[meteorite-list] Fossil Meteorite Unearthed From Crater

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jun 15 12:14:08 2006
Message-ID: <200606151611.JAA23976_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/June06/Morokweng.html

Fossil Meteorite Unearthed From Crater
Planetary Science Research Discoveries
June 14, 2006

--- A meteorite of unusual chondritic composition was found in a highly
unlikely place, challenging how we think about colossal impact events on
Earth.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

A team of scientists lead by Wolf Maier (University of Quebec, Canada
and University of Pretoria, South Africa and soon at University of
Western Australia, Perth) and Marco Andreoli (University of the
Witwatersrand and South African Nuclear Energy Corp.) and colleagues who
also hail from Canada, South Africa, the United Kingdom, and the United
States, have announced the discovery of a 25-centimeter-wide chondritic
meteorite unearthed from the 145-million-year-old Morokweng impact crater
in South Africa. Found within the crater's impact melt sheet about 770
meters (half a mile) down a drilling borehole, the hefty meteorite's
existence would seem improbable given its low chance of surviving the
high shock pressures and temperatures normally associated with large
impact events. Its unusual composition could mean it is a sample from a
previously unknown part of the LL chondrite parent body or maybe it is
from an entirely different asteroid population than other known meteorites.

Reference:

    * Maier, W. D., M. A. G. Andreoli, I. McDonald, M. D. Higgins, A. J.
      Boyce, A. Shukolyukov, G. W. Lugmair, L. D. Ashwal, Pl. Gr??ser, E.
      M. Riples, and R. J. Hart (2006) Discovery of a 25-cm Asteroid
      Clast in the Giant Morokweng Impact Crater, South Africa. Nature,
      v. 441, p. 203-206.

------------------------------------------------------------------------

One of Earth's Largest Impact Craters

Africa The Morokweng crater, centered at 26o32' S and 23o32' E, is at
the edge of the Kalahari Desert in northern South Africa. It has a
diameter of at least 70 kilometers and is one of the largest terrestrial
impact craters known today. Worn by erosion and obscured by sediments
and Kalahari sands, the crater is unrecognizable on the surface but was
discovered by Marco Andreoli as a circular pattern of magnetic anomalies
during early-1990s mining explorations (as documented in the book A
Century of Geological Endeavour in Southern Africa 1895-1995.)

[Image]
LEFT: This satellite view shows the location of the Morokweng impact
crater in South Africa.

In 1997 researchers from University of the Witwatersrand and their
colleagues reported that boreholes drilled into the center of the crater
hit an impact melt sheet at least 870 meters thick. The melt sheet is
rock at the base of the crater that was liquified by heat of the impact.
This melt sheet has high abundances of chromium, nickel, cobalt, and the
platinum-group elements.

The age of the crater was determined by isotopic age dating of zircons
plucked from the impact melt rock. Ion microprobe analyses (see PSRD
article: Ion Microprobe for uranium-thorium-lead isotopic compositions
place the age of the crater at 146.2 ?? 1.5 million years--the same
age as a major geological boundary, the Jurassic-Cretaceous.

------------------------------------------------------------------------

A Cosmochemical Record Breaker

The Morokweng melt sheet is out of the ordinary for a few reasons.
First, it contains more meteorite fragments than melt sheets of most
other impact craters--fragments thought to be relicts of the impactor
that created the crater. In Morokweng the fragments are pebble-sized (<1
centimeter) and represent 5-6% chondritic contamination of the melt
sheet--a percentage so high that only one other crater's melt sheet
(Clearwater East crater in Quebec, Canada) has come close to that
figure. Second, the impact melt is unusually rich in nickel (up to 0.25%
NiO in pyroxene and chromium (up to 0.35% Cr2O3 in the orthopyroxene, and
0.69% in clinopyroxene).

