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

From: K. Ohtsuka <ohtsuka_at_meteoritecentral.com>
Date: Wed Feb 23 11:49:06 2005
Message-ID: <000801c519c7$2df5d8c0$587e76da_at_LocalHost>

Hello List members,

See also "Mineralogy of dark inclusions in CV" by Brearly & Jones in
"Planetary Materials, Reviews in Mineralogy vol. 36", p3-225, which is a
well-written review for CV DI, I guess.

K. Ohtsuka, Tokyo

----- Original Message -----
From: <bernd.pauli_at_paulinet.de>
To: <Meteorite-list_at_meteoritecentral.com>
Sent: Thursday, February 24, 2005 1:26 AM
Subject: [meteorite-list] NWA 3118 CV3 Chondrule-Field Update


> > 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
>
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Received on Wed 23 Feb 2005 11:46:10 AM PST