Petrogenesis of melt rocks, Manicouagan Impact Structure, Quebec

Abstract

Abstract. The 65‐km‐diameter Manicouagan impact structure has an eroded 230‐m‐thick sheet of clast‐laden, impact melt rock with an estimated preerosional volume of >270 km3. All samples are characterized by mineral and lithic clasts or their incompletely digested remnants. Drawing upon previous theoretical studies of shock waves, we suggest that the Manicouagan melt formed in 1 or 2 s in a 5‐km‐radius hemisphere near the point of impact. The melt accelerated to a few kilometers per second, and the melt and the less shocked debris surrounding it flowed downward and outward for a few minutes until the melt formed a lining of a 5‐ to 8‐km‐deep, 15‐ to 22‐km‐radius cavity. Extremely turbulent flow thoroughly homogenized the melt and promoted the incorporation and progressive digestion of slower moving, less shocked, cooler debris surrounding the melt. This debris had been finely fragmented, but not melted, to grain sizes of less than 1 mm by the passage of the shock waves. Because of the fine grain size, the melt and fragmented debris equilibrated thermally in about 100 s. During thermal equilibration, virtually all clastic debris (i.e. alkali feldspar, biotite, hornblende, garnet, and scapolite), other than highly refractory quartz and plagioclase as well as many of the centimeter size lithic clasts other than anorthosite, were digested. The preservation of quartz and plagioclase mineral clasts implies that the clasts and melt equilibrated to temperatures near but not above the liquidus. Plagioclase nucleation was initiated by the drop in temperature and possibly by direct nucleation on undigested debris. The initiation of crystallization vastly increased the melt viscosity, preventing settling of 10‐mm clasts of basement. Flow of melt through basement fractures is evidence that readjustment of the crater floor took place during the period of clast‐melt thermal equilibration.

Mineralogy and petrology of melt rocks from the Popigai impact structure, Siberia

“…The presence of an inner augite corona and an outer feldspar-bearing corona in the Manicouagan melt rocks led Simonds et al (1978) to suggest that these coronas developed progressively. The limited occurrence of the alkali feldspar coronas at Popigai suggests that either the Popigai melt rocks crystallized more rapidly than those at Manicouagan, thereby restricting the development of these coronas, or that the melt composition did not favour alkali feldspar growth.…”

Numerical modeling of impact‐induced hydrothermal activity at the Chicxulub crater

“…The initial temperature of the model's central melt sheet is uniform, as petrologic studies of impact-melt-derived rocks at a variety of terrestrial impact craters show that clasts of diverse provenance tend to be evenly mixed throughout the sheet, probably due to turbulent mixing during melt aggregation (Phinney and Simonds 1977). Although melt sheet margins are rapidly cooled during the final stages of crater formation, as indicated by an abundance of undigested clasts within a few meters of the basal contact (e.g., Simonds et al 1976), the temperature of the interior is expected to be largely homogeneous as a result of this turbulent mixing. Postimpact convection in the central melt sheet is unlikely to have significantly contributed to the cooling of the melt due to substantial viscous damping in systems like Chicxulub that occur in siliceous continental crust (Onorato et al 1978;Kring 1995), and is not modeled here.…”

An in-depth look at the lunar crater Copernicus: Exposed mineralogy by high-resolution near-infrared spectroscopy

“…Dence, 1971;Grieve, 1987;Grieve et al, 1981). The mobile melt would have progressively assimilated various clast materials, thereby leading to faster cooling of the mix and the formation of a heterogeneous array of distinct units ranging from clast-rich impact melt breccia (Dence, 1971;Grieve et al, 1977) to clast-poor melt (Simonds et al, 1976(Simonds et al, , 1978. Figure 12A[ENTER]) shows signs of fluidic flow patterns of melt materials emplaced along the northern rim terraces (indicated by arrows) and draining 23 downwards onto the crater floor.…”