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

[Doug Ross]

I haven’t had much time to post lately, but am really appreciating these informative discussions. Thanks!

Doug Ross
doug at dougross.net


On 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