Depth profiling of noble gases in ilmenite and olivine substrates

Abstract

Solar wind noble gases in extraterrestrial materials provide important insights into past solar activity and space weathering processes. These noble gases are concentrated within the uppermost 100 nm of material surfaces, requiring in situ analysis with high depth and lateral resolution. Additionally, the multi-isotope analysis of noble gases is useful in evaluating the degassing of materials. To address these requirements, we developed a depth profiling method for multi-noble gas isotopes by time-of-flight secondary neutral mass spectrometry. We prepared ilmenite and olivine substrates co-implanted with ⁴He, ²⁰Ne, ²²Ne, ³⁶Ar, and ⁴⁰Ar as standards for solar wind-irradiated materials and surveyed the optimal analytical conditions to improve the substrate detection limits. As a result, we achieved ⁴He, ²⁰Ne, ²²Ne, ³⁶Ar, and ⁴⁰Ar detection limits of 7 × 10¹⁶, 3 × 10¹⁶, 9 × 10¹⁵, 2 × 10¹⁶, and 5 × 10¹⁶ cm⁻³ for ilmenite and 2 × 10¹⁷, 4 × 10¹⁶, 2 × 10¹⁶, 2 × 10¹⁶, and 9 × 10¹⁷ cm⁻³ for olivine, respectively. The reproducibility for the concentration and elemental/isotope ratio measurements ranged from 5% to 31%. This method allowed for the visualization of nano-scale solar wind noble gas distributions on extraterrestrial material surfaces, providing insights into the solar wind activities and space weathering of celestial bodies.