Abstract
Columbite crystals from niobium-yttrium-fluorine (NYF) pegmatites lacking zircon or containing metamict cyrtolite were analyzed for major and minor elements (Electron Microprobe (EMP)), trace elements (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)), and U-Pb geochronology (Laser AblationMulti-Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS)). All four pegmatite localities sampled are hosted by the Proterozoic Fordham gneiss and/or Paleozoic Bedford gneiss (Columbite-(Fe); Kinkel and Baylis localities) and the Manhattan Schist of Lower Paleozoic age (Columbite-(Mn); Fort George and Harlem River Drive localities) and yield Neoacadian ages. The weighted average U-Pb ages are 372.2 ± 8.2 Ma (Baylis Quarry), 371.3 ± 7.3 and 383.4 ± 8.9 Ma (Kinkel Quarry); 383 ± 15 Ma (185th St. and Harlem River Drive); and 372 ± 10 Ma (Fort George). A partial metamict zircon (“cyrtolite”) from the Kinkel Quarry yielded a weighted average U-Pb age of 376.9 ± 4.3 Ma. The Neoacadian ages obtained agree with those determined by thermal ionization mass spectrometry (TIMS) for zircon from Lithium-Cesium-Tantalum (LCT) pegmatites from Connecticut and Maine. No pegmatites temporally associated with the Taconic orogeny were found. The size, lack of common Pb, uniform U concentrations across crystal cross-sections, sufficient but moderate uranium concentrations, lack of metamictization, and consistency in U-Pb isotopic ratios for columbite samples BCB-COL, NYSM #25232, and NYSM #525.8 suggests they show promise as potential standards for oxide mineral LA-MC-ICP-MS geochronological analyses, however, additional characterization using ID-TIMS would be necessary to develop as such.
Highlights
Columbite has the general formula AB2 O6 where the “A” site is occupied mainly by Fe, Mn, and Mg and the “B” site by Nb and Ta
The Neoacadian ages obtained agree with those determined by thermal ionization mass spectrometry (TIMS) for zircon from Lithium-Cesium-Tantalum (LCT) pegmatites from Connecticut and Maine
We present major and trace element concentrations by electron microprobe (EMP) and LA-ICP-MS, respectively
Summary
Columbite (designated the columbite-group within) has the general formula AB2 O6 where the “A” site is occupied mainly by Fe, Mn, and Mg and the “B” site by Nb and Ta. Three end-members, columbite-(Fe), columbite-(Mn), and columbite-(Mg), are known to occur in nature. Columbite-(Fe) is the most common, whereas columbite-(Mg) is rare [1]. The columbite minerals occur in magmatic deposits of W, Sn, and rare earth elements (REE) [2], alkaline and carbonatitic intrusions [3], Geosciences 2018, 8, 169; doi:10.3390/geosciences8050169 www.mdpi.com/journal/geosciences. A dark ore of columbite-group minerals, is an important source for the strategic elements niobium and tantalum and, for this reason, ignited and funded armed conflicts in some African countries [5]. Despite relatively low uranium concentrations, columbite-group minerals can be used for high resolution U-Pb geochronology because they have sufficient uranium concentrations and low common
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