http://www.psrd.hawaii.edu/Oct02/firstRock.html
The First Rock in the Solar System
Planetary Science Research Discoveries
October 24, 2002
--- An aggregate of corundum, hibonite, and
perovskite may be among the first rocks to form in
the Solar System.
Written by Steven B. Simon
Department of Geophysical Sciences, University of Chicago
My colleagues Andrew Davis, Lawrence Grossman (both of University of
Chicago), Kevin McKeegan (UCLA), and I have discovered an exceptionally
refractory inclusion in the Murchison carbonaceous chondrite. It is an
aggregate of corundum, hibonite, and perovskite, the three minerals expected
to condense first in a hot, cooling gas of solar composition. This inclusion
was one of the first rocks to form in the solar system 4.5 billion years
ago. It was preserved by being sequestered rapidly from the gas and enclosed
in a growing carbonaceous chondrite asteroid.
Reference:
Simon, S. B., A. M. Davis, L. Grossman, and K. D. McKeegan (2002) A
hibonite-corundum inclusion from Murchison: A first-generation
condensate from the solar nebula. Meteoritics and Planetary Science, v.
37, p. 533-548.
--------------------------------------------------
Samples From the Developing Solar System
A major area of interest in planetary science is the origin and early
history of the solar system. We know a great deal about the solar system,
but we continue to strive to learn more details, such as the early
temperature and pressure conditions. We know that the solar system formed
from a large cloud of gas and dust known as the solar nebula. The sun
contains 99.9% of the mass of the solar system and we know the composition
of the sun from spectroscopic analysis of the light it emits; therefore we
basically know the composition of the solar nebula. We can use known
physical properties and stabilities of minerals to calculate what minerals
will form from a gas of this composition as it cools, and derive a
theoretical, equilibrium condensation sequence of minerals that formed as
the solar nebula cooled. Finding the predicted minerals and studying them
would support the condensation model and improve our understanding of the
early solar system.
For detailed study of materials from the early solar system, we need to have
samples. The Earth is too active a planet to provide the necessary samples,
as its rocks are weathered, eroded, folded, and remelted. We therefore look
to meteorites for a record of the early solar system. Specifically, we study
carbonaceous chondrites, which have never been melted or changed much at all
since the formation of the solar system. In these meteorites can be found
many of the very same minerals predicted to form from a gas of solar
composition. They occur in small (approx. 5-10 mm across) assemblages, known
as refractory inclusions because of the relatively high formation
temperature of the minerals in them. They can be thought of as small,
individual rocks that formed in space and became enclosed in a later-formed
matrix. They probably spent billions of years on a small asteroid, escaping
the weathering, erosion, and plate tectonics that destroy rocks on Earth,
before being ejected into space and eventually captured by Earth.
--------------------------------------------------
Condensation
According to calculations, pressures that would be reasonable for the solar
nebula are between one-thousandth and one-millionth of the atmospheric
pressure at the Earth's surface. At such pressures, minerals condense at
temperatures that are below their melting points, so they condense as
solids, much like solid H2O (frost) may condense from the air on a cold
winter night. In a solar gas at 1/1000 atmosphere, corundum (Al2O3) is the
first major mineral to form. It condenses at 1770 Kelvin (K), or 1497ºC
(water boils at 373 K or 100ºC). The next mineral to form is hibonite,
CaAl12O19, at 1743 K, followed by perovskite, CaTiO3, at 1688 K.
[mineral aggregate]
This animation shows
the minerals corundum,
hibonite, and
perovskite condensing
from a hot, cooling gas
and forming an
aggregate rock.
Temperature in Kelvin
is shown in the upper
left corner.
We have found, in the Murchison carbonaceous chondrite, a refractory
inclusion that consists of corundum, hibonite, and perovskite - it is
perhaps one of the first rocks to form in the history of the solar system,
even older than the Earth, the Moon, and all the planets.
--------------------------------------------------
Freezing and Thawing
Murchison is a CM chondrite, and probably the
best way to find inclusions in that type of
meteorite is by freeze-thaw disaggregation. We immerse a sample of the
meteorite in water, freeze it, let it thaw, then freeze again. The expansion
of water when it freezes breaks apart the meteorite, loosening the
inclusions from the matrix. The minerals of interest to us are much denser
than the matrix, so we put the disaggregated meteorite into a liquid that is
denser than the matrix. The lighter material floats and the objects of
interest sink. We recover the dense particles, and each one is examined
under a microscope.
