Pyroxenes from Governador Valadares and Lafayette: a Nitrogen and Noble

Introduction: The Martian meteorites Governador Valadares (GV hereafter) and Lafayette (Laf. hereafter) are finds [1]. GV was found in the Minas Gerais Region (Brazil) in 1958. Laf. was recognized in the collections of Purdue University in Lafayette (Indiana) in 1931. Both belong to the group of nakhlites, which consists of 7 meteorites, including 2 from Antarctica and 2 from Northwest Africa. The nakhlites are igneous rocks composed of clinopyroxene with minor amounts of orthopyroxene, olivine and mesostasis. From textural and chemical lines of evidence they are regarded as cumulates. For detailed information see the comprehensive review by [2]. Here we discuss six components contributing to bulk noble gas signatures: radiogenic He, cosmogenic nuclides (in Ne and Ar), Martian interior, fractionated and unfractionated Martian atmosphere, and fractionated terrestrial atmosphere. We also present nitrogen data. Nitrogen and noble gases from literature: The only study on nitrogen reports stepwise combustion data for both meteorites [3]. For GV noble gas data (He, Ne and Ar only) first were published by [4]. A set of all five noble gases was measured by [5]. For Laf. four data sets exist: In 1966 [6] reported Xe data on Nakhla and Laf. He, Ne and Ar in Laf. were measured by [7], Ar, Kr and Xe data are provided by [8] on bulk and by [9] on iddingsite. Samples: We investigated bulk samples and pyroxene separates (purity checked by SEM). In the bulk samples only noble gases were analysed, whereby the heavy noble gases were compromised by the sample preparation. As a same sample aliqot was required for a parallel study, the samples had been ground, which is known to introduce elementally fractionated heavy noble gases [10]. Results: Only sums and selected isotopic ratios are shown in Tab. 1. Helium. In principle, helium is expected to be lost due to shock during launch. [11] calculated loss of radiogenic He of 42 % and 58 % for Laf. and GV, respectively. For this the age of the meteorite and U and Th concentrations are needed. Unfortunately no elemental data exist for the pyroxenes. Only phosphates of Laf. have been analysed for U and Th [12]. Comparing the concentrations of U and Th in phosphate with concentrations in bulk [13] and the amount of phosphate in the whole rock given by petrological observations (~0.5 % [14], 0.97 % [2]), it can be seen that the amount of U and Th from phosphates already is adequate to (or even exceeds) the whole rock abundance. In contrast, the observed He content of pyx is 5 % (Laf.) and 16 % (GV) lower than that of the bulk rock. The almost uniform disribution of He in the rock is, at least in part, a result of the small grain size (20–60 μm [13]) of the phosphate, which allows implantation of the α-particles into neighbouring phases. Two additional effects may have contributed: The phosphates, described to be surrounded by pyx or plagioclase [13], were unintentionally remaining in the pyx separate or the Herad was redistributed by the shock related to the meteorites’ launch. Nominal calculated cosmic ray exposure ages are slightly higher for pyroxenes than for bulk. With chemical data given by [2] we calculate 12.2 Ma for pyx compared to 10.4 Ma for bulk (GV) and 11.6 Ma for pyx compared to 11.2 Ma for bulk (Laf.). Neon. Comparing pyx and bulk, concentrations and isotopic ratios are similar. 92 % and 96 % of Ne is released in the 1200 °C step in Laf. and GV, respectively. Ne is essentially purely cosmogenic. The (Ne/Ne)c ratio agrees well with calculations based on production rates [15] for the chemistry of the pyx and bulk rocks [2] assuming a radius in the range 5 to 10 cm. Argon, Krypton and Xenon. As mentioned above, our bulk samples were contaminated by the introduction of elementally fractionated air. Like neon, argon in GV and Laf. is dominated by the cosmogenic component, Ar being completely cosmogenic in all but the lowest (400 °C) T-steps. Fig. 1 shows Xe/Xe vs. Kr/Xe for the pyx of this study, mineral separates from Nakhla [16] as well as other nakhlite literature data. For Nakhla, where several measurements of bulk rock are available from literature, the sums of pyx and olivine from [16] fall well into the region defined by these data. For all samples mixing of a Chassignylike endmember with fractionated Martian atmosphere can explain the results, with olivine containing less fractionated atmosphere than pyx and bulk. As ~74 (Laf.) to 81 % (GV) of the rock are augite [1, 2] pyx resembling the bulk is not surprising, if the considered element is contained in the main and not in an accessory phase. For GV and Laf. few data exist. Nevertheless, comparison of pyx, which falls onto the mixing line Chassigny–Martian atmosphere, with bulk results, which are shifted to higher Xe/Xe and lower Kr/Xe [8], indicates that pyx is not necessarily the carrier of the fractionated component in Laf. This question is addressed in detail in a second abstract [22]. Lunar and Planetary Science XXXVII (2006) 1612.pdf