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

From: Ruben Garcia <rubengarcia85382_at_meteoritecentral.com>
Date: Tue, 18 Mar 2014 21:10:14 -0700
Message-ID: <CAJet4mP699G4fD_J2uBJkhjuCmP6hV3rk5+hVxu2DmfXEHRN6w_at_mail.gmail.com>

After being on Facebook for a week I gotta say "LIKE" to Karen's post.



On Tue, Mar 18, 2014 at 8:08 PM, Karen Ziegler <kziegler at unm.edu> wrote:
> Hi Mendy and list,
>
> Here is my input on the oxygen isotopes:
>
> Oxygen isotopes in unequilibrated samples will show a large range of
> values, because they do retain their initial oxygen isotope values of
> their individual components. Magmatic crystallization temperatures, for
> example, will give different minerals-pairs certain fractionations (that
> are dependent on the crystallization temperature) (e.g. Friedman & O'Neil,
> 1977). So, there is a certain expected range of oxygen isotope
> compositions in "bulk" samples, depending on how much of each mineral is
> in your "bulk" sample.
> Once metamorphism sets in, this inter-mineral fractionation decreases more
> and more - as temperature goes up. So, you'd expect the range of oxygen
> isotope values to shrink/collapse in their range as metamorphism
> increases.
> The same way you would expect the chemical characteristics, e.g.,
> Fe-content, to become more homogeneous, to have a smaller range, with
> increasing degree of metamorphism.
> O-isotope values per se will not tell you the metamorphic grade, but the
> "range" of individual analyses of a given sample will be an indicator of
> the degree of metamorphism.
>
> The oxygen isotope values of UOCs depends on how you have selected you
> sample. As Jeff said, "oxygen heterogeneity in these objects bulk
> samplests will be a function of sample size, as fine matrix grains
> equilibrate much more quickly than coarse ones." The proportion of
> chondrule to coarse to fine matrix is important ?..
> The best way to approach this is to do a detailed
> component/mineral-separation of the UOCs, analyze the chondrules vs. the
> matrix, analyze the olivines and the pyroxenes, etc. Comparing olivine
> O-isotopes, e.g., is much more useful that comparing "bulk" O-isotope
> values.
>
> Karen
>
>
> On 3/16/14 6:39 PM, "Jeff Grossman" <jngrossman at gmail.com> wrote:
>
>>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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>>
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-- 
Rock On!
Ruben Garcia
http://www.MrMeteorite.com

Received on Wed 19 Mar 2014 12:10:14 AM PDT