AW: [meteorite-list] NWA 2058 and pseudotachylite

From: Jörn Koblitz <koblitz_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:53 2004
Message-ID: <E5E6112EA31FA24CB448E091C6883C050EBB84_at_server2000.microfab.de>

> Shock stage: S6 plus ...

S7, S8...?

The problem with further increasing shock pressures is that the rock is =
extremely heated up to melting by the energy of the shock wave and the =
compression and shear friction involved. This will vanish all signs of =
shock the material experienced before the subsequent annealing and =
healing of shock features happened. That's why impact melt rocks doesn't =
show the typical features of heavy shock which are necessary to properly =
classify the shock stage. Observed shock features in impact melt rocks =
or IMBs originated from secondary (later) shock events (imagine heavily =
bombardment of the chondrite parent body and further processing of the =
material). So, I think S6 is the limit. Beyond that, nobody can tell for =
sure.

Joern

> -----Urspr=FCngliche Nachricht-----
> Von: bernd.pauli_at_paulinet.de [mailto:bernd.pauli@paulinet.de]
> Gesendet: Mittwoch, 24. M=E4rz 2004 21:44
> An: Meteorite-list_at_meteoritecentral.com
> Betreff: [meteorite-list] NWA 2058 and pseudotachylite
>=20
>=20
> > It is the most shocked chondrite yet discovered with no chondritic
> > features that can be measured for metamorphism meaning ...
>=20
> 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".
>=20
> > Shock stage: S6 plus ...
>=20
> According to St=F6ffler, S6 means: very strongly shocked,=20
> recrystallization
> of olivine and phase transformations of olivine (ringwoodite).
>=20
> I wonder if this will lead to an extension of the shock stage=20
> scale to S7 ?!
>=20
> > this meteorite was formed by a hypervelocity impact on the=20
> surface of an=20
> > asteroid and cooled within milliseconds as it was ejected=20
> out into space.
>=20
> Fast cooling and complete melting also happened to Yanzhuang.=20
> F. Begemann
> et al. write: "The metal in the FeNi-FeS blobs shows the=20
> distinct dentritic
> structure characteristic of fast cooling. Indeed, the typical=20
> dentrite width
> of some 30 micrometers is comparable to that observed in a=20
> metal regulus of
> vacuum-melted H-chondrite Pantar that was cooled down to 800=B0C =
within
> approximately 3 minutes."
>=20
> Adam, do NWA 2058 FeNi-FeS blobs also show this dentritic structure?
>=20
> > NWA 2058 is an H (Pseudotachylite) ...
>=20
> According to Rubin, the Elbert LL6 chondrite contains narrow=20
> glassy pseudo-
> tachylite-like veins. If these veinlets are "type-A", they=20
> may be coesite-
> and stishovite-bearing.
>=20
> Adam, any reports of these high-pressure polymorphs in NWA 2058?
>=20
> Here on Earth, The Vredefort Dome and Sudbury (Sudbury=20
> breccia) structures
> are the type areas for this unusual rock type called=20
> pseudotachylite but it has
> also been observed in other impact structures (Rochechouart,=20
> Manicouagan,
> Slate Islands).
>=20
>=20
> Best wishes,
>=20
> Bernd
>=20
>=20
> References:
>=20
> ST=D6FFLER D. et al. (1991) Proposal for a revised petrographic shock
> classification of chondrites (Meteoritics 26-4, 1981, A398-A399).
>=20
> BEGEMANN F. et al. (1992) On the thermal history of heavily shocked
> Yanzhuang H-chondrite (Meteoritics 27-2, 1992, 174-178).
>=20
> XIANDE X. et al. (1991) The new meteorite fall of Yanzhuang,=20
> a severely
> shocked H6 chondrite with black molten materials (Meteoritics=20
> 26-4, 1991, A411).
>=20
> FRENCH B.M. (1998) Traces of Catastrophe (LPI Contribution=20
> 954, pp. 65-69).
>=20
> RUBIN A.E. (2003) Post-shock annealing and post-annealing=20
> shock: implications
> for the thermal and shock histories of ordinary-chondrite=20
> parent bodies (Lunar and
> Planetary Science XXXIV (2003), 1263.pdf).
>=20
> To: adamhupe_at_comcast.net
> Cc: meteorite-list_at_meteoritecentral.com
>=20
>=20
> ______________________________________________
> Meteorite-list mailing list
> Meteorite-list_at_meteoritecentral.com
> http://www.pairlist.net/mailman/listinfo/meteorite-list
>=20
Received on Thu 25 Mar 2004 03:06:10 AM PST