Third, and unique to Morokweng's melt sheet is the new discovery
reported by Wolf Maier and colleagues of a large (25-cm) fossil
meteorite. Never before has such a boulder-sized chuck of the impactor
been found within a large crater. Previously, researchers have found
fossil meteorites in crater ejecta deposits (for an example, see PSRD
article: Tiny Traces of a Big Asteroid Breakup) but finding a large
fragment inside a huge crater has been deemed nearly impossible. We
explore the ideas of why the survival of this meteorite fragment is so
extraordinary in the next section, after we examine the meteorite
itself in more detail.

[Image]
meteorite in core

Imagine the surprise when the drill sliced through an obviously
different kind of rock 770 meters down. This is a photograph of the core
with the dark-colored fossil meteorite before it was cut for analysis.
The melt sheet is also shown for comparison. Pen is shown for scale.

Maier and his colleagues report that the Morokweng meteorite is
chemically unaltered except for a thin (1 millimeter) coating of brown
alteration minerals. Their laboratory analyses show that the meteorite
has chondritic chromium isotope ratios and identical platinum-group
element ratios to the bulk impact melt.

The research team found diagnostic features of a highly equilibrated
chondrite breccia, including well-preserved chondrules of various
textures: porphyritic, excentroradial (see images below), and barred.
These textures are produced by different degrees of melting and are
typical of chondrules in chondrites. Olivines with 120o triple junctions
indicate that recrystalization occurred in the parent asteroid as the
result of thermal metamorphism.

[Image]
chondrule

[Image]
chondrule

These are photomicrographs taken in polarized transmitted light of
chondrules in the Morokweng meteorite. On the top is porphyritic
orthopyroxene. On the bottom is excentroradial orthopyroxene.

Maier and coauthors report that the fossil meteorite resembles an LL6
chondrite breccia, yet its atypical composition and texture do not fit
exactly into any of the known chondrite groups. The platinum-group
element contents of the Morokweng meteorite are lower than in normal LL
chondrites. It contains unusually iron-rich silicates and iron-nickel
sulfide, but does not have troilite (an iron sulfide) and there is no
metal, which would be expected in this type of meteorite. It seems they
have found some thing a little different.

Morokweng vs. chondrite groups The olivines and orthopyroxenes in the
Morokweng meteorite (plotted as the star symbol) have higher iron
contents than most other LL, L, and H chondrites. Axes are labeled Fs
and Fa. Fs (ferrosilite, FeSiO3) is the iron-rich end-member of the
enstatite-ferrosilite solid-solution series. Fa (fayalite, Fe2SiO4) is
the iron-rich end-member of the olivine solid-solution series.)

------------------------------------------------------------------------

Does This Challenge Our Ideas of Impact Cratering?

The Morokweng fossil meteorite is a rare find. It is a surviving remnant
of a much larger projectile that blasted out the crater. Its existence
challenges the accepted idea that large bodies hit with such energy that
they are melted or vaporized within seconds of impact. Laboratory
modeling experiments and numerical simulations of the cratering process
support this idea. For example, at high impact angles (close to
vertical), the predicted peak shock pressures are 200-500 GPa. Predicted
temperatures exceed 2,000K (1,700 oC). If any bits survived, they would
be melted and chemically altered.

Until now craters larger than four kilometers in diameter have not
yielded any large remnant meteorites. In these cases, the composition of
the original impacting body is usually determined indirectly by
analyzing chemical tracers of metals, such as nickel, cobalt, and the
platinum-group elements. What were the special conditions that made it
possible to preserve this unaltered meteorite in the Morokweng melt
sheet? Was it slower than the normal 15-20 kilometers/second (say
Earth's escape velocity of 11 kilometers/second)? Was it an asteroid
rubble pile, hence weak? (For example, see PSRD article: Honeycombed
Asteroids.) Running more cratering
experiments and finding similar fossil meteorites on Earth in large
impact craters, particularly those with melt sheets that contain an
elevated dissolved platinum-group element component, would help to
answer such questions. Perhaps future explorers will find projectile
pieces in the impact melts of large lunar craters, such as Tycho
pictured below.

