Thermobarometry of Sphene-bearing Reactions: Calculations With New Thermodynamic Data

Sphene is a common titanium-bearing accessory mineral in many metamorphic and igneous environments. It is formed in greenschist facies rocks according to reaction (1), rutile + calcite + quartz = sphene + CO2, and is also commonly found at pressures in the blueschist and eclogite facies above 15 kbar. Sphene is a commonly dated phase, and it can be useful to determine metamorphic conditions with a mineral which is used for dating. New measurements on the low temperature heat capacity of sphene (Manon et al. 2004, 2005) have allowed a better determination of the thermodynamics of sphene used in calculating metamorphic phase equilibria. In this work, these data are used to re-determine the conditions of metamorphism in high pressure assemblages. Manning & Bohlen (1991) applied several sphene-bearing equilibria as barometers in eclogites facies rocks. Their experimental reversals on reaction (2), rutile + anorthite = sphene + kyanite, agreed well with the curve derived from the internally consistent data base of Berman (1988). They calculated metamorphic conditions for three eclogite localities based on reaction (3), grossular + rutile + quartz + H2O = clinozoisite + sphene, and (4), grossular + rutile + coesite = sphene + kyanite. Tropper et al. (2002) re-calculated these reactions using a molecular activity model derived from their experiments on the coupled substitution of fluorine and aluminum in sphene. Due to the nonideality of the mixing model at high dilutions, they determined pressures up to 10 kbar higher than previous calculations for highly aluminous sphenes from diamond-bearing eclogites of the Kokchetav Massif (Sobolev & Shatsky, 1991). In this study, reactions (1)-(4) were recalculated utilizing the refined thermodynamic data for sphene. End-member curves for reactions (3) and (4) are shifted by more than a kbar at low geothermal gradients. Page et al. (2003) determined pressures in the Bakersville eclogite, North Carolina, using reaction (3) and the data from Holland & Powell (1998). Application of the new data shifts the locus of the curve by 1 kbar at 500°C. The effect is more pronounced at lower temperatures, indicating that applications of reaction (3) to blueschist assemblages are in need of revision. Changes in calculated pressures for reactions (2) - (4) are even larger relative to calculations done with the data base of Berman (1988). The reaction, grossular + rutile = sphene + corundum is of potential use in metabauxites and some marbles, which may contain significant amounts of titanium, but lack other assemblages suitable for thermobarometry. Rutile + lawsonite + grossular = sphene + zoisite + H2O, is a potentially important dehydration reaction in subducting slabs. It limits the stability of lawsonite further than titanium-absent reactions. Accessory minerals such as rutile and sphene, which tend to be neglected in metamorphic thermobarometry, are important especially in high-variance rocks such as eclogites and some blueschists.