Experimental Investigations of the Partitioning of Nb, Mo, Ba, Ce, Pb, Ra, Th, Pa, and U between Immiscible Carbonate and Silicate Liquids

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Geochimica Et Cosmochimica Acta
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Liquid carbonate/liquid silicate partition coefficients (D) for a suite of incompatible elements (Nb, Mo, Ba, Ce, Pb, Th, U) have been determined experimentally. For one experiment, D's for Pa and Pa were also determined. The partitioning behavior is shown to be a regular function of the ionic field strength (z/r) of the partitioned elements. At constant temperature, increasing pressure from 7 to 20 kbar caused little change in the measured D. At constant pressure, increasing temperature causes all D's to approach unity, as the temperature nears the critical temperature and the two liquids become compositionally similar. Addition of P or F to the system enlarges the stability field of carbonate but does not affect the partitioning behavior of our suite of elements. However, both P and F are strongly partitioned into the carbonate liquid.Omphacite/carbonate Liquid D's at 50-60 kbar have also been determined. Our results for D-U and D-Th are similar to those found previously for augite/silicate liquid. Thorium is more compatible than U in pyroxene, indicating that excesses of Th-230/U-238 found in MORE cannot be explained by fractionation between pyroxene and melt.The Soret effect in carbonate liquids is quite small, in contrast to the effect in silicate liquids. In general, we see no evidence for Soret separation in our experiments, implying that endmember Ca-, Na-, and Mg-carbonate liquids have simple speciations and mix nearly ideally. This observation suggests that the results of our liquid/liquid partitioning studies do not strongly depend on the exact composition of the carbonate. Our experimentally determined liquid-liquid partition coefficients can explain recent fractionations of Pb-210/U-238 and Ra-226/U-238 in carbonatitic lavas from the Oldoinyo Lengai volcano, but cannot explain similar fractionations in Th-230/U-238. Pb-210 has the shortest half-life of these nuclides, and is, therefore, the most sensitive to the exact timing and mechanism of fractionation. Because we can explain Pb-210/U-238 in terms of recent, instantaneous liquid immiscibility, we favor this model for the origin of the deviations of Pb-210/U-238 and Ra-226/U-238 from those expected under secular equilibrium. If our interpretation is correct, some other fractionation process(es) must have fractionated Th (and perhaps Pa) from U, Pa, and Pb in the Oldoinyo Lengai carbonatites.


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