Ti-in-biotite geothermometer of Henry et al. (2005)

Click on this link to obtain a larger version of the Ti-X_Mg plot.

The Ti-in-biotite (TIB) geothermometer of Henry et al. (2005) is based on the Ti-saturation surface of near-isobaric natural biotite data for peraluminous metapelites equilibrated at 4-6 kbar. This surface is evaluated in terms of temperature (T) vs. X_Mg vs. Ti, and is solved specifically for T.

Temperatures can be determined either by plotting biotite Ti and Mg/(Mg+Fe) values on the simple binary diagram (see above) or by calculating T from the expression:

T = ([ln(Ti) - a - c(X_Mg)³]/b)^0.333

where T is temperature in ºC, Ti is the apfu normalized to 22 oxygens, X_Mg is Mg/(Mg+Fe), and the a, b and c parameters are given below.

Coefficient	Value
a	-2.3594
b	4.6482e-9
c	-1.7283

This expression is valid in the range X_Mg = 0.275 -1.0, Ti = 0.04 - 0.6 apfu and T = 480-800 ºC. Due to isotherm spacing, temperatures are more uncertain at low temperatures (< 600 ºC) than at high temperatures. Based on the standard deviations of the temperatures in our calibrated data set from the saturation surface, the uncertainty of the Ti-in-biotite geothermometer is estimated to be ±24ºC at lower temperatures (< 600 ºC), improving to ±12 ºC at high temperature (> 700 ºC).

A TiInBiotiteThermometer Excel file is given for the user's convenience.

To apply the TIB geothermometer expression in the strictest sense, the following criteria should be met:

Ilmenite or rutile is present in the rock - sets Ti at maximal levels in biotite for a given PT condition and biotite X_Mg. (Absence of these minerals will result in a minimum calculated temperature)
Graphite is present in the rock - restricts conditions to low and relatively constant f_O2 and biotite to Fe³⁺ in biotite to~12% of Fe_total.
Quartz is present in the rock - sets Si at maximal levels in coexisting biotite
Aluminous minerals such as staurolite, cordierite or Al₂SiO₅ polymorphs are present - sets Al at maximal levels in biotite. (Absence of aluminous minerals in the presence of Ti minerals generally enhances Ti concentration in biotite and will generally result in calculated T that overestimates actual conditions.)
Pressure range of the rocks should be 4-6 kbar - this is the P range of the calibration data set. (Based on evidence from a number of biotite experiments, samples equilibrated at lower P will have higher Ti and yield higher TIB temperatures whereas samples equilibrated at P>6 kbar will have lower Ti and will yield low TIB temperatures.)
Biotite compositions should be in the range X_Mg=0.275-1.0 and Ti=0.04-0.6 apfu - this is the compositional range of the biotite data in the calibration data set. (Application of the TIB thermometer beyond this composition range will produce greater than the ±24ºC uncertainties in calculated temperatures.)

An added feature of the systematics of the Ti-in-biotite is that it can also serve as a very sensitive monitor of local equilibrium and disequilibrium in a rock (see right hand panel of image at top).

Darrell Henry publications related to TIB geothermometry and Ti substitutions in biotite

Henry, D. J., Guidotti, C. V. and Thomson, J. A. (2005) The Ti-saturation surface for low-to-medium pressure metapelitic biotite: Implications for Geothermometry and Ti-substitution Mechanisms. American Mineralogist, 90, 316-328. (pdf of article)

Henry, D.J. and Guidotti, C.V. (2004) “Ground truthing” biotite in peraluminous and metaluminous metamorphic rocks. Geological Society of America Abstracts with Program, 36. (abstract)

Henry, D.J. and Guidotti, C.V. (2004) Ti substitution mechanisms in biotite: Perspectives from a biotite Ti-saturation surface. Abstracts of the Goldschmidt Conference.

Henry, D. J. and Guidotti, C. V. (2002) Isobaric T-X Ti-saturation surfaces for metapelitic biotite: implications for geothermobarometry. International Mineralogical Association Abstracts.

Henry, D. J. and Guidotti, C. V. (2002) Ti in biotite from metapelitic rocks: Temperature effects, crystallochemical controls and petrologic applications. American Mineralogist, 87, 375-382. (pdf of article)

Henry, D. J. and Guidotti, C. V. (2000) Ti in biotite from metapelitic from metapelitic rocks: some petrologic applications. Geological Society of America Abstracts with Program, 32, A53.

LaGrange, A., Henry, D. J. and Guidotti, C. V. (1992) Ti in biotite from graphite-bearing metapelites of NW Maine: interrelationship of crystal chemistry, mineral assemblage and temperature. Transactions of the American Geophysical Union, 73, 326.

Guidotti, C. V., Cheney, J. T. and Henry, D. J. (1988) Compositional variation of biotite as a function of metamorphic reactions and mineral assemblage in the pelitic schists of western Maine: American Journal of Science-Wones Memorial Volume, v. 288A, 270-292.

Darrell Henry is the Campanile Charities Professor of Geology and Geophysics at Louisiana State University whose research specialty is metamorphic petrology. Further details of his professional background are included in an accompanying vita or faculty profile.

To contact Darrell Henry call (225)-578-2693, fax (225)-578-2302 or e-mail dhenry@geol.lsu.edu . Address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803.

This page was last updated on 04/17/06.