[M-TT44] Frontiers in Geochemistry

[MTT44-P04] Development of precise Cr isotope analysis for extraterrestrial materials

*Tetsuya Yokoyama¹, Koki Sumiya¹, Yusuke Ohkuma¹, Keiji Nishikawa¹ (1.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology)

Keywords:Chromium isotopes, Extraterrestrial materials, Mn-Cr chronology, 54Cr isotope anomaly

Chromium isotopic compositions in extraterrestrial materials are becoming one of the most important cosmochemical tracers in recent planetary science. In particular, nucleosynthetic isotope anomaly of 54Cr (ε54Cr) in meteorites suggests the origin and formation location of planetary materials in the early solar system [1]. Additionally, the 53Mn-53Cr dating method, which utilizes the short-lived nuclide 53Mn (T1/2 = 3.7 Myr) and its daughter nuclide 53Cr, is effective for elucidating the processes that occurred within ~30 Myr after the formation of the solar system [2]. In this study, we revisited the analytical methods for the determination of Cr isotopes in rock samples, and developed a new, simple procedure that enables precise and accurate measurement of ε54Cr values and 53Mn-53Cr ages in extraterrestrial materials. First, we improved the chemical separation method via ion-exchange chromatography and developed a new procedure that separates Cr with high recovery and high purity in fewer separation steps than previous methods. Next, we developed a method that measures the 55Mn/52Cr ratios in samples using a conventional quadrupole ICP-MS instrument. Additionally, we have devised a method to suppress secondary isotope fractionation during the isotope analysis of Cr with TIMS. The developed method was applied to three carbonaceous chondrites (Murchison, Allende, Kainsaz), three ordinary chondrites (Forest City, Saratov, Tuxtuac), a Martian meteorite (Tissint), and a terrestrial sample (JP-1). The ε54Cr values of these samples were consistent with those obtained in previous studies [3]. The 53Mn-53Cr isochron for the six chondrites yielded an age of 4564.5 ± 1.9 Ma. This age suggests the occurrence of Mn-Cr fractionation within ~3 Myr after the beginning of the solar system, resulting in the different Mn/Cr ratios in individual chondrites. However, chondrites consist of various constituents including CAIs, chondrules, metal, and matrix, each of which have different origin with different age and initial 53Cr/52Cr ratio. Therefore, the obtained 53Mn-53Cr isochron could be a mixing line that may not have age significance [3]. More analyses of precise Cr isotopes in extraterrestrial materials, specifically for the 53Mn-53Cr ages for their constituents, are required to further understand the origin and evolutionary processes of planetary materials.

References
[1] Kleine T. et al. (2020) Space Sci. Rev., 216, 55. [2] Trinquier et al. (2008) GCA, 72, 5146. [3] Zhu K. et al. (2021) GCA, 301, 158.