Fluidization and hydrothermal alteration of the Suevite deposit at the Ries Crater, West Germany, and implications for Mars

Abstract

The formation, cooling, and alteration of the impact ejecta deposits at the Ries crater are an analog for impact processes and the formation of regoliths and soils on Mars. We have studied the suevite deposit, an impact‐melt‐bearing breccia found outside of the crater rim. This suevite contains large numbers of chimney‐like degassing pipes, apparently formed by high gas pressures in the hot (>500°C) fluidized suevite immediately after deposition of the suevite. The likely sources of the gas are volatiles from the melted and shocked basement inclusions. The suevite outside of the crater rim contains about 15 wt % clay, indicating a large amount of alteration of the suevite. The clay is an Fe‐rich montmorillonite with less than 5% interstratified illite, indicating that the temperature at which most of the alteration occurred was less than 100°–130°C. The duration of this low‐temperature period may have been extended by additional heating from the exothermal alteration reactions. The surfaces of the degassing pipes are not altered to clays, however, but have a thin Fe‐rich oxide or hydroxide coating. The absence of alteration in the pipes suggests that the main alteration event outside the crater occurred under unsaturated conditions, such that the walls of the pipes were dry. Suevite deposits are likely to exist on Mars, because of the presence of volatile‐rich target rock. Martian suevites may have also been fluidized and hydrothermally altered, contributing significantly to the formation of the clay‐like phases inferred to exist in the Martian soil.

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

“…Alteration of Ries suevites has been noted since the early 1970s (e.g., Engelhardt 1972;Chao et al 1978;Newsom et al 1986;Osinski 2005). Indeed, the IHT alteration in crater-fill suevites is well defined.…”

“…Newsom et al 1986;Osinski et al 2004;Osinski 2005). It has been generally accepted that these clays formed by postimpact hydrothermal alteration of impact-generated glasses and/or finely comminuted crystalline basement material (Engelhardt 1972;Stähle 1972;Engelhardt and Graup 1984; Newsom et al 1986Newsom et al , 1990.…”

“…He suggested that the interior surfaces of the degassing pipes are coated with (hydrothermal) clays. Newsom et al (1986) studied the degassing pipes in detail and found that the surfaces of degassing pipes are not coated with clay minerals, but a thin (<2 µm) layer of iron oxide or hydroxide. They suggested that the absence of clay in the pipes indicates that degassing pipes have probably remained mostly dry since the original degassing event occurred, indicating that the suevite was in the unsaturated or vadose zone during the main period of the hydrothermal alteration.…”

“…Newsom et al (1986) suggested that ∼10-15 vol% of the montmorillonite is of hydrothermal origin in the groundmass of surficial suevites. However, Engelhardt (1997) and Osinski et al (2004) reported that the suevite groundmass contains up to ∼50 vol% of "clayey" material that is evidently a hydrous silicate (montmorillonite, according to Engelhardt 1997); however, the exact nature and origin of this clayey groundmass remains poorly constrained.…”

Traces of catastrophe: a handbook of shock-metamorphic effects in terrestrial meteorite impact structures

“…The source(s) of this carbon has not been estab- The thermal energy deposited in an impact structure as shock heating and impact-melt formation can produce hydrothermal activity and related ore deposits similar to those that result from more conventional geological processes. Such postimpact activity is frequently expressed in the secondary alteration of the impact melts themselves (Dence, 1971;Newsom et al, 1986;McCarville and Crossey, 1996). However, in larger structures, the combination of large volumes of melt and extensive hydrothermal circulation may produce new sedimentary deposits and associated ore bodies, e.g., the Vermillion Formation at Sudbury (Canada) (Grieve and Masaitis, 1994).…”