IMPACT CRATER EXPLORATIONS
by: Charles O'Dale
MISTASTIN IMPACT CRATER
a Laser ablation 40Ar/39Ar measurements on impact melt rocks (89 individual laser heatings) yield a date of 36.6 ±2.0(2σ)Ma while (U–Th)/He thermochronology on zircons from the basement rocks at Mistastin yields a date of 35.8 ±1.0(2σ)Ma (Upper Eocene) (Young, 2014).
b Maskelynite: A type of naturally occurring glass having the composition of plagioclase series feldspar, created by the vitrification of plagioclase by shock melting in meteorites and meteorite impacts. Image courtesy of NASA.
complex meteorite crater is technically a “central peak basin structure”. The structure’s rim includes a 26 km diameter ring of low hills that have an elevation of up to 670 m above sea level. These hills surround a depression, the centre of which is filled by the 16 km diameter Lake Mistastin. The lake surface is 338 m above sea level, and about 150 m lower than the mean regional elevation. It contains a horseshoe shaped island ~3 km in diameter, which rises ~130 m above lake level. The island consists largely of shocked basement lithologies and is the eroded remanent of a central uplift (Grieve 2006).
The crater was originally thought to be volcanic in origin. In 1968 it was confirmed as a meteorite impact site with the discovery of shock metamorphism features, specifically PDF in quartz, and poorly developed shatter cones (Taylor and Dence, 1969).
The impact meteorite type may have been of iron composition (Wolfe et al., 1980). The target rock type is Precambrian, crystalline containing Proterozoic anorthosites, mangerite, granodiorite and quartz monzonite and exhibits a full range of shock features from brecciation to diaplectic glasses. If you ignored the lakes, the area almost looks like a moonscape.
(Courtesy of NASA/LPI) Eastward moving glaciers have drastically reduced the surface expression of this structure, removing most of the impact melt sheet and breccias and exposing the crater floor. Glacial erosion has also imparted an eastward elongation to the crater that is particularly evident in the shape of the lake that occupies the central area of the structure. Isolated patches of fill and sub floor target rocks are preserved (Taylor and Dense, 1969). At the time of the impact 36 million years ago, the continents were approximately in their present positions and a moderate biological extinction had occurred which is associated with a microtektite and an iridium impregnated geological layer. The earliest apes made their appearance 10 million years later.
Plateaus surrounding the lake up to 5 km away from the shore-line are interpreted as terraces that formed by collapse during the modification stage of the crater (Mader et al., 2011).
The sheet of impact melt (image right - Courtesy of GISP)overlies a thin layer of breccia on the crater floor and has an exposed thickness of 80 m at its base. The melt is fine grained, glassy with numerous country rock inclusions. At higher levels it is medium grained with micro-porphyritic and poikilitic textures similar to many Apollo 16 melt rocks. Reaction, assimilation and local partial melting of the inclusions occur and the partial melts show varying degrees of mixing with the impact melt#.
# The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.
Grieve, R.A.F., 1975. Petrology and chemistry of the impact melt at Mistastin Lake crater, Labrador. Geol. Soc. Am. Bull.86, 1617–1629
Grieve, R.A.F., Impact Structures in Canada. Geological Association of Canada, 2006.
Mader, M.M., Osinski, G.R., Marion, C.L., 2011. Impact ejecta at the Mistastin Lake impact structure, Labrador, Canada. In: 42nd Lun. Plan. Sci. Conf, p.2505.
Marion, C.L., Sylvester, P.J., 2010. Composition and heterogeneity of anorthositic im-pact melt at Mistastin Lake crater, Labrador. Planet. Space Sci.58 (4), 552–573.
Taylor, E. C., Dence, M. R., A probable meteorite origin for Mistastin Lake, Labrador. Canadian Journal of Earth Sciences, v. 6, pp. 39-45. 1969.
Wolf, R., Woodrow, A.B. and Grieve,R.A.F., Meteoritic material at four Canadian impact craters. Geochimica et Cosmochimica Acta, v. 44, pp. 1015-1022. 1980.