Ion microprobe evidence for the mechanisms of stable-isotope retrogression in high-grade metamorphic rocks


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Ion microprobe evidence for the mechanisms of stable-isotope retrogression in high-grade metamorphic rocks
Contributions to Minerlaogy and Petrology
Eiler J.M., Valley J.W., Graham C.M., Baumgartner L.P.
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Retrograde interdiffusion is widely proposed as the dominant factor in
producing the stable isotopic fractionation among minerals in slowly
cooled igneous and metamorphic rocks. Mineral zonation consistent with
interdiffusion of stable isotopes has never been directly observed,
however, leaving doubt as to the mechanism responsible for the
bulk-mineral isotopic compositions commonly measured. Ion microprobe
analyses of oxygen isotope ratios in magnetite were combined with
conventional bulk mineral analyses and diffusion modeling to document
the relationship between mineral zonation and the mechanism of
retrogression inferred from bulk mineral data. Two samples of
magnetite-bearing, quartzo-feldspathic Lyon Mountain gneiss from the
Adirondack mountains, N.Y. were studied in detail. Conventional stable
isotope analysis of both samples indicates that isotopic thermometers
are discordant and were reset by as much as 200 degrees C from the
estimated peak temperature of 750 degrees C. The relative order of
apparent temperatures recorded by various thermometers differs between
the two samples, however, with T-gtz-fsp much greater than T-mt-qtz and
T-mt-fsp in one sample and T-qtz-fsp < T-mt-qtz and T-mt-fsp in the
other. Diffusion modeling using the Fast Grain Boundary model shows that
the former pattern of apparent temperatures is consistent with closed
system interdiffusion during cooling, whereas the latter is not. The
modeling predicts that 0.5 mm diameter magnetite grains common to this
rock type will contain isotopic zonation of 1 parts per thousand (rims
lower in delta(18)O than cores), and that the cores of smaller (0.1 mm)
grains will be similarly lower than to the cores of large (0.5 mm)
grains. Ion microprobe analysis reveals that the zoning patterns of
magnetite grains from the first sample contain clear core to rim
zonation in multiple grains (Delta core-rim = 1.1 +/- 0.4 parts per
thousand) and predicted grain-size vs core composition variations,
consistent with diffusion-controlled resetting of bulk mineral
fractionations. In contrast, the second sample shows irregular inter-
and intra-granular variations over an 8 parts per thousand range,
consistent with open system alteration. These results provide direct
documentation of the importance of interdiffusion in affecting stable
isotope distributions in slowly cooled rocks. The correlations of
bulk-mineral resetting with zonation show that bulk mineral data, when
interpreted with detailed modeling, can be used to determinate what
processes controlling retrogression.
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02/10/2012 19:34
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