EVALUATING Kr- AND Xe-DATA IN THE NAKHLITES AND ALHA

Results on nakhlite Martian meteorites and on ALHA84001 have led to a debate on the noble gas components contributing to the inventory of these meteorites and the ways to introduce them [1-7]. In a diagram Xe/Xe vs. Kr/Xe they fall off the mixing line Martian interior – Martian atmosphere tending towards lower Kr/Xe-ratios. This was discussed already in 1988 by [8], who suggested incorporation of elementally fractionated Martian atmosphere, possibly as a result of adsorption. Here we evaluate the components contributing to the signatures in Lafayette from a unique set of samples: Lafayette is the only SNCmeteorite from which Kr and Xe data for bulk [9], pyroxene [10], and the alteration product iddingsite [11] are available. We estimate the Kr/Xe-ratio of the elementally fractionated Martian atmosphere (EFM hereafter) component, apply a correction procedure for elementally fractionated air (EFA as first discussed by [12]) on results from other nakhlites and ALHA84001 and discuss the results in the context of an iddingsite formation model based on petrological and geochemical observations [13–15]. Evaluation of components: For our calculation we use Martian interior (MI hereafter) from [4] and unfractionated Martian atmosphere (MA hereafter) from [16]. Xe isotopic ratios of EFA and EFM are assumed to be identical to those in air and MA, respectively. All percentages in the decomposition refer to Xe. An attempt to disentangle Lafayette. Lafayette bulk in principle contains MI, MA, EFM and EFA. We evaluate the amount of EFA in Lafayette from the 1200 °C-step and the sum of our pyx measurement [10] to be 22 % (for Xe). For this we assume the 1200 °C-T-step as free from terrestrial contamination. This results in Kr/Xe of EFA=3.6 in Lafayette. Furthermore the 1200 °C-T-step of pyx, falling onto the mixing line in Figs 1–2, can be split into 96 % MI and 3 % MA. Taking the pyx as the basis for the following calculation requires that it is representative for bulk rock minus terrestrial contamination. We regard this as justified by the facts that a) ~74 % of Lafayette are pyx, and olivine plus pyx add to >90 % of the sample [17], b) olivine and pyx are rather similar in their Xe/Xe ratios in Nakhla [18]. Iddingsite is a Martian alteration product [13–15], thus it should contain EFM. In the analysis of [11] iddingsite was melted by laser fusion. Therefore the data include also the terrestrial contamination. If we assume the terrestrial contamination being equal in iddingsite and pyroxene (which is by no means necessarily true) we obtain Kr/Xe=6.6, and Xe/Xe=2.34 for EFAcorrected iddingsite. Thus, the corrected sample can be nominally split up into 17 % MI and 83 % EFM (Ignoring EFA and splitting the uncorrected iddingsite results in 36 % MI + 64 % EFM). Finally we arrive at 73 % MI, 3 % MA, 2 % EFM, and 22 % EFA for Lafayette bulk. Kr/Xe of EFM. From Fig. 1 can be seen that EFM extrapolated with the reported errors of the iddingsite measurement [11] can range between 4 and 13.5, values readily observed in terrestrial weathering environments. Taken at face value, the uncorrected iddingsite would require an EFM endmember of Kr/Xe=8.7, the corrected 7.7. For the further calculations we assume a constant Kr/Xe for EFM of 8±1.