[meteorite-list] The scientific importance of subtype 3.00 meteorites and oxygen isotope analysis

From: Jeff Grossman <jngrossman_at_meteoritecentral.com>
Date: Sun, 16 Mar 2014 18:39:44 -0500
Message-ID: <006c01cf4171$02d33620$0879a260$_at_gmail.com>

Mendy and list,

My comments:

Oxygen: I would say that O isotope heterogeneity as described here is not a
good measure of metamorphism. Oxygen heterogeneity in these objecbulk
samplests will be a function of sample size, as fine matrix grains
equilibrate much more quickly than coarse ones. If you analyze small
aliquants of sample, most UOCs will be heterogeneous. If, on the other
hand, we were talking about the O isotope heterogeneity of individual
olivine grains, akin to how we measure FeO in olivine, you might be able to
devise a metamorphic parameter. But so far, I'm not aware of anybody
devising a way to use O isotopes to measure metamorphic grade.

The meaning of type 3.00: you said, "A subtype of 3.00 means that the
material has survived unchanged by heat (radioactive decay, pressure,
impact/shock, etc.) or aqueous alteration since its formation." This is
incorrect. It means the material is unaffected by thermal metamorphism.
Semarkona is shock stage S2, so it has been seen elevated pressures due to
impacts on the parent body. It also shows abundant evidence for light
aqueous alteration. You can think of all these things as independent
processes. Semarkona saw little heat, but got a little shocked and a little
bit wet. Many CM chondrites saw little heat, but a lot of water. I would
call these CMs type 3.00 as well, but traditional usage has coined another
term for really wet chondrites, namely type 2. Oh well. Metamorphically,
they are type 3.00. Some chondrites saw little shock and a lot of thermal
metamorphism. Anyway, all type 3.00 means is that the object saw little
prolonged secondary heating. The parent body may have been too small to
differentiate, or it may have formed too late to take advantage of heat
sources like Al-26 (and there may be other possibilities).

We are always looking for material that escaped processing on asteroids to
learn about the origin of the solar system. Type 3.00 chondrites are good
for doing such studies. CAIs are also important for early solar system
studies, and we're fortunate that the meteorites richest in CAIs tend to be
low petrologic types that escaped heating on asteroids as well; many
carbonaceous chondrites are like this.

I hope this is a start at answering your questions.

Jeff


> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com [mailto:meteorite-list-
> bounces at meteoritecentral.com] On Behalf Of Mendy Ouzillou
> Sent: Sunday, March 16, 2014 12:46 PM
> To: Met-List
> Subject: [meteorite-list] The scientific importance of subtype 3.00
meteorites
> and oxygen isotope analysis
>
> Well, with the LPSC going on starting this week, I sure hope we get some
> participation from our scientific contributors to these questions.
>
> Someone asked me to explain the scientific importance of meteoritic
material
> with a 3.00 subtype. Reading through "The onset of metamorphism in
ordinary
> and carbonaceous chondrites" by Grossman and Brearley 2005, I realized
that a
> key tool used in the analysis of NWA 7731 and NWA 8276 was not present in
> the literature.
>
> So, I'll start with this first part of questions: In my discussions with
Dr. Agee, he
> mentioned that the heterogeneity of the oxygen isotope results is
important
> because it indicates that the material has not been metamorphosed by heat
or
> shock. Any heating would have caused the oxygen to begin to equilibriate.
So, is
> the oxygen isotope analysis something that should be added to the list of
factors
> used in evaluating low sub-types? Or is it a proxy for more complex tests?
I am
> hoping that Karen Ziegler can also add some insights.
>
> The second set of questions is perhaps more complex. What is the
scientific
> importance of the 3.00 subtype??I can get this one kicked off, but would
> appreciate a more nuanced answer than what I can provide.
> The subtype 3.00 represents the earliest glimpse of the properties of
proto-
> planetary material in our solar system. A subtype of 3.00 means that the
> material has survived unchanged by heat (radioactive decay, pressure,
> impact/shock, etc.) or aqueous alteration since its formation. An
implication of
> the unequilibrated nature of this material is that the parent body had to
be quite
> small for it not to differentiate in any way.
>
> Though both scientifically important, what different types of insights do
we gain
> from CAIs versus subtype 3.00 material? The answer is I am sure that they
> complement each other, but in what way. Which is oldest?
>
> The rarity of this type of material cannot be underestimated since between
the
> only 3 known (Semarkona, NWA 7731 and NWA 8276), there is only 1,561g
> available for research and/or collectors. Of that total weight,
Semarkona's 691g
> is almost unattainable. So, once again NWA delivers the goods!
>
> Regards,
>
> Mendy Ouzillou
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Received on Sun 16 Mar 2014 07:39:44 PM PDT