CO2 degassing and carbonate-silicate liquid immiscibility as alternative mechanisms of evolution of CO2-rich alkaline melts: Evidence from microinclusions

The rapid ascent of alkaline mafic-ultramafic magmas is usually accompanied by decompression degassing and loss of volatiles (primarily, CO2). In contrast, the development of magma chambers, where mantle melts can be stored under slowly changing temperature and pressure conditions, is favorable for extensive fractionation and separation of immiscible carbonatite melt. We considered the two scenarios of the evolution of volatile-rich magmas by studying melt and fluid inclusions in minerals of two groups of alkaline igneous complexes: (I) mafic complexes without carbonatites from Germany (Mahlberg melanephelinites) and Russia (alkali basalts of the Chukchi Peninsula) and (II) carbonatite-bearing complexes of Italy (Monticcio Lake, Vulture Volcano) and Tajikistan (Dunkeldyk Massif). Phenocrysts from the rocks of group I contain melt and abundant CO2 fluid inclusions, whereas those from group II rocks contain melt inclusions only. The melt inclusions in minerals of both rock rocks are homogenized at high temperatures (1200-1240oC), which are consistent with low H2O contents in the melts. The silicate melts encapsulated in the inclusions are rich in incomaptible trace elements and have high agpaitic indexes increasing in the sequence Vulture (0.5) - Mahlberg (0.6) - Chukchi (0.6) - Dunkeldyk (0.9). The melts from group II rocks show very high contents of volatile components (CO2, S, F, and Cl). Phenocrysts from group I rocks often contain hermetic or partly decrepitated CO2-dominated fluid inclusions. These inclusions show significant variations in CO2 density, which suggests complex crystallization histories at different depth levels (e.g. for Mahlberg: 0.8 GPa - 0.5 GPa - 0.3 GPa - 0.1 GPa - 2 MPa). Phenocrysts from group II rocks contain cogenetic silicate and carbonate melt inclusions. The immiscible carbonate liquid separated from a parental carbonated silicate magma at temperatures up to 1200oC and pressures of at least 0.5-0.7 GPa for the Dunkeldyk complex and 0.2 GPa for Vulture Volcano. The separation of carbonate liquid and subsequent crystallization of carbonates and hydrous minerals prevented magma degassing till the latest stages of magma crystallization. The difference in the behavior of carbon species in deeply derived magmas (i.e. degassing versus silicate-carbonate immiscibility) depends on the composition and fractionation path of the primary magma.