Nitrogen and argon release profiles in Luna 16 and Luna 24 regolith samples: The effects of regolith reworking

Abstract

Fines, microbreccias and agglutinates from the Luna 16 mature regolith 1635 and fines from the immature/submature Luna-24 regolith have been analysed for N and argon isotopes in order to understand the origin of isotopically distinct N released at different temperatures. All high-resolution runs reveal a similarity in the release of 36Ar, 40Ar and N over a wide temperature interval. The similarity in the 40Ar and 36Ar releases and the near coincidence in the 1635 agglutinates implies that the implanted species were redistributed and homogenised during regolith processing such that, regardless of the huge difference in ion implantation energy between solar 36Ar and non-solar 40Ar, their present distribution and their release temperatures are now essentially equal. A small amount of 40Ar released in the lower temperature steps with elevated 40Ar/36Ar is considered to be trapped after reworking. While such mixing and homogenisation may also be expected for N components of different origins, to date all known stepped runs regularly demonstrate a reproducible variation in δ15N, suggesting no homogenisation. We consider regolith N to be a mixture of several components trapped at different times, and some nitrogen that was not involved in the reworking. Relatively heavy N released around 500°C appears to be the most pure form of the component trapped after reworking, probably from accreted meteoritic matter. Middle temperature isotopically lighter N appears to be a mixture of solar and non-solar N largely homogenised, and therefore solar N can not be seen in its pure form. Bulk δ15N as well as formally deconvoluted δ15N thermal profiles imply that the non-solar N has a variable δ15N value. Several non-solar N sources are considered with their input resulting in increasing regolith δ15N with time. Because N from meteorites and IDPs appear to be dominant, a mechanism is required to reduce the C/N ratio typical of meteoritic matter to that approaching the low value observed in the lunar regolith. Preferential loss of methane appears to be a viable explanation, following generation either by proton sputtering or in reducing vapour plumes.