[M-ZZ45] Frontiers in geochemistry: discussing its appeal and future prospects

[MZZ45-08] In-situ analysis of helium, neon, and argon isotopes in ilmenite and olivine

*Yuta Otsuki¹, Ken-ichi Bajo¹, Tomoya Obase¹, Rainer Wieler², Hisayoshi Yurimoto¹ (1.Earth and Planetary System Science, Department of Natural History Sciences, Graduate school of science, Hokkaido University, 2.ETH Zürich)

Keywords:Noble gas, Depth profiling, TOF-SNMS

Introduction
Solar wind (SW) is irradiated to the regolith of airless bodies such as the Moon and asteroid Itokawa [e.g., 1, 2]. The energy distribution of the SW can be analyzed by depth profiling of implanted noble gases in those regolith grains [3]. However, SW depth profiling has been limited to the case of the Genesis target materials [3]. In order to apply this method to natural samples, we prepared noble gases (He, Ne, and Ar) implanted ilmenite and olivine as analogues of soil minerals on Moon and Itokawa. In this study, we developed analytical conditions of LIMAS to analyze ⁴He, ^20,22Ne, and ^36,40Ar in the analogue standards.

Materials and Methods
Isotopes of ⁴He, ^20,22Ne, and ^36,40Ar were implanted into San Carlos olivine (USA) and Monastry Diamond ilmenite (South Africa) polished sections. Depth profiling was performed by LIMAS using the same measurement conditions as in [3] except for TOF conditions. We set the time-of-flight (TOF) in the MULTUM II mass spectrometer of LIMAS to 454 to 461 µs.

Results and Discussion
The mass resolution (MR) measured by the ⁴He⁺ peak was 28,000 (FWHM), which is sufficient to separate the isobaric peaks of ¹⁶O⁴⁺ and ¹²C³⁺. The peaks of ²²Ne²⁺ and ³⁶Ar⁴⁺ were also separated from their isobaric ions (⁵⁵Mg⁵⁺ and ²⁷Al³⁺). Although the ²⁰Ne^{+ or 2+} and ⁴⁰Ar^{2+ or 4+} are isobaric at m/z 10 or 20, their peaks were sufficiently separated under the MR in this study.
The depth profile shows that the ²⁰Ne⁺ was significantly interfered by a tail of ²⁴Mg¹⁶O²⁺ peak. On the other hand, the depth profile of ²⁰Ne²⁺ was decreased close to the blank intensity from residual ²⁰Ne, suggesting ²⁴Mg¹⁶O⁴⁺ was not detected. This suggests that ²⁰Ne²⁺ is suitable for ²⁰Ne depth profiling for samples containing MgO component. The depth profile of ²⁰Ne shows that ⁴⁰Ca^{2+ or 4+} was an interference for ⁴⁰Ar^{2+ or 4+}, and ³⁰Si³⁺ was for ⁴⁰Ar⁴⁺. The MR in this study is much smaller than the required MR (190,000) to separate ⁴⁰Ca^{2+ or 4+} peaks, and insufficient to separate from the left tail of the ³⁰Si³⁺ peak. The intensity of ⁴⁰Ca²⁺ was 20 times higher than that of ⁴⁰Ca⁴⁺ while the ionization efficiency of ⁴⁰Ar^{2+ and 4+} was similar. The ³⁰Si³⁺ peak was smaller than ⁴⁰Ca⁴⁺ peak. Therefore, the ⁴⁰Ar⁴⁺ is suitable for ⁴⁰Ar depth profiling of ilmenite and olivine samples.
We found a TOF condition of LIMAS showing interference-free noble gas ion detections of ⁴He⁺, ²⁰Ne²⁺, ²²Ne²⁺, and ³⁶Ar⁴⁺ with simultaneous detections of major element ions of ¹⁶O²⁺, ²⁴Mg²⁺, ⁴⁸Ti⁴⁺, ⁵⁶Fe²⁺, and ²⁸Si⁺ from the samples. Note that the interference of ³⁰Si³⁺ and ⁴⁰Ca⁴⁺ were remained. The detection limits of ⁴He, ^20,22Ne, and ^36,40Ar were 3 × 10¹⁷, 3 × 10¹⁶, 1 × 10¹⁶, 4 × 10¹⁶, 3 × 10¹⁷ cm^-3 for ilmenite and 2 × 10¹⁷, 9 × 10¹⁶, 2 × 10¹⁶, 3 × 10¹⁶, 3 × 10¹⁸ cm^-3 for olivine, respectively.

References [1] Wieler R. (2016) Geochemistry 76, 463. [2] Nagao K. et al. (2011) Science 333, 1128. [3] Bajo K. et al. (2015) GJ. 49, 559.