Hydrothermal alteration at the Lonar Lake impact structure, India: Implications for impact cratering on Mars

Abstract

Abstract-The 50,000 year old, 1.8 km diameter Lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock (the 65 million year old Deccan Traps). The composition of the Lonar basalts is similar to martian basaltic meteorites, which establishes Lonar as an excellent analogue for similarly sized craters on the surface of Mars. Samples from cores drilled into the Lonar crater floor show that there are basaltic impact breccias that have been altered by post-impact hydrothermal processes to produce an assemblage of secondary alteration minerals. Microprobe data and X-ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite, with minor celadonite, and carbonate. Thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130°C and 200°C. By comparing the Lonar alteration assemblage with alteration at other terrestrial craters, we conclude that the Lonar crater represents a lower size limit for impact-induced hydrothermal activity. Based on these results, we suggest that similarly sized craters on Mars have the potential to form hydrothermal systems, as long as liquid water was present on or near the martian surface. Furthermore, the Fe-rich alteration minerals produced by post-impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil.

A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology

“…Celadonite is a dioctahedral mica mineral that occurs mostly within vesicles of basaltic rocks and tends to form during their hydrothermal alteration (Deer et al, 1992;Meunier, 2005). Celadonite has been found to precipitate with Fe-rich smectites in the Lonal Lake impact structure, a classic Mars analog hydrothermal system activated by a bolide impact (Hagerty and Newsom, 2003). Geochemical modeling of Lonal Lake alteration carried out by these same authors indicates that this hydrothermal assemblage was deposited at temperatures of 130-200°C under non-ambient conditions.…”

First petrographic results on impactites from the Yaxcopoil‐1 borehole, Chicxulub structure, Mexico

“…Many fragments, however, and especially all mafic melt particles, show variable degrees of alteration, in accordance with the pervasive interaction of fluid phases after deposition. Such alteration has been observed at several impact sites, notably at the Ries, Lonar, and Mistastin Craters, where the impact melt particles and glasses were converted to saponite and montmorillonite (e.g., Osinski 2003;Hagerty and Newsom 2003). Variable degrees of K-metasomatism are also indicated by the K contents of mafic particles (Fig.…”

Post‐impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?

“…IHT is a well known phenomenon that has been found at several terrestrial craters varying in size and target compositions (e.g., Koeberl et al 1989;McCarville and Crossey 1996;Ames et al 1998;Sturkell et al 1998;Naumov 2002;Osinski et al 2001Osinski et al , 2005Kirsimäe et al 2002;Hagerty and Newsom 2003;Hode et al 2003;Versh et al 2005); similar features have also been suggested for impacts on extraterrestrial planetary bodies (e.g., Allen et al 1982). At the Ries crater, IHT has been addressed in detail by Newsom et al (1986) and Osinski (2003Osinski ( , 2005, and Osinski et al (2004).…”