Dating Magmatism on Mars: Application of In-Situ and Microsampling Techniques for Shergottite Geochronology
Creators: Sheen, Alex I-Fan
Publisher: name: University of Alberta Library
Year: 2022

<p>Establishing an absolute time scale of Mars is critical to understanding the planet’s geological evolution. Currently, martian meteorites are the only samples of the planet available for radiometric geochronology analysis in terrestrial laboratory settings. The record of known martian absolute ages is dominated by shergottites, the largest group of martian meteorites, which represent mafic to ultramafic extrusive to hypabyssal bodies. Shergottites are conventionally dated using isochron methods (e.g., Rb–Sr, Sm–Nd, Lu–Hf) involving mineral separation and chemical chromatography. On the other hand, advances in secondary ion mass spectrometry (SIMS) analysis have enabled in-situ U–Pb baddeleyite (ZrO2) geochronology, which has the potential not only to preserve the petrographic context but also to overcome challenges of resolving igneous versus secondary signatures in the major minerals and of the bulk rock. In-situ radiometric chronology of shergottites is therefore crucial for refining the martian meteorite age record as well as developing strategies for dating future returned samples from Mars. This dissertation presents three studies aimed at 1) investigating constraints associated with SIMS U–Pb baddeleyite geochronology and 2) assessing the feasibility of micromill extraction as a less destructive alternative to bulk crushing for Rb–Sr and Sm–Nd geochronology of shergottites.<br>
A comprehensive survey of petrography, mineralogy, and geochemistry in a suite of eight basaltic shergottites confirms an igneous origin of baddeleyite, which crystallized in association with evolved late-stage melts. Pyroxene major element compositions demonstrate that baddeleyite crystallization in these shergottites is controlled primarily by fractional crystallization; samples with pyroxene compositions evolving beyond the stability boundary at 1-bar pressure generally contain more abundant baddeleyite than those with pyroxene compositions terminating at or before the stability boundary. These observations demonstrate that pyroxene composition can be a useful proxy for evaluating baddeleyite distribution in unknown shergottite samples, enabling a preliminary feasibility assessment for SIMS U–Pb analysis prior to conducting detailed baddeleyite search. <br>
Electron backscatter diffraction (EBSD) analysis of baddeleyite microstructures in the enriched basaltic shergottites Jiddat al Harasis (JaH) 479, Northwest Africa (NWA) 10299, and NWA 12919 reveals widespread phase transformation to high-pressure orthogonal polymorphs (o-ZrO2) before reverting to the monoclinic structure (m-ZrO2). A larger proportion of grains preserving magmatic orientations in JaH 479 suggests it experienced lower bulk shock pressure compared to NWA 10299 and NWA 12919. <br>
SIMS U–Pb baddeleyite analysis yields igneous crystallization ages of 210±9 Ma (JaH 479; 2σ), 196±11 Ma (NWA 10299), and 188±11 Ma (NWA 12919). The results show no resolvable Pb loss in the analyzed shergottites, indicating that post-shock heating associated with the observed microstructures was insufficient to drive significant Pb diffusion in baddeleyite. <br>
The minimum volumes of olivine, pyroxene, plagioclase, and merrillite that need to be micromilled for an isotopic analysis (ID-TIMS or ICP-MS) are estimated to be between 10^5 and 10^7 μm^3 for Sr and between 10^5 and 10^9 μm^3 for Nd. Physical characteristics of shergottites, including grain size, morphology, compositional zoning, inclusions, and shock features, present significant limitations for obtaining pure mineral fractions using the maximum micromill sampling resolution of ~40 μm (at a drilling depth of 20 μm). In addition, variations in mineral composition may result in realistic sample volumes several orders of magnitude greater than these minimum estimates. These technical and physical constraints make micromill sampling unsuitable for conducting Rb–Sr or Sm–Nd geochronology of shergottites. <br>
The findings presented in this dissertation demonstrate that the physical characteristics of shergottites present unique challenges that must be considered when applying analytical methods that have been developed for terrestrial analogs, especially with respect to preserving the samples and resolving igneous signatures from secondary effects such as impact shock and alteration. SIMS U–Pb baddeleyite dating benefits from the use of pyroxene composition trends for assessing baddeleyite distribution and of EBSD microstructural analysis for constraining U–Pb isotope systematics under shock metamorphism. Although micromill sampling is not currently feasible for shergottite Rb–Sr and Sm–Nd geochronology, the identified technical and physical limitations provide guidelines for designing future analytical strategies. The findings presented in this dissertation contribute to a refined approach for applying in-situ and microanalysis techniques to dating shergottites and potentially future returned samples, with wider implications for expanding the absolute time scale of Mars.</p>