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

A carbonatite-derived fluid trapped under closed-system conditions is prese rved as fluid inclusions in quartzitic country rocks of the Kalkfeld carbon atite complex, Namibia. The fluid inclusion population provides a unique op portunity of investigating in situ the composition of a natural, volatile-r ich carbonatitic fluid with all components trapped and preserved, and its r elationship to the volatile-deficient, parental Kalkfeld carbonatite. indiv idual fluid inclusions display all intermediate compositions between three end-members: (1) low-density CO2; (2) a Na,K,Ca,Cl-, HCO3--bearing aqueous brine; and (3) an assemblage of solid phases comprising nahcolite, halite, burbankite, sylvite, fluorocarbonate (rouvilleite ?), cryolite, Mn-Fe-calci te, feldspar/feldspathoid, fluorite, base metal sulfides, and phosphate. Ca thodoluminescence imaging of the host quartzites features a network of flui d migration pathways. In situ exsolution of invasive fluids and necking-dow n processes of fluid inclusions, triggered by rapid cooling of interconnect ed fluid ponds, caused heterogeneous entrapment of the various fluid compon ents. 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 c onsistent rare-earth element (REE)(cn) patterns. Analysis of the bulk carbo natitic fluid yielded the following element ratios: Na/K = 1.5 to 2.7, Na/C a = 1.5, Na/Sigma REE = 4 to 17, Fe/Mn = 5.2 to 7.2, TNU = 3 to 13, and Y/H o = 25 to 65. An estimation of the volatile components in the fluid populat ion at CO2 = 20 wt% and H2O = 20 wt% permits a quantitative assessment of t he composition of the Na-Ca-REE-Sr alkali-carbonatitic fluid with 40 wt% H2 O and CO2, 28 to 29 wt% Na2O + K2O, 13 to 16 wt% CaO, 3.0 to 4.1 wt% FeOtot , up to 3 wt% Sigma REE, up to 3 wt% Sr, 1 to 1.7 wt% MgO, 1 wt% TiO2, 0.6 wt% MnO, and Th and Ba reaching 1600 and 8000 ppm, respectively. The REE pa tterns 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 thos e 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 fraction ation. A comparison between this alkali-carbonatitic fluid with the volatil e-deficient, sovitic Kalkfeld carbonatite suggests that virtually all alkal i metals and Cl, and a major proportion of F, Th, U, and Ti were preferenti ally partitioned into this fluid. This fluid was also able to accommodate s ignificant 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 l eft to right to partition into the fluid relative to the crystallizing carb onatite melt. As this fluid migrates through and interacts with invaded hos t rocks, elements will tend to precipitate in the same qualitative sequence from left to right. This selective precipitation of elements from a migrat ing fluid accounts for observations made in metasomatized crustal and mantl e-derived rocks. Copyright (C) 1999 Elsevier Science Ltd.