[meteorite-list] Making Sense of Droplets Inside Droplets (Chondrules & CAIs)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jun 1 13:30:47 2005
Message-ID: <200506011727.j51HRXa21076_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May05/chondrulesCAIs.html

Making Sense of Droplets Inside Droplets
Planetary Science Research Discoveries
May 31, 2005

--- The vexing presence of chondrules inside supposedly older
calcium-aluminum-rich inclusions (CAIs) in chondrites makes sense if the
CAIs were remelted.

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

Chondrules and calcium-aluminum-rich inclusions (CAIs) in stony
meteorites called chondrites are
silicate objects only fractions of a millimeter to several millimeters
in diameter. Both formed during rapid heating events at the dawn of the
solar system, before there were planets. Conventional wisdom, based on
numerous observations and isotopic
analyses, indicates that CAIs formed before chondrules. CAIs contained
more radioactive aluminum-26 (26Al, which has a half-life of only
730,000 years) when they formed than did chondrules, indicating that
they formed 1-3 million years earlier. Relict pieces of CAIs have even
been found inside chondrules, and so must have formed earlier. However,
Shoichi Itoh and Hisayoshi Yurimoto of the Tokyo Institute of Technology
found a chondrule inside a CAI, the reverse of the normal situation,
which indicated that some chondrules must have formed before CAIs, a
blow to the conventional wisdom.

Alexander (Sasha) Krot (University of Hawaii), Professor Yurimoto from
Tokyo, Ian Hutcheon (Lawrence Livermore National Laboratory), and Glenn
MacPherson (Smithsonian Institution) report two additional cases of
chondrules inside CAIs. They show that in both cases the CAIs contained
less 26Al when they crystallized than did most CAIs. The CAIs are also
depleted in oxygen-16 (16O), a characteristic associated with
chondrules. Durable minerals located in the central parts of the two
CAIs have 16O-rich compositions. Krot and his co-workers conclude that
the two chondrule-bearing CAIs had chondrule material added to them
during a reheating event about 2 million years after they had originally
formed. The conventional wisdom that CAIs are older than chondrules
remains intact, at least for now, but this work shows that CAIs, like
most solar system materials, can be reworked after they form.

Reference:

    * Krot, A. N., H. Yurimoto, I. D. Hutcheon, and G. J. MacPherson
      (2005) Chronology of the early Solar System from chondrule-bearing
      calcium-aluminum-rich inclusions. Nature, vol. 434, p. 998-1001.

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

Chondrules and CAIs: Hot Stuff in the Early Solar System

Chondritic meteorites are composed of materials that formed before
planets roamed the solar system. The oldest of these materials are
calcium-aluminum-rich inclusions (CAIs), light-colored objects rich in
refractory elements (that condense at
a high temperature). Besides calcium and aluminum, this includes
magnesium, titanium, and rare earth elements. CAIs range in size from
about a millimeter to a centimeter. Meteoriticists have identified
several distinct varieties of CAIs, but all share a high temperature
origin. Some might be condensates from the solar nebular; for example,
see the PSRD article: First Rock in the Solar System
<http://www.psrd.hawaii.edu/Oct02/firstRock.html>. Other CAIs might be
evaporation residues.

Allende meteorite
Slab of the Allende CV carbonaceous chondrite. Large light-colored
objects are CAIs. Smaller, round, dark objects are chondrules.

------------------------------------------------------------------------
Efremovka meteorite
A calcium-aluminum-rich inclusion (CAI) in the carbonacious chondrite
Efremovka with anorthite (an), melilite (mel), and pyroxene (px).

Chondrules are millimeter-sized frozen droplets of molten silicate. They
are less refractory than CAIs, but are still relatively high-temperature
products of solar system formation. Like CAIs, they come in a wide
variety of types, but all share a history of having been melted
(requires a temperature of more than 1400oC) and cooled rapidly (5 to
1000oC/hour).

