[meteorite-list] Ghubara Revisited

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:37:34 2004
Message-ID: <3A39510B.63C04831_at_lehrer1.rz.uni-karlsruhe.de>

Dan wrote:

> The lighter colored section also seemed
> to have more defined chondrules.

Bernd responded:

> a regolith breccia with a dark matrix and light
> clasts. The lighter colored sections have more
> defined chondrules because there are L3 frag-
> ments set in an L5 host.

James Baxter (private mail):

> I recently obtained a slice of Ghubara in which the DARK
> portion appears to be unequilibrated and has prominent
> chondrules and the light portion looks more equilibrated
> and has fewer and less distinct chondrules. This seems to
> be the reverse of what you described.

Hello List,

James' observations are confirmed by a passage from Wasson's 1974 book
on meteorites [WASSON J.T. (1974) Meteorites, Springer Verlag 1974, p.
196]:

"The difference in petrologic grade between xenoliths and host may be
related to their relative friabilities. The material of lower petrologic
type was more friable and was comminuted to form the host; the stronger
materials of higher petrologic type were fragmented, but generally
survived as larger clasts."

Lower petrologic type obviously implies more chondrules - thus the dark
Ghubara matrix should show abundant chondrules. James' scanned picture
clearly shows just that and so does a look at my specimen. Higher
petrologic type means a higher degree of recrystallization, and,
consequently fewer chondrules. In a nutshell: The host matrix is
chondrule-rich, the clasts should be chondrule-poor.

But Dan wrote to the List:

> The lighter colored section also seemed
> to have more defined chondrules.

Perhaps Wasson can again help (page 196):

"If this explanation is correct, some cases should be found where the
clast is of lower petrologic type than the host, since the correlation
between friability and petrologic type is not a perfect one."

Wasson does not mention if such "cases" may occur simultaneously in one
and the same meteorite (specimen). Does James have the "run-of-the-mill"
version of Ghubara and Dan the more "exotic" version? Or is Dan
mistaken?

With the help of a 12x loupe, I took another look at my Ghubara slice
and noticed about an equal number of chondrules in both "lithologies",
whereas James' xenolithic component almost seems to be devoid of any
chondrules; so now I am at my wit's end :-(

Folks, grab your Ghubaras, look at them closely, take your Ghubara thin
sections and examine them in plane and polarized light and report to the
list your findings. I think this is a great occasion for this list to do
invaluable scientific, observational work.

By the way, the entry in the Blue Book also supports James' observation:

"... the host is unequilibrated, with olivine Fa21.6
to Fa26.5, but olivine of xenoliths is Fa24 ..."


Best wishes and
thank you very
much James,

Bernd
Received on Thu 14 Dec 2000 06:00:27 PM PST