[meteorite-list] Little Chondrules and Giant Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Oct 24 12:43:09 2005
Message-ID: <200510241641.j9OGfmq14578_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Oct05/chondrules_impacts.html

Little Chondrules and Giant Impacts
Planetary Science Research Discoveries
October 21, 2005

--- Chondrules in metal-rich meteorites formed a couple of million years
after most other chondrules, possibly by impact between moon-sized or
larger objects.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Alexander (Sasha) Krot (University of Hawaii), Yuri Amelin (University
of Toronto), Pat Cassen (SETI Institute), and Anders Meibom (Museum
National d'Histoire Naturelle, Paris) studied and then extracted frozen
droplets of molten silicate (chondrules from unusual meteorites rich in
metallic iron-nickel. Called CB (Bencubbin-like) chondrites,
these rare but fascinating meteorites contain chondrules with different
properties than those in other types of chondrites. Most notably, the
chondrules contain very small concentrations of volatile elements and
variable concentrations of refractory elements. (Volatile elements
condense from a gas at a relatively low temperature, or are boiled out of
solids or liquids at relatively low temperature. Refractory elements are
the opposite.) Some of the metal grains in CB chondrites are chemically
zoned, indicating that they formed by condensation in a vapor cloud.

The most intriguing feature of chondrules in CB chondrites is their
relatively young age. Lead-lead isotopic dating of chondrules separated
from two CB chondrites show that they formed 5 million years after
formation of the first solids in the solar system (calcium-aluminum-rich
inclusions), which is about at least two million years after formation
of other chondrules, and after energetic events in the solar nebula
stopped. Krot and his colleagues suggest that the CB chondrules formed
as the result of an impact between Moon- to Mars-sized protoplanets.
Such impacts were so energetic that huge amounts of material were
vaporized and then condensed as chondrules or chemically zoned metal
grains. This event enriched refractory elements and depleted volatile
elements. Such large impacts appear to play important roles in planet
formation, including the formation of the Moon.

Reference:

    * Krot, Alexander N., Yuri Amelin, Patrick Cassen, and Anders Meibom
      (2005) Young chondrules in CB chondrites from a giant impact in
      the early Solar System. Nature, vol. 436, p. 989-992.

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

Normal Chondrules and their Formation

Most meteoriticists think that normal chondrules (if "normal" is the
right word to use for millimeter-sized frozen droplets of silicate melts
that formed before the planets) formed by flash heating events in the
solar nebular, the flattened cloud of gas and dust in which the Sun and
planets formed. (See PSRD article: Flash Heating
<http://www.psrd.hawaii.edu/Mar00/flashHeating.html>.) Numerous ideas
for the source of flash heating were devised over the years, but the
favorite now is shock waves in the solar nebula. There are, as usual in
science, a lot of ideas for the origin of the shock waves.

    * Accretion shocks, in which energetic waves are set up as dust
      falls onto the growing accretion disk around the proto-Sun.
    * Waves set up by infalling clumps of dust and gas.
    * Bow shocks in front of planetesimals moving through the dusty nebula.
    * Formation of spiral arms and clumps in the protoplanetary disk.
    * Eruption of X-ray flares from the young Sun.

[image of type I chondrule]

This backscattered electron image shows a typical normal chondrule in a
CR carbonaceous chondrite. Most of the grains are olivine crystals
(black, labeled ol). They are surrounded by a glass (grey, labeled gl).
Pyroxene (px) and droplets of metallic iron (white, labeled met) are
also visible.

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

[photo of experimental shock wave]

The image above shows the shock wave in front of a blunt object (moving
from right to left) in a wind tunnel during an experiment at NASA Ames
Research Center. This may resemble the shock wave preceding a
planetesimal moving through the solar nebula.

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

[photo of shock waves from ship guns]

Spherical shock waves generated by the firing of the huge guns of the
USS Iowa are clearly visible on the ocean surface. While not exactly a
simulation of a shock wave in the early solar system, the photograph
gives a good view of the shock wave generated by the explosion.

