Experimental study of intracrystalline Fe2+-Mg exchange in three augite crystals: effect of composition on geothermomentric calibration

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 degrees 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, Fe-M2(2+) + Mg-M1 = Fe-M1(2+) + Mg-M2, in terms of the partition coefficient K-D{= [(Fe-M1(2+))(Mg-M2)/(Fe-M2(2+))(Mg-M1)]}, were then used to calibrate the following geothermometric equations: -lnK(D)= 2727 (+/-132)/T(K) - 0.383 (+/-0.113); (r = 0.99) -lnK(D)= 4204 (+/-214)/T(K) -1.570 (+/-0.183); (r = 0.98) -lnK(D)= 5305 (+/-351)/T(K) - 2.328 (+/-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: T(K) = [12100 (750) - 27700 (1700)(Ca + Na + R3+) + 20400 (1000)(Ca + Na + R3+)(2)]/[-lnK(D) + 7.1 (0.6) - 20.3 (1.4)(Ca + Na + R3+) + 15.2 (0.8)(Ca + Na + R3+)2]