PCA91082 meteorite
The chondritic meteorite PCA 91082 contains both chondrules and CAIs.
This X-ray map shows the elemental abundances in the meteorite: red is
magnesium, green is calcium and blue is aluminum.

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

Oxygen Isotope Fingerprint

The relative abundances of the isotopes of oxygen are very informative
about the origin of solar system materials. There are three stable
(non-radioactive) varieties of oxygen isotopes. Each has the same number
of protons in the nucleus, but different numbers of neutrons, resulting
in atomic masses of 16, 17, and 18. These different isotopes are called
oxygen-16 (16O), oxygen-17 (17O), and oxygen-18 (18O).

On Earth, rocks vary in the proportions of the three oxygen isotopes,
but they vary in a simple way. Two rocks with the same 18O/16O ratio
will have the same 17O/16O ratio. If their 18O/16O ratios differ by,
say, 0.2%, their 17O/16O ratios will differ by half this amount, 0.1%.
Rocks from Mars and igneous (melted) meteorites (which come from
asteroids) follow the same pattern, though the lines are offset from the
Earth line. Moon rocks lie on the Earth line. Thus, on a plot of 17O/16O
vs 18O/16O, planetary rock data lie along a line with a slope of 0.5.

CAIs and chondrules do not obey this well established planetary slope
0.5 behavior. They plot along a slope 1 line. Change 18O/16O by 0.1% and
17O/16O also changes by 0.1%. This is consistent with addition or loss
of pure 16O. There are several proposed sources for the 16O [see PSRD
article: Oxygen Isotopes Give Clues to the Formation of Planets, Moons,
and Asteroids <http://www.psrd.hawaii.edu/Dec01/Oisotopes.html>] but
let's just use the amount of 16O as a marker. In general, CAIs have a
higher abundance of 16O than do chondrules, as shown in the diagram below.

oxygen ratios
Plot showing the 18O/16O and 17O/16O ratios in chondrules and CAIs in
meteorites. These particles define a line with much steeper slope than
the Earth line, consistent with loss or addition of 16O. Chondrules
contain less 16O than do CAIs. Cosmochemists measure the 18O/16O and
17O/16O ratios in terms of deviations in parts per thousand from a
standard (delta 18O and delta 17O). The usual standard is mean ocean
water, abbreviated SMOW, for Standard Mean Ocean Water. Pure 16O would
plot at -1000 parts per thousand on both axes.

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

Ages from Vanished Isotopes

Cosmochemists can determine the relative ages of objects formed more
than 4.5 billion years ago by using the abundances of the decay products
of isotopes that no longer exist. The isotopes vanished because their
half-lives were so short that they have completely decayed. A prime
example of this is 26Al, which has a half life of only 730,000 years. It
decays to magnesium-26 (26Mg). The trick, of course, is to figure out
how to measure the abundance of something that no longer exists.
Cosmochemists perform this feat of isotopic magic by measuring the
aluminum and magnesium isotopes in different minerals in the same
samples. If 26Al was present when a sample formed, as the concentration
of aluminum increases, so should the abundance of 26Mg relative to 24Mg.
An example of this technique is shown in the diagram below. For more
information, see PSRD article: Using Aluminum-26 as a Clock for Early
Solar System Events <http://www.psrd.hawaii.edu/Sept02/Al26clock.html>.

Mg isotopic ratios
Magnesium isotopic ratios measured in different minerals with different
ratios of aluminum to magnesium from a refractory inclusion in the
meteorite Allende. Magnesium shows excesses in the isotope 26 that are
correlated with the aluminum/magnesium ratio, indicating that the 26Mg
excesses originated from the decay of the radioactive isotope 26Al. This
finding is evidence for the initial presence of 26Al in early solar
system objects.

