Mathematics – Logic
Scientific paper
Mar 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47..631m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 47, Issue 3, pp.631-644
Mathematics
Logic
39
Scientific paper
Oxygen isotope fractionations between wollastonite, diopside, jadeite, hedenbergite and water have been experimentally studied at high pressures (1<- P H 2 O 24 kbar) and temperatures (400/ dg T <- 800/ dgC ) using the three-isotope method (Matsuhisa et al. , 1978). Initial 18 O/ 16 O fractionations were made close to equilibrium and initial 17 O / 16 O ratios were well removed from equilibrium, allowing accurate determinations of the equilibrium 18 O/ 16 O fractionations and of the extent of isotopic exchange. Scanning electron microscope and rate studies show that the wollastonite-water and diopside-water exchange reactions occur largely by solution-precipitation (Ostwald Ripening) mechanisms. Equilibrium 18 O/ 16 O fractionations between water and the minerals wollastonite, diopside, and hedenbergite are in close agreement with one another, whereas significantly more positive fractionations are found for jadeite-water. These isotopic substitution effects can be ascribed to replacement of Si---O---M bonds (M is a divalent metal cation in octahedral coordination) by higher frequency Si---O---Al bonds. The fractionations determined in this study can be combined with quartz- and feldspar-water data of Matsuhisa et al. (1979) and revised magnetite-water data of (1963), to provide a coherent set of mineral-pair fractionations satisfactorily represented by straight lines through the origin on a conventional graph of In /ga versus T -2 . Mineral-water data, on the other hand, cannot readily be fitted to the simple relationship suggested by and Javoy (1973). Coefficients " A " for the mineral-pair fractionations 1000 ln = A × 10 6 T -2 are: Geologically reasonable temperatures are found for appropriate natural assemblages.
Clayton Robert N.
Goldsmith Julian R.
Matthews Alan
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