[meteorite-list] Tagish Lake -- A Meteorite from the Far Reaches of the Asteroid Belt (Part 2 of 2)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:34 2004
Message-ID: <200212130149.RAA07524_at_zagami.jpl.nasa.gov>

Tagish Lake -- A New Type of Primitive Meteorite

Now back to Tagish Lake. Mike Zolensky and friends found that Tagish Lake is
actually composed of two somewhat different rock types. The major difference
between the two lithologies is in the abundance of carbonate minerals, one
is poor in carbonates and the other is rich in them (see the images below).
Both are composed of low temperature minerals pseudomorphing original high
temperature phases, but some residual high temperature minerals are
nevertheless preserved. This leads to a petrologic classification of type 2.
However, Zolensky and company noted that Tagish Lake contained relatively
few pseudomorphed chondrules -- much less than in the other type 2
chondrites, the CM2 and CR2 chondrites. They also noted that Tagish Lake has
a much lower density than any other type of chondrite. They posited that
Tagish Lake was formed further out in the solar system where fewer
chondrules were formed and so was composed of a higher proportion of low
density matrix material.

       [carbonate-poor rock type]
       A back-scattered electron image of the carbonate-poor
       lithology showing one of the rare, pseudomorphed chondrules.
       Almost none of the original high-temperature minerals are
       left in the chondrule, but the original texture is well
       preserved. Very few of the mineral grains in this lithology
       are carbonates. The scale bar is 100 micrometers.

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

            [carbonate-rich rock type]
            A back-scattered electron image of the
            carbonate-rich lithology showing its typical
            texture. The abundant light gray grains are
            carbonate minerals. The scale bar (lower right) is
            10 micrometers.

Well, we now know that Tagish Lake is a petrologic type 2 chondrite, but to
which chemical class does it belong? The chemical classes are defined
primarily by characteristic features of the bulk composition plus the
isotopic composition of oxygen. Oxygen is the third most abundant element in
the solar system, after H and He. Thus, it may come as some surprise that
the oxygen isotopic composition of the solar nebula was not everywhere the
same. About thirty years ago, Robert Clayton of the University of Chicago
and colleagues showed that different fractions of the Allende carbonaceous
chondrite had distinct oxygen isotopic compositions that demonstrated this
heterogeneity. In a series of subsequent publications, Clayton and
colleagues showed that many meteorite classes have their own distinctive
oxygen compositions. Now days, whenever an unusual meteorite falls or is
found, its oxygen isotopic composition is determined to see how it fits in
the solar pantheon. Brown and colleagues presented analyses of the oxygen
isotopic composition of two bulk samples of Tagish Lake. They found that it
is distinct from any other chondrite class, although it is close in oxygen
isotopic composition to the primitive CI chondrites. Thus, Tagish Lake
seemed to represent a new type of chondrite. This is borne-out by other
compositional studies.

Several research groups have done bulk elemental composition studies of
Tagish Lake -- Brown and colleagues, Friedrich and colleagues at Purdue
University, and me, your humble narrator. The bulk elemental compositions of
chondrites record the fractionation processes that occurred during formation
of the rocky matter in the solar system. At the start, the solar nebula was
composed of a cloud of gas and dust. As formation of the solar system
started, gravitational attraction caused the cloud to begin to collapse to
form the proto-sun. This collapse heated the cloud to the point that the
dust in the inner regions vaporized. Subsequently, the gas cloud cooled, and
minerals began to condense out of the gas phase much the way snowflakes
condense from the water vapor contained in Earth's atmosphere. This process
led to fractionations -- that is separations -- of some elements relative to
others based on the minerals they condense into. Different chemical classes
of chondrites show differing types and degrees of these fractionations.

Geochemists divide elements into four basic types. Lithophile elements are
those that are contained primarily in silicate minerals -- the rocky bits of
meteorites. Siderophile elements are found mostly in the iron metal phase.
Chalcophile elements follow sulfur into the sulfide minerals. Finally,
atmophile elements remain largely in the gas phase. Different silicate
minerals, metal and sulfide minerals condense out of the solar nebula at
different temperatures. If some process separates minerals from the gas
phase before condensation is complete, elemental fractions occur and these
can be preserved in chondritic meteorites. The CI chondrites are nearly
unfractionated relative to the bulk matter of the solar system. For a wide
range of lithophile, siderophile and chalcophile elements, CI chondrites are
very similar in composition to the solar photosphere (the visible "surface"
of the sun). Since the sun contains about 99.9% of the mass of the solar
system, its composition is effectively that of the solar system. Thus, it
appears that the CI chondrites acquired all but the atmophile elements in
the same proportions as in the primitive solar nebula.

The bulk composition of Tagish Lake is shown compared to CI and CM
chondrites in the abundance diagram shown below. The CI chondrites are shown
as a straight line because I have normalized the data to CI chondrites --
that is, I divided the concentration of each element in each meteorite type
by its concentration in CI chondrites. (I also did a double normalization to
magnesium in each meteorite type. This corrects for differing degrees of
dilution by volatile constituents in the rocks -- water and carbon dioxide.)
The elements are ordered by decreasing temperature at which each element is
calculated to have condensed out of the gas phase in the solar nebula, and I
have used different shading for lithophile, siderophile and chalcophile
elements. Tagish Lake is closest in composition to CI and CM chondrites, but
as can be seen, it matches neither of these chondrite types exactly. For
those elements that condense above about 1340 K (about 1067 oC, or 1953 oF),
Tagish Lake is a good match for CM chondrites. However, for elements that
condensed below about 1080 K (about 807 oC, or 1485 oF), Tagish Lake lies
between the CI and CM chondrites. This is true independent of whether the
element is lithophile, siderophile or chalcophile in character. Here again,
the bulk composition shows that Tagish Lake is its own beast, although it is
closer to CM chondrites in composition.

