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Sodium and potassium in cordierite - a potential thermometer for melts?

Department of Geology and Geophysics, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, U.K.
^** Online News Editor, New Scientist, 151 Wardour St., London W1F 8WE, U.K.

^* corresponding author: e-mail: Pauline. Thompson{at}glg.ed.ac.uk

K₂O and Na₂O contents of a low-Be-Li cordierite coexisting with a granitic melt, characterised by a near constant Na/K ratio of 1.39 (in atomic proportions), have been determined as a function of P, T, aH₂O and aCO₂ in the range 3–7 kbar and 800–1000 °C. aH₂O and aCO₂ were calculated for individual experiments using measured H₂O contents of cordierite and melt, and CO₂ contents of cordierite. Na in the cordierite does not vary with P, T aH₂O or aCO₂ within this experimental range. The Na₂O content is within analytical error of the value 0.120 wt% Na₂O in all these experiments and thus does not support previous experimental results that showed more variable Na contents. K in cordierite increases (from 0.03 to 0.18 wt% K₂O) with increasing T and decreasing aH₂O (H₂O content in cordierite and melt) but is not influenced by aCO₂.

The measured variations in K/Na in cordierite (in atoms per formula unit) are systematic with T and aH₂O. The following expression describing the temperature dependence of cordierite-melt K, Na exchange for a melt with a fixed alkali composition of 4.79 wt% K₂O and 4.37 wt% Na₂O can therefore be derived

where T is in degrees Kelvin. This can be used, with due caution, as a relative or absolute thermometer for approximating the temperature (e.g. absolute uncertainty of ±45 °C at the centre of the experimental range studied) at which cordierite and granitic melt last equilibrated. The effect of variations in the melt composition from that used in our experiments may add to the uncertainty in any absolute temperature estimates. However, by comparison with experimentally determined minimum melt compositions, the thermometer is applicable to any melt formed at a water activity of less than 0.5 over the entire pressure range of cordierite-melt stability (1–8 kbar). At higher water activities approaching unity the equation above is only applicable at low pressures. Application of this thermometer requires the availability of high-quality analytical data for H₂O, K and Na in low-Li and -Be cordierite, and its precision is limited at lower temperatures by the difficulties in measuring the low K₂O contents expected in cordierites formed with granitic melts.

Key-words: sodium, potassium, cordierite, granitic melt, thermometer.

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				S. Erdmann, R. A. Jamieson, and M. A. MacDonald Evaluating the Origin of Garnet, Cordierite, and Biotite in Granitic Rocks: a Case Study from the South Mountain Batholith, Nova Scotia J. Petrology, August 1, 2009; 50(8): 1477 - 1503. [Abstract] [Full Text] [PDF]

				R. KAINDL, P. TROPPER, and I. DEIBL A semi-quantitative technique for determination of CO2 in cordierite by Raman spectroscopy in thin sections European Journal of Mineralogy, June 1, 2006; 18(3): 331 - 335. [Abstract] [Full Text] [PDF]