[meteorite-list] Tambo Quemado

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Fri Aug 20 09:41:52 2004
Message-ID: <DIIE.0000002B000026DB_at_paulinet.de>

Hello List,

One if the irons that have always been high on my wish list, apart
from Cape York, is Tambo Quemado, a member of the IIIAB irons
that must have been artificially reheated to about 1000?C at some
point of its history and the result: fused schreibersite crystals
and transformed kamacite grains are clearly visible under a microscope.

Eric Twelker has a batch of beautiful Tambos and so I was finally able to
acquire a museum-quality 80.6-gram slice from him. My slice (that I am so
very proud of and so happy to own) can be seen at:

http://www.meteoritemarket.com/TAM.htm

The troilite nodule has a whopping diameter of 17 millimeters, has
a mottled appearance and is surrounded by a seam of schreibersite.

It's an incredibly beautiful iron - see Martin Horejsi's and Marlin Cilz's
article in the Meteorite Magazine, May 1998, Volume 4, No. 2, p. 13 or go
to BUCHWALD V.F. (1975) Handbook of Iron Meteorites, Vol. 3, pp. 1174-1177.

When I closely inspected the troilite nodule under my microscope at 16x and at
32x magnification, I was "dumbfounded" when I found something that should not be
there. I instantly took some pics with my digital camera but will have to ask
Jeff Kuyken if he can upload them for me on his website for everyone to muse about.

It is a "stony", maybe a silicate-related inclusion at the upper edge of the troilite
nodule. Wondering what I was looking at, I searched for information both in O. Richard
Norton's Encyclopedia, in Buchwald, and in McSween. On page 205 of "Meteorites and Their
Parent Planets", Mc Sween says that "silicates in IIIAB irons are similar in composition
to HED chondrites".

Now, my inclusion doesn't look like it could be of HED origin. Visually, it looks more
like the Cumberland aubrite texture - a very improbable, unlikely provenance though.

Vagn Buchwald says that silicate inclusions in IIIAB irons are extremely rare and
in Appendix I, pp. 152-153, he mentions only 2 irons that may be silicate-bearing:
Avoca (olivine?) and Grant (tridymite, glass or silicates?)

In "Astronomy Now" for October 1996, p. 4, I found this:

Silicate inclusion is meteorite mystery

Type IIIA and IIIB iron meteorites are thought to have formed in the cores of asteroids.
They often contain small inclusions of chromite and one or more iron-bearing phosphate
minerals which probably formed in the late stages of core crystallisation. The recent
discovery by American scientists of a silicate inclusion in a type IIIA meteorite Puente
del Zacate is much harder to explain. According to the authors of the report in Science,
"How a graphite-bearing silicate inclusion was introduced into a low-carbon IIIA iron core
is difficult to envision." One possible answer is that the inclusion originated in the lower
mantle of the asteroid close to its iron core. Another possibility is that some small masses
of iron formed and cooled inside the silicate-rich mantle (by Peter Bond).

See also:

OLSEN E.J. et al. (1996) A silicate inclusion in Puente del
Zacate, a IIIA iron meteorite (Science 273, 1365-1367).

*******************************************************************************************

Puzzling questions:

Does my Tambo Quemado slice harbor such a graphite-bearing
silicate inclusion which would make it no. 3 among the IIIAB irons?

Is it something glassy, quartz/tridymite-related?

Is it a stony component from the lower mantle-core interface?

Is it preterrestrial or (unlikely) terrestrial?

Is it something chondritic or achondritic?

How, after all, did it get there ???

*******************************************************************************************

Parameters of my digital pics:

Tambo Quemado, IIIABx16-01a
Magnification: 16x
Exposure: 1/20
Eyepiece projection
Aperture: 6.7

Tambo Quemado, IIIABx32-01b
Magnification: 32x
Exposure: 1/25
Eyepiece projection
Aperture: 3.5
Received on Fri 20 Aug 2004 09:41:49 AM PDT