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

From: Karen Ziegler <kziegler_at_meteoritecentral.com>
Date: Wed, 19 Mar 2014 03:08:09 +0000
Message-ID: <CF4E6E96.22C27%kziegler_at_unm.edu>

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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Received on Tue 18 Mar 2014 11:08:09 PM PDT