| Popis: |
Three augite crystals with differing chemical compositions [Fonualei dacite lava flow, Tonga (Fon39) (En 36 Fs 27 Wo 37 ), Alicudi andesitic dike (KC) (En 46 Fs 11 Wo 43 ), and Vulcano basalt dike (PD30) (En 42 Fs 12 Wo 46 )] 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 Fe 2+ -Mg between M1 and M2 sites of clinopyroxene was determined by single-crystal structure refinement. The Fe 2+ -Mg partitioning data of the order-disorder reaction, Fe 2+ M2 + Mg M1 = Fe 2+ M1 + Mg M2 , in terms of the partition coefficient K D {= [(Fe 2+ M1 )(Mg M2 )/ (Fe 2+ M2 )(Mg M1 )]}, 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 R 3+ 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*} |
