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The role of the volatile phase for REE and Y fractionation in low-silica carbonate magmas: implications from natural carbonatites, Namibia

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Summary

Two bimodal carbonatite complexes in Namibia of Cretaceous age are explored as to the presence and composition of a coexisting carbonatitic fluid. The Kalkfeld and Ondurakorume complexes contain both Ca- and Mg/Fe-carbonatites, composed of calcite alone or calcite with ferroan dolomite, fluorapatite and strontianite. The major element evolution in the bulk rocks from sövites to beforsites is due to crystallization of calcite and fluorapatite. All carbonatites show a negative Y anomaly in normalised REE plots. Fractionation is accompanied by successively lower HREE contents between Tb and Yb, expressed by the ratios Nd/Ho and Ho/Lu. The evolution of this downward-facing hump goes along with decreasing Y contents in bulk rocks and minerals. All this requires an additional phase coexisting with the carbonate liquid during fractionation. Comparison between the bulk rocks and the expelled fluid shows that the latter had preferentially accumulated the HREE and Y. Further evidence for this process are hydrothermal, HREE, Y-rich fluorites in other carbonatite complexes which reflect the composition of the expelled fluid. The high strength of fluoride complexes suggests that fluoride complexing in the carbonatitic fluid is the process responsible for extracting HREE and Y from the carbonatite magma, leaving fractionated carbonatite rocks depleted in these elements. The geochemical evolution of carbonatite magmas along fractionation has therefore to be considered in a melt-mineral-fluid system.

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  1. Instituto de Geociências, University of Brasilia, Brasilia, Brazil

    B. Bühn

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Correspondence: B. Bühn, Instituto de Geociências, Universidade de Brasilia, Campus Universitário Darcy Ribeiro, Brasilia 70910-900, Brazil

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Bühn, B. The role of the volatile phase for REE and Y fractionation in low-silica carbonate magmas: implications from natural carbonatites, Namibia. Miner Petrol 92, 453–470 (2008). https://doi.org/10.1007/s00710-007-0214-4

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