ANALYSES OF NITROGEN AND ARGON IN SINGLE LUNAR GRAINS: TOWARDS A QUANTIFICATION OF THE ASTEROIDAL CONTRIBUTION TO PLANETARY SURFACES

Abstract

We performed nitrogen and argon isotopic analyses in single 200-μm-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 15N and 38Ar correlate among different ilmenite grains, yielding a mean 15Nc/38Arc production ratio of 14.4±1.0 atoms/atom. This yields a 15N 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 δ15N values show large variations (up to 300‰) among different grains of a given soil, reflecting complex histories of mixing between different end-members. The 36Ar/14N 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 δ15N values correlate roughly with the 36Ar/14N ratios from a non-solar end-member characterized by a 36Ar/14N ratio close to 0 and variable but generally positive δ15N values, to lower δ15N values accompanied by increasing 36Ar/14N ratios, supporting the claim of Hashizume et al. (2000) that solar nitrogen is largely depleted in 15N relative to meteoritic or terrestrial nitrogen. Nevertheless, the 36Ar/14N ratio of the 15N-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 δ15N values is comparable to the flux of micrometeorites and interplanetary dust particles estimated for the Earth. Hence we propose that the variations in δ15N 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 δ15N 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.