[meteorite-list] aubrite vesicles

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:48:56 2004
Message-ID: <3B98AD5D.87E086E8_at_lehrer1.rz.uni-karlsruhe.de>

James Baxter wrote:

> Check out the photo of the 2.354 gram specimen of Norton County
> at Rob Elliot's site.I purchased this from him and the black melt
> phase has what definitely appear to be vesicles in it.They have
> nice perfectly spherical interiors under the microscope and do
> not appear to be a weathering phenomenon.


Hello, Jim, Dave, and List

I checked it /them out and yes, there they are. Very interesting!
It's even more evident in the "part slice with l a r g e central
gas vesicle - 4.664 g" that Rob offers for sale.

Hmm, ... on the one hand, aubrites are brecciated (except for
Shallowater) and several are regolith breccias containing solar wind
implanted noble gases, on the other, Norton County does not contain
solar wind implanted noble gases and thus it is not a regolith breccia,
at least according to Okada A. et al., 1988.

Furthermore, in an abstract, Miura Y.N. et al., 2000) write that the
six aubrites* they examined didn't contain a significant amount of
solar He and Ne.

* Bishopville, Cumberland Falls, Mayo Belwa,
  Mt. Egerton, Norton County, Pena Blanca Springs

Now back to Okada (Okada A. et al. 1988). At the end of this
comprehensive paper on the Norton County aubrite, the authors
conclude: "We suggest that parent body collisional disruption
and gravitational reassembly of most of the debris is required
to explain the coexistence of slowly cooled and r a p i d l y
cooled lithologies.

This might imply the presence of "gas holes" - trapped near-surface
gases (igneous history/explosive volcanism) which burst open while
the meteorite plunged through the atmosphere. These holes then
solidified when the crust cooled.

There is a b & w picture of Norton County in Rolf Bühler's (SML)
out-of-print "Meteorite, Urmaterie Aus Dem Interplanetaren Raum"
(p. 118): This picture also shows a glassy crust with those hair-like
gas holes and cracks.

The paper by Dickinson et al. (1997) is the only one where I found
something like "vesicles" or "vesicular". One of the experimental
charges they produced had a highly vesicular nature but this was
not genuine meteoritic (aubritic) material but material produced
artificially to carry through their partitioning experiments.


References:

OKADA A. et al. (1988) Igneous history of the aubrite parent
asteroid: Evidence from the Norton County enstatite achondrite
(Meteoritics 23-1, 1988, 59-74).

MIURA Y.N. et al. (2000) Noble gas studies of aubrites
(MAPS 35-5, 2000, Suppl., A112).

T.L. DICKINSON et al. (1997) Experimental rare-earth-element
partitioning in oldhamite: Implications for the igneous origin
of aubritic oldhamite (Meteoritics 32-3, 1997, 395-412).
Received on Fri 07 Sep 2001 07:19:57 AM PDT