Postimpact hydrothermal alteration of the Manson impact structure

Details Postimpact hydrothermal alteration of the Manson impact structure Postimpact hydrothermal alteration of the Manson impact structure

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.499m&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 499-500

Mathematics

Logic

2

Core Sampling, Geotemperature, Iowa, Meteorite Craters, Structural Properties (Geology), Temperature Effects, Breccia, Lithology, Mineralogy, Scanning Electron Microscopy, Textures, X Ray Diffraction

Scientific paper

Core materials from the Manson impact structure (MIS), Manson, Iowa, are examined in order to evaluate postimpact alteration processes. Interpretation of the high-temperature postimpact hydrothermal system is based on mineralogic investigation. MIS rocks from the M1, M7, M8, and M10 cores obtained by the continental scientific drilling project (CSDP) in 1991 and 1992 are used in this study. All lithologies, including the sedimentary clast breccias (SCB), crystalline clast breccias (CCB), and central peak crystalline peaks (CPC), have been described previously. Emphasis is placed on fluid conduits that cross-cut all these lithologies. Analytical techniques include petrography, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). The minerals are grouped according to their temperatures of occurrence in modern geothermal systems. The highest temperatures in the MIS are represented by a garnet and ferroactinolite assemblage (assemblage I). Assemblage II contains epidote, prehnite, and wollastonite, which represents slightly lower temperatures in the system. The existence of laumontite, quartz, and adularia defines a third assemblage. Assemblage IV is defined by calcite and clays, and represents the lowest alteration temperature at the MIS. These temperature-sensitive calc-silicates serve to constrain the fluid temperatures of the MIS hydrothermal system. Assemblage I suggests that the system reached over 300 C. Successively decreasing temperatures through time, approaching ambient temperatures, are suggested by the lower temperature assemblages II, III, and IV. A model for the cooling history of the MIS is reported elsewhere. The distribution of these high-temperature minerals points to the central uplift, not the melt sheet, as being the heat source for the system.

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