[meteorite-list] Tiny Traces of a Big Asteroid Breakup (Fossil Meteorites)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:48 2004
Message-ID: <200403101549.HAA20931_at_zagami.jpl.nasa.gov>


Tiny Traces of a Big Asteroid Breakup
Planetary Science Research Discoveries
March 9, 2004

     --- Fossil meteorites and chromite
     grains record a hundred-fold increase
     in the number of meteorites that fell
     480 million years ago compared to the
     meteorite influx today.

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

Ancient geologic conditions in southern Sweden were ideal to preserve
meteorites that fell to Earth about half a billion years ago. Researcher
Birger Schmitz (working as a visiting professor at Rice University and now
at the University of Lund, Sweden) and his colleagues in Göteborg, Sweden
have analyzed over 40 of these rare fossil meteorites along with relict
chromite grains collected from sites in a 250,000-square-kilometer area of
480-million-year-old limestone. They attribute the abundance and wide
distribution of this space debris to a meteorite influx at least one hundred
times more intense than the influx today. Rather than a smorgasbord of
different types, cosmochemical evidence shows that the fossil meteorites are
L or LL chondrites leading the team to conclude that these meteorites and
chromite grains derived from a major collision in the asteroid belt. The age
of the limestone is very close to the impact age of many L chondrites
suggesting that this major collision was the breakup of the L chondrite
parent body, possibly the largest impact in the asteroid belt in the last
few billion years.


     Schmitz, B., Häggström, T., and Tassinari, M. (2003) Sediment-dispersed
     extraterrestrial chromite traces a major asteroid disruption event,
     Science, v. 300, p.961-964.


What Fell on the Seafloor

480 million years ago (during the Ordovician period), a large relatively
shallow sea covered parts of what are now the eastern Baltic countries.
Limestone beds formed in this tranquil environment at average sedimentation
rates of one to a few millimeters per thousand years. Researchers know from
studying the hardgrounds (cemented horizons) that the sedimentation rates
varied from long periods of non-deposition to periods of more rapid
sedimentation. This Ordovician limestone with beautifully preserved fossils
is a popular decorative and building stone that has been quarried in Sweden
since the 1100s. One inactive quarry in central Sweden, near Rättvik, is
renowned for its rock...as well as jazz and opera since its transformation
in 1991 into an open-air concert amphitheater called Dalhalla.

Aside from its obvious economic value, arguably the most extraordinary fact
about the Ordovician limestone is fossil meteorites. Quarry workers
originally considered them blemishes in the limestone and discarded the
sawed plates that contained them. The first fossil meteorite (Brunflo) was
discovered in 1952 when the rock plate was set aside as a curiosity. But it
was not identified as a meteorite until 1979.

                 [Brunflo fossil meteorite and chondrules]
      Brunflo fossil meteorite in limestone (left) with close-up views
      of relic chondrules, both barred (center) and radiating (right).

The second fossil meteorite (Österplana) found in a discarded plate was
announced in 1988. Professor Schmitz's group has now analyzed more than 40
additional fossil meteorites collected at the Thorsberg quarry at Kinnekulle
since systematic searching began there in the early 1990s. Their collection
of space debris from Thorsberg quarry also includes sand-sized grains of
chromite, dropped from disintegrated meteorites. As we'll see in a following
section, the preserved structure of the meteorites and the chemical
composition of the relict chromite grains are critical in proving these
"blemishes" are genuine meteorites.

                        [Thorsberg limestone quarry]
         This 1999 photo shows the basins where the ground water is
                       pumped out to remove the rock.


      [Distribution of fossil meteorites in limestone]

Fossil Meteorites from Thorsberg Quarry

The active part of the Thorsberg quarry spans a 3.2-meter vertical
sequence of rock showing 12 prominent beds that range in thickness from
11-62 centimeters. Based on fossil assemblages and sedimentation rates,
the researchers estimate that the sequence represents 1 to 2 million years
of sedimentation. Fossil meteorites have been recovered from half of
the beds in the sequence. So far, the most meteorite-rich bed has produced
26 specimens found over a seafloor area of 2,700 square meters.

The fossil meteorites range in length from about 1 to 20 centimeters
and typically occur on hardgrounds where they accumulated with nautiloid
shells. In work reported in 2001, Schmitz and colleagues observed that
the delicate shells show no preferred orientation and are usually well
preserved and concluded that the meteorites and shells were concentrated
due to sediment winnowing by bottom currents. They ruled out that the
meteorites were simply transported into position by strong bottom currents.
In other words, the meteorites settled where they fell.

