MISTASTIN IMPACT CRATER

MISTASTIN IMPACT CRATER

by: Charles O’Dale

  • Type: Central peak basin
  • Age (ma): 36.6 ± 2a
  • Diameter: 28 km
  • Location: Newfoundland/Labrador, Canada. N 55° 53′ W 63° 18′
  • Shock Metamorphism: Shatter cones are poorly developed. PDF in quartz and feldspars (Taylor and Dence, 1969). Maskelyniteb .

a Dating Method:

-40Ar-39Ar by J. Whitehead (pers. comm. 2001)

-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.

Mistastin impact crater, centre right. (LandSat)
Mistastin impact crater. (Google)
Mistastin impact crater – east (looking west).

General Area:  Moderate relief, 200-300 m, close to the tree-line in the Canadian Shield. Area has been glaciated, with ice moving west to east. The target rocks are crystalline.

Specific Features:  Structure is defined by a ring of low hills, 28 km in diameter, surrounding a depression filled by Lake Mistastin. A horseshoe-shaped island, rising 130 m above the lake and -3 km in diameter represents a central uplift. Although this is a fairly young crater, much of the original topography has been removed by glacial erosion. A weak fracture halo surrounds the crater in the target rocks and is best expressed in the west.

RADARSAT radar image of the Mistastin impact crater.
Mistastin Impact Crater geophysical.
The red dot represents the approximate area of the Mistastin impact 36.4 million years ago in the Paleogene Period.

The Mistastin Impact Crater, a heavily eroded 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).

Aerial Exploration

The large depression made by the impact that created the Mistastin Meteorite Crater is illustrated in this image. When we cleared the western rim and entered the crater we were forced to fly below the crater rim level to stay out of the clouds.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.

Mistastin impact crater –  taken from about 1000’ over the western rim of the crater (looking east). Discovery Hill is visible in the centre background. The island in the crater lake is the central peak of the impact.
An outcrop of impact melt rocks known as “Discovery Hill”, in the Mistastin impact crater

On the margins of the lake are vestiges of the 80 m thick impact melt sheet that contain evidence of meteoritic features in quartz, feldspar and diaplectic glasses (maskelynite). The impact melt sheet is visible in this image as the plateau just to the left of center at the edge of the lake. In the far background the eastern rim is visible between the clouds.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#.

An outcrop of impact melt rocks known as “Discovery Hill”, in the Mistastin impact crater
An outcrop of impact melt rocks known as “Discovery Hill”, in the Mistastin impact crater.

# 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.

Horseshoe Island, the central peak of the Mistastin impact crater.
Basement rocks consist of granodiorite, man-gerite (hypersthene monzonite) and anorthosite. Maskelynite and diaplectic quartz glass found within basement rocks at Horseshoe island (Grieve, 1975) are typical shock metamorphic features and indicate peak pressures of 30–45GPa. U/Pb geochronology on zircon extracted from basement rocks at Horseshoe Island yields a Mesoproterozoic age of 1440 ±20Ma (Marion and Sylvester, 2010).

Horseshoe Island, the remnant of the central peak in the center of the lake, is 130 m above the lake level with a diameter of 3 km. In this image the northern rim of the Mistastin Crater is clearly visible in the background. It is part of the central uplift composed primarily of Precambrian crystalline adamellite with small amounts of anorthosites. These target rocks have experienced various grades of shock metamorphism before being incorporated into the melt sheet. Geologically, the closest comparison to Mistastin Crater is the Clearwater West Crater, which has a similar distribution of igneous rocks with shocked inclusions around central mounds of moderately shocked basement rocks. The arch shape of Horseshoe Island resembles the ring islands of the Clearwater West Meteorite Crater. Mistastin is the more deeply eroded and younger than Clearwater. The igneous rocks at Mistastin and Clearwater have many textural features in common. They are considered to be the product of shock melting of the country rocks with admixtures of fragments of less strongly shocked materials. The variations in texture and composition are probably the result of mixing in different proportions of shocked fragments and melts of the adamellite and anorthosites host rocks, together with variations in cooling history. (Taylor, Dence, 1968).

Mistastin impact crater, central Labrador.
Yours truly at the Mistastin impact crater.

We had to gain over 1000’ in altitude to safely clear the northern rim of the crater/basin. We continued our flying explorations further north.

