CAN THE TI-IN-BIOTITE THEMOMETER BE ACCURATELY APPLIED OUTSIDE ITS 4-6 KBAR CALIBRATION RANGE?

The Ti-in-biotite (TiB) geothermometer has been extended to pressures (P) as low as 2.5 kbar. Chemical data from natural biotite, gathered from well-characterized contact metamorphic aureoles, were used to evaluate the applicability of the TiB geothermometer of Henry et al. (2005) at P lower and higher than the 4-6 kbar range of the original calibration. Biotite mineral analyses normalized on a 22 oxygen basis (58) from low-P (2.5-4.0 kbar) ilmenite-bearing, aluminous, silica-saturated, and generally graphite-bearing metapelites of the Ballachulish aureole, Scotland, and the Bugaboo and Nelson aureoles, Southeast British Columbia, along with analyses from higher-P terrains (20) Mica Creek, British Columbia, and Lepontine Alps, Switzerland, were compiled and used to test P dependence. Independent temperature (T) estimates for each biotite were assigned by comparing sample locations to mapped isograds.

The new, low P data, plotted as T (°C) (x), Mg/(Mg+Fe) (y), and Ti (apfu) (z) for the low P contact aureoles, were fit with a surface expression. Comparing the geometry and fit quality of this surface to the Ti-saturation surface used to calculate the TiB provided evidence that at these different P settings, the Ti solution is essentially the same. Rearranging the low P surface equation for T and applying it to 529 biotites from the original TiB calibration dataset predicted T within an average of 1 ºC with a standard deviation of ±26 ºC. The original TiB geothermometer has an uncertainty of ±25 ºC. This implies that the 2.5-4.0 kbar surface is similar to the 4.0-6.0 kbar surface. Application of the TiB to lower P settings is appropriate. TiB produces less accurate (mean error = -29 ºC) and more imprecise (standard deviation = 48 ºC) T when applied to the high P dataset. Mica Creek T data alone, however are underestimated but acceptably precise (mean error = -36 ºC and standard deviation = ±26 ºC). This is consistent with experiments showing that at higher P, Ti content for a given T is significantly reduced. An additional factor may be that, in natural settings increased αH2O with increased pressure in graphitic systems could inhibit the deprotonation reaction, the dominant mechanism for Ti substitution in biotite at high T. Because of reduced Ti content, TiB is likely to produce low apparent T determinations in higher P settings.