[Image]
Tycho Crater from orbit

Tycho Crater on the Moon is about 85 kilometers across and is visible in
the southern highlands. This orbital image shows the floor's central
peak and smooth impact melt sheet.

------------------------------------------------------------------------

The Low-Down on This Crater

The discovery of the Morokweng fossil meteorite is a new piece of
information that may help us better understand the bombardment history
in the inner solar system. The unusual composition of Morokweng might
suggest that the nature of meteorites may have changed through time
--types of impactors hitting Earth 145 million years ago were not the
same as bodies hitting more recently. Morokweng may represent a sample
of a different asteroid population from any other meteorite collected so
far.

Maier and colleagues state that they found no evidence to suggest that
the absence of metal and abundance of sulfide are the result of
contamination from interaction with the impact melt. They attribute the
mineralogy to metamorphism in the parent body. If the mineralogy
reflects metamorphism in the parent body, then the Morokweng fossil
meteorite could have come from a previously unknown interior portion of
the LL chondrite parent body.

------------------------------------------------------------------------

ADDITIONAL RESOURCES

    * Andreoli, M. A. G., L. D. Ashwal, R. J. Hart, C. B. Smith, S. J.
      Webb, M. Tredoux, F. Gabrielli, R. Cox, and B. B. Hambleton-Jones
      (1995) The Impact Origin of the Morokweng Ring Structure, Southern
      Kalahari, South Africa. Geol. Soc. South Africa Centennial
      Geocongress, Johannesburg, Extended Abstr. Vol. 1, p. 541-544.
    * Anhaeusser, C. R., (ed.) 1997. A Century of Geological Endeavour
      in Southern Africa 1895-1995. Geol. Soc. South Africa,
      Johannesburg, 587 pp.
    * Earth Impact Database
      <http://www.unb.ca/passc/ImpactDatabase/index.html>, entry for
      Morokweng
      <http://www.unb.ca/passc/ImpactDatabase/images/morokweng.htm>.
      Maintained by the Planetary and Space Science Centre, University
      of New Brunswick, Canada.
    * Hart, R. J., M. Cloete, I. McDonald, R. W. Carlson, M. A. G.
      Andreoli (2002) Siderophile-rich Inclusions from the Morokweng
      Impact Melt Sheet, South Africa: Possible Fragments of a
      Chondritic Meteorite. Earth and Planet. Sci. Letters, v. 198, p.
      49-62.
    * Koeberl, C., R. A. Armstrong, W. U. Reimold (1997) Morokweng,
      South Africa: A Large Impact Structure of Jurassic-Cretaceous
      Boundary Age. Geology,, v. 25, p.731-734.
    * Maier, W. D., M. A. G. Andreoli, I. McDonald, M. D. Higgins, A. J.
      Boyce, A. Shukolyukov, G. W. Lugmair, L. D. Ashwal, Pl. Gr??ser, E.
      M. Riples, and R. J. Hart (2006) Discovery of a 25-cm Asteroid
      Clast in the Giant Morokweng Impact Crater, South Africa. Nature,
      v. 441, p. 203-206.
    * Martel, L. (2004) Tiny Traces of a Big Asteroid Breakup. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html
    * Martel, L. and G. J. Taylor (2006) Ion Microprobe. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/Feb06/PSRD-ion_microprobe.html
      <../Feb06/PSRD-ion_microprobe.html>
    * "Shock Discovery of Ancient Space Invader
      <http://www.sciencemuseum.org.uk/antenna/asteroidimpact/>"
      Antenna: science news by the Science Museum, London.
    * Taylor, G. J. (1999) Honeycombed Asteroids. Planetary Science
      Research Discoveries.
      http://www.psrd.hawaii.edu/Aug99/asteroidDensity.html
      <../Aug99/asteroidDensity.html>
    * University of Quebec news item with photograph
      <http://www.uqac.ca/uqactualite/categorie.php?type=Portail%20UQAC&date=20060609085805>.
Received on Thu 15 Jun 2006 12:11:46 PM PDT