SUB-NANOGRAM NEODYMIUM ISOTOPE MEASUREMENTS ON A NEW ISOTOPX PHOENIX TIMS USING 10^11 AND 10^12 OHM RESISTORS

The Earth and Environmental Sciences Dept. at Boston College recently acquired a new Isotopx Phoenix Thermal Ionization Mass Spectrometer (TIMS). The instrument was constructed in late 2015 into early 2016 and was delivered to Boston College in spring 2016. Initial isotope analyses of Nd standard JNdi-1 yield consistent and accurate results across differing aliquot sizes (300 ng Nd, 4 ng Nd, and 200-400 pg Nd), filament geometries (triple vs. single), analysis types (metal vs. oxide), and resistor arrays (10^11Ω vs. 10^12Ω). For 300 ng runs, standards are loaded on triple filaments with HNO₃ and analyzed as Nd+ using an automated ramp sequence and a multidynamic method. The program was set to ramp and maintain a 4 volt 144Nd+ beam. The first 13 analyses yield an average 143Nd/144Nd = 0.512109±0.000005 (i.e. 10 ppm; 2SD) with an average internal precision of 3 ppm (2σ). The smaller 4ng and 400pg standards were loaded on outgassed single Re filaments with an H₃PO₄+Ta₂O₅ slurry activator solution and analyzed as NdO+ using a multidynamic method. The 4ng loads were also ramped automatically, and signal intensities of 3 volts 144NdO+ were maintained during measurements. The first 14 analyses of 4 ng loads yield 143Nd/144Nd = 0.512103±0.000004 (i.e. 8 ppm; 2SD) with an average internal precision of 5 ppm (2σ). The sub-nanogram sized loads were analyzed using a 10^12Ω resistor array. The filaments were ramped manually and signal intensities of approximately 2 volts (200mV 10^11Ω equivalent) 144NdO+ were utilized for measurements. The first fourteen analyses, spanning measurements made in the factory and after the instrument was installed at Boston College, yield 143Nd/144Nd = 0.512112±0.000014 (i.e. 28 ppm; 2SD) with an average internal precision of 13 ppm (2σ).

Our results demonstrate the ability of the Phoenix TIMS to accurately and consistently measure Nd isotopes of various aliquot sizes, including sub-nanogram loads on 10^12Ω resistors. We anticipate further improvements as we refine our run protocols, loading procedures, and calibrate our oxygen isotope composition for NdO+. High precision Nd isotopic analysis on sub-nanogram samples is crucial for precise Sm-Nd garnet geochronology. Garnet geochronology dating tectonometamorphic processes should represent another important component of the EARTHTIME effort.