Not everyone agrees with an origin by shock waves in the solar nebula,
of course. Some meteoriticists think chondrules originated during
impacts between solid or molten small bodies. Everyone agrees, however,
that chondrules made their appearance early in the history of the solar
system. Isotopic dating of chondrules shows that they formed within
three million years of the formation of calcium-aluminum-rich inclusions
(CAIs), the first solids to be produced in the solar nebula. This was a
busy period. Most meteoriticists and astrophysicists believe that not
only did shock waves create CAIs and normal chondrules from fluffy balls
of dust, but chondrules and dust accreted into asteroid-sized
planetesimals, some of which were melted by short-lived isotopes (mostly
26Al), forming metallic cores, mantles, and basaltic crusts. Other
planetesimals were just heated a bit. All this in a period lasting only
3 million years. (Only geologists and astronomers attach the adjective
"only" to 3 million years. But 3 million years is only 0.07% of the age
of the Solar System.)

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

CB Chondrites and Their Unusual Chondrules

There is an unusual group of five chondrites, named CB chondrites (for
Bencubbin-like, one of their members). Their most notable feature is
that they are loaded with round chunks of metallic iron-nickel. Many of
the metal particles in two members of the group, HH 237 and QUE 94411,
are chemically zoned in a way that indicates formation from a hot, but
cooling, gas (see PSRD article: The Oldest Metal in the Solar System
<http://www.psrd.hawaii.edu/Sept00/primitiveFeNi.html>). Besides big,
abundant metal particles, the CB chondrites have distinctive chondrules
in them. Instead of chondrules that contain abundant large crystals in a
fine-grained matrix, those in CB chondrites are much more uniform in
grain size. One type of chondrule is called "cryptocrystalline," which
means that the crystals are too small to be seen even in a microscope. A
second type contains olivine crystals shaped like a small, skinny
skeleton, called "skeletal" (e.g. see insert figure below).

[CB chondrites]

Photograph of a polished slab of Gujba, with insert of closer view of
another CB chondrite, Hammadah al Hamra 237. Note the large abundance of
metallic iron (bright). The chondrule in HH 237 is a skeletal olivine
(SO) chondrule; it is surrounded by metallic iron.

A striking chemical feature of CB chondrules is that they are very
depleted in volatile elements. Even moderately volatile elements such as
sodium and potassium are very low in abundance. In contrast, refractory
elements (these condense from a gas at a high temperature and include
calcium, titanium, aluminum, and the rare earth elements) are all in the
same relative abundance as in average solar system materials (given by
CI carbonaceous chondrites and measurements of the Sun), but their
abundance varies from 4 times higher than CI to only 1% of CI.

Meteoriticists do not agree on how these unusual chondrites formed. Some
suggest energetic events in the solar nebula; others suggest impacts
between asteroids. Nebula models require that the chondrules in CB
chondrites be old, while the nebula was still around. On the basis of
observations of disks around other stars and theoretical calculations,
astronomers say that the solar nebula lasted only about 3 million years.
Thus, CB chondrules ought to have formed within 3 million years of the
origin of the solar system. Normal chondrules are no more than 3 million
years younger than calcium-aluminum-rich inclusions (CAIs), the first
solid objects formed when the Solar System formed. Krot and his
colleagues wanted to determine the ages of the unusual chondrules in CB
chondrites.

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

The Young Ages of CB Chondrules

Krot separated chondrules from two CB chondrites, Gujba and HH 237. He
characterized these carefully using scanning electron microscopy and
electron microprobe analysis, then passed them on to Yuri Amelin for
lead-lead age dating. These are painstaking analyses to make, especially
on such small samples. It requires a special clean room (to isolate the
samples from lead all around us) and ultra-clean chemicals.

Lead-lead dating uses the fact that 207Pb-206Pb are produced at
different rates from their parent isotopes, 235U and 238U, respectively.
By also measuring 204Pb, which is not produced by radioactive decay,
cosmochemists can make a diagram called an isochron. The samples, if all
have the same age, lie on a line whose slope defines the age. It takes a
bit of math and knowledge of the decay rates of uranium isotopes.

