[meteorite-list] NWA 2058 and pseudotachylite

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:53 2004
Message-ID: <DIIE.0000003F00001C9E_at_paulinet.de>

> It is the most shocked chondrite yet discovered with no chondritic
> features that can be measured for metamorphism meaning ...

So the Yanzhuang (H6, S6) chondrite that fell 1990, Oct 31, and is
one of the most severely reheated and the most severely shocked
H chondrites has got a new "brother".

> Shock stage: S6 plus ...

According to St=F6ffler, S6 means: very strongly shocked, recrystallization
of olivine and phase transformations of olivine (ringwoodite).

I wonder if this will lead to an extension of the shock stage scale to S7 ?!

> this meteorite was formed by a hypervelocity impact on the surface of an
> asteroid and cooled within milliseconds as it was ejected out into space.

Fast cooling and complete melting also happened to Yanzhuang. F. Begemann
et al. write: "The metal in the FeNi-FeS blobs shows the distinct dentritic
structure characteristic of fast cooling. Indeed, the typical dentrite width
of some 30 micrometers is comparable to that observed in a metal regulus of
vacuum-melted H-chondrite Pantar that was cooled down to 800=B0C within
approximately 3 minutes."

Adam, do NWA 2058 FeNi-FeS blobs also show this dentritic structure?

> NWA 2058 is an H (Pseudotachylite) ...

According to Rubin, the Elbert LL6 chondrite contains narrow glassy pseudo-
tachylite-like veins. If these veinlets are "type-A", they may be coesite-
and stishovite-bearing.

Adam, any reports of these high-pressure polymorphs in NWA 2058?

Here on Earth, The Vredefort Dome and Sudbury (Sudbury breccia) structures
are the type areas for this unusual rock type called pseudotachylite but it has
also been observed in other impact structures (Rochechouart, Manicouagan,
Slate Islands).


Best wishes,

Bernd


References:

ST=D6FFLER D. et al. (1991) Proposal for a revised petrographic shock
classification of chondrites (Meteoritics 26-4, 1981, A398-A399).

BEGEMANN F. et al. (1992) On the thermal history of heavily shocked
Yanzhuang H-chondrite (Meteoritics 27-2, 1992, 174-178).

XIANDE X. et al. (1991) The new meteorite fall of Yanzhuang, a severely
shocked H6 chondrite with black molten materials (Meteoritics 26-4, 1991, A411).

FRENCH B.M. (1998) Traces of Catastrophe (LPI Contribution 954, pp. 65-69).

RUBIN A.E. (2003) Post-shock annealing and post-annealing shock: implications
for the thermal and shock histories of ordinary-chondrite parent bodies (Lunar and
Planetary Science XXXIV (2003), 1263.pdf).

To: adamhupe_at_comcast.net
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Received on Wed 24 Mar 2004 03:27:07 PM PST