Post-Impact Hydrothermal Alteration of the Manson Impact Structure
Abstract
Core materials from the Manson Impact Structure (MIS; Manson, Iowa) are examined in order to evaluate post-impact alteration processes. Interpretation of the high temperature post-impact 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 rocks (CPC) have been described previously [1]. Particular emphasis is placed on fluid conduits (i.e., fractures) that cross-cut all these lithologies. Analytical techniques include petrography, SEM, and XRD. The presence of andradite garnet, ferroactinolite, epidote, prehnite, wollastonite, laumontite, quartz, adularia, pyrite, clays, and calcite, define a hydrothermal system, which developed at a temperature of at least 275 degrees C (possibly up to 360 degrees C), and ended at approximately ambient temperatures. These 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 define a third assemblage. Assemblage IV is defined by calcite and clays, and represents the lowest alteration temperature at the MIS. Garnet and ferroactinolite occurs in lithologies associated with the central uplift (i.e., CCB-SM and CPC lithologies). According to textural associations, visible with the SEM and petrographically, garnet forms before ferroactinolite. Garnet and ferroactinolite occurs in modern geothermal systems where measured minimum temperatures range from 275 degrees C [2] to 360 degrees C [5] and 280 degrees C [2] to 325 degrees C [5], respectively. Epidote, prehnite, and wollastonite are also preferentially found in the CCB-SM and CPC lithologies. They are not observed in the CCB-MM lithology. Petrographic analyses reveal that epidote and prehnite both occur after assemblage I minerals. There is no textural evidence to indicate the paragenesis of wollastonite, but x-ray diffraction shows epidote always occurring with wollastonite. Similar coeval assemblages of wollastonite, epidote, and prehnite are noted in the Larderello (Italy) [3] and Krafla (Iceland) [4] geothermal systems. The prehnite/epidote/wollastonite assemblage in geothermal systems around the world represents a minimum temperature of occurrence ranging from 200 degrees C to 300 degrees C [5]. Textural relationships reveal laumontite growing after prehnite. Laumontite within Icelandic geothermal systems occurs over a temperature range of 100 degrees C to 230 degrees C [4]. Although quartz and adularia have a wide temperature range over which they occur in geothermal systems [5], they appear to post-date assemblage II minerals in the MIS. Textural evidence reveals that calcite, which is coeval with clays, is the last mineral to form in this system. Assemblage IV (calcite/clays) is interpreted to represent the lowest temperatures in the system, possibly similar to the Valles (New Mexico) geothermal system in which calcite and clays occur together at temperatures as low as 40 degrees C [6]. These temperature-sensititve calc-silicates serve to constrain the fluid temperatures of the MIS hydrothermal system. Assemblage I suggests that the system reached over 300 degrees 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 [7]. The distribution of these high temperature minerals points to the central uplift, not the melt sheet, as being the heat source for the system. Acknowledgments: This project is supported by DOE/BES. The SEM and XRD Laboratories at the Department of Earth and Planetary Sciences, University of New Mexico were utilized for this research. References: [1] Anderson et al. (1993) LPSC XXIV, 35-36. [2] Gianelli G. and Teklemariam M. (1993) J. Volcanology and Geothermal Research, 56, 429-445. [3] Cavaretta et al. (1982) Economic Geology, 77, 1071-1084. [4] Kristmannsdottir H. and Tomasson J. (1976) Natural Zeolites: Occurrence, Properties, and Use, 277-284. [5] Bird et al. (1984) Economic Geology, 79, 671-695. [6] Hulen J. B. and Nielson D. L. (1986) JGR, 91, 1867-1886. [7] Crossey et al. (1994) LPS XXV, 299-300.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29Q.499M
- Keywords:
-
- Antarctic Regions;
- Collection;
- Contamination;
- Micrometeoroids;
- Ice;
- Melting;
- Pipes (Tubes);
- Lunar and Planetary Exploration;
- ASTROBLEMS; HYDROTHERMAL ALTERATION; IMPACT CRATERS; IMPACTS; SHOCK EFFECTS