Molybdenum Isotopic Composition of Iron Meteorites, Chondrites and Refractory Inclusions

Introduction. Recent Mo isotopic studies of meteorites reported evidence for differences in isotopic compositions for whole rocks of some primitive [1-3] and differentiated meteorites [2,4], relative to terrestrial materials. Enrichments of rand p-process isotopes of up to 3-4 ε units (ε unit = parts in 10) over s-process dominated isotopes are the most prominent features. Certain types of presolar grains show large enrichments in s-process isotopes [5], however, it was concluded on grounds of mass balance that incomplete digestion of such grains cannot explain the enrichments of of rand p-process isotopes in whole rocks of primitive chondrites [2]. If the reported variability in rand p-process isotope enrichments reflects the true isotopic characteristics of the whole rocks, the implications are quite profound. It would suggest the presence of large scale Mo isotopic heterogeneity within the solar accretion disk with likely collateral effects for other elements. However, such effects were not found for Ru isotopes [6,7], nor for Zr isotopes [8]. Another recent Mo isotopic study by multi collector ICP-MS could not confirm the reported deviations in Allende, Murchison or iron meteorites [9]. Here, we present new results for the Mo isotopic composition of iron meteorites, chondrites and CAIs obtained by negative thermal ionization mass spectrometry (NTIMS). We discuss analytical aspects and the homogeneity of Mo isotopic compositions in solar system materials. Analytical Techniques. Iron meteorites (1-2.7 g) were dissolved in conc. HCl. Chondrites (~1g) were digested overnight at 235oC using reverse aqua regia in borosilicate Carius tubes. CAIs were also digested in Carius tubes and Mo was collected during Re separation by solvent extraction. Molybdenum was separated from sample matrices by a three-step ion exchange procedure using HF-HCl on AG1-X8 resin and HCl on AG50-X8 resin. Molybdenum was loaded on outgassed Re filaments, metalized for 1 h, and covered with La nitrate. Mo isotopic compositions were measured on the UMD Sector 54 multi collector TIMS in the form of MoO3, at 1370-1420oC, and corrected for oxygen isotopic compositions. Signal intensities were 200-1900 mV for Mo standards, and 100-900 mV for samples on mass 146 (Mo). Data were collected in two cycles. In the first cycle all Mo isotopes were measured. In the second cycle, Ru interferences were monitored on mass 149 (RuO3). Mass discrimination was corrected using the exponential law and a value of 92Mo/98Mo = 0.607898, consistent with 94Mo/98Mo = 0.3802 proposed by [10]. The 92Mo/98Mo ratio was chosen because it covers the large mass range necessary to obtain accurate mass bias corrections on other Mo isotopes. The Ru interference (101Ru/95Mo = 0.000068±18) required corrections of 0.2, 0.05, and 0.8 ε units on Mo, Mo, and Mo, respectively. No interferences from Zr isotopes were detected. Filament blanks for Mo range between 0.5 and 0.9 ng. Carius tube blanks were 5-6 ng. Chemistry blanks range between 2 and 3 ng. Total blanks are insignificant for the present study because minimum quantities of processed meteoritic Mo were near 1 μg.