[meteorite-list] CAIs in Allende

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:14 2004
Message-ID: <3B3644B4.4A94DF81_at_lehrer1.rz.uni-karlsruhe.de>

Dave wrote:

> I notice that in my Allende slice, some of the CAIs appear to be zoned, that
> is they have a roughly central area of a greyer material than the rest of
> the CAI that surrounds it. Can any of you erudite people afford me an
> explanation as to how the gray inner region varies chemically/lithologically
> to the outer portion.

Hi Dave and List,

RUZICKA A.M. et al. (1994) Origin of CAI rims by vaporization and
metasomatism (abs. Meteoritics 29-4, 1994, 526):

On their margins, most Ca, Al-rich inclusions (CAIs) contain thin "rims"
comprised of multiple mineral layers. Previous work has shown that the
bulk composition of the rims can be explained by a combination of
vaporization during flash heating of the CAIs and a metasomatic event*
that involved an influx into CAIs of Mg and Si. The layering structure
of the rims may have formed by a coupled reaction/diffusion process
during metasomatism, similar to that which produced mineralogically
zoned olivine coronas in mesosiderites ... Rim layers consist mainly
of spinel, forsteristic olivine, Ti, Al-rich pyroxene, Al-rich diopside,
melilite, anorthosite, and lesser amounts of perovskite, Fe-Ni metal,
and feldspathoids.

* Metasomatism:

A series of metamorphic processes whereby chemical changes occur
in minerals or rocks as a result of the introduction of material,
often in hot aqueous solutions, from external sources.


Best wishes,

Bernd
Received on Sun 24 Jun 2001 03:51:16 PM PDT