Element partitioning between immiscible carbonatite–kamafugite melts with application to the Italian ultrapotassic suite

Abstract

Trace element partition coefficients between conjugate carbonatite–silicate melts are a suitable tool to test whether such natural magmas originated by liquid immiscibility. In the Intra-Apennine Magmatic Province, Central Italy, highly silica-undersaturated ultrapotassic kamafugites coexist with carbonate-rich rocks of controversial nature: a mantle origin and magmatic nature of the carbonatites are indicated by the primitive nature of some kamafugites and by mantle-debris occurring within the kamafugite–carbonate/carbonatite deposits. An alternative hypothesis suggests that the CO2 originates from crustal carbonate assimilation by a basaltic magma, resulting in silica undersaturation of the kamafugite rocks. This experimental study investigates whether carbonatite and kamafugite melts such as the Italian ones can be immiscible at upper mantle conditions. Partition coefficients between conjugate melts determined in this study are compared with calculated element partitioning between natural carbonatite/kamafugite pairs to test a possible origin of the carbonatites by liquid immiscibility.The experiments demonstrate that kamafugite and alkali-rich carbonatite melts are immiscible at 1.7GPa, 1220°C. Trace element partition coefficients obtained from static- and centrifuging piston cylinder experiments are within factor 5 from unity and deviate less than a factor of 2 from unity for 26 out of 37 trace elements investigated. The alkali- and earth alkali-elements have Di≥1, indicating that carbonatite melts in equilibrium with alkaline silicate melts should be at least as alkali-rich as the silicate melts. HFSE partition preferentially into the silicate melt.A comparison of the experimental partition coefficients with natural carbonatite–kamafugite pairs from the Intra Apennine Province reveals good agreement (except for the alkalis), supporting that liquid immiscibility could indeed explain the observed rock suites. Original carbonatite compositions are not well preserved due to secondary leaching that almost completely removed the alkalis leading to significant uncertainties and compositional variations and potentially modified trace element concentrations too. Based on the observed trace element concentrations, large volume assimilation of sedimentary calcite is unlikely. We thus propose that CO2 was already present in the parental primitive magmas forming the kamafugites. The carbonatites formed by unmixing from the silicate magmas and the related CO2-saturation were most probably responsible and triggered the explosive eruptions in the Intra Apennine Province.