Oxygen isotope fractionations involving pyroxenes: The calibration of mineral-pair geothermometers

Alan Matthews*, Julian R. Goldsmith, Robert N. Clayton

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

190 Scopus citations

Abstract

Oxygen isotope fractionations between wollastonite, diopside, jadeite, hedenbergite and water have been experimentally studied at high pressures (1<- PH2O ≥ 24 kbar) and temperatures (400/dg ≤ T <- 800/dgC) using the three-isotope method (Matsuhisa et al., 1978). Initial 18O 16O fractionations were made close to equilibrium and initial 17O 16O ratios were well removed from equilibrium, allowing accurate determinations of the equilibrium 18O 16O 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 18O 16O 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 SiOM bonds (M is a divalent metal cation in octahedral coordination) by higher frequency SiOAl 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 O'NEIL (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 Bottinga and Javoy (1973). Coefficients "A" for the mineral-pair fractionations 1000 ln α = A × 106T-2 are: {A table is presented} Geologically reasonable temperatures are found for appropriate natural assemblages.

Original languageEnglish
Pages (from-to)631-644
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume47
Issue number3
DOIs
StatePublished - Mar 1983
Externally publishedYes

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