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 >200 m thick crater-fill deposit, which currently covers an ∼60 km2 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 >10 GPa and >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.