I haven’t had much time to post lately, but am really appreciating these informative discussions. Thanks!
Doug Ross d...@dougross.net On Mar 18, 2014, at 8:08 PM, Karen Ziegler <kzieg...@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 ______________________________________________ Visit the Archives at http://www.meteorite-list-archives.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list