Almost all the data gathered up to now indicate that the initial ratio
of 26Al/27Al is higher in CAIs than in chondrules. This ratio varied
with time in the early solar system because 26Al is radioactive. Data
for CAIs uniformly give an initial 26Al/27Al ratio of 5 x 10-5. Every
half life (730,000 years) decreases 26Al/27Al by a factor of two.
Chondrules tend to have 26Al/27Al lower than the values in CAIs. Using
the half life of 26Al, the 26Al/27Al ratio, and the equation for
radioactive decay, cosmochemists calculate that chondrules are between 1
and 3 million years younger than CAIs.

The story is not completely clear cut, of course. Martin Bizzarro and
colleagues at the Geological Museum, Denmark, and Victoria University of
Wellington, New Zealand, made very accurate isotopic analyses of
chondrules and CAIs drilled out of polished slabs of the Allende
chondrite. The five CAIs analyzed fell on a single line that indicated
the usual value of 5 x 10-5 for the 26Al/27Al ratio. The chondrules,
however, scattered more, indicating a range of initial 26Al/27Al ratios.
Some were equal to the typical CAI value; others were lower. Taken
together Martin Bizzarro's data suggest that formation of chondrules and
CAIs began at the same time, but that chondrule formation continued for
1-2 million years after production of CAIs stopped. However, Sasha Krot
and his colleagues argue that Bizzarro dated the formation of the
chondrule precursor dust, not the time chondrules formed by melting.
Dating the time of formation of individual chondrules cannot be done
unambiguously from a bulk isotopic analysis of a chondrule--magnesium
and aluminum isotopes must be measured on separate mineral grains in a
chondrule.

Nevertheless, Martin Bizzarro and his colleagues raise an important
issue that must be settled. One important line of evidence is the
presence of CAIs inside chondrules. These have been observed by several
meteoriticists, including Sasha Krot and Hisayoshi Yurimoto and their
co-workers. To be incorporated into a molten chondrule, a CAI must
already exist, hence is older. All the cases reported were of CAIs
inside chondrules. All, that is, until Itoh and Yurimoto found a
chondrule inside a CAI, implying contemporaneous formation of chondrules
and CAIs, in accord with the interpretation Martin Bizzarro and his
colleagues made from their isotopic data.

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

Chondrules Inside CAIs

Sasha Krot was intrigued by the unsettling data reported by Bizzarro and
co-workers and by Itoh and Yurimoto. The data appeared to upset a
perfectly good applecart. He applied his very astute eye to some
chondrites and found more cases of chondrules inside CAIs. Then, working
with Hisayoshi Yurimoto, Ian Hutcheon, and Glenn MacPherson, they
studied the chondrules in detail with electron microscopy and electron
and ion microanalysis, and analyzed oxygen, magnesium, and aluminum
isotopes. The evidence they assembled suggests that the CAIs containing
chondrules were remelted in the chondrule-forming region.

Allende meteorite
The top picture is of a CAI (blue and green) in the Allende meteorite.
The image was made by combining x-ray counts from magnesium (red),
calcium (green), and aluminum (blue) in an electron microprobe. The area
in the square labeled c is shown in an electron microscope image in the
lower photograph. The minerals olivine (ol) and orthopyroxene (opx) are
common in chondrules. Compositions and the minerals present point to
this area being a little piece of a chondrule. It was included in the
CAI melt, so must have existed already--that is, it is older.

Krot and co-workers describe two CAIs that contain chondrule fragments.
The photographs above show what one of them looks like if your eyes
could see electrons and x-rays. The chondrules are clearly identified by
the presence of iron-bearing olivine and orthopyroxene, common minerals
in chondrules but not in CAIs.

