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

摘要

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 hydro-thermal 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.