NEW CRYSTALLIZATION THERMOMETERS FOR ZIRCON, RUTILE AND SPHENE: CALIBRATIONS, DIFFUSION CONSIDERATIONS, AND APPLICATIONS

The broad P-T stability range of some accessory minerals makes them excellent candidates for use in geothermometry. Zircon, rutile and sphene are particularly promising because: 1) they are found in a wide variety of rocks; 2) they can be used for radiometric age determinations; and 3) they have generally sluggish diffusion characteristics. Crystallization thermometers for these minerals thus have the potential to provide absolute time frames for thermal histories of crustal systems. Thermometer calibrations for zircon, rutile and sphene were achieved using laboratory experiments in combination with analyses of natural accessory mineral grains whose crystallization temperatures are independently constrained. The new thermometers are based upon the limited interchangeability of Zr and Ti—that is, Ti content of zircon and Zr content of rutile and sphene. In all cases the concentrations of these elements in their respective hosts vary systematically with temperature:

log(Ti_zcn) = 6.03 - 5080/T

log (Zr_rut) = 7.36 - 4470/T

log(Zr_sph) = 11.30 - 9615/T

(T is in Kelvins; Ti and Zr refer to the concentrations of these elements in ppmw; all calibrations are for a nominal pressure of 1 GPa). The Ti-in-zircon thermometer is insensitive to pressure; Zr in rutile shows minor sensitivity to pressure, and Zr in sphene decreases by a factor of ~6 in response to a pressure increase of 1 GPa. These thermometers can be implemented using available in situ analytical techniques (EMPA; LA-ICP/MS; SIMS); in general, however, LA-ICP/MS or SIMS is needed for zircons crystallized below ~ 850°C and sphenes below ~725°C. The EMPA can be used for Zr-in-rutile thermometry on crystals formed at temperatures as low as ~500°C. Preliminary diffusion data for Ti in zircon indicate a high closure temperature broadly consistent with existing diffusion data for other tetravalent ions. However, Zr in rutile is relatively fast-diffusing, raising the possibility of partial diffusive re-equilibration to temperatures below 600°C in some retrograde environments. Diffusion of Zr in sphene is intermediate between the other two, implying generally closed-system behavior. Applications and intercomparisons of the new thermometers will be made with reference to natural rocks ranging from glassy rhyolites to subduction-zone migmatites.