LUNAR NITROGEN: INDIGENOUS SIGNATURE AND COSMIC-RAY PRODUCTION RATE

Abstract

Indigenous lunar nitrogen composition and abundances have been determined in old ferroan anorthosite 60025 and in anorthositic breccia 67915 from North Ray Crater, as well as in 55 cm deep volcanic glasses of 74001 double-drive core from Shorty Crater. Also included in the set is the well-documented lunar basalt 75075, collected near the Camelot Crater. Indigenous lunar N abundances are low (at ppm level), but there is some variation between glass-rich cores, mare basalts and anorthosites. The uniform indigenous N isotopic signature of δ15N=+13.0+/-1.2%%, is consistent with data reported previously for Shorty Crater samples. The indigenous δ15N cannot account for the light nitrogen component, observed in the lunar regolith samples. We have determined cosmic-ray production rates P(15N) for the above rocks and the drill core samples. The average production rate estimate (for low shielding) of P(15N)=5.8+/-0.6 pg 15N/g/Ma is ~60% higher than published lunar 15N production rates, but consistent with the meteoritic production rate derived from silicates in the Enon meteorite, when normalized to 2π-irradiation geometry. From the observed cosmogenic 15N excesses and the reported cosmogenic 21Ne abundances in core 74001 we derive a (15NC/21NeC) production rate ratio of 4.0+/-0.3 for silicates.