LSU Petrologist establishes a Means to Determine Temperature from Titanium Contents in Biotite (Mica)

Dr. Darrell Henry has been developed a method to easily determine the temperature of formation of certain metamorphic and igneous rocks based on the titanium (Ti) contents in biotite (dark mica). It has been known for a number of years that as the temperature of a rock increases the Ti contents generally increase in minerals such as amphibole, tourmaline, biotite and, even, quartz. In these minerals Ti is typically present minor to trace amounts and exists in a 4+ oxidation state. One of the consequences of Ti⁴⁺ being associated with Fe²⁺ in minerals (e.g. amphibole, tourmaline, biotite) is that Ti shares its electrons with Fe in adjacent sites resulting in an absorption of portions of visible light such that a brown coloration develops. The higher the Ti content (for a given Fe content), the more intense the brown coloration in thin section (30 μm slice of rock on a glass slide). For example (to the right), biotite formed in garnet zone biotites (T~500ºC) is a light orange-brown, but biotite formed in the lower sillimanite zone (T~650ºC) is dark brown.

For biotite, Darrell Henry (LSU), Charles Guidotti (Univ. of Maine) and Jennifer Thomson (E. Washington Univ) quantified this relationship using a large natural biotite data sets from metamorphosed mudstones (metapeiltes) of Maine and Massachusetts. They were able to generated a Ti-saturation surface (TºC - Mg/(Mg+Fe) – Ti surface) for biotites from graphitic, peraluminous metapelites that contain ilmenite (FeTiO₃) or rutile (TiO₂) and have equilibrated at roughly 4-6 kbar. The surface fit equation was reformulated to produce an empirical Ti-in-biotite geothermometer for peraluminous metamorphic and igneous rocks. This geothermometer is estimated to have precision of better than ±25ºC. For more information on the geothermometer and a spreadsheet for solving temperature see the following link: http://www.geol.lsu.edu/dhenry/Research/biotite/TiInBiotiteGeothermometer.htm

Further reading:

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