As shown in the oxygen isotope diagram above, CAIs and chondrules have
different amounts of 16O. Krot and his colleagues reasoned that if the
CAIs were remelted and had chondrules added to them, this ought to show
up in the oxygen isotopic compositions of mineral grains in the CAIs and
in their included chondrules. This is exactly what they found. Using an
ion microprobe they measured the isotopic compositions of minerals in
each CAI and its chondrule chip. They found that the chondrule material
had the normal chondrule oxygen, which is low in 16O. Relict,
hard-to-melt grains like spinel had more typical CAI-like compositions
much richer in 16O. Minerals that occur in the outer zones of the CAIs
have low amounts of 16O. All this suggests that both CAIs could have
been remelted, and a pre-existing chondrule was added to the melt.

oxygen ratios
Oxygen isotopic compositions of minerals in two CAIs that contain
chondrule fragments. The minerals in the chondrules and in the outer
portions of the CAI have relatively low amounts of 16O (they plot close
to the intersection with the terrestrial line). Minerals in the interior
and minerals that melt at high temperatures (e.g., spinel) preserve the
typical composition richer in 16O.

Krot and co-workers also measured magnesium and aluminum isotopes in
individual minerals using an ion microprobe. They found that when the
latest melting took place the chondrules contained much less 26Al than
typical for CAIs. This could mean that the CAIs formed later than other
CAIs. Or, it might mean that the CAIs formed at one time and were then
remelted some time later. According to the 26Al abundance, this second
melting would have taken place about 2 million years after the CAIs with
the initial 26Al/27Al ratio (5 x 10-5) formed (see diagram below). Krot
favors the second explanation on the basis of typical 16O abundance in
minerals in the interiors of the CAIs.

Mg and Al ratios
Magnesium isotope abundances measured in different minerals with
different aluminum to magnesium ratios in two chondrule-bearing CAIs
(labeled ABC and TS26). The slopes of the lines fitted to the data for
these two inclusions are more than ten times less than the 5 x 10-5
value characteristic of most CAIs, suggesting the two chondrule-bearing
inclusions formed at least 2 million years later. Sasha Krot and his
colleagues suggest that the younger age for these CAIs was caused by a
heating event that remelted them and incorporated chondrule materials
inside the molten glob of CAI.

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

Looking Back in Time

This trip in a time machine to events that took place before the planets
formed would not be possible without high-tech analytical tools. The
CAIs and their included chondrules were identified by optical microscopy
and characterized by scanning electron microscopy and electron
microprobe analysis. The isotopic compositions of oxygen, magnesium, and
aluminum were measured with an ion microprobe, an amazing device that
can measure isotopes and trace elements on the scale of less than a
millimeter. These tools and the experience and intuitive powers of the
cosmochemists involved allow us to look back in time to when gas and
dust surrounded the young Sun, but before the planets accreted out of
the dusty cloud. In fact, the melting events recorded by the CAIs that
Krot and his team describe may be part of the planet-forming process.

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

ADDITIONAL RESOURCES

    * Bizzarro, M., J. A. Baker, and H. Haack, 2004, Mg isotope evidence
      for contemporaneous formation of chondrules and refractory
      inclusions. Nature, vol 431, p. 275-278.


    * Itoh, S. and H. Yurimoto (2003) Contemporaneous formation of
      chondrules and refractory inclusions in the early Solar System.
      Nature, vol. 423, p. 728-731.


    * Krot, A. N., H. Yurimoto, I. D. Hutcheon, and G. J. MacPherson
      (2005) Chronology of the early Solar System from chondrule-bearing
      calcium-aluminum-rich inclusions. Nature, vol. 434, p. 998-1001.


    * Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976)
      Demonstration of 26Mg excess in Allende and evidence for 26Al.
      Geophys. Res. Lett., v. 3, p. 41-44.


    * Scott, E. R. D. (2001) Oxygen Isotopes Give Clues to the Formation
      of Planets, Moons, and Asteroids. Planetary Science Research
      Discoveries. http://www.psrd.hawaii.edu/Dec01/Oisotopes.html.

    * Simon, S. B. (2002) First Rock in the Solar System. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/Oct02/firstRock.html
      <http://www.psrd.hawaii.edu/Oct02/firstRock.html>.

    * Zinner, E. (2002) Using Aluminum-26 as a Clock for Early Solar
      System Events. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Sept02/Al26clock.html.
Received on Wed 01 Jun 2005 01:27:33 PM PDT