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

From: Rob Wesel <nakhladog_at_meteoritecentral.com>
Date: Thu Jun 2 01:40:29 2005
Message-ID: <002901c56735$95c2daf0$998caa43_at_robewcufk0z2s3>

Someone on the list had also mentioned the possibility that a CAI could be
conical allowing chondritic material to fill the cone. The cone, when sliced
would look like chondritic material completely encapsulated within a CAI.
Context is everything.

Rob Wesel
http://www.nakhladogmeteorites.com
------------------
We are the music makers...
and we are the dreamers of the dreams.
Willy Wonka, 1971



----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, June 01, 2005 10:27 AM
Subject: [meteorite-list] Making Sense of Droplets Inside
Droplets(Chondrules & CAIs)


>
>
> 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.
>
> ______________________________________________
> Meteorite-list mailing list
> Meteorite-list_at_meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
Received on Thu 02 Jun 2005 01:40:28 AM PDT