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

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 Ar-36, Ar-40 and N over a wide temperature interval. The similarity in the Ar-40 and Ar-36 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 Ar-36 and non-solar Ar-40, their present distribution and their release temperatures are now essentially equal. A small amount of Ar-40 released in the lower temperature steps with elevated Ar-40/Ar-36 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 delta(15)N, 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 degreesC 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 delta(15)N as well as formally deconvoluted delta(15)N thermal profiles imply that the non-solar N has a variable delta(15)N value. Several non-solar N sources are considered with their input resulting in increasing regolith delta(15)N with time. Because N from meteorites and interplanetary dust particles 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.