10:45 AM - 11:00 AM
[MTT28-07] Development of highly precise molybdenum isotop analysis by negative thermal ionization mass spectrometry (N-TIMS) for the study of isotope anomalies in bulk meteorites
Keywords:meteorite, TIMS, molybdenum
A Mo standard solution for atomic absorption spectrometry (Kanto Chem.) was used as an in-house standard. Iron meteorites (Tambo Quemado (IIIAB), Henbury (IIIAB), and Alibion (IVA)) were leached in 6 M HCl and digested with 16 M HNO3–12 M HCl. After recovering solutions including Mo, the sam- ples were dissolved in 0.4M HCl–0.5M HF to separate Mo by employing the anion exchange resin (Eichrom AG1-X8) through HCl–HF and HF–HNO3 media . Molybdenum isotopes (MoO–) were measured by negative-TIMS using TRITON plus (Thermo-Fisher Scientific) installed at Tokyo Tech. The instrument was equipped with 9 Faraday cups with 1011 Ω amplifiers. Approximately 3 μg of Mo was loaded on a zone-refined Re filament together with La(NO3)3 as an activator (La/Mo ~5). The results were obtained by averaging 360 ratios collected in the static multicollection mode.
For achieving highly precise Mo isotope analysis, it is important to determe the oxygen isotopic composition of MoO3 ions in each measurement by monitoring masses 149 (100Mo16O217O ) and 150 (100Mo16O218O ) ions and to use the data for correcting for the O isotope interferences. After correcting the O isotopic interference and performing mass-dependent fractionation during the TIMS measurement, the acquired Mo isotopic ratios yielded the following reproducibilities (2SD; n = 21): 47, 16, 10, 13, and 33 ppm for 92Mo/96Mo, 94Mo/96Mo, 95Mo/96Mo, 97Mo/96Mo,and 100Mo/96Mo, respectively. The reproducibilities have been improved by 1.3–2.7 times compared tothose obtained in previous studies using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).The accuracy of our technique was confirmed by measuring Mo isotopic compositions for two iron meteorites, i.e., Henbury (IIIAB) and Albion (IVA). Molybdenum isotope anoamlies for these meteorites are consistent with those obtained in the previous study . Moreover, we determined positive Mo isotope anomalies for a new iron meteorite, Tambo Quemado (IIIAB). Our N-TIMS technique can be applied to the studies of nucleosynthetic isotope anomalies in extraterrestrial materials as well as mass-dependent Mo isotopic shift in environmental samples.
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