Geology, distribution and geochemistry of impact melt at the Mistastin lake impact crater, Labrador

Marion, Cassandra Lorraine
Abstract

The Mistastin Lake crater in Labrador, Canada (55°53’N; 63°18’W) contains a 3 km wide central uplift within a 19 x 12 km wide lake and has a rim diameter of 28 km. The projectile impacted Mesoproterozoic crystalline target rocks approximately 36 Ma ago. — This study consists of detailed field observations; geology, geochemistry, and geochronology of impact melt and target rocks of the Mistastin impact crater. To determine (1) the significance of the relationship between preserved melt thickness and vesicularity in the melt rocks; (2) the scale and origin of compositional heterogeneities in impact melts produced in craters of moderate size and the relationship between entrained mineral clasts and impact melt composition; and (3) the origin of zircon clasts in the impact melts. — Melt rocks that are distributed around two thirds of the lake in patchy outcrops vary in thickness from <1 m to 80 m. Previous estimates suggested that a coherent melt sheet up 10 200m thick formed in the crater and that the much smaller preserved unit thicknesses are the result of glacial erosion. New field observations and laboratory measurements identify a relationship between distribution, thickness and vesicularity of melt rock units. The thickest melt-rock occurrence, at Discovery Hill, is massive, crystalline, non-vesicular and 80 m thick. In contrast, 1-2 m thick melt-rock occurrences elsewhere in the crater are glassy and vesicular. Measured vesicularities vary from 0.1 to 31 % and follow an empirical relationship (ϕ = 30±2 h-0.8±0.1) whereby vesicularity ϕ increases with decreasing melt rock thickness h. Plagioclase microlite crystallization temperatures of thin melt rock outcrops are very high (>1300°C), indicating rapid cooling rates. Lower crystallization temperatures (~1245°C) for the Discovery Hill melt are consistent with slower cooling rates. The data suggest that the pre-erosional melt sheet at Mistastin was not uniform and, consequently, previous estimates for the level of erosion and the volume of the melt produced have been overestimated. — Target rocks which contributed to the impact melt consist principally of anorthosite, mangerite and granodiorite. Chemical compositions of bulk samples of thirty-three melt rocks and fourteen target rocks were measured by XRF and SN-ICPMS. Matrix compositions of nine samples of impact melt rocks were determined by EPMA and LA-ICPMS. Zircon grains from four samples of target rock and zircon clasts from three samples of impact melt rock were measured for multi-element composition, U-Pb age and Hf-isotopic composition by LA-(MC)-ICPMS. — The data reveal compositional heterogeneities in the impact melts on the scales of both bulk samples and matrices. Bulk samples can be divided into compositions with high and low concentrations of high-field strength elements (HFSE; Ti, Zr, Nb) and Fe, Ba, Ce and Y. High HFSE-type melt rocks formed when impact melt entrained large quantities of clasts from mangerite, which is rich in HFSE. Matrix compositions of bulk samples do not show the HFSE distinction but are affected by the introduction of low-temperature melts from the clasts to form dispersed, micron-scale silica-rich heterogeneities. Both clast entrainment and melting are more extensive for the thicker flow units which had a higher heat capacity for melting and cooled more slowly than thinner flows. — The best estimate of the sources of the initial impact melt is ~73% anorthosite, ~7% mangerite and ~20% granodiorite, based on least-squares modeling of major element compositions of the matrices of thinner flows. Zircon derived from anorthosite can be distinguished from zircon from mangerite and granodiorite on the basis of higher Nb/Ta and Eu/Eu* ratios and more negative initial ε Hf values. Zircon clasts greater than 40 microns in size in the impact melt rocks are dominantly or exclusively derived from mangerite and granodiorite. Hence zircon may be a poor provenance indicator for target rock contributors to impact melts. (2009) Geology, distribution and geochemistry of impact melt at the Mistastin lake impact crater, Labrador. Masters thesis, Memorial University of Newfoundland.

Side Note

The Fraser River Valley in Labrador.

North of the Mistastin Impact Crater is some fascinating geology. In particular I wanted to share with you this image of the Fraser River Valley in Labrador, typical of the scenery we flew over in our exploration. This is one area of Canada I want to explore by foot!!


References

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

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.

 Marion, Cassandra Lorraine .,Geology, distribution and geochemistry of impact melt at the Mistastin lake impact crater, Labrador (2009) Geology, distribution and geochemistry of impact melt at the Mistastin lake impact crater, Labrador. Masters thesis, Memorial University of Newfoundland.

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.

Earth Impact Database