Constraints on the flux of meteoritic and cometary water on the Moon from volatile element (N-Ar) analyses of single lunar soil grains, Luna 24 core

We report new nitrogen and argon isotope and abundance results for single breccia clasts and agglutinates from four different sections of the Luna 24 drill core in order to re-evaluate the provenance of N trapped in lunar regolith, and to place limits on the flux of planetary material to the Moon's surface. Single Luna 24 grains with 40Ar/Ar-36 ratios <1 show delta N-15 values between -54.57. and +123.3 parts per thousand relative to terrestrial atmosphere. Thus, low-antiquity lunar soils record both positive and negative delta N-15 signatures, and the secular increase of the delta N-15 value previously postulated by Kerridge (Kerridge, J.F. [1975]. Science 188(4184), 162-164. doi:10.1126/science.188.4184.162) is no longer apparent when the Luna and Apollo data are combined. Instead, the N isotope signatures, corrected for cosmogenic N-15, are consistent with binary mixing between isotopically light solar wind (SW) N and a planetary N component with a delta N-15 value of +100%, to +160%,. The lower delta N-15 values of Luna 24 grains compared to Apollo samples reflect a higher relative proportion of solar N, resulting from the higher SW fluence in the region of Mare Crisium compared to the central near side of the Moon. Carbonaceous chondrite-like micro-impactors match well the required isotope characteristics of the non-solar N component trapped in low-antiquity lunar regolith. In contrast, a possible cometary contribution to the non-solar N flux is constrained to be <= 3-13%. Based on the mixing ratio of SW to planetary N obtained for recently exposed lunar soils, we estimate the flux of micro-impactors to be (2.2-5.7) x 10(3) tons yr(-1) at the surface of the Moon. Our estimate for Luna 24 agrees well with that for young Apollo regolith, indicating that the supply of planetary material does not depend on lunar location. Thus, the continuous influx of water-bearing cosmic dust may have represented an important source of water for the lunar surface over the past similar to 1 Ga, provided that water removal rates (i.e., by meteorite impacts, photodissociation, and sputtering) do not exceed accumulation rates. (C) 2011 Elsevier Inc. All rights reserved.