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

From: Graham Ensor <graham.ensor_at_meteoritecentral.com>
Date: Wed, 19 Mar 2014 09:11:19 +0000
Message-ID: <CAJkn+kbcR=Y3uwN4rJM_irXT4VBPZHw18GEgy+TGFh6CAbwVzA_at_mail.gmail.com>

Ditto Ruben.

Graham

On Wed, Mar 19, 2014 at 4:10 AM, Ruben Garcia
<rubengarcia85382 at gmail.com> wrote:
> 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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>>
>> ______________________________________________
>>
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>
>
> --
> Rock On!
>
> Ruben Garcia
> http://www.MrMeteorite.com
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Received on Wed 19 Mar 2014 05:11:19 AM PDT