The carbon and nitrogen stable isotope geochemistry of two lunar meteorites: ALHA-81005 and Y-86032

Abstract

The carbon and nitrogen stable isotope geochemistry of two lunar meteorites, ALHA-81005 and Y-86032 has been compared with that of an Apollo 16 regolith breccia, 60016. Although much of the carbon present in all three samples is terrestrial organic contamination, the meteorites have higher carbon abundances and lighter isotopic compositions than 60016. The non-contaminant carbon in ALHA-81005 and Y-86032 occurs as two distinct components, combusting between 550-700℃ and 900-1100℃. Since these components are absent from the pristine lunar breccia, they must have been added (i) from the impactor which ejected the meteorites from the Moon; (ii) in the Antarctic or (iii) be representative of a lunar environment not sampled by Apollo missions. At temperatures over 1100℃, spallogenic carbon combusts, with elevated δ^C, greater than 0‰. Nitrogen systematics are less-well resolved than carbon, partly due to the lower amounts of nitrogen gas liberated by the meteorites. Nitrogen abundance of ALHA-81005 and Y-86032 fall in the range of values from lunar breccias and δ^N values follow the heavy-light-heavy pattern characteristic of such samples. Spallogenic carbon and nitrogen are more abundant in ALHA-81005 than Y-86032,in keeping with its longer exposure age. Nitrogen data are consistent with identification of ALHA-81005 and Y-86032 as lunar highland breccias compacted from immature regolithic material