[meteorite-list] Impact Melt Rocks

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:51 2004
Message-ID: <DIIE.0000004300000FDD_at_paulinet.de>

I am sitting here and enjoying the last two days of our summer vacations here.
Three different meteorite classes keep my weary eyes open at 01:50 hrs a.m.

a) The lunar meteorites in my collection
b) The LL3.x chondrule wonders
c) The impact melts

There are presently at least 7 chondritic impact melt rocks in my collection -
among these are (not sure about Park Forest, Portales Valley or NWA 66):

Chico - Dhofar 10 - Gao (B) - NWA 722 - NWA 1652 - Shaw - Spade

.. and a motley crowd they are :-)

The L melt rocks - like Chico - suffered at least 2 catastrophic collisions about
500 mio. years ago as shown by their high shock stages (S6 for Chico), silicate
blackening (Orvinio), maskelynite (Ramsdorf), shock veining or mosaicism under
crossed polars, high pressure phase such as majorite [Mg3(MgSi)Si3O12] and
diamond - to name a few characteristical features of some highly shocked L's.

Of course, they are also heavily depleted in volatile components.My 6.5-gram Chico
that I acquired from Dieter Heinlein in 1999 neatly shows cm-size globules (oval blebs)
of metal and sulfide, and several dark roundish patches in the surrounding impact melt.

Gao (b) - I got that beauty from Eric Olson - and NWA 722 (El Kachla) from Carine
and Bruno, could almost be twins. They show unshocked chondritic textures along
with shocked and highly shocked phases, "stream channels" with FeNi particles
showing a preferred orienation the closer they are to these "channels".

My latest additon, NWA 1652 (from Stefan Ralew), is another piece in the jigsaw
puzzle of chondritic IMBs. Although my little specimen does not show those metal
blebs of my Chico piece - the metal is distributed more evenly, it does show those
dark, roundish patches. If you are interested in viewing my little specimen, there it it
is - you'll have to scroll down a bit:

http://www.meteoriten.com/special.html

My 2.4-gram Shaw is another interesting representative of the L-impact suite
because it is usually classified as an L6 but R.T. Dodd et al. (1975) assigned
it to petrologic grade 7 (!). Other meteoriticists stated it was a complex impact
melt breccia - G.J. Taylor et al. (1979).

Enough spouted ... time to hit the bunk :-)

Bernd
Received on Fri 05 Sep 2003 07:52:28 PM PDT