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Coupled cation and oxygen isotope exchange
between alkali feldspar and aqueous chloride
solution
Labotka T. C., Cole D. R., Fayek M., Riciputi L. R., and
Stadermann F. J. (2004)
Amer. Mineralogist 89, 1822-1825..
ABSTRACT
Nanoscale isotope and chemical images of grains of Amelia
albite that were reacted with 2 m 18O-enriched
solution of KCl show a correspondence between O-isotope
exchange and K-Na exchange. Experiments were conducted for
4-6 d at 600 ºC and 200 MPa. After 6 d, the 150
micrometer diameter albite grains had 5-20 micrometer rims
in which Na was nearly completely replaced by K and in which
the O was strongly enriched in 18O. The boundary
between the core albite and the K-feldspar replacement is
sharp and decorated with numerous pores. The distribution of
Na and K, determined by electron probe microanalysis, is
uniform within the core and rim and has an abrupt
discontinuity at the interface. There is no evidence for
K-Na interdiffusion at the resolution of electron probe. The
NanoSIMS shows that the interface is also sharp in the
distribution of 18O and 16O. The
NanoSIMS image data and the electron probe data were
coregistered; principal components analysis of the merged
data set shows that 86% of the total variance in the data
result from a single principal component loaded by the
replacement of Na by K and 18O. The combined
electron probe and NanoSIMS analyses indicate that both
cation and isotope exchange occurred during solution and
reprecipitation of the feldspar.
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