Low T heat capacity measurements and new entropy data for titanite (sphene): implications for thermobarometry of high-pressure rocks |
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| Authors: | M R F Manon E Dachs E J Essene |
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| Institution: | 1. Department of Geological Sciences, University of Michigan, 2534 CC Little Building, 1100 N University, Ann Arbor, MI, 48109-1005, USA
2. Abteilung Mineralogie und Meterialwissenschaften, Fachbereich Geographie, Geologie und Mineralogie, Universit?t Salzburg, Hellbrunnerstr. 34, 5020, Salzburg, Austria
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| Abstract: | The accepted standard state entropy of titanite (sphene) has been questioned in several recent studies, which suggested a
revision from the literature value 129.3 ± 0.8 J/mol K to values in the range of 110–120 J/mol K. The heat capacity of titanite
was therefore re-measured with a PPMS in the range 5 to 300 K and the standard entropy of titanite was calculated as 127.2
± 0.2 J/mol K, much closer to the original data than the suggested revisions. Volume parameters for a modified Murgnahan equation
of state: V
P,T
= V
298° × 1 + a°(T − 298) − 20a°(T − 298)] × 1 – 4P/(K
298 × (1 – 1.5 × 10−4 T − 298]) + 4P)]1/4 were fit to recent unit cell determinations at elevated pressures and temperatures, yielding the constants V
298° = 5.568 J/bar, a° = 3.1 × 10−5 K−1, and K = 1,100 kbar. The standard Gibbs free energy of formation of titanite, −2456.2 kJ/mol (∆H°f = −2598.4 kJ/mol) was calculated from the new entropy and volume data combined with data from experimental reversals on the
reaction, titanite + kyanite = anorthite + rutile. This value is 4–11 kJ/mol less negative than that obtained from experimental
determinations of the enthalpy of formation, and it is slightly more negative than values given in internally consistent databases.
The displacement of most calculated phase equilibria involving titanite is not large except for reactions with small ∆S. Re-calculated baric estimates for several metamorphic suites yield pressure differences on the order of 2 kbar in eclogites
and 10 kbar for ultra-high pressure titanite-bearing assemblages. |
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| Keywords: | Sphene Titanite Low T heat capacity Third law entropy Thermodynamic data Geobarometry Thermobarometry High-pressure metamorphism |
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