Burbankite, a (Sr,REE,Na,Ca)-carbonate in fluid inclusions from carbonatite-derived fluids: Identification and characterization using Laser Raman spectroscopy, SEM-EDX, and synchrotron micro-XRF analysis

Burbankite, ideally (Na,Ca)(3)(Sr,REE,Ba)(3)(CO3)(5), is a rare REE carbona te mineral that until now had been encountered only at a few localities inc luding highly alkaline silicate rocks, carbonatites, and lacustrine sedimen ts. It was identified as an abundant solid phase in fluid inclusions that r epresent fluids derived from the Kalkfeld carbonatite complex (Namibia). Bu rbankite occurs in association with other solids including nahcolite, halit e, sylvite, rouvilleite (?), fluorite, calcite, cryolite, base metal sulfid es, and phosphates. The carbonatite-derived fluids were trapped in quartzit e country rocks close to the carbonatite contact. The optical and geochemic al identification of burbankite has been confirmed by confocal Laser Raman spectrometry. The burbankite crystals show a Raman shift at 1078 cm(-1), wh ich is significantly displaced relative to peaks for other common carbonate s and is much broader. The elemental composition of burbankite was determin ed by a combination of SEM-EDX on opened inclusions and synchrotron-XRF ana lysis on unopened wafers. The SEM-EDX analyses of the burbankite crystals y ielded a compositional range (in wt%) of Na2O 10.6-17.5, CaO 3.6-17.4, SrO 12.0-26.7, BaO 2.5-5.5, La2O3 3.5-7.0. Ce2O3 4.7-9.0, Nd2O3 0.9-2.1, and CO 2 (calc.) 29.8-35.2. The Na/Ca ratios are between 1.0 and 4.3, which is hig h in comparison with rock-forming burbankite occurrences, and clearly disti nguishes the burbankite crystals from carbocernaite. Synchrotron micro-XRF spectra yielded REE patterns decreasing from La to Yb over 2.5 orders of ma gnitude with small negative Eu anomaly [(Eu/Eu*)(cn) = 0.5-1.0] in some cas es. The Y/Ho ratios range from 1 to 5, and Th/U ratios are between 1 and 10 . The fluids trapped are interpreted to represent a highly evolved but pris tine, alkali-rich, hydrous, carbonate melt, which had not lost alkalis to t he country rocks by fenitization processes. The common occurrence of burban kite crystals in the fluid inclusions shows the high capability of carbonat e melts and fluids to transport high-field-strength and large-ion-lithophil e elements.