Gordon R. Osinski

Impact-generated carbonate melts: evidence from the Haughton structure, Canada

Evidence is presented for the melting of dolomite-rich target rocks during formation of the 24 km diameter, 23 Ma Haughton impact structure on Devon Island in the Canadian high Arctic. Field studies and analytical scanning electron microscopy reveal that the s 200 m thick crater-fill deposit, which currently covers an V60 km 2 area in the center of the structure, comprises fragmented target rocks set within a carbonate^silicate matrix. The silicate component of the matrix consists of Si^Al^Mg-rich glass. The carbonate component is microcrystalline calcite, containing up to a few wt% Si and Al. The calcite also forms spherules and globules within the silicate glass, with which it develops microtextures indicative of liquid immiscibility. Dolomite clasts exhibit evidence of assimilation and may show calcite and rare dolomite overgrowths. Some clasts are penetrated by calcite and silicate injections. Along with the carbonate^ silicate glass textures, the presence of pigeonite and spinifex-textured diopside suggests that the matrix to the crater-fill deposit was originally molten and was rapidly cooled. This indicates that the impact event that generated Haughton caused fusion of the predominantly dolomitic target rocks. It appears that the Ca^Mg component of the dolomite may have dissociated, with Mg entering the silicate melt phase, while the Ca component formed a CaCO3-dominant melt. The silicates were derived by the fusion of Lower Paleozoic sandstones, siltstones, shales and impure dolomites. Evidence for melting is corroborated by a review of theoretical and experimental work, which shows that CaCO3 melts at s 10 GPa and s 2000 K, instead of dissociating to release CO2. This work indicates that carbonate-rich sedimentary targets may also undergo impact melting and that the volume of CO2 released into the atmosphere during such events may be considerably less than previously estimated. fl 2001 Elsevier Science B.V. All rights reserved.

Microbial colonization in impact- generated hydrothermal sulphate deposits, Haughton impact structure, and implications for sulphates on Mars

Hydrothermal gypsum deposits in the Haughton impact structure, Devon Island, Canada, contain microbial communities in an endolithic habitat within individual gypsum crystals. Cyanobacterial colonies occur as masses along cleavage planes, up to 5 cm from crystal margins. The crystals are transparent, so allow transmission of light for photosynthesis, while affording protection from dehydration and wind. The colonies appear to have modified their mineral host to provide additional space as they expanded. The colonies are black due to UV-screening pigments. The relative ease with which microbial colonization may be detected and identified in impact-generated sulphate deposits at Haughton suggests that analogous settings on other planets might merit future searches for biosignatures. The proven occurrence of sulphates on the Martian surface suggests that sulphate minerals should be a priority target in the search for life on Mars. Received 12 May 2004, accepted 7 July 2004

Thermal alteration of organic matter in an impact crater and the duration of postimpact heating

The 24-km-diameter Tertiary Haughton impact structure formed in rocks that contained preexisting liquid hydrocarbons. Biomarker ratios in the hydrocarbons show a consistent pattern of variation in degree of heating across the structure. The heating reached a maximum at the crater center and is attributed to hydrothermal activity following impact. Kinetic modeling suggests a time scale of ∼5 k.y. for the heating, at a maximum temperature of 210 °C. The short time scale suggests that in moderate-sized craters, which are abundant on Mars, heating is not so extensive that fossil or extant organic matter would be obliterated.

Evidence for the shock melting of sulfates from the Haughton impact structure, Arctic Canada

Abstract Sulfate minerals can form an important component of the matrix to carbonate-rich impact melt breccias at the 24 km diameter, 23 Ma Haughton impact structure, Arctic Canada. The textural and chemical features of the matrix-forming sulfates indicate that these phases, in addition to co-existing carbonates and silicates, crystallized directly from an impact-generated melt. Evidence for this includes (1) the matrix-supported nature of the crater-fill lithologies, (2) sulfate–carbonate–silicate liquid immiscible textures, (3) possible quench textures in anhydrite, and (4) flow textures developed between anhydrite and silicate-rich glasses. Further supporting evidence includes the presence of Si, Al and Mg in the anhydrite structure, which were probably ‘trapped’ by quenching from a melt. Irregular blebs and globules of shock-melted carbonates within anhydrite also suggest a common origin for the two phases. Field studies reveal that clasts of anhydrite-bearing target material are also present in the crater-fill deposits. Several clasts of anhydrite–quartz lithologies exhibit evidence for incipient shock melting in both phases. Previous assumptions about the response of sulfates and carbonates to hypervelocity impact (i.e., lack of melting) may, therefore, be incorrect.

Geomicrobiology of Impact-Altered Rocks

Rocks shocked by asteroid or comet impact events can be made more porous by the shock volatilization of minerals, and they can be fractured by the intense heat and pressures of impact. New spaces within the rock provide access points and surfaces for the growth of microbial communities, illustrating an example of how shock metamorphism can generate new habitats for microbial colonization. We review data on the colonization of shocked gneiss from the Haughton impact structure by phototrophs and heterotrophs. Shocked rocks can preferentially trap water and protect against wind-induced desiccation. The interior of shocked rocks is often warmer than the air temperature, and protects against ultraviolet radiation. Because impact events are a ubiquitous process on solid planetary surfaces, the shocked-rock habitat may be important on other planets, and it may have been important on the early Earth when primitive microorganisms lived under a much higher impact flux than today.