Abstract
Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree. In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content (ppm by weight), pressure (GPa) and reciprocal absolute temperature: \( {\text{log}}({\text{Zr}}^{{{\text{sphene}}}} ,{\text{ppm}}) = 10.52( \pm 0.10) - \frac{{7708( \pm 101)}} {{T(K)}} - 960( \pm 10)\frac{{P({\text{GPa}})}} {{T(K)}} - {\text{log}}(a_{{{\text{TiO}}_{2} }} ) - \log (a_{{{\text{SiO}}_{2} }} ). \)The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization.
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Notes
An alternative method is to combine the TitaniQ and TiO2 solubility equations to create an equation for temperature that depends only on Ti content of quartz and Ti content and composition of the host glass. However, there are relatively large errors associated with this approach due to the similarity of the slopes for the equations describing Ti solubility in quartz and in melts. Another alternative is to simultaneously solve the Zr-in-sphene (for a given P) and TitaniQ equations for both temperature and \( a_{{{\text{TiO}}_{{\text{2}}} }} ; \) this approach assumes that sphene and quartz crystallized at the same temperature.
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Acknowledgments
This work was supported by NSF grant no. EAR-0440228 grant to E. B. Watson. The authors wish to thank Graham D. Layne for his assistance with the ion microprobe, Eric H. Christiansen for providing us with samples, and John Ferry and Gerhard Franz for their helpful reviews of this manuscript.
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Communicated by T.L. Grove.
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Hayden, L.A., Watson, E.B. & Wark, D.A. A thermobarometer for sphene (titanite). Contrib Mineral Petrol 155, 529–540 (2008). https://doi.org/10.1007/s00410-007-0256-y
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DOI: https://doi.org/10.1007/s00410-007-0256-y