HR: 0830h
AN: V51I-0395    [PDF]
TI: Mass-Dependent Molybdenum Isotopic Variations in Molybdenites Determined by MC-ICP-MS
AU: * Pietruszka, A J
EM: apietrus@geology.sdsu.edu
AF: Univ. of Maryland, Dept. of Geology, College Park, MD 20742 United States
AU: Walker, R J
AF: Univ. of Maryland, Dept. of Geology, College Park, MD 20742 United States
AU: Candela, P A
AF: Univ. of Maryland, Dept. of Geology, College Park, MD 20742 United States
AB: We have developed analytical techniques for the measurement of mass-dependent molybdenum isotopic variations in geological materials using a Nu Instruments plasma ionization mass spectrometer. Our methods are modified from the procedures developed by Barling et al. (2001; EPSL; v. 171) using Zr as an external inter-element monitor of instrumental mass bias. For the chemical separation and purification of Mo, we use a low blank, high yield ($>$98%) two-column procedure to maximize the removal of matrix elements and reduce the chromatographic fractionation of Mo. The effect of the solution Mo/Zr upon the measured Mo isotope ratios was evaluated by analyzing solutions with a range of Mo/Zr. Matrix effects due to Zr were significant at low Mo/Zr (up to an 0.8\permil range for 97Mo/95Mo), but were negligible for the high Mo/Zr that we used for our analyses. Matrix effects for a number of other elements (Na, Mg, Nb, Cd, Sb, W, and Tl) were evaluated and found to be below detection even for extremely high amounts, except for Mg. In this case, matrix effects on the Mo isotope ratios were not observed at Mg/Mo below 0.015, which is at least a factor of 3 higher than typical samples. In contrast, chromatographic fractionation of Mo was found to be significant. The 97Mo/95Mo of the Mo eluted from the first column in small cuts varied by a range of 2\permil. However, this effect was negligible ($<$0.05\permil) for yields greater than 96%. The precision and accuracy of three different mass bias corrections were evaluated using analyses of synthetic Mo solutions: the "graphical" (Marechal et al., 1999; Chem. Geol.; v. 156) and the "inter-element" methods using Zr to correct the Mo isotopic ratios for mass bias, and a simple sample-standard bracketing approach that ignores the presence of Zr. All three methods gave similar accuracy, but the last approach was found to give a superior precision by a factor of 2 (typically 0.15\permil, 2 sigma, on 97Mo/95Mo). Five molybdenites that formed under a range of redox conditions were analyzed using this procedure. They display a total 97Mo/95Mo range of 1.4\permil, which is significantly larger than the 0.4\permil range found in a previous study of molybdenites (Barling et al., 2001). More detailed studies are required to determine the origin of these mass-dependent isotopic variations of Mo.
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting