abstracts on carbonaceous chondrites at LPSC

[Peter Abrahams <telscope@europa.com>]

Abstracts for the 29th Lunar & Planetary Science Conference, March 1998,
are at:
http://cass.jsc.nasa.gov/meetings/LPSC98/pdf/sess42.pdf
This site requires Adobe Acrobat software (free).  Papers on carbonaceous
chondrites include these from a session on March 17:

The matrix in CM meteorites contains chondrules, calcium aluminum
inclusions, and other constituents, most of which have an opaque rim that
probably formed in an aqueous process.  Rims are found in other chondrites,
but the CM rims are distinguished by their composition (mostly hydrated
minerals in a fine grain), and by the smaller individual grains (mostly
calcite) in CMs.  Thin sections of four CM chondrites, including Murchison,
were studied using optical & electron imaging.  The mineral composition of
the rims is presented in detail, and their structure divided into single
layer or concentric layers.  Sometimes it appears as if the rims
precipitated onto dissolving calcite from the grain.  Other rims appear to
have formed as vugs, lining a cavity, which were later filled in by
calcite.  Either scenario is considered plausible.  (Browning)

The Leoville meteorite is one of the CV3 carbonaceous chondrites that
contains dark inclusions.  In Leoville, one of these inclusions contains
irregular fragments of olivine and pyroxene, a few of which have chondritic
textures, with fine grained rims.  They are set in a shocked matrix of
olivine grains, many crystals of magnetite, and a few olivine crystals.
This inclusion is unique among those studied from CV chondrites, in the
quantity of magnetite, and the lack of chondrules.  If the fragments are
from chondrules, they were of a type that was much different than
chondrules found in chondrites.  This inclusion is relatively unaltered.
Another DI from Leoville is described, this one a highly altered, fine
grained olivine.
No conclusions are drawn from these unusual inclusions.  (Brearley)

A very few samples of Allende contain inclusions with crystals of red
spinel.  Crystals that are sometimes found in Murchison are similar in
size, zoning, chemistry, and silicate inclusions.  The largest of these
deep red spinel crystals found in Allende is a 1 mm by .75 mm fragment, set
in a fine matrix of nepheline and olivine.  This crystal is now in a thin
section, where it appears pale pink, and under an electron microscope shows
zones in a chevron shape.  It probably crystallized in an oxidizing
atmosphere, not in the solar gas, because of its high chromium and iron
content.  Slow cooling with fluctuating temperatures could produce the
observed zones.  These spinel crystals are probably relict in Allende.
Allende & the CV3s are very different material than Murchison and the CM2s,
in the nature of their chondrules and CAIs.  Therefore, their similar
spinel crystals probably mean that there was a large quantity of spinel
crystals as condensate in the early nebula.  (Simon)

The Allende meteorite releases ‘planetary noble gases’ when heated to 1200
- 1600 degrees C.  The mineralogy of the carrier of the gas is a mystery,
and is called “Phase Q”.  Possibly, atoms of noble gas occupy sites on the
surface of Q.  It is proposed that Q is a carbon nanotube, since similar
structures have been observed in Allende.  These tubes might be some
micrometers in length and nanometers in diameter.  (Heymann)

______________________________________
Peter Abrahams, telscope@europa.com

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