The genesis of natrocarbonatites: Constraints from experimental petrology and trace element partitioning.

摘要

Carbonatites have been widely studied because of their unusual composition and as a key to understanding the geochemical evolution of the mantle. Natrocarbonatites from Oldoinyo Lengai (Tanzania) received special attention because this is the only active carbonatite volcano. Although carbonatites from Oldoinyo Lengai are exceptionally sodic compared to others, many authors have tried to include these lavas in a broader framework of carbonatite petrogenesis which explains the majority of carbonatites.;Various aspects of the petrogenesis of natrocarbonatites from Oldoinyo Lengai have been previously studied. However, previous experimental studies provided little trace element partitioning data between silicate and carbonate liquids at conditions suitable for a comparison with the natural lavas. Moreover, no study has been done on the crystallisation of natrocarbonatites once exsolved.;The aim of this study was two-fold. The first part was to demonstrate that silicate-bearing natrocarbonatites fractionate to silicate-free natrocarbonatites. The second part focussed on how liquid immiscibility produces silicate-bearing, not silicate-free, natrocarbonatites, and on constraining the conditions of liquid immiscibility between silicate-bearing natrocarbonatite and conjugate wollastonite nephelinite.;The experiments were prepared using natural lavas as starting materials. The comparison between phase assemblages, and major and trace element data on natural lavas and on experimental run products was used to constrain the conditions of formation of the natural lavas. The fractionation of the silicate-bearing natrocarbonatite to produce silicate-free natrocarbonatite was studied using experiments at 100 and 20 MPa, 550--900°C. Results showed that silicate-free natrocarbonatites could be the product of in situ crystallisation of silicate-bearing natrocarbonatites at ∼20 MPa, 600°C. Liquid immiscibility between silicate-bearing natrocarbonatites and wollastonite nephelinites was studied using experiments prepared with different mixtures of natrocarbonatite and nephelinite at 20--200 MPa, 700--900°C. Silicate-bearing natrocarbonatites were suggested to exsolve from wollastonite nephelinite at ∼100 MPa, 750°C.;The degree of polymerisation of the silicate melt is important in describing the liquid immiscibility process. Therefore, major and trace element partitioning between immiscible liquids was discussed as a function of the structure of the silicate liquid. However, it was shown that the formation of carbonato- and halogen-complexes in both liquids makes the determination of the role of each liquid difficult.;Natrocarbonatites or their parental magma (melilite nephelinite) are more likely not to reach the surface than calciocarbonatites or their parental magma (olivine nephelinite). Consequently, they must be widespread in the lithospheric mantle as efficient metasomatic agents.