Oxygen isotope fractionation between rutile and water and geothermometry of metamorphic eclogites

Abstract

Summary: Oxygen isotope fractionation between rutile and water has been studied from 300 °C to 700 °C, PH₂O = 1 kb, using aqueous oxidation of titanium metal as the equilibration reaction. The mechanism of rutile formation (which is critical to the assessment of isotopic equilibrium) is an ‘armouring’ reaction in which rutile grows around grains of titanium metal by solution-precipitation processes. Mean fractionation factors expressed as 10³ In αTiO₂-H₂O obtained in the present study are:−6.20±0.23‰ at 304±5 °C−6.64±0.27‰ at 405±6 °C−6.11±0.16%. at 508±6 °C−4.45±0.28%. at 608±6 °C−3.38±0.15%. at 698±6 °C.These data agree with those obtained at temperatures above 500 °C by Addy and Garlick (1974) but do not accord with theoretical predictions by Bottinga and Javoy (1973). A minimum in the calibration curve 10³ ln α versus 10⁶T⁻² occurs between 300 °C and 500 °C but from 500 °C to 700 °C ¹⁸O fractionation between rutile and water may be expressed by the equation:10³ ln α = −(4.72±0.40)10⁶T⁻²+(1.62±0.53).Oxygen isotope analyses of rutile and quartz from metamorphic eclogites and schists from the Tauern Window, Austria, yield isotopic temperatures at about 550 °C in agreement with results obtained on similar rocks from the Sesia Zone (Western Alps, Italy) and elsewhere by other workers. Petrologic studies indicate that the latest metamorphism of the Tauern eclogites reached about 450 °C Thus the measured partitions of ¹⁸O between rutile and quartz indicating temperatures around 550 °C have been inherited from an earlier metamorphic event.