Nitrogen isotopes in lunar highlands breccias

Abstract

Some breccias from the lunar highlands have probably trapped solar wind gases at a very early epoch in the history of the moon, as implied by their high contents of parentless fissiogenic xenon and sometimes, of parentless radiogenic 129Xe. Four samples of this type, on which noble gas data already exist, have been selected for analysis of nitrogen contents and isotopic composition, by using step-wise heating techniques: 14047, 14055, 14307, 60255. Since uncertainties in the evolution of the solar wind 15N/ 14N ratio with time are due in part to uncertainties in the measurement of the epoch of exposure, those samples provided the opportunity to measure the isotopic composition of nitrogen which has been trapped in the remote past, avoiding the problems inherent in the use of spallogenic nuclides. Results show that, in the samples studied from the Apollo 14 landing site, nitrogen is not particularly light, and has not been acquired, as a whole, in very ancient times. The conflicting presence of both parentless xenon and nitrogen of relatively “recent” isotopic signature can be explained if the hypothetical light nitrogen is diluted by more abundant, heavier nitrogen. Accordingly, the very ancient soil components which are implied in these objects by the presence of excess fission xenon have been re-exposed at a much later epoch, or mixed with some younger soil components, before the compaction event. The present data do not question the hypothesis of a secular isotopic variation of lunar trapped nitrogen, but cannot prove that very light nitrogen was trapped together with parentless fission xenon in the soil components of the highlands soil breccias. The very unusual release pattern of nitrogen in breccia 60255 can result from nitrogen isotopic homogenization with gas loss.