Thesis Format

Integrated Article

Mineralogy, Petrology, and Shock History of Martian Regolith Breccia

Author

Fengke Cao, Western UniversityFollow

Degree

Doctor of Philosophy

Program

Geology

Collaborative Specialization

Planetary Science and Exploration

Supervisor

Flemming, Roberta L.

2nd Supervisor

Izawa, Matthew R.M.

Affiliation

Institute for Planetary Materials, Okayama University

Co-Supervisor

Co-Supervisor

Abstract

Martian polymict regolith breccia meteorites (NWA 7034, 7475, 8171, 11220) formed through the accumulation of diverse lithologies, including igneous, sedimentary, and impact-derived products. These meteorites represent the oldest Martian rock (~ 4.5 Ga) and recorded complex multi-shock histories including pre-launch shock events and the ejection impact. Some clasts endured multiple shock events, while others seem to have avoided the shock effects altogether. Quantitative investigations into the shock effects remain limited, leaving the complex impact history not fully understood. The degree of cumulative shock experienced by the ancient Martian crust, as preserved in its minerals like feldspar, pyroxene remains inadequately explored. We utilized micro-XRD to investigate experimentally shocked plagioclase at 0−56 GPa and quantitatively evaluated shock-metamorphic effects in these minerals. Our observations revealed partial amorphization occurring in andesine at 29.6–33.8 GPa, complete amorphization at ~ 50 GPa. Bytownite becomes partial amorphization at ~ 25.5–27 GPa, complete amorphization at ~ 49 GPa. The measured FWHMχ (or ΣFWHMχ), representing the preserved shock levels in minerals, spans from 0.54°–10.93° in plagioclase, 2.31°–9.67° in alkali feldspar, 0.88°–9.76° in orthopyroxene, and 0.89°–10.65° in clinopyroxene. Only ~ 7% of plagioclase clasts exhibit shock pressure >14.4 GPa, with the highest of ~ 35.2 GPa. Our research suggests small fractions of feldspar and pyroxene minerals might have experienced frequent or intense collisions on Mars' surface. Plagioclase ranges from 20−60% An, where the predominant phase is andesine. Pyroxene has two clusters: high-calcium augitic and low-calcium pyroxenes. Olivine is identified in spherules, shard, and igneous clast. Phosphates include apatite, merrillite, YREE-merrillite, and monazite. The average water-equivalent hydroxyl concentration in apatite is ~ 0.36 wt%, YREE-merrillite has a high concentration of (Y+REE)₂O₃ (up to ~12.8 wt%). Some apatite grains are shocked, but some are not. Micro-XRD demonstrates efficacy in assessing the shock levels of minerals, particularly in the low to medium shock range. This study contributes to a deeper comprehension of Mars' crustal impact history and regolith evolution by delving into shock metamorphism in feldspar and pyroxene. The calibration work bears significance in quantifying shock pressure in plagioclase across various meteorites and future returned Mars samples.

Summary for Lay Audience

Rocks hailing from the surface of Mars have traversed vast distances to Earth as Martian regolith breccia meteorites. These specimens serve as time capsules from the Red Planet, concealing within them valuable insights regarding its composition and history. Researchers have been examining these meteorites to uncover fascinating details about the past evolution of Mars' surface. These meteorites include the oldest Martian mineral which crystallized at ~ 4.5 billion years ago - 100 million years older than the earliest known Earth rocks. An exceptional facet of these rocks is their exposure to impact processes, the result of high-velocity collisions of asteroids and comets onto the Martian surface. Such impact events produce a range of structural, textural, and chemical changes that can be used to reveal the history of the Martian surface. In the Martian regolith meteorites, some materials suffered severe shocks, while others were nearly unaffected. X-ray diffraction (XRD) is a powerful method employing the scattering of X-rays from crystalline matter to reveal the atom-scale structures. Micro-XRD extends the power of X-ray diffraction to very small targets and allows the study of individual components within complex rocks such as the Martian regolith breccias. Micro-XRD can also reveal the extent of shock-produced strain in crystalline matter. Employing micro-XRD, we measured the shock signatures recorded in both synthetically shocked mineral standards and in the Martian breccia meteorites. This approach enabled us to quantify the shock pressures experienced by these materials. Intriguingly, not all minerals exhibited uniform responses to shock. Most minerals seemingly bypassed the impacts entirely, but a small portion of the common rock-forming minerals feldspar and pyroxene did not. In addition to feldspar and pyroxene, we also found other minerals such as olivine from different types of clasts and four distinct phosphates. The insights gained from this study go beyond just understanding Martian mineralogy; they help us piece together the larger puzzle of Mars' crustal evolution across epochs by deciphering its impact history.

Recommended Citation

Cao, Fengke, "Mineralogy, Petrology, and Shock History of Martian Regolith Breccia" (2023). Electronic Thesis and Dissertation Repository. 9874.
https://ir.lib.uwo.ca/etd/9874