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The nature of petrogenetic links between carbonatites and associated silicate rocks is a matter of discussion for several decades and still remains controversial among igneous petrologists. The Belaya Zima plutonic complex in southern Siberia is a typical intrusion, in which carbonatites are spatially and temporally associated with the ijolite series rocks and nepheline syenites. In this study we use whole-rock compositions and trace element characteristics of the major and accessory minerals for reconstructing the magmatic evolution of the complex and clarifying the origin of carbonatites. We conclude that the observed gradual increase of Zr, Nb and REE concentrations in the magma is consistent with the process of extensive fractional crystallization and gradual transition from silicate rocks to carbonatites, and inconsistent with the formation of carbonatites by liquid immiscibility. Magma differentiation by fractional crystallization continued during the carbonatite formation. The textural and analytical evidence indicates that the early calcitic carbonatites evolved to dolomitic and ankeritic carbonatites. In addition, maximum Nb and Zr concentrations occur in the calcitic carbonatite, whereas the REE content increases in the dolomitic and ankeritic facies. [ABSTRACT FROM AUTHOR]

Abstract: Liquid–liquid partitioning of 42 elements between synthetic silicate melts and immiscible fluoride, chloride, carbonate, phosphate and sulfate liquids was studied at temperatures of 650–1100°C, pressures 72–100MPa, with 0–11wt.% H2O. One series of experiments was performed in a rotating internally-heated autoclave where separation of the immiscible liquids was assisted by centrifugal forces. An analogous series of experiments was done in static rapid-quench cold-seal pressure vessels. The experimentally determined liquid–liquid distribution coefficients (D’s) vary over several orders of magnitude, as a result of variable Coulombic interactions between cations and anions. For alkaline, alkaline earth and rare earth elements ther is a strong and systematic dependence of the liquid/liquid D values on the ionic potential Z/r for all the examined systems. In contrast, highly charged cations (e.g., HFSE) show no systematic relationships between the D’s and Z/r. New experimental constraints on the carbonate/silicate liquid–liquid D values presented here confirm that rare metals such as Nb, Zr, REE, Th and U concentrate in silicate liquids, and therefore carbonatites that carry economical rare metal mineralization are not likely to have formed by liquid immiscibility. The comparison between experimentally-determined carbonate–silicate liquid–liquid D values and bulk-rock natrocarbonatite vs. nephelinite compositions at the Oldoinyo Lengai in Tanzania reveals significant discrepancies for Cs, Rb, Ba, Be, Zn, heavy REE, Ti, Mo and W, thus rendering a simple, one-stage immiscibility model for Oldoinyo Lengai questionable. [Copyright &y& Elsevier]

Investigates the origin of natrocarbonatite magmas from Oldoinyo Lengai volcano, Tanzania. Fluid condensate origin; Liquid immiscibility origin; Relationship between natrocarbonatite and related silicate rock compositions; Miscibility gap; Fractional crystallization.

Provides information on a study which analyzed the element distribution between carbonatite melts and hydrous fluids. Experimental and analytical methods; Experimental results and discussions.

Here we present an experimental study of the distribution of a broad range of trace elements between carbonatite melt, calcite and fluorite. The experiments were performed in the CaCO3 + CaF2 + Na2CO3 ± Ca3(PO4)2 synthetic system at 650-900 °C and 100 MPa using rapid-quench cold-seal pressure vessels. Starting mixtures were composed of reagent-grade oxides, carbonates, Ca3(PO4)2 and CaF2 doped with 1 wt% REE-HFSE mixture. The results show that the distribution coefficients of all the analyzed trace elements for calcite and fluorite are below 1, with the highest values observed for Sr (0.48-0.8 for calcite and 0.14-0.3 for fluorite) and Y (0.18-0.3). The partition coefficients of REE gradually increase with increasing atomic number from La to Lu. The solubility of Zr, Hf, Nb and Ta in the synthetic F-rich carbonatitic melts, which were used in our experiments, is low and limited by crystallization of baddeleyite and Nb-bearing perovskite. [ABSTRACT FROM AUTHOR]