[meteorite-list] COMETS AND CARBONACEOUS CHONDRITES

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu Sep 21 20:54:35 2006
Message-ID: <002001c6dde1$a187f5e0$cd2ce146_at_ATARIENGINE>

Hi, E.P.,


    The truth is we really don't know what comets
and asteroids actually are, or whether there's a real
distinction between them, or if they are just keywords
derived (mistakenly) from the two extremes of a
continuous spectrum of bodies with every intermediate
state fully represented.

    There are "comets" that "die" and turn into
"asteroids," and there are "asteroids" that suddenly
develop a coma and become "comets." But the
two terms may not be a descriptions of two
essentially different classes of bodies at all. After
we sample and/or visit 50 or 100 of them, we'll have
a much better idea...

    The association of carbonaceous chondrites with
"comets" is supposed by many, but not ever demonstrated.
No meteorite has ever been definitively linked to a comet.
There are no "known" samples of cometary material. (We
may have it, but if we do, we don't know it...) On the
chance that CC's may be linked to cometary material
or be similar to it...

    Here's a summary on Carbonaceous Chondrites
(quickly ripped from the Net, not my data-leaky
brain). The metal content runs from 50% for
Bencubbinites, 15% for CH type, down to about
1% for other classes. Some classes have clearly
never been warmed about 50 degrees absolute;
some people have suggested that the CH class
formed intra-Mercurially. Obviously, all carbon
containing meteorites didn't start out in the same
single nursery! Another indicator that the heresy
that the early system was very well stirred might
be true.

    Carbonaceous chondrites account for about
3% of all known chondrites. They are classified
according to the proportion and size of the chondrules
they contain (one rare subclass lacks chondrules).
The average contents of CC's are: Carbon, 2.0%;
Metals, 1.8%; Nitrogen, 0.2%; Silicates, 83.0%;
Water, 11.0%. At most, they can be 20% water and
can contain as much as 4% carbon. Carbonaceous
Chondrites contain around 5% kerogen.

The sub-classes are:

CI chondrites, only a handful of which are known, are
named for the Ivuna meteorite. They have very few
chondrules and are composed mostly of crumbly,
fine-grained material that has been changed a lot by
exposure to water on the parent asteroid. As a result
of this aqueous alteration, CI chondrites contain up
to 20% water in addition to various minerals altered
in the presence of water, such as clay-like hydrous
phyllosilicates and iron oxide in the form of magnetite.
They also harbor organic matter, including polycyclic
aromatic hydrocarbons (PAHs) and amino acids,
which makes them important in the search for clues
to the origin of life in the universe. It remains uncertain
whether they once had chondrules and refractory
inclusions that were later destroyed during the formation
of hydrous minerals, or they lacked chondrules from
the outset. CIs have never been heated above 50?C,
indicating that they came from the outer part of the
solar nebula. They are especially interesting because
their chemical compositions, with the exception of
hydrogen and helium, closely resemble that of the
Sun's photosphere. They thus have the most primitive
compositions of any meteorites and are often used as
a standard for gauging how much chemical fractionation
has been experienced by materials formed throughout
the solar system.

CM chondrites are named for the Mighei meteorite
that fell in Mykolaiv province, Ukraine, in 1889.They
contain small chondrules (typically 0.1 to 0.3 mm in
diameter) and similar-sized refractory inclusions.
They also show less aqueous alteration than, and
about half the water content of, CI chondrites. Like
CIs, however, they contain a wealth of organic material -
more than 230 different amino acids in the case of the
famous Murchison meteorite. Comparisons of
reflectance spectra point to the asteroid 19 Fortuna
or, possibly, the largest asteroid, 1 Ceres, as
candidate parent bodies.

CV chondites are named for the Vigarano meteorite
that fell in Italy in 1910. They resemble ordinary
chondrites and have large, well-defined chondrules
of magnesium-rich olivine, often surrounded by iron
sulfide, in a dark-gray matrix of mainly iron-rich olivine.
They also contain calcium-aluminum inclusions (CAIs) -
the most ancient minerals known in the solar system -
that typically make up more than 5% of the meteorite.

CO chondrites are named for the Ornans meteorite
that fell in France in 1868. They some similarities in
composition and chemistry to the CV chondrites and
may have formed with them in the same region of
the early solar system. As in the CV group, CAIs
are present but are commonly much smaller and
spread more sparsely in the matrix. Also typical
of COs are small inclusions of free metal, mostly
nickel-iron, that appear as tiny flakes on the polished
surfaces of fresh, unweathered samples.

CK chondrites are named for the Karoonda meteorite
that fell in Australia in 1930. They were initially thought
to be members of the CV group but are now grouped
separately since they differ in some respect from all
other carbonaceous chondrites. Their dark gray or
black coloration is due to a high percentage of
magnetite dispersed in a matrix of dark silicates
consisting of iron-rich olivine and pyroxene. This
shows they formed under oxidizing conditions, yet
there is no sign of aqueous alteration. Elemental
abundances and oxygen isotopic signatures suggest
that CKs are closely related to CO and CV types.
Most CK chondrites contain large CAIs and some
show shock veins that point to a violent impact history.

CR chondrites are named for the Renazzo meteorite
that fell in Italy in 1824. They are similar to CMs in
that they contain hydrosilicates, traces of water, and
magnetite. The main difference is that CRs contain
reduced metal in the form of nickel-iron and iron
sulfide that occurs in the black matrix as well as in
the large chondrules that make up about 50% of the
meteorites. A possible parent body is Pallas, the
second largest asteroid. The CH and CB chondrites
are so closely related to the CRs that all three groups
may have come from the same parent or at least from
the same region of the solar nebula.

CH chondrites are named for their High metal content.
They contain up to 15% nickel-iron by weight and are
closely related in chemical composition to the CRs and
CBs. They also show many fragmented chondrules,
most of which, along with the less abundant CAIs, are
very small. As with the CRs, the CHs contain some
phyllosilicates and other traces of alteration by water.
One theory suggests that the CHs formed very early
in the solar system's history from the hot primordial
nebula inside what is today the orbit of Mercury, later
to be transported to outer, cooler regions of the nebula
where they have been preserved to this day. Mercury
may have formed from similar, metal-rich material, which
would explain its high density and extraordinary large
metal core.

CB chondrites, also known as bencubbites, are named
for the prototype found near Bencubbin, Australia, in
1930. Only a handful of these unusual meteorites are
known. All are composed of more than 50% nickel-iron,
together with highly reduced silicates and chondrules
similar to those found in members of the CR group.

C ungrouped chondrites (C UNGRs) fall outside the
other groups and probably represent other parent
bodies of carbonaceous chondrites or source regions
of the primordial solar nebula.


Sterling K. Webb
----------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine_at_yahoo.com>
To: <meteorite-list_at_meteoritecentral.com>
Sent: Thursday, September 21, 2006 5:48 PM
Subject: Re: [meteorite-list] 2003 EL61, IN PERSON


> Hi Sterling -
>
> With Chiemgau under "challenge", the only evidence of
> heavy elements in comets that I can easily point to is
> the increased iridium at the KT boundary.
>
> I can't really comment on metals in carbonaceous
> chondrite meteorites, and right now I would be most
> interested in data from others on these.
>
> good hunting,
> Ed
>
Received on Thu 21 Sep 2006 08:54:16 PM PDT