Xenon and Argon Isotopes in Irradiated, Etched Nakhla: Characterising the Host of Martian Atmospheric Xenon

Workshop on the Issue of Martian Meteorites 7022.pdf

XENON AND ARGON ISOTOPES IN IRRADIATED, ETCHED NAKHLA: CHARACTERISING THE HOST OF MARTIAN ATMOSPHERIC XENON. J. D. Gilmour, J. A. Whitby, R. Burgess and G. Turner. De- partment of Earth Sciences, University of Manchester, Manchester, M13 9PL, England (email Ja- [email protected])

Introduction: A noble gas component with the and pyroxene but concentrated in mesostasis[3]. elevated 129Xe/132Xe ratio characteristic of the Martian Combining our data with literature abundances of the atmosphere is found in Nakhla (and the other Nak- major minerals [4] suggests that around 10% of the hlites)[1,2]. It is elementally fractionated with Martian atmosphere derived component is found in 84Kr/132Xe values depleted by factor of ~5 relative to olivine, approximately half the remainder being asso- the Martian atmospheric value (as measured in ciated with pyroxene and half with mesostasis. We EET79001 lithology C). We believe this fractionation have now supplemented these data with analyses of reflects the processes by which it was derived from the Nakhla samples crushed and etched in water and acid. Martian atmosphere and incorporated in the meteor- Experimental: 210 mg of Nakhla was lightly ite. If, as has been suggested, aqueous alteration is ground to <400 mm and divided into 3 subsamples: responsible for this fractionation the xenon component Nak1 (61.42 mg), Nak2 (97.99 mg), Nak3 (57.96 mg). should be associated with ‘iddingsite’[2]. Our aim is A finer grain size was not used because of the antici- to locate the host phase(s) of the Martian atmosphere pated difficulty in handling and analyzing powders xenon component and so determine what led to its after irradiation. Samples Nak2 and Nak3 were de- incorporation and whether the same process can ac- rived from the untreated sample by 20 hours’ dissolu- count for the elemental fractionation. tion in water and 0.3 molar HNO3 respectively. The We have previously reported analyses of mineral solutions above samples Nak2 and Nak3 were ana- separates from Nakhla that showed that a component lyzed for a selection of cations using ICP-OES, while with elevated 129Xe/132Xe is associated with olivine anions from the solution above Nak2 were analyzed by ion chromatography (the ionic strength of the acid solution above Nak3 prevented use of this technique). Aliquots of each sample were prepared and sub- 10 jected to neutron irradiation. After such irradiation, potassium, chlorine, calcium, iodine, barium and ura- nium concentrations can be measured as enrichments 1 in the daughter isotopes of argon and xenon produced after neutron capture. Xenon isotopic analysis using RELAX and conventional argon isotopic analysis of these samples are ongoing. Here we report results of 0.1 'Rust' laser step heating analyses of aliquots of Nak2 and Nak3. Results: Chemical analysis of the solutions above 0.01 Nak2 and Nak3 revealed that the major element removed during the water etch was chlorine (20% of the expected budget), traces of sodium and sulphur also being present. Acid treatment removed around 6% of 0.001 Silicon Normalised Concentration the sample mass, the elemental concentrations being consistent with olivine dissolution and a contribution Olivine from iddingsite and phosphate (figure 1). 40 39 .0001 Argon: There is no variation among the Ar/ Ar Si Fe Mg Ca Al Na P S age data from the three separates. The most dramatic difference is a reduction in the chlorine content of Nak2 (as expected from analysis of the solution) and its near-complete removal from Nak3 (figure 2). Figure 1. The solution above the acid treated sample Xenon: There is no evidence for any depletion in (Nak3) is dominated by the dissolution products of oli- the Martian atmosphere-derived component (as meas- vine. Minor elements suggest additional dissolution of 129 ured by excess Xe) between Nak2 and Nak3, and phases including aqueous alteration products. Olivine both are consistent with previously reported bulk val- and rust compositions from [5]. ues [2]. In Nak2, there is a good correlation between Workshop on the Issue of Martian Meteorites 7022.pdf

XENON AND ARGON ISOTOPES IN ETCHED NAKHLA: J. D. Gilmour et al.

likely that it is fine grained and mesostasis is again implicated. The failure of acid etching to remove xe-

0.6 non in spite of its dissolution of olivine suggests that, although some excess 129Xe is present[6], iddingsite is not a major carrier. This work reveals differences between the Nakhlite component and the similarly elementally fractionated noble gas component present in ALH84001. In par- 129 132 0.4 ticular, the Xe/ Xe ratio of the trapped component in ALH84001 is 2.0 ± 0.2 [7] while Nakhla yields 2.4. Furthermore, the dominant release of excess 129Xe from ALH84001 occurs at high temperature and is associated with orthopyroxene[7]. It seems that simi- lar Kr/Xe fractionations in the Nakhlites and 0.2 ALH84001 do not indicate a common origin. References: [1] Ott U. and Begemann F. (1985) Na- Chlorine/Potassium (nominal g/g) ture 317 509-512. [2] Drake M. J. et al., (1994) Meteoritics 26 854-859. [3] Gilmour J. D. et al. (1998) E.P.S.L. (sub- mitted). [4] Treiman A. H. (1986) Geochim. Cosmochim. Acta 50 1061-1070. [5] Gooding J. L. et al.(1991) Mete- 0 oritics 26 135-143. [6] Swindle T D. at al. (1995) L.P.S.C. 0 20 40 60 80 100 XXVI (1385-1386). [7] Gilmour J.D. et al. (1998) Geo- Percentage K Release chim. Cosmochim. Acta (in press).

Figure 2: Water treatment (Nak2 - solid blue bars) removes around 20% of the chlorine content of (Nak1 - open bars). Acid treatment removes nearly all of the 4 chlorine in these samples (Nak3 - solid red bars). 3 1.0 iodine and 129Xe at low temperature (Figure 3). The iodine concentration is reduced by a factor close to 2 2 129

by acid treatment. Somewhat surprisingly this does Xe 129 0.5 XS

not remove excess Xe and so destroys the correla- Barium (ppm) tion. Around 50% of the Martian atmospheric xenon 1 (/10

is released at low temperature in this sample. -12

Discussion: One Martian atmosphere component 0 0 cm 2

is released at low temperature and is associated with 3 STP g iodine, one is released during the high temperature 1.0 steps in which barium-derived gas is evolved. In the -1

light of our work on separates, it is reasonable to as- ) sign the first release to mesostasis and the second to 1 0.5 pyroxene, though further work will be required to Iodine (ppb) confirm this conclusion. The partial removal of iodine during the acid etch stage of our analyses (along with near-total removal of 0 0 chlorine) and the associated destruction of the Martian 0 5 10 15 20 atmosphere xenon-iodine correlation is difficult to Release Step explain. We do not have accurate temperature data, but comparison of the release patterns suggests iodine Figure 3: Laser step heating of Nak2 shows that ~50% and chlorine may not be associated. The low tem- of the excess 129Xe correlates with iodine in a low temperature correlation between iodine and xenon is re- perature release. The remaining, high temperature markably good and seems to require a common host xenon seems associated with a barium-rich phase. phase. Leaching of iodine from this host phase may Acid treatment removed ~50% of the iodine without be capable of producing this effect; if so, it seems removing xenon, and so destroyed the correlation.