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

Blue-green nickeloan tourmaline from a micaceous enclave of a marble from Samos, Greece, contains unusually high concentrations of Ni (up to 3.5 wt% NiO), Co (up to 1.3 wt% CoO), and Zn (up to 0.8 wt% ZnO). The polymetamorphic karstbauxite sample has an uncommon assemblage of nickeloan tourmaline, calcite, zincian staurolite, gahnite, zincohogbomite, diaspore, muscovite, paragonite, and rutile. The complex geologic history is reflected in multi-staged tourmaline growth, with cores that represent detrital fragments surrounded 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 developed 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-foitite overgrowths developed at the (+) c-pole. Volumetrically dominant Zone-2 overgrowths are strongly zoned nickeloan dravites with a continuous increase in Mg. Co, Ca, and F at the expense of Fe, Zn, Cr, and V from the Zone-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 exhibit compositional polarity such that, at the (-) c-pole, overgrowths are enriched in Mg, F, Na, Ca, and Cr relative to overgrowths at the (+) c-pole that are, in turn, enriched in Al, Fe, Ni, Co, and (x)square. Element partitioning involving tourmaline rims and coexisting minerals indicates that relative partitioning of Ni is tourmaline much greater than staurolite > gahnite; Co is tourmaline > staurolite > gahnite; and Zn is gahnite > staurolite much greater than tourmaline.