NITROGEN AND HEAVY NOBLE-GASES IN ALH-84001 - SIGNATURES OF ANCIENT MARTIAN ATMOSPHERE

Nitrogen and noble gases have been studied in a bull; sample and three density separates of the Martian orthopyroxenite ALH 84001. The delta (15)N values which lie between 85 parts per thousand and -18 parts per thousand (and after correcting for cosmogenic contribution, between 4 6 parts per thousand and -23 parts per thousand), define a two compone nt mixing trend in a plot of delta(15)N VS. 1/N, With Chassigny as one endmember and another component with delta(15)N greater than or equal to 46 parts per thousand. This trend is different from the one define d by the data from EET 79001,C and glass from Zagami. Most of the kryp ton and xenon are of trapped origin; the ratios Xe-129/X-132 and Xe-13 6/Xe-132 being similar to the Martian atmospheric values as found in E ET 79001,C. In addition, small contributions from in situ U-238 fissio n and live I-129 decay are evident in some high temperature steps, the later observation attesting to the antiquity of this Martian meteorit e. Excesses at Kr-80,Kr-82 and Xe-128 due to neutron capture effects o n bromine and iodine, respectively, are observed in all the samples. T hese 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 entered the meteorite as a trapp ed component. The lower delta(15)N (greater than or equal to 46 parts per thousand) and Ar-40/Ar-36 less than or equal to 1400 in the trappe d component of ALH 84001, as compared to the values from EET 79001,C, together with the fact that radiogenic Ar-40 and trapped Ar-36, Kr-84, and Xe-132 have similar release pattern, are strongly suggestive that the trapped component in ALH 84001 represents Martian atmosphere of s imilar to 4Ga ago. The noble gas elemental ratios Ar-36/Xe-132 and Kr- 84/Xe-132 show an elemental fractionation trend, enriching the heavy n oble gases, similar to what has been observed in Nakhla (Drake et al., 1994). Comparing the nitrogen and xenon isotopic records and the radi ogenic and stable isotope ratios (Ar-40/Xe-129 and Ar-36/N-14) from AL H 84001 representing Martian atmospheric component of similar to 4 Ga ago, with those from EET 79001,C representing Martian atmospheric comp onent of recent past, we infer the following on the evolution of the M artian atmosphere: (a) Xenon isotopic composition, as well as the amou nts of xenon have been completely evolved at 4 Ga in Martian atmospher e and almost remained unchanged to the present; (b)The radiogenic Ar-4 0 has not been completely degassed into the atmosphere at 4 Ga; (c) Ni trogen has been lost in a continuous process, leading to an increase i n the ratio of Ar-36/N-14 as well as the delta(15)N in the present Mar tian atmosphere as compared to 4 Ga ago. These inferences are consiste nt with the model predictions (Pepin, 1994). Copyright (C) 1997 Elsevi er Science Ltd.