Evidence for a predominantly non-solar origin of nitrogen in the lunar regolith revealed by single grain analyses

We measured the amounts of N-14 and Ar-36 in single mineral and glass grain s from a lunar soil by laser extraction, with the goal of studying the cont roversial origin of trapped nitrogen in the lunar regolith. The average N-1 4/Ar-36 ratio of 29 ilmenite grains is 379, similar to the value determined previously on a large ilmenite separate from the same soil and 10 times la rger than the solar ratio of 37. However, the N-14/Ar-36 ratios in the indi vidual grains vary between 1 and 440 times the solar ratio. Ar-36 amounts i n the ilmenite grains scatter by more than two orders of magnitude, N amoun ts by less than a factor of 6. The variability of the N-14/Ar-36 ratio form s a striking contrast to the very uniform relative abundances of Ar, Kr, an d Xe trapped from the solar corpuscular radiation observed earlier in ilmen ite and other mineral grains from the same soil. This strongly suggests tha t, on average, some 90% of the N in the grains has a non-solar source, cont rary to the often expressed view that essentially all N in the lunar regoli th has been trapped from the solar wind. The conclusion that the lunar rego lith testifies to a secular variation of the N isotopic composition in the solar wind of similar to 30% becomes thus highly questionable. The origin o f the bulk of trapped lunar nitrogen remains unknown. (C) 1999 Elsevier Sci ence B.V. All rights reserved.