Nitrogen and heavy noble gases in ALH 84001: Signatures of ancient Martian atmosphere

Abstract

Nitrogen and noble gases have been studied in a bulk sample and three density separates of the Martian orthopyroxenite ALH 84001. The δ 15N values which lie between 85%. and −18%. (and after correcting for cosmogenic contribution, between 46%. and −23%.), define a two component mixing trend in a plot of δ 15N vs. 1/N, with Chassigny as one endmember and another component with ° 15N ≥ 46%.. This trend is different from the one defined by the data from EET 79001,C and glass from Zagami. Most of the krypton and xenon are of trapped origin; the ratios 129Xe/132Xe and 136Xe/132Xe being similar to the Martian atmospheric values as found in EET 79001,C. In addition, small contributions from in situ 238U fission and live 129I decay are evident in some high temperature steps, the later observation attesting to the antiquity of this Martian meteorite. Excesses at 80, 82Kr and 128Xe due to neutron capture effects on bromine and iodine, respectively, are observed in all the samples. These neutron effects are not consistent with in situ production in the meteoroid during cosmic ray exposure and hence should be produced in the Martian atmosphere or surface and enterd the meteorite as a trapped component. The lower δ 15N (≥46%.) and 40Ar/36Ar ≤ 1400 in the trapped component of ALH 84001, as compared to the values from EET 79001,C, together with the fact that radiogenic 40Ar and trapped 36Ar, 84Kr, and 132Xe have similar release pattern, are strongly suggestive that the trapped component in ALH 84001 represents Martian atmosphere of ∼4Ga ago.The noble gas elemental ratios 36Ar/132Xe and 84Kr/132Xe show an elemental fractionation trend, enriching the heavy noble gases, similar to what has been observed in Nakhla (Drake et al., 1994). Comparing the nitrogen and xenon isotopic records and the radiogenic and stable isotope ratios (40Ar/129Xe and 39Ar/14N) from ALH 84001 representing Martian atmospheric component of ∼4 Ga ago, with those from EET 79001,C representing Martian atmospheric component of recent past, we infer the following on the evolution of the Martian atmosphere: (a) Xenon isotopic composition, as well as the amounts of xenon have been completely evolved at 4 Ga in Martian atmosphere and almost remained unchanged to the present; (b) The radiogenic 40Ar has not been completely degassed into the atmosphere at 4 Ga; (c) Nitrogen has been lost in a continuous process, leading to an increase in the ratio of 36Ar/14N as well as the δ 15N in the present Martian atmosphere as compared to 4 Ga ago. These inferences are consistent with the model predictions (Pepin, 1994).