HR: 1330h
AN: V22D-0609    [PDF]
TI: Oxygen Isotope Thermometry and Speedometry
AU: * Ni, H
EM: hni@umich.edu
AF: The University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AU: Zhang, Y
EM: youxue@umich.edu
AF: The University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AB: Oxygen isotope fractionation depends on temperature and has been extensively applied to geothermometry. However, unless the cooling is rapid enough, formation or peak temperature may not be recorded, and retrograde reaction and diffusion among minerals may lead to inconsistent temperatures for different mineral pairs. Dodson (1973) examined the isotopic exchange of a single crystal with an infinite reservoir and developed the concept of closure temperature (Tc). Giletti (1986) treated closure temperature as an innate property of a mineral and examined the isotopic exchange in a closed system. Eiler et al. (1992a, b, 1994) pointed out that this method is not strictly correct, and developed an FGB model for isotopic evolution of a multi-mineral system. They concluded that the apparent equilibrium temperature (Tae) calculated from isotopic fractionation between two phases is strongly dependent on mineral proportions and could even be negative. Although the FGB model can in principle be applied to determine the cooling history by matching observed and calculated isotopic compositions of the minerals by varying the cooling history, in practice the application is very difficult. In this contribution, we apply the FGB model to look for a simple method for oxygen isotope thermometry and speedometry. Our results can be summarized as follows. For a multi-mineral system, it is best to use two minerals with the greatest isotopic fractionation (such as quartz and magnetite, or quartz and rutile) to calculate Tae. Tae obtained from such a mineral pair almost always lies between the closure temperatures of the two minerals. The significance of this thermometry (calculation of Tae) is actually in speedometry: Using Tae as a proxy for Tc of each of the two minerals, the range of cooling rate can be estimated using the method of Dodson. If the two minerals happen to have diffusion properties so that they have similar closure temperatures (e.g., quartz and magnetite), Tae would be an excellent approximation of Tc of both minerals, from which cooling rate of the rock can be estimated. In summary, a mineral pair with the largest fractionation and similar closure temperatures is the best for obtaining cooling rate. San Jose tonalite (Giletti, 1986) serves as a good example to apply this method. It contains 5 minerals: quartz, plagioclase, hornblende, biotite, and magnetite, in the order of oxygen isotopic ratio (hornblende, quartz, magnetite, biotite and plagioclase, in the order of decreasing closure temperature). Tae between quartz and magnetite is 817 K (Zheng, 1995). Using diffusion data from Giletti and Yund (1984) for quartz and from Giletti and Hess (1988) for magnetite, the cooling rate at Tae ranges from 16 to 65 K/Myr.
DE: 1040 Isotopic composition/chemistry
DE: 1099 General or miscellaneous
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting