Abstract

Three augite crystals with differing chemical compositions [Fonualei dacite lava flow, Tonga (Fon39) (En36Fs27Wo37), Alicudi andesitic dike (KC) (En46Fs11Wo43), and Vulcano basalt dike (PD30) (En42Fs12Wo46)] were equilibrated over the temperature range 700–1100 °C through both disordering and ordering reactions (“reversal technique”) and then quenched at room temperature. Following each experiment, the partitioning of Fe2+-Mg between M1 and M2 sites of clinopyroxene was determined by single-crystal structure refinement. The Fe2+-Mg partitioning data of the order-disorder reaction, Fe2+M2 + MgM1 = Fe2+M1 + MgM2, in terms of the partition coefficient KD{= [(Fe2+M1)(MgM2)/ (Fe2+M2)(MgM1)]}, were then used to calibrate the following geothermometric equations:

\[{-}ln\mathit{K}_{D}\ =\ 2727\ ({\pm}132)/\mathit{T}(K)\ {-}\ 0.383\ ({\pm}0.113);\ (r\ =\ 0.99)\]
\[{-}ln\mathit{K}_{D}\ =\ 4204\ ({\pm}214)/\mathit{T}(K)\ {-}\ 1.570\ ({\pm}0.183);\ (r\ =\ 0.98)\]
\[{-}ln\mathit{K}_{D}\ =\ 5305\ ({\pm}351)/\mathit{T}(K)\ {-}\ 2.428\ ({\pm}0.289);\ (r\ =\ 0.98)\]

respectively for Fon39, KC, and PD30. A situation in which thermodynamic parameters depend on crystal composition may be envisaged. It should be noted that such dependence is essentially controlled by the (Ca+Na) and R3+ contents of the clinopyroxene. On the basis of compositional dependence, the following geothermometric equation is proposed:

\begin{eqnarray*}&&\mathit{T}(K)\ =\ [12100\ (750)\ {-}\ 27700\ (1700)(Ca\ +\ Na\ +\ R^{3+})\ +\ 20400\ (1000)(Ca\ +\ Na\ +\ R^{3+})^{2}]/[{-}ln\mathit{K}_{D}\ +\\&&7.1\ (0.6)\ {-}\ 20.3\ (1.4)(Ca\ +\ Na\ +\ R^{3+})\ +\ 15.2\ (0.8)(Ca\ +\ Na\ +\ R^{3+})^{2}]\end{eqnarray*}
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