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 δ¹⁵N 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 δ¹⁵N 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, 82}Kr 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 δ¹⁵N(≥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 δ¹⁵N in the present Martian atmosphere as compared to 4 Ga ago. These inferences are consistent with the model predictions (Pepin, 1994).