In Situ Helium Isotope Microimaging of Meteorites

Isotope imaging is commonly used to investigate the localization of trace elements and their isotopes. In situ noble gas analysis of meteorites revealed the distribution of primordial noble gases that were trapped in the building blocks of asteroids and planets during the early stage of the solar system evolution. Solar wind noble gases are among the primordial gases present in meteorites and were trapped through exposure to solar wind. Micrometer-resolution in situ noble gas analysis has not been achieved due to the lack of sensitivity and spatial resolution. The microscale imaging technique is crucial for identifying the carrier phase of the solar wind noble gases. We have developed ⁴He isotope imaging utilizing secondary neutral mass spectrometry with strong field postionization. This technique achieved a lateral resolution of 2 μm and a ⁴He detection limit of 2 × 10¹⁷ cm^-3. This development allows for the study of a solar wind gas-rich meteorite, Northwest Africa 801 carbonaceous chondrite, with micrometer resolution. The solar wind ⁴He carriers are fine-grained particles and are sparsely scattered in the matrix region.