Analyses of nitrogen and argon in single lunar grains: towards a quantification of the asteroidal contribution to planetary surfaces

We performed nitrogen and argon isotopic analyses in single 200-mum-sized ilmenite grains of lunar regolith samples 71501, 79035 and 79135. Cosmogenic and trapped components were discriminated using stepwise heating with a power-controlled CO2 laser. Cosmogenic N-15 and Ar-38 correlate among different ilmenite grains, yielding a mean N-15(c)/Ar-38(c) production ratio of 14.4 +/- 1.0 atoms/atom. This yields a N-15 production rate in bulk lunar samples of 3.8 - 5.6 pg (g rock)(-1) Ma(-1), which agrees well with previous estimates. The trapped delta(15)N values show large variations (up to 300parts per thousand) among different grains of a given soil, reflecting complex histories of mixing between different end-members. The Ar-36/N-14 ratio, which is expected to increase with increasing contribution of solar ions, varies from 0.007 to 0.44 times the solar abundance ratio. The trapped delta(15)N values correlate roughly with the Ar-36/N-14 ratios from a non-solar end-member characterized by a Ar-36/N-14 ratio close to 0 and variable but generally positive delta(15)N values, to lower delta(15)N values accompanied by increasing Ar-36/N-14 ratios, supporting the claim of Hashizume et al. (2000) that solar nitrogen is largely depleted in N-15 relative to meteoritic or terrestrial nitrogen. Nevertheless, the Ar-36/N-14 ratio of the N-15-depleted (solar) end-member is lower than the solar abundance ratio by a factor of 2.5-5. We explain this by a reprocessing of implanted solar wind atoms, during which part of the chemically inert rare gases were lost. We estimate that the flux of non-solar N necessary to account for the observed delta(15)N values is comparable to the flux of micrometeorites and interplanetary dust particles estimated for the Earth. Hence we propose that the variations in delta(15)N values observed in lunar regolith can be simply explained by mixing between solar wind contributions and micrometeoritic ones infalling on the Moon. Temporal variations of delta(15)N values among samples of different antiquities could be due to changes in the micrometeoritic flux through time, in which case such flux has increased by up to an order of magnitude during the last 0.5 Ga. (C) 2002 Elsevier Science B.V. All rights reserved.