be used to constrain the Ca and Na contents respectively of the carb onatite melt responsible for metasomatism.