      [abundance plot]
      A plot of the abundances of a wide range of elements in Tagish
      Lake compared to the primitive carbonaceous chondrite types CI
      and CM. Each symbol represents a different element, and they
      are shaded according to geochemical behavior -- lithophile,
      siderophile and chalcophile. The elements are plotted according
      to the temperature at which they are calculated to condense out
      of the nebular gas into mineral phases. For the more volatile
      elements (those on the right) Tagish Lake is between CI and CM
      chondrites in composition.

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

Diamonds and Star Dust

Monica Grady of the Natural History Museum in London and her colleagues did
detailed analyses for carbon and nitrogen in Tagish Lake, and made estimates
of the various light-element components in the meteorite. First off, they
found that Tagish Lake contains by far more total carbon than either CI or
CM chondrites. However, they studied only a small chip of the meteorite, and
they estimated that between 22% and 47% of the carbon was derived from
carbonate minerals. It is possible that their sample was of the minor,
carbonate-rich lithology identified by Zolensky and friends, in which case
the C content determined by Grady and colleagues may overestimate the true
abundance. A large fraction of the total carbon is contained in organic
molecules -- Grady and company estimate that about 44% of the C is so
contained. Currently it is believed that organic compounds in primitive
meteorites either are compounds formed in the interstellar medium, or
remnants of those compounds modified during alteration on asteroids. Either
way, the presence of abundant organic material in Tagish Lake implies that a
portion of the material that went into forming its parent asteroid was never
strongly heated in the nascent solar nebula. Finally, Grady and colleagues
noted that a portion of the carbon is contained in what are called
nanodiamonds -- very tiny diamond grains at most only a few micrometers in
size. In fact, Tagish Lake contains more of the nanodiamonds than any other
meteorite. Nanodiamonds are believed to have formed in the expanding shell
of a type II supernova. Hence, they are literally the dust of other stars
that traveled through the interstellar medium to end up in our forming solar
system. Again, because Tagish Lake contains more of these grains of
stardust, it seems likely that this meteorite was formed farther out in the
solar nebula than other meteorite types.

The emerging picture, then, is that Tagish Lake represents a new type of
primitive meteorite, similar to, but distinct from the CI and CM chondrites.
In some properties Tagish Lake is more similar to CI chondrites, in others
to CM chondrites. The early studies discussed above point out numerous
characteristics of Tagish Lake that suggest that it came to us from the
outer reaches of the asteroid belt -- a reflectance spectrum like the
distant D asteroids, high carbon and water contents, abundant interstellar
nanodiamonds and organic molecules, the low density and low abundance of
chondrules -- all bespeak an origin far from the sun. Without doubt, further
study of this unique stone will tell much about the events that transpired
roughly 4.56 billion years ago that ultimately led to us, sitting in front
of our computers, wondering how it all came to be.
             --------------------------------------------------

[ADDITIONAL RESOURCES]

     Book recommendation for general audiences: Meteorites: Their Impact on
     Science and History by Brigitte Zanda and Monica Rotaru (Editors),
     2001, Cambridge University Press, 128 p.

     Brown, Peter, G. and others (2000) The fall, recovery, orbit, and
     composition of the Tagish Lake meteorite: a new type of carbonaceous
     chondrite, Science, v. 290, p. 320-325.

     Friedrich, J.M., Wang M-S., and Lipschutz, M.E. (2002) Comparison of
     the trace element composition of Tagish Lake with other primitive
     carbonaceous chondrites, Meteoritics and Planetary Science, v. 37,
     p.677-686.

     Grady, Monica M. and others (2002) Light element geochemistry of the
     Tagish Lake C12 chondrite: comparison with CI1 and CM2 meteorites,
     Meteoritics and Planetary Science, v. 37, p.713-735.

     Hiroi T., Zolensky, M.E., and Pieters, C.M. (2001) The Tagish Lake
     meteorite: A possible sample from a D-type asteroid, Science, v.293, p.
     2234-2236.

     Mittlefehldt, David W. (2002) Geochemistry of the ungrouped
     carbonaceous chondrite Tagish Lake, the anomalous CM chondrite Bells,
     and comparison with CI and CM chondrites, Meteoritics and Planetary
     Science, v. 37, p. 703-712.

     Tagish Lake Meteorite/fireball investigation homepage from the
     consortium study by the University of Calgary, University of Western
     Ontario and NASA/JSC.

     Tagish Lake Meteorite news release (May 2000) from the University of
     Calgary.

     Zolensky, M.E. and others (2002) Mineralogy of Tagish Lake: an
     ungrouped type 2 carbonaceous chondrite, Meteoritics and Planetary
     Science, v. 37, p. 737-761.

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Received on Thu 12 Dec 2002 08:49:24 PM PST