Composition of natural, volatile-rich Na–Ca–REE−Sr carbonatitic fluids trapped in fluid inclusions

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A carbonatite-derived fluid trapped under closed-system conditions is preserved as fluid inclusions in quartzitic country rocks of the Kalkfeld carbonatite complex, Namibia. The fluid inclusion population provides a unique opportunity of investigating in situ the composition of a natural, volatile-rich carbonatitic fluid with all components trapped and preserved, and its relationship to the volatile-deficient, parental Kalkfeld carbonatite. Individual fluid inclusions display all intermediate compositions between three end-members: (1) low-density CO 2; (2) a Na,K,Ca,Cl −, HCO 3 −-bearing aqueous brine; and (3) an assemblage of solid phases comprising nahcolite, halite, burbankite, sylvite, fluorocarbonate (rouvilleite ?), cryolite, Mn–Fe–calcite, feldspar/feldspathoid, fluorite, base metal sulfides, and phosphate. Cathodoluminescence imaging of the host quartzites features a network of fluid migration pathways. In situ exsolution of invasive fluids and necking-down processes of fluid inclusions, triggered by rapid cooling of interconnected fluid ponds, caused heterogeneous entrapment of the various fluid components. Consequently, synchrotron–XRF analyses of individual inclusions show a broad range of compositions with Th/U = 1 to 45 and Y/Ho = 1 to 28, but consistent rare-earth element (REE) cn patterns. Analysis of the bulk carbonatitic fluid yielded the following element ratios: Na/K = 1.5 to 2.7, Na/Ca = 1.5, Na/ΣREE = 4 to 17, Fe/Mn = 5.2 to 7.2, Th/U = 3 to 13, and Y/Ho = 25 to 65. An estimation of the volatile components in the fluid population at CO 2 = 20 wt% and H 2O = 20 wt% permits a quantitative assessment of the composition of the Na−Ca−REE−Sr alkali-carbonatitic fluid with 40 wt% H 2O and CO 2, 28 to 29 wt% Na 2O + K 2O, 13 to 16 wt% CaO, 3.0 to 4.1 wt% FeO tot, up to 3 wt% ΣREE, up to 3 wt% Sr, 1 to 1.7 wt% MgO, 1 wt% TiO 2, 0.6 wt% MnO, and Th and Ba reaching 1600 and 8000 ppm, respectively. The REE patterns of individual fluid inclusions and of the bulk fluid extend over two orders of magnitude from La to Lu, and in this respect are similar to those of the parental Kalkfeld carbonatite, but are distinguished by a negative (Eu/Eu∗) cn anomaly of 0.5 to 0.6. The data suggest that this fluid is a direct sample of those expelled during a late stage of carbonatite fractionation. A comparison between this alkali–carbonatitic fluid with the volatile-deficient, sövitic Kalkfeld carbonatite suggests that virtually all alkali metals and Cl, and a major proportion of F, Th, U, and Ti were preferentially partitioned into this fluid. This fluid was also able to accommodate significant concentrations of Rb, Cs, Cu, Pb, and Zr in individual samples. The qualitative sequence: Fe = Mn > Sr = REE > Mg = F > Ba = Y > Ti > Th = U > (Zr,Cu,Pb,Rb,Cs) > K > Na = Cl represents an increasing tendency from left to right to partition into the fluid relative to the crystallizing carbonatite melt. As this fluid migrates through and interacts with invaded host rocks, elements will tend to precipitate in the same qualitative sequence from left to right. This selective precipitation of elements from a migrating fluid accounts for observations made in metasomatized crustal and mantle–derived rocks.