Principal features of impact‐generated hydrothermal circulation systems: mineralogical and geochemical evidence

Abstract

Any hypervelocity impact generates a hydrothermal circulation system in resulting craters. Common characteristics of hydrothermal fluids mobilized within impact structures are considered, based on mineralogical and geochemical investigations, to date. There is similarity between the hydrothermal mineral associations in the majority of terrestrial craters; an assemblage of clay minerals-zeolites-calcite-pyrite is predominant. Combining mineralogical, geochemical, fluid inclusion, and stable isotope data, the distinctive characteristics of impact-generated hydrothermal fluids can be distinguished as follows: (i) superficial, meteoric and ground water and, possibly, products of dehydration and degassing of minerals under shock are the sources of hot water solutions; (ii) shocked target rocks are sources of the mineral components of the solutions; (iii) flow of fluids occurs mainly in the liquid state; (iv) high rates of flow are likely (10 )4 to 10 )3 m s )1 ); (v) fluids are predominantly aqueous and of low salinity; (vi) fluids are weakly alkaline to near-neutral (pH 6-8) and are supersaturated in silica during the entire hydrothermal process because of the strong predominance of shock-disordered aluminosilicates and fusion glasses in the host rocks; and (vii) variations in the properties of the circulating solutions, as well as the spatial distribution of secondary mineral assemblages are controlled by temperature gradients within the circulation cell and by a progressive cooling of the impact crater. Products of impact-generated hydrothermal processes are similar to the hydrothermal mineralization in volcanic areas, as well as in modern geothermal systems, but impacts are always characterized by a retrograde sequence of alteration minerals.

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

“…The range of temperature variations, in space and time, results in specific secondary paragenetic mineral associations. The latest stages of the cooling occurs at temperatures below 300 °C, when temperatures last for exponentially longer times, results in a small number of mineral phases (mineral associations), typically characterized by clay mineral-zeolite-calcite-(pyrite) association (Naumov 2005). This alteration is rockdominated and dependent on target lithologies .…”

“…There is an additional early phase of K-metasomatism accompanied by minor albitization and chloritization at temperatures of approximately 200-300 °C. The predominance of alkali and calcic zeolites is indicative of weakly alkaline hydrothermal solutions, which is typical for impact-induced hydrothermal systems in general (Naumov 2002(Naumov , 2005Osinski 2005). The source of the fluids for the Ries hydrothermal system was likely a combination of surface (meteoric) waters that percolated down from the overlying crater lake and groundwater that flowed in from the surrounding country rocks into the hydrostatic space created by the impact event (Osinski 2005).…”

“…Smectite is one of the major alteration products of volcanic glass (e.g., Tomita et al 1993;Fuente et al 2002) and it is recognized as an important secondary phase in impact glasses (e.g., Naumov 2005). Usually, the formation of smectite from a siliceous glass proceeds through removal of excess of Si and alkali compounds.…”

“…The same spatial structure and specific alteration gradients of the hydrothermal alteration are described for IHT systems. There the most intense alteration with thermal gradients in order of 100 °C km −1 occurs within the hottest central parts of an impact structure (Masaitis and Naumov 1993;Naumov 2005;Versh et al 2005).…”

“…Impact-induced hydrothermal activity (IHT) occurs following meteorite impacts in water-or ice-rich targets due to differential temperatures in the impact heated and/or melted crater rocks (see Naumov [2005] for a review). 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).…”