The results for Gujba and HH 237 are shown in the isochron diagram
below. The chrondrule and chondrules fragments from each sample fall on
well-defined lines that indicate the same age within analytical
uncertainty, 4,562.7 (plus/minus 0.5) million years for Gujba, and
4,562.8 (plus/minus 0.9) million years for HH 237. This indicates that
they formed at essential the same time. More important, their age is 5
million years younger than the age of CAIs, 4,567.2 (plus/minus 0.7)
million years. CB chondrules are not only younger than CAIs, they are so
much younger that solar nebula processes should have shut off by the
time they formed.

lead isochron diagram of three chondrules 204Pb /206Pb vs 207Pb /206Pb
diagram for chondrules separated from Gujba. The ellipses show
two-standard-deviations uncertainty around each data point. The data lie
on a well-defined line that indicates and age of 4,562.68 million years.
This age is about 5 million years younger than the oldest materials in
the Solar System.

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

Chondrule Formation by Impact?

So, the CB chondrites (at least Gujba and HH 237) formed after the solar
nebula had finished being energetic. Yet their formation indicates an
energetic process was involved. The metal was melted and some seems to
have formed from a vapor. Chondrules were totally molten droplets that
contained no pre-existing debris and cooled rapidly. All these features
point a cosmic finger at formation during an impact event. In a
collision between large objects, there is a lot of vaporization and
melting. These hot conditions provide settings in which metal nodules
and skeletal olivine chondrules form by melting, and cryptocrystlline
chondrules (which are smaller) are made by condensation from a vapor.
There are no unmelted remnants of the original materials left in these
meteorites. This is consistent with their formation by impact between
two objects the size of Earth's Moon. Such a monumental collision would
separate melt and vapor from unmelted materials.

Painting of two-body collision

Collision between two Moon-sized (or larger) objects in the early Solar
System would have produced vast amounts of melt and vapor, from which
the components in CB chondrites could have formed. Such impacts may have
been common for a few million years early in the evolution of the Solar
System.

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

Crash, Bam, Whack, Slam: The Early Solar System

Astrophysical models of the very early Solar System suggest that the
asteroid belt (where CB chondrites come from) contains only 0.1% as much
material as was present initially. Within a few million years of
formation of the solar nebula most of that mass was in the form of Moon
to Mars-sized planetary embryos. How fast they were depleted by mutual
collisions (a critical part of the planet-forming process) depended on
exactly when Jupiter and Saturn formed because gravitational
interactions between the embryos and the giant planets caused mixing
throughout the asteroid belt. Collisions among these big rocky balls
would have been extremely energetic, producing the conditions necessary
to form CB chondrites.

Sasha Krot and his colleagues tell an interesting story. An appealing
aspect of it is that it fits in with the current paradigm for planet
formation. This involves impacts between planetary embryos until they
either formed the inner planets or were scattered to the far reaches of
the Solar System, never to return. Some of the large objects in the
asteroid belt must have collided with each other. Perhaps there are
other products in other types of chondrite that we ought to examine. In
fact, perhaps non-chondrites, which we think formed by melting inside
asteroids, formed as the result of giant impacts.

Most planetary scientists think that the Moon formed as the result of an
impact between a Mars-sized planetary embryo with the young Earth. Like
CB chondrites, the Moon is depleted in volatile elements and enriched in
refractory elements. Perhaps the CB impact event (if there was one, of
course) is a small example of the Moon-forming giant impact.

The early Solar System was a violent place! Carl?? Pieters' (Brown
University) poem about the Moon, published in Origin of the Moon, has a
particularly appropriate line that reads, "Moon holds secrets of ages
past when planets dueled for space." Apparently CB chondrites also hold
some secrets about an epoch when planets dueled for space.

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

ADDITIONAL RESOURCES

    * Fastlight < http://www.fastlight.demon.co.uk> Artwork for
      aerospace and spacecraft projects.

    * Krot, Alexander N., Yuri Amelin, Patrick Cassen, and Anders Meibom
      (2005) Young chondrules in CB chondrites from a giant impact in
      the early Solar System. Nature, vol. 436, p. 989-992.

    * Origin of the Moon (1986) W. K. Hartmann, R. J. Phillips, G. J.
      Taylor (eds.), Lunar and Planetary Institute, 781 p.

    * Taylor, G.J. (2000) Flash Heating. Planetary Science Research
      Discoveries. http://www.psrd.hawaii.edu/Mar00/flashHeating.html
      <http://www.psrd.hawaii.edu/Mar00/flashHeating.html>

    * Taylor, G.J. (2000) The Oldest Metal in the Solar System.
      Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Sept00/primitiveFeNi.html
Received on Mon 24 Oct 2005 12:41:47 PM PDT