Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

• Published in: Journal of geophysical research. Planets 2016-10, Vol.121 (10), p.2120-2149
• Main Authors: McCubbin, Francis M., Boyce, Jeremy W., Novák‐Szabó, Tímea, Santos, Alison R., Tartèse, Romain, Muttik, Nele, Domokos, Gabor, Vazquez, Jorge, Keller, Lindsay P., Moser, Desmond E., Jerolmack, Douglas J., Shearer, Charles K., Steele, Andrew, Elardo, Stephen M., Rahman, Zia, Anand, Mahesh, Delhaye, Thomas, Agee, Carl B.
• Format: Article
• Language: English
• Subjects: Annealing; Apatite; Basalt; Bed load; Breccia; clast; Crusts; Crystallization; Deposition; Deposits; Devitrification; Earth Sciences; Fragmentation; Geological history; Geological time; Geology; Glass; High temperature; Impact analysis; Lithification; Lithology; Load distribution; Mars; Mars (planet); Mars surface; Mars volcanoes; Oxidation; Particle size distribution; Petrography; Planetary geology; Pulverized; Radiometric dating; Rainout; Regolith; Rocks; Sciences of the Universe; sedimentary; Stress concentration; Surface layers; Texture; transport; Volcanic activity; Zircon
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1002/2016JE005143

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali‐rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K‐Ar, U‐Pb in apatite, and U‐Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre‐Amazonian high‐temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification. Key Points The Martian breccia NWA 7034 is an air fall deposit that was likely formed by impact processes on Mars The earliest crust on Mars formed through alkali‐rich volcanism under oxidizing conditions Regolith formation processes on Mars were driven by impact/volcanic processes, whereby unconsolidated materials were fused together at high T