Cold Desert Alteration of Martian Meteorites: Mixed News from Noble Gases, Trace Elements and Oxygen Isotopes

Introduction: Currently, about 30% of the known Martian meteorites have been found in Antarctica, with some rare samples such as the orthopyroxenite AHL84001 amongst them [1]. We have previously shown [2, 3] that terrestrial weathering in hot and cold deserts can cause elemental fractionation of air incorporated into those meteorites, which complicates the quantification of the noble gas components. This can cause serious contamination, partially obscuring the extraterrestrial heavy noble gas signatures [e.g., 2] by, for example, mimicing the Martian interior component [4]. This is a critical problem because key inferences have been made on the basis of these noble gas signals (e.g. Martian atmosphere in EETA79001, fractionated or ancient Martian atmosphere in ALH84001 (see [5]), and they are the finds that have a well documented history). In this study we investigate the rim and interior of ALHA77005 and RBT 04261 Martian meteorites to characterise terrestrial air contamination, and derive potential correction procedures. Samples and methods: We obtained rim and interior samples of ALHA77005 (splits 225 (rim), 226 (close to rim), and 227 (interior), thin section 54, thick section 230); and of RBT 04261 (splits 34 (rim), 35 (close to rim) and 36 (interior), thin section 43, thick section 42). Note, that the terrestrial residence ages for the meteorites are different: 190±70 kyr for ALHA77005 [6], and less than 60 kyr for RBT 04261 [7]. Microscopy, virtual microscope documentation, and electron microprobe analyses (Cameca SX 100) were done at the OU. Each split was gently crushed. For Ar-Ar analyses ~40 bulk rock grains were handpicked, irradiated at McMaster reactor (Canada), and measured by melting grains individually by laser heating for Ar isotope analysis (MAP215-50). In addition stepwise heating was performed on a larger grain of each sample. For the five-noble gas measurements ~50 mg were heated in five steps and measured using the standard protocol [2]. Mineral trace-element measurements were made using an Agilent 7500s ICP-MS coupled with a UV (213 nm) laser system. Synthetic doped glass NIST 612 was used for external calibration. Oxygen isotope analyses were undertaken at the OU using an infrared laser-assisted fluorination system [8].  17 O values have been calculated using a linearized format [9]. Samples were analyzed both untreated and after leaching in a solution of ethanolamine thioglycollate (EATG), which has proved to be efficient at removing terrestrial weathering products, without significantly disturbing the primary oxygen isotope composition of the sample [10]. Mineralogy: Microscopic and microprobe investigation confirmed that weathering effects in these meteorites from Antarctica are minimal. Along cracks Ca, Si and S enrichment, and Fe and Mg depletion occur in ALHA77005. RBT 04261 has some fusion crust, which is intact and visible alteration is minimal, which is in line with its shorter terrestrial residence time. Trace elements: A number of silicates (olivine, pyroxene, feldspar) were analyzed from both meteorites for their trace element contents. We specifically analyzed cores, rims and other areas of mineral grains located near large cracks to cover the possible spectrum of trace element variations. Our preliminary analyses of the data suggest limited variations in trace element concentrations from different areas of mineral grains from both meteorites although there are subtle compositional differences which could be ascribed to the effects of terrestrial weathering.