Martian atmosphere-like nitrogen in the orthopyroxenite ALH84001

Abstract

Isotopic and elemental compositions of nitrogen, neon, and argon in the Martian meteorite ALH84001 were studied. Two whole-rock samples of ALH84001,57 were analyzed with detailed stepped combustion experiments (from 200 to 1200°C with every 100°C and 200°C, respectively, with laser ablation experiments for melting the samples). Nitrogen with δ 15N = 418 ± 44‰ and 366 ± 15‰ after correction for cosmogenic nitrogen were obtained at 800°C, which are the highest δ 15N values ever found among Shergottites, Nakhlites, Chassignite meteorites. The heavy nitrogen in ALH84001 may have the same origin as that in the glass portions of two shergottites, EETA79001 and Zagami, and is likely to have been derived from the Martian atmosphere. However, when and in which phase Martian atmosphere-like nitrogen was trapped in ALH84001 is still unknown. One possibility inferred from the released temperatures is that nitrogen is sited in maskelynite. On the plot of δ 15N versus Ar/N (diagrams of δ 15N vs. 40Ar/ 14N and 36Ar/ 14N), the data points for ALH84001 in this study form a linear trend, which apparently differs from that defined by glass portions of the shergottites EETA79001 and Zagami. The present trend also differs from the trend seen by Grady et al. for ALH84001. The 40Ar/ 14N and 36Ar/ 14N ratios obtained in this work are relatively smaller than those of the shergottites on such plots, indicating the following possibilities: (1) Martian atmospheric Ar/N ratio changed throughout geologic time (e.g., ≈4 Ga) without a large change in δ 15N, that is, ALH84001 trapped ancient Martian atmospheric nitrogen, and Ar/N ratio was initially lower, or (2) different trapping sites or mechanisms between ALH84001 and the shergottite glasses made an apparent difference in the Ar/N ratio, and elementally fractionated gases were trapped in ALH84001. In the case (1), either increase in Ar concentration in the Martian atmosphere due to preferential Ar degassing from the Martian interior or preferential escape of atmospheric nitrogen must have occurred. Nitrogen with δ 15N from −13‰ to +5‰, and with δ 15N ≥ 200‰ were released at low and high temperatures, respectively. The former may have been derived from the Martian lithosphere or terrestrial atmosphere, and the latter must be of cosmogenic nitrogen. The largest amount of 40Ar was released at temperatures similar to the largest release of heavy nitrogen of ≈800°C. According to reported K abundance in ALH84001, the majority of 40Ar in our samples is considered to be of radiogenic origin.