NITROGEN AND NOBLE GAS COMPONENTS IN THE MARTIAN ORTHOPYROXENITE ALH 84001.  S. V. S. Murty, R. K. Mohapatra, and C. J. Clement, Physical Research Laboratory, Navragpura, Ahmedabad 380 009, INDIA.

Published in Lunar and Planetary Science XXVI,pp. 1019-1020, LPI, Houston, TX.

A bulk sample and three density separates of ALH 84001 have been studied for nitrogen and noble gases by stepwise extraction. Neon is mostly cosmogenic and gives an exposure age of 16.4 Ma. The elemental ratio 84/132 >/= 4, while 129/139 varies between 1.52 to 2.22. The delta 15N varies between -21‰ in the density separate 3 g/cc and 201‰ in the bulk sample. In addition to a martian atmospheric component, at least two additional components are indicated by these results.

Introduction:  Soon after the realization that ALH 84001 is a martian meteorite [1] a number of studies have been initiated to characterize the chemical, isotopic, and mineralogical characteristics of the martian reservoirs [2-9]. ALH 84001 is different from the SNCs and is much older and probably represents the ancient martian crust [1]. We undertook a simultaneous study of nitrogen and noble gases in an effort to look for the atmospheric and crustal components of Mars and to delineate the cosmogenic history.

Experimental:  From a sample crushed to ~100 µm size, we separated three density fractions by using sodium polytungstate solution of density 3 g/cc. A bulk sample and the three density separates have been studied for nitrogen and noble gases by stepwise pyrolysis using standard procedures [10]. An initial 400°C combustion in 2 torr O2 is carried out to remove surficial contamination. All subsequent extractions are done by pyrolysis.

Results and Discussion:  Nitrogen and noble gas data for the bulk sample are given in Table 1. Nitrogen data for ALH 84001 is being reported for the first time.

Table 1:  Nitrogen and noble gases in a bulk sample of ALH 84001 (conc. 10-8 ccSTP/g). Sample weight = 692.38 mg.

delta 15N
    ±.0195.0051.0025.007.0013    .83

Neon and Argon:  Neon is mostly dominated by the cosmogenic component. Using 21Ne production rate [3,5] an exposure age of 16.4 Ma is obtained. This exposure age is the highest among the presently known martian meteorites. The measured value of 40/36 = 1662 for the bulk sample has been corrected for the cosmogenic component giving (40/36)corrected = 4210, which is much higher than the trapped value of ~2400 [11] in EET 79001,C, indicating a large amount of radiogenic 40Ar. Assuming the trapped 40/36 = 2400 is valid for ALH 84001, we calculate the radiogenic 40 = 391 × 10-8 ccSTP/g. With K = 80 ppm for the bulk [1], this translates into a K-Ar age of 3.74 Ga, which is much higher than the crystallization age of 1.3 Ga for the SNCs.

Nitrogen:  The bulk sample has 0.75 ppm N, with delta 15N = 85.7‰. Among the samples analysed delta 15N varies between a minimum of -21‰ in the 1000°C step of 3 g/cc density sample and a maximum of 201‰ in the 1200°C step of bulk sample. Part of this large positive delta 15N is due to the cosmogenic nitrogen. Using cosmogenic 21Ne and the ratio (15N/21Ne)c ~ 4 [12] we estimated the correction to the total delta 15N to be 39‰ and obtained the corrected delta 15N = 45.9‰. Since cosmogenic nitrogen is mostly released at the melting step only, the 201‰ value in the 1200°C fraction of the bulk sample is unlikely to be dominated by cosmogenic component and might indicate the presence of martian atmospheric component. Even the corrected bulk value of 45.9‰ is the largest positive value among the SNCs, after EET 79001,C [11], indicating the presence of some trapped N of martian atmosphere in ALH 84001. The lightest delta 15N value of -21‰ most probably represents the martian indigenous component.

Krypton and Xenon:  The isotopic ratios 129/132 and 136/132 are as high as 2.22 and 0.347 respectively, very close to the values found in EET 79001,C [13] but the value 84/132 ~ 6 is very low, unlike in EET 79001,C. In a three-isotope plot of 129/132 vs. 136/132 (Fig. 1) the ALH 84001 data fall on a mixing trend defined by the Chassigny and EET 79001,C end members, but not so in the plot of 129/132 vs. 84/132 (Fig. 2). Data for the other SNCs in the figures is taken from [11,13-15]. These trends most probably indicate the presence of an elementally fractionated component, which is most probably produced in the process of trapping atmospheric noble gases in the weathering products on Mars [14].

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