[meteorite-list] impact melts

From: Frank Prochaska <fprochaska_at_meteoritecentral.com>
Date: Sun Apr 3 13:18:11 2005
Message-ID: <0IED00HDFS25Y6D0_at_vms040.mailsrvcs.net>

        I haven't seen a reply to your post yet, so I'll take a shot.

        The terms chondrite and achondrite are not used to denote texture
any more. They were originally coined for that purpose, but that was long
before the very accurate chemical analysis techniques we have today, and the
greater understanding of the formation of meteorites.
        A chondrite is a meteorite that has a 'chondritic' composition, that
is a composition similar to that found in the Sun. (This is assumed to be
the composition of protoplanetary disk, except for some changes due to a few
billion years of nuclear fusion.) So a CI chondrite is still a chondrite
even though there are no chondrules to be seen.
        An achondrite is a meteorite whose composition has been altered
significantly from the chondritic precursor material by, I guess I have to
call it "geological" processes. In other words, they are differentiated
rocks. Just completely melting a meteoroid doesn't make it an achondrite.
Something like a cumulate eucrite is essentially a basalt or lava rock that
can be found on Earth. It formed from a precursor rock that was completely
melted and grains of particular minerals precipitated and settled out of the
melt, probably settling to the bottom of the magma chamber. Meanwhile other
material in the melt, with a different composition since the remaining
minerals would crystallize at a different temperature (different time),
would perhaps float on top and become a different rock above or perhaps be
drawn off or 'move' somewhere else while still fluid to crystallize
somewhere else etc. A cumulate eucrite is a highly differentiated rock.
You can envision scenarios where a melt would differentiate slightly, but
not nearly so dramatically. These would be a more primitive differentiated
rock or achondrite.

        An impact melt is just that -- a rock that melted from an impact. A
chondrite that melted from an impact is still a chondrite, because the bulk
composition has not changed, just the texture of the rock since it melted
and solidified again. There are certainly impact melts that appear to be
entirely melted precursor material. Happy Canyon was originally considered
an E6, then an E7, and now an E - impact melt (but a chondrite nonetheless).
A breccia is a rock that is made up of broken pieces of rock that
re-lithified some way or another. A breccia could be made up entirely of
pieces of the same rock (pieces of the impact melt), or be a mixture of
pieces of different rocks (pieces of the melt, unmelted precursor material,
etc.).

        Sorry for the long answer, but some background into "why" seemed
more useful than a "yes" or "no."



Frank Prochaska




-----Original Message-----
From: meteorite-list-bounces_at_meteoritecentral.com
[mailto:meteorite-list-bounces_at_meteoritecentral.com] On Behalf Of stan .
Sent: Saturday, April 02, 2005 6:42 PM
To: meteorite-list_at_meteoritecentral.com
Subject: [meteorite-list] impact melts


are all impact melts breccias of melt and original matrix? if the matrix is
totally melted is is still considered an impact melt, or does the stone
become an achonderite (a non primative achonderite, young achonderite, etc)

thanks for any clarification


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Received on Sun 03 Apr 2005 01:18:05 PM PDT