Microanalysis of Niobium in Iron Meteorites

Introduction: Niobium and tantalum are generally regarded to be lithophile incompatible elements occurring in chondritic relative proportions in planetary materials [1]. Precise Nb/Ta data have indicated that Nb may be depleted in the Earth's mantle [2, 3] which may be evidence for sequestering in an eclogite reservoir [2], or for its removal into the core [3, 4]. Metal-silicate partitioning experiments have indicated that Nb, but not Ta, may behave in a weakly siderophile manner, and predict abundances of 15-50 ppb in iron meteorites [4]. Bulk analysis of iron meteorites by spark source mass spectrometry reported Nb abundances of 7-40 ppb correlated with V and Cr abundances [5], apparently consistent with the predictions above. We report the results of a microanalytical investigation of V, Cr, Nb, Zr, Hf and Ta abundances in metal, troilite, schreibersite and cohenite from iron meteorites using laser ablation ICP-MS. The elements above (with the exception of Cr) were found to be below detection limits in metal, phosphide and carbide phases, but V, Cr and Nb were found to be present in troilite, particularly from IAB irons. Analytical Methods: Polished slabs of iron meteorites were obtained, and phases identified using a JEOL JSM5800LV Scanning Electron Microscope. Laser ablation ICP-MS analysis of mineral phases was performed at The University of Chicago using a CETAC Transgenomics LSX-200 laser ablation system coupled to a ThermoFinnigan Element1 high resolution ICP-MS. A large spot size (50-100 µm) was used in line scan mode [6] to produce a bright, stable ion signal from the sample. Peaks monitored in low resolution included 182 W. Intensities were normalized to Fe, and converted to abundances using intensities determined on NIST SRM 1263a, a VCr steel doped with Zr, Nb, Mo, Hf, Ta and other trace elements. A terrestrial pyrite was used for S calibration. Blanks were determined prior to each measurement, and the 3σ variability of the blank used to determine the detection limits which were 1-3 ppm for V, Cr and Mn, and 0.1-2.0 ppb for Y, Zr, Nb, Hf, and Ta, depending on the spot size used. Following the identification of Nb in IAB troilites at very substantial count rates (up to 8x10 3 cps), the Canyon Diablo troilite mass spectrum was scanned at mass resolving power of M/∆M~5000 at m/e= 88-96 to confirm the identity of the peaks. Mass calibration was