This photo shows me with Rebecca Elsenheimer at the scanning
electron microscope at the University of Chicago. Rebecca was a
high school student who worked in our lab through a mentorship
program with the Illinois Mathematics and Science Academy. She
disaggregated the meteorite and selected the sample for study.
Murchison hibonite has a sky blue color, so hibonite-rich inclusions can be
readily identified. Calculations show that if it were not removed from the
nebular gas, corundum would react with the gas to form hibonite. Apparently,
most of it did so, because corundum is very rare. Most hibonite-bearing
inclusions contain spinel (MgAl2O4), which should condense at 1501 K, rather
than corundum.
--------------------------------------------------
The First Rock
It's exciting to find a corundum-hibonite-perovskite inclusion, because
there are conditions under which these would be the first three minerals to
condense from the solar nebula, but we need to study the inclusion closely
to find evidence of whether it is indeed a condensate or whether it is
something that was heated and melted before it was trapped within the
meteorite and preserved. The inclusion, numbered M98-8, was mounted in epoxy
and polished, yielding a smooth, flat surface, which is needed for
microscopic study. We viewed the sample with a scanning electron microscope.
A backscattered electron image is shown below. The higher the average atomic
weight of a mineral, the more efficiently it reflects electrons to the
detector, and the lighter it looks in the image. Thus, the epoxy is black,
corundum is dark gray, hibonite is light gray, and perovskite is white.
[M98-8 image]
A backscattered electron image of M98-8. Corundum
(cor) is the dark gray phase seen at the upper left
and as isolated, rounded grains enclosed in hibonite
(hib). Perovskite (pv) is white, void spaces and
epoxy are black.
This image shows two important features. First, we note that there are gaps
between many of the hibonite grains, as might be expected for an aggregate
of individually formed condensate grains. If the inclusion had solidified
from a molten droplet, we would expect to find tightly intergrown crystals,
with the shapes of late crystals conforming to the shapes of early crystals.
Second, from the gaps we can see or infer grain boundaries, and it appears
that many of the hibonite crystals enclose rounded corundum grains. This is
consistent with the corundum having formed first, and becoming rounded as it
reacted to form hibonite, but the reaction stopped before all of the
corundum was consumed. If this object had crystallized from a melt having
the composition of the inclusion, we might expect corundum, the first phase
to crystallize, to mainly be found around the edge of the inclusion, and
hibonite, the second phase, to mainly occur in the core, assuming it would
have cooled (and crystallized) from the outside in. This is not observed.
A closer look (below), with a secondary electron (surficial) image, shows
that the voids are angular, their triangular or trapezoidal shapes
determined by hibonite crystal faces.
[M98-8 secondary electron image]
A secondary electron image of M98-8, showing angular
gaps between grains, bounded by straight crystal
faces. Some plates of hibonite, a crystal habit
typical of this mineral, can be seen in the gap at
the center of the photo (shown by the arrow).
Some of the voids contain plates of hibonite, below the polished surface.
This further suggests that crystals were individually formed and brought
together, rather than grown together. It turns out that there are problems
with any model for the formation of M98-8 that includes a liquid stage. It
would require very high temperatures to melt, which would make evaporation
likely, and we know from isotopic analysis that the inclusion did not
experience significant degrees of evaporation. In addition, conditions
required to keep a liquid from evaporating at those temperatures, such as
high pressures, are thought to be unrealistic. From our observations and
analyses, we conclude that M98-8 did not crystallize from liquid, and that
it is a primary condensate that has remained virtually unchanged since its
formation in the early solar nebula over 4.5 billion years ago.
ADDITIONAL RESOURCES
Simon, S. B., A. M. Davis, L. Grossman, and K. D. McKeegan (2002) A
hibonite-corundum inclusion from Murchison: A first-generation
condensate from the solar nebula. Meteoritics and Planetary Science, v.
37, p. 533-548.
University of Chicago news release: Geophysical sciences scholars
mentor high school student in meteorite study.
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