Mineralogy, Petrology, and Shock History of Martian Regolith Breccia
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.