SODIUM AND POTASSIUM IN CORDIERITE - A POTENTIAL THERMOMETER FOR MELTS?

Abstract

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, aH(2)O and aCO(2) in the range 3-7 kbar and 800-1000 degreesC. aH(2)O and aCO(2) 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 aH(2)O or aCO(2) 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 aH(2)O (H2O content in cordierite and melt) but is not influenced by aCO(2). The measured variations in K/Na in cordierite (in atoms per formula unit) are systematic with T and aH(2)O. 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 T = -10890 (+/- 3400) / [ln (K/Na) - 9.539 (+/- 2.850) + 1.303 (+/- 0.427) rootaH(2)O], 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 degreesC 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.