Oxygen isotope exchange and closure temperatures in cooling rocks

Abstract

Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of 6"O within grains, and the bulk 6"O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 6lXO values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ('slow cooling'), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures (TJ for the two phases and the mode of the rock. Adding a third phase into a system which undergoes 'slow' cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not 'slow', then the mineral 6"O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral 6'*0 values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems.

Oxygen isotopic compositions of IVA iron meteorites: implications for the thermal evolution derived from in situ ultraviolet laser microprobe analyses

“…For a given mineral, Tc is higher for larger grain sizes and fast cooling rates. In a two-mineral assemblage, oxygen isotopes will stop exchanging when the system cools down to a temperature equal to the higher Tc of the two minerals (Giletti, 1986;Jenkin et al, 1994). If this specific isotopic composition is maintained without further disturbance, the oxygen isotope equilibrium temperature estimated from the 18 O/ 16 O fraction- In our case, pyroxene with a lower diffusion coefficient has a higher Tc than tridymite and stops diffusive exchange first during cooling (Giletti and Yund, 1984;Farver, 1989).…”

The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland

“…Useful interpretation of the measured K-feldspar ä 18 O in the context of this paper is predicated on the assumption that there has not been post-formational oxygen isotope resetting during cooling (Jenkin et al 1994) or by influx of a later external fluid of different ä 18 O. For the former situation, we note that even though feldspar exhibits a very rapid oxygen volume diffusion rate compared with other silicate mineral phases (Jenkin et al 1994), the closure temperature for oxygen isotope exchange is estimated to be about 200 8C (see Jenkin et al 1991), which coincides with the maximum temperature estimated from the fluid-inclusion observations.…”

Estimation of oxygen diffusivity from anion porosity in minerals.

“…Giletti (1986) illustrated the cooling rate model in which oxygen diffusion data can be used in con junction with the oxygen isotope composition of minerals to interpret apparently disequilibrium oxygen isotope fractionations in a slowly cooled mineral assemblage. The technique developed by Giletti (1986) has since been applied to place constraints on cooling rates, on the degree of de formation during retrogression, and on the pres ence or absence of a fluid phase during metamor phism (e.g., Jiang et al, 1988;Sharp et al, 1988;Jenkin et al, 1991Jenkin et al, , 1994Valley and Graham, 1991;Eiler et al, 1992). With the availability of much more oxygen diffusion data derived from mineral anion porosity in this study, the application of the cooling rate model can be extended to various rocks.…”