Abstract
An experimental study initiated to calibrate the distribution coefficient \(K_D = \frac{{({\text{FeO}}/{\text{MgO}})_{{\text{ga}}} }}{{{\text{(FeO}}/{\text{MgO)}}_{{\text{cpx}}} }}\) in eclogites as a geothermometer has been done on (a) a mineral mis, (b) a glass of the typical tholeiite composition and (c) a series of glasses of tholeiite compositions with \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 93.\)
The mineral mix was found to be unsuitable as reactant due to incomplete equilibration but the minimum K D of the mineral mix and the K D from glass of tholeiite composition are identical within experimental uncertainty. These data constitute a reversal of the garnet/clinopyroxene partition relationship and provide justification of the use of glass as a reactant for the project. To eliminate any uncertainty in interpretation of mineral analyses due to possible variation in Fe+++/Fe++ between runs, experiments were carried out in iron capsules on the nine tholeiite glasses, thus maintaining iron as Fe++.
Microprobe analytical techniques yielded mineral analyses of comparable accuracy to analyses of natural phases for experiments within the temperature range from 600° C to 1500° C and a pressure range from 20 kb to 40 kb.
It has been shown that for \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 85\), the bulk chemical composition does not perceptibly affect the K D value. At 30 kb the K D value ranges from 18.0 at 600° C to 1.45 at 1400° C, defining the linear relationship in a ln K D vs 1/T(°K) plot. The pressure dependence of the K D -value has been shown to be greater than previously predicted. There is a straight line relationship in ln K D vs Pressure (Kb) between 20 and 40 kb at constant temperature (1100°C). This enables us to determine K D =fn (T, P) and \(T(^\circ {\text{K}}) = \frac{{3686 + 28.35 \times P({\text{Kb}})}}{{\ln K_D + 2.33}}\).
This expression uniquely determines the temperature of equilibration of natural eclogites of basaltic bulk composition when the K ga,cpx D is known and a pressure estimate can be given.
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Råheim, A., Green, D.H. Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and clinopyroxene. Contr. Mineral. and Petrol. 48, 179–203 (1974). https://doi.org/10.1007/BF00383355
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DOI: https://doi.org/10.1007/BF00383355