Evidence of solar wind irradiation on mineral grains embedded in matrix of the Northwest Africa 801 CR chondrite

Abstract

Solar wind (SW) is preserved in meteorites as abundant solar noble gases. We performed in situ ⁴He isotope imaging of mineral grains in the CR2 chondrite matrix of Northwest Africa 801 using time-of-flight secondary neutral mass spectrometry with strong-field post ionization. ⁴He⁺ signals were detected along the surfaces of individual grains of Fe-Ni metal, ferrihydrite, olivine, pyroxene, and troilite. The high ⁴He concentrations along the surfaces indicate implantation of SW into the mineral grains. We determined the SW-⁴He fluence of eight mineral grains from the line profiles across the grain boundaries. SW-⁴He fluence ranged from 2.7 × 10¹⁶ to 58 × 10¹⁶ atoms cm⁻². These fluences were then used to calculate the SW irradiation durations. Assuming irradiation occurred at 4 astronomical units, the durations ranged from 3.8 to 82 kyr. These durations correspond to the residence time of individual mineral grains on the surface of the parent body. The variation in residence time for the mineral grains suggests variable durations for local mixing and burial processes on the parent body. The SW exposure ages provide insights into the gardening rate driven by small-scale impact mixing processes on the parent body.