A study of nitrogen isotopic systematics in lunar soils and breccias.
The Open University.
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A variety of lunar soils (bulk soils, grain size fractions and mineral separates) and regolith breccias are studied for nitrogen abundance and isotopic composition. in order to investigate the isotopic variation in the lunar regolith and to constrain the origin of the nitrogen. The lunar soils and breccias are analysed using a high sensitivity static-vacuum mass spectrometer. capable of measuring nanogram quantities of nitrogen with a δ15N precision of ±0.5‰. In addition to nitrogen abundance measurements, conjoint carbon and argon abundances (allthough the latter are semi-quantitative) are also measured. Lunar soils and breccias are analysed by stepped combustion and pyrolysis extraction. Typically employing temperature resolution of 25-50°C. However, some soils and breccias are analysed by high-resolution extraction using temperature increments of 10°C. constituting the highest resolution studies performed to date.
The high-resolution extractions of lunar soils and breccias has confirmed the existence of the heavy-light-heavy-light-heavy (W-shaped) isotopic profile observed by previous workers for pyrolysis extractions of lunar breccias. The current study has confirmed that the W-shaped profile is restricted to pyrolysis extractions, but is present in both soils and breccias. The well documented heavy-light-heavy (V-shaped) isotopic profile is observed in stepped combustion extractions.
The nitrogen in the lunar regolith has been shown to be of both solar and non-solar origin. The former is confirmed from the solar-like C/N ratios in lunar soils. and the latter is confirmed from the excess N/36Ar ratio. between 5 and 39 times the solar value. Although neither nitrogen source is believed to be a minor component, the solar wind is believed to contribute -30% of the total nitrogen, although the relative proportions of solar and non-solar nitrogen are not well constrained.
The surface-correlation of the low temperature nitrogen release (LTN), from the <10μm fraction from soil A12023. has enabled calculation of the isotopic composition of this component with δ15N = +35.3 ±3.6‰. It is believed that this value is representative of the isotopic composition of the recent solar wind. Furthermore, the absence of isotopically light nitrogen in agglutinates separated from A12073. and the increasing contribution of high temperature nitrogen (HTN) with increasing grain size from the same soil. has suggested that the volume-correlated nitrogen is released at temperatures ≥1050°C. This is believed to be representative of the ancient solar wind, and has a mean δl5N = +77.0 ± 6.0‰. This work has shown that the maximum secular variation of 15N/14N in the lunar regolith is -2.8%. significantly lower than previous estimates.
The low temperature. isotopically heavy nitrogen component. NLT. and the isotopically light nitrogen components. NMT and NHT (although the latter is only observed during stepped pyrolysis extractions). display a l5N/l4N variation of 38%. This cannot be accounted for by a secular variation of 15N/l4N in this work. The origin of NLT is not well constrained. but may be due to re-implantation of atmospheric nitrogen. or fractionation of solar wind gases during the formation of amorphous rims around soils grains. A proportion 1-3% I of NHT has been observed in soil A12023. with δ15N = -186‰, the lightest observed in a lunar soil. The similarity of this value with that obtained for lunar breccia A79035. has suggested the possibility of a relict component. implanted prior to breccia consolidation. No evidence of presolar grains could be identified in A79035, although there is some evidence for amorphous carbon. This may also be associated with the presence of fine-grained metallic iron as a significant proportion of this in lunar soils is believed to originate from meteorite contamination.
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