^1,2,3A. I. Sheen,¹C. D. K. Herd,^2,3K. T. Tait
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70002]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada
³Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

Accurate dating of Martian meteorites is crucial for understanding key events in the planet’s evolution. However, not all Martian meteorites are amenable to dating techniques currently in use for these rocks. The priority of sample preservation precludes mineral separation methods for low-volume specimens, whereas the less destructive in situ SIMS U-Pb method depends on the availability of U-bearing accessory minerals. Micromilling allows for spatially guided sampling of target phases down to the sub-mm scale, therefore enabling chromatography-based analysis while preserving the overall specimen. This study presents an evaluation of micromill sampling for extracting individual mineral fractions in situ from shergottites, the most common group of Martian meteorites, for Rb-Sr and Sm-Nd geochronology. Based on trace element content in major minerals in shergottites (pyroxene, plagioclase, olivine, and merrillite) and assuming that a minimum load size of 0.25 ng Sr and 1 ng Nd is required to achieve baseline isotopic precision (2σ of ~240 ppm on 87Sr/86Sr and ~100 ppm on 143Nd/144Nd), the minimum required sample volume ranges in the orders of 105–107 μm3 for one Sr isotopic analysis and 105–109 μm3 for one Nd isotopic analysis. Considering the need for sample purity, significant limitations exist in the maximum sampling resolution of the micromill instrument (~40 μm for the conical carbide drill bit chosen for this study) with respect to shergottite petrography. Insufficient grain size, irregular morphology, and the presence of small inclusions may reduce the area that can be drilled per grain. Shock-induced fractures, which sometimes act as pathways for terrestrial alteration, are pervasive in shergottites and create additional challenges for effective high-purity sampling of the target phase. In addition, variation in trace element content in the target phases may result in the realistically required drilling volumes being orders of magnitude greater than the minimum estimates. Lastly, estimated drilling time per fraction may reach over 5 h for pyroxene (Sr, Nd), plagioclase (Nd), and olivine (Sr, Nd), increasing the susceptibility to a larger procedural blank as well as requiring constant, labor-intensive monitoring for long durations. Based on these technical and physical constraints, we do not consider micromill sampling to be currently compatible with Sr isotopic analysis of olivine and Nd isotopic analysis of pyroxene, plagioclase, and olivine in shergottites. The feasibility of geochronology applications may be improved with future advances in analytical development, such as increasing the micromill sampling resolution and reducing the load size required for isotopic analysis.

¹Libby D. Tunney,²Aaron B. Regberg,¹Christopher D. K. Herd,³Richard E. Davis,⁴Christian L. Castro
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70008]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, Texas, USA
³Texas State University, NASA Johnson Space Center, Houston, Texas, USA
⁴JES Tech, NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Meteorites are easily contaminated at the Earth’s surface by microbial activity. Here, DNA extracts from two meteorite specimens and samples from curation laboratory surfaces are analyzed with amplicon sequencing, to understand microbial communities that contaminate meteorites and that may be resident in curation facilities. In addition, two different DNA extraction kits, the PowerSoil DNA Isolation Kit and the QIAamp UCP Pathogen Mini Kit, are utilized to determine if certain kits are more favorable for low biomass studies of meteorites. We find that, regardless of the type of kit used, the majority of microbial taxa that dominate meteorite and meteorite curation environments include those that are prevalent in soils or in the human microbiome. Our results have implications for advanced curation methods to protect the intrinsic properties of meteorites, such as extraterrestrial organics and minerals, from microbes. Preserving meteorites in pristine states and understanding the complex relationship between meteorites and terrestrial microbes can inform our search for the origin of life or life elsewhere in the universe.