A feasibility assessment of micromill sampling for Rb-Sr and Sm-Nd geochronology of shergottites

Abstract

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.