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Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.390m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 390
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Hydrothermal Alteration, Impact Craters, Impact Heating, Puchezh-Katunki Impact Crater
Scientific paper
Research (using the results of deep drilling) of hydrothermal alteration in the Puchezh-Katunki Crater [1,2] enables us to construct a preliminary model of a hot-water circulation system in this giant astrobleme. Unlike previous reconstructions [3] that consider a circulation system originated in connection with the mass of impact melt in an ideal astrobleme, we examine this process in the impact crater as a whole. Considerable hydrothermal alteration of rocks is restricted by central uplift of the Puchezh-Katunki Crater. The preimpact temperature of the uplift's crystalline rocks (those that occur at 5-6 km in depth before impact) could be more than 100 degrees C. The principal factors that caused the creation of the circulation system are (1) the thermal field of the massif of brecciated and heated rocks and (2) crater lake formation. Shock- and friction-enhanced heating coupled with the influence of injecting impact melt masses generated an ellipsoidal-shaped thermal anomaly for 5-6 km in depth and near 600 km in volume. The heated massif was characterized by temperature values after equilibration from 500 degrees-600 degrees C in the center to 100 degrees-200 degrees C at the edges. The porosity of rocks decreased at depth and outward from the center as well. Hydrothermal convection took place when water from a ring trough lake infiltrated the lens of hot and porous impact breccia and basement rocks, reaching the surface at the uplift's margins and in the bottom of the central pit. A meteoric origin of circulated water is corroborated by isotopic values of fracture-filling calcite (delta ^18O = 21-24 per mil SMOW; delta ^13C = -20-3 per mil PDB) and anhydrite (delta ^18O = 8-10 per mil SMOW). There is no reliable information about addition of any juvenile substance in the circulation system. These facts support the subsurface origin of hydrothermal circulation. The united regressive hydrothermal process may be subdivided into three successive stages (Fig. 1): 1. Inital stage. The temperature decreased gradually downward and outward from impact center, remaining above 400 degrees-500 degrees C for several kilometers in depth. The circulation front was disposed at the borders of the zone of superheated (in comparison with water boiling point) rocks. 2. Main stage. The circulation system comprised a maximal amount of rocks. Inversion of the thermal field took place because the upper zone of central uplift cooled much more rapidly than the massif's center. Thermal maximum (200 degrees-300 degrees C) occurred at a depth of 2.5-4 km. Vertical zonation of hydrothermal mineral distribution [4] created during this stage is caused by both thermal gradient (30 degrees-100 degrees C/km) and evolution of compositions of ascending solutions. 3. Final stage. Circulation was restricted by the uppermost zone of basement and overlain polymict breccia, suevites, and coptomict sediments. It subsided gradually as the decreasing of permeability caused by the deposition of clay sediments and cementation of fractures took place as well as the temperature falling from 100 degrees-150 degrees C. Formation of zeolite-calcite cement matter in a 100-m layer of crater lake deposits supposes that the circulation remained active for a long time. This formation ceased as the temperature in the central uplift fell until it was equilibrated with the geothermal gradient. A proposed model demonstrates the principal trend of an impact-generated hydrothermal circulation process. Subsequently, working out of this model could give a quantative estimation of postimpact processes acting in astroblemes. References: [1] Masaitis V. L. and Mashchak M. S. (1990) Meteoritics, 25, 383. [2] Pevzner L. A. et al.(1992) LPS XXIII, 1063-1064. [3] Newsom H. E. (1980) Icarus, 44, 207-216. [4] Naumov M. V., this volume. Fig. 1, which appears here in the hard copy, shows the three stages of the hydrothermal circulation process.
Masaitis Victor L.
Naumov M. V.
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