Compositional zoning and element partitioning in nickeloan tourmaline froma metamorphosed karstbauxite from Samos, Greece

Blue-green nickeloan tourmaline from a micaceous enclave of a marble from S amos, Greece, contains unusually high concentrations of Ni (up to 3.5 wt% N iO), Co (up to 1.3 wt% CoO), and Zn (up to 0.8 wt% ZnO). The polymetamorphi c karstbauxite sample has an uncommon assemblage of nickeloan tourmaline, c alcite, zincian staurolite, gahnite, zincohogbomite, diaspore, muscovite, p aragonite, and rutile. The complex geologic history is reflected in multi-s taged tourmaline growth, with cores that represent detrital fragments surro unded by two-staged metamorphic overgrowths. Zone-1 metamorphic overgrowths , which nucleated next to detrital cores, are highly asymmetric and exhibit compositional polarity such that narrow overgrowths of brown schorl develo ped at the (-) c-pole are enriched in Mg, Ti, and F, and depleted in Al, Fe , and X-site vacancies ((x)square) relative to wider, gray-blue schorl-to-f oitite overgrowths developed at the (+) c-pole. Volumetrically dominant Zon e-2 overgrowths are strongly zoned nickeloan dravites with a continuous inc rease in Mg. Co, Ca, and F at the expense of Fe, Zn, Cr, and V from the Zon e-1 interface to the outermost rim. Within Zone 2, Ni reaches a maximum of 0.5 apfu before decreasing in the outer 20-40 gm. Zone-2 overgrowths also e xhibit compositional polarity such that, at the (-) c-pole, overgrowths are enriched in Mg, F, Na, Ca, and Cr relative to overgrowths at the (+) c-pol e that are, in turn, enriched in Al, Fe, Ni, Co, and (x)square. Element par titioning involving tourmaline rims and coexisting minerals indicates that relative partitioning of Ni is tourmaline much greater than staurolite > ga hnite; Co is tourmaline > staurolite > gahnite; and Zn is gahnite > staurol ite much greater than tourmaline.