[meteorite-list] NWA 3118 CV3 Chondrule-Field Update

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Wed Feb 23 11:26:57 2005
Message-ID: <DIIE.0000002D000033AC_at_paulinet.de>

> What does all this mean?
 
Hello Walter, Bill, and List,

I'll try to go through this in little steps:

> examination of the inclusion ...shows it to consist of small chondrules
> and chondrule fragments composed of very magnesian olivine (Fa1.1)

The chondrules in the lower right part of the 32-gram slice are much smaller
than chondrules in the remaining CV3 matrix like chondrules in CO3 chondrites
that are much smaller.

See also O.R. Norton's Encyclopedia, pp. 135-136 and p. 136, Fig. 7.19:

"While the average chondrule-diameter of a CV is about 1 mm, the average for
a CO chondrite is about 0.15 mm".

> small chondrules ... composed of very magnesian olivine (Fa1.1)

O.R. Norton, 135: "most [chondrules in CO chondrites] are porphyritic
olivine chondrules with olivine made of nearly pure forsterite".

O.R. Norton, p. 312, Appendix C, Minerals in Meteorites:

"Forsterite = the magnesium end member of the olivine ... (Mg2SiO4)"

The chemical formula shows it is *magnesian* olivine and Fa1.1 shows there
is hardly any trace of iron. The opposite end member is fayalite (Fe2SiO4),
the iron end member (here it is iron that is missing).

> with marginal Fe-rich reaction zones (Fa36.9)

Imagine a small rim around the chondrules. These outer zones reacted with
FeO-rich (iron-rich) olivine in the surronding matrix and thereby became
enriched in Fe (not just Fa1.1 [iron-poor] but Fa36.9 [iron-rich]).

> against a porous matrix composed of felted blades of relatively
> ferroan olivine (Fa40.9-45.9) with accessory troilite and pentlandite.

Here is what I already said the matrix is rich in olivine that contains
a lot of iron and not as much magnesium as those "magnesian chondrules".

> This clast has the attributes of Type A/B dark inclusions ...

Dark inclusions are lithic fragments up to about 5 cm in size. They have
been found in several CV3 chondrites, for example in Allende, Vigarano,
Leoville, etc.). Their main mineral component is fayalitic olivine. Some
are chondrule-rich, some are chondrule-free (consist almost entirely of
matrix)*.

*Reference:

WEISBERG M.K. et al. (1998) Fayalitic olivine in CV3 chondrite matrix
and dark inclusions: A nebular origin (MAPS 33-5, 1998, 1087-1099).

> This clast has the attributes of *Type A/B* dark inclusions ...

HUTCHISON R. (2004) Meteorites: A Petrologic, Chemical, and
Isotopic Synthesis (Cambridge Planetary Science Series, p. 62):

Porphyritic chondrules may be type I and FeO-poor
                          or type II and FeO-rich

They may additionally be subdivided into two categories:

- silica-poor A
- silica-rich B

AB is intermediate.

> Oxygen isotope analysis ... gave replicate (= duplicate) values of:

d17O = +0.08, -0.18;
d18O = 5.14, 4.99;
D17O = -2.63, -2.81 per mil, respectively, which plot on
       the best fit line for whole rock CV chondrites.

See O.R. Norton, p. 135 or McSWEEN H.Y. (1999) Meteorites and
Their Parent Planets, p.51: "The oxygen isotopic compositions
provide one means of classifying chondrites into clans and groups".

When you look at these plots in Norton or McSween, you'll see that
these values are in the range for CV chondrites but very close to
the area for CM chondrites (which are much more primitive than
CVs and point toward aqueous activity and aqueous alteration on
their parent bodies.

So this clast may represent a more hydrated relict part of a CV parent
body the other parts of which are more dehydrated due to parent body
metamorphism.

A piece from a boundary area between more and less dehydrated
material?

A pocket (or clast) of more primitive material embedded into the
higher metamorphosed CV3 material?

> a specimen that clearly demonstrates what the above report describes:

 http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=6513101418


Best wishes,

Bernd
Received on Wed 23 Feb 2005 11:26:53 AM PST