The left-hand column represents the 12 limestone beds being quarried at
Thorsberg. Numbers inside this column show how many meteorites were
found in a bed. The right-hand column shows the distribution of

              [quarry worker sawing limestone with meteorite]
      Quarry worker saws a plate of limestone. Below the blade is a
      meteorite. The photograph on the right shows a typical fossil
      meteorite with alteration halo in the red limestone.


     [limestone with fossil meteorite]
     This plate of limestone was cut parallel to the seafloor surface.
     Nautiloid shells accumulated near the 4.5-cm-wide meteorite seen
     in the center of the scene. Iron in the red limestone around the
     meteorite was reduced resulting in the halo of lighter gray

                     [limestone with fossil meteorite]
     This plate of limestone was also cut parallel to the seafloor
     surface. A Nautiloid shell is seen to the left of the 6 cm x 8 cm
     meteorite. Tiny white spots in the meteorite are relict
     chondrules. The ragged edge of the meteorite shows where the
     fusion crust was partly peeled off.


Summary of Chromite Chemistry

The fossil meteorites contain only two relict mineral phases: chromite and
chromian spinel. All other minerals were altered during fossilization.
Elemental analyses of the relict chromite as well as bulk-meteorite platinum
group element or osmium isotope analyses allowed Schmitz and his colleagues
to identify the type of meteorite. We'll focus on their chromite analyses.

They found that the majority of the chromites from fossil meteorites plot in
the low MgO and Al2O3 and high TiO2 fields characteristic of L and LL
                            [chromite analyses]
     Chromite grains from the fossil meteorites (red dots) compared to
     a representative sampling of H, L, and LL chondrites.

Schmitz and his team analyzed chromites from the fossil meteorites and
chromite grains found separately in the limestone. They searched for
chromite grains at other localities where they could sample the same
sequence of Ordovician limestone beds as in the Thorsberg quarry. The
limestone was crushed to 0.5-centimeter fragments and then dissolved in
acid. The bits left behind, in the size range from 63 to 355 microns, were
searched for chromite grains. At the nearby Hällekis locality, they examined
150 kilograms of limestone from the bed below the meteorite-rich base at
Thorsberg but found no chromite grains or fossil meteorites. [See PSRD
supplementary map showing collection sites.]

The dispersed chromite grains collected over the 250,000 square-kilometer
area have similar chemistry to the chromite found in the fossil meteorites.
The researchers conclude that these dispersed grains are remnants of some,
perhaps most, of the meteorites that fell 480 million years ago and
disintegrated over time. The chemical data support the interpretation that
the fossil meteorites are L chondrites.


Meteorite Influx Rate

Schmitz and colleagues determined an Ordovician meteorite flux rate based on
the abundance and distribution of the fossil meteorites and dispersed
chromite grains. These samples were collected in limestone beds representing
1 to 2 million years of deposition. They've ruled out longer time spans
because the evolutionary changes they see in the trilobite and conodont
fossils through the section are quite small.

The fossil meteorites are too altered to be paired with any certainty, so
the researchers used total meteorite mass rather than number of falls to
estimate the influx rate. They report, for example, that the 26 fossil
meteorites found over the 2,700 square meter area in the basal bed in
Thorsberg quarry represent a total original mass of 2.2 kilograms. This same
bed contains on average 2.5 chromite grains per kilogram of limestone. They
calculate that these dispersed chromite grains represent 15 kilograms of
original meteoritic mass. On the basis of the total original mass of
recovered fossil meteorites as well as dispersed chromite grains, Schmitz
and coauthors estimate that the flux of meteorites in the mass range from 10
to 1,000 grams was enhanced at least by a factor of 120±50 in the Ordovician
period compared to today. This enhanced flux applied to the entire 250,000
square kilometer search area.


Where did the Fossil Meteorites Come From?

   Gaspra, irregular in shape, measures
    19x12x11 kilometers and is littered
 with impact craters.

Professor Schmitz and his colleagues suggest that when the fossil meteorites
arrived on Earth 480 million years ago they were an early surge of debris
from a massive collision in the asteroid belt. The field data support the
idea that the surge, 100 times higher than today's influx, continued
throughout the 1 to 2 million years of limestone deposition. About half the
meteorites that fall today are L chondrites and many of them were shocked
480±20 million years ago. Meteoriticists simply call the disrupted asteroid
that produced these meteorites the L chondrite parent body. Some astronomers
have suggested that remnants of the L chondrite parent body are the Flora
family of S-type asteroids. The Flora family, one of the largest asteroid
families with more than 800 members, is thought to have formed by the
destruction of a 200-kilometer-sized asteroid sometime during the last
billion years. So the fossil meteorites may come from an asteroid like
Gaspra, a member of the Flora family that was photographed by the Galileo
spacecraft on its way to Jupiter.

The terrestrial age of the fossil meteorites is consistent with
theoretically modeled lag times between the breakup of the asteroid and when
the early surge of meteorites would rain down on Earth. Schmitz and
colleagues from Zurich, Switzerland report in an abstract to the 2004 Lunar
and Planetary Science Conference that the fossil meteorites they've
collected in southern Sweden had very short cosmic ray exposure ages,
ranging from a few 100,000 years to about one million years, and that the
meteorites found in the upper, younger limestone beds have the higher
exposure ages.


The Challenge to Find More

By looking in Earth's own ancient rock record, researchers in Sweden are
collecting fossil meteorites to understand the bombardment history of the
inner solar system. More samples would help support the idea that these
meteorites are evidence of a global hundred-fold increase in the bombardment
rate following the breakup of the L chondrite parent body. Since shallow
seas were spread over many areas of Earth during the Ordovician period,
Professor Schmitz and his colleagues think that similar concentrations of
fossil meteorites and chromite grains should be present worldwide in
Ordovician limestone. Future work will take these fossil meteorite hunters
to China. They say there are promising sites in South America as well. In
North America, the best place to search might be in New York state. Look
around, if you live near Ordovician limestone you might find a fossil



     Dalhalla. Enjoy a fresh air concert at the dramatic festival stage
     Dalhalla built in a former Swedish limestone quarry.

     Haack, H., Farinella, P., Scott, E.R.D., and Keil, K. (1996)
     Meteoritic, asteroidal, and theoretical constraints on the 500 Ma
     disruption of the L chondrite parent body, Icarus, v. 119, p. 182-191.

     Heck, Ph. R., Baur, H., Schmitz, B., and Wieler, R. (2004) Very short
     delivery times of meteorites after the L-Chondrite parent body break-up
     480 MYR ago, Lunar and Planetary Science XXXV Conf. abstract 1492.

     Kring, D. A., and Cohen, B. A. (2002) Cataclysmic bombardment
     throughout the inner solar system 3.9-4.0 Ga, J. Geophys. Res.,
     107(E2), 10.1029/2001JE001529, 2002.

     Nesvorný, D., Morbidelli, A., Vokrouhlický, D., Bottke, W. F., and
     Bro?, M. (2002) The Fora Family: A case of the dynamically dispersed
     collisional swarm?, Icarus, v. 155, p. 155-172.

     Nyström, J. O., Lindström, M., and Wickman, F. E. (1988) Discovery of a
     second Ordovician meteorite using chromite as a tracer, Nature, v. 336,
     p. 572-574.

     Schmitz, B., Häggström, T., and Tassinari, M. (2003) Sediment-dispersed
     extraterrestrial chromite traces a major asteroid disruption event,
     Science, v. 300, p.961-964.

     Schmitz, B., Tassinari, M., Peucker-Ehrenbrink, B. (2001) A rain of
     ordinary chondritic meteorites in the early Ordovician, Earth and
     Planetary Science Letters, v. 194, p. 1-15.

     Schmitz, B., and Tassinari, M. (2001) Fossil meteorites, in Accretion
     of Extraterrestrial Matter Throughout Earth's History, B.
     Peucker-Ehrenbrink, B. Schmitz (eds.), Kluwer Academics, New York, p.

     Schmitz, B., Peucker-Ehrenbrink, B., Lindström, M., and Tassinari, M.
     (1997) Accretion rates of meteorites and cosmic dust in the early
     Ordovician, Science, v. 278, p. 88-90.

     Scott, E.R.D. (2002) Meteorite evidence for the accretion and
     collisional evolution of asteroids, Chapter in Asteroids III, W.
     Bottke, A. Cellino, P. Paolicchoi, and R. Binzel, (eds.), University of
     Arizona Press, p. 697-709.

     Thorslund, P. and Wickman F. E. (1981) Middle Ordovician chondrite in
     fossiliferous limestone from Brunflo, central Sweden, Nature, v. 289,
     p. 285-286.

     Web site on Meteorite Parent Bodies from the American Museum of Natural
Received on Wed 10 Mar 2004 10:49:47 AM PST

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