Omphacite microstructures as time-temperature indicators of blueschist- and eclogite-facies metamorphism

Abstract

Omphacites from a wide range of geological environments have been examined by transmission electron-microscopy. Their microstructures are sufficiently variable as to be potential indicators of thermal history for blueschist and eclogite metamorphism. In particular, the average size of equiaxed antiphase domains (APD's) arising from cation ordering appears to be a characteristic feature of each environment and increases in the sequence: Franciscan, blueschist (1) ≈ Turkey, blueschist (2) < Guatemala, jadeitic blocks in serpentinite (3) < Syros, blueschist (9) ≈ Red Wine Complex, Canada, amphibolite (1) < Maksyutov Complex, Urals, blueschist (3) ≈ Zermatt-Saas, blueschist (5) ≈ Allalin, metagabbro (4) < Tauern, eclogite (1) ≈ Franciscan, eclogite (5) < Nybö, Norway, eclogite (2) (numbers in brackets indicate the number of hand specimens for which omphacite microstuctures are known).

A relationship between APD size, annealing time and temperature has been derived by analogy with the known APD coarsening behaviour in other systems where: (APD size)ⁿ

$$({\text{APD size)}}^{\text{n}} \propto {\text{e}}^{{\text{(}} - {\text{Q/RT)}}} \cdot {\text{ }}time{\text{.}}$$

. Most omphacites fit into a self-consistent scheme with n=8±2 if the activation energy (Q) is assumed to be that of cation disordering (75 kcal mole⁻¹), available estimates of peak metamorphic temperature (T) are used, and a reasonable geological time-scale is taken as 10⁴–10⁸ years. According to this model, APD sizes are set in a relatively short interval of the total history of a rock when its temperature is close to its peak value. APD sizes are much more sensitive to temperature than to time and may be used as a geothermometer which has the advantage of not being reset by re-equilibration at low temperatures. Petrological implications arising from the model are that Allalin metagabbros were metamorphosed at a similar peak temperature to Zermatt-Saas blueschists, Franciscan eclogites reached higher temperatures than has been previously supposed and that the microstructures in some Sesia-Lanzo omphacites are consistent with a high temperature, pre-blueschist origin. Deviation from an ideal coarsening law with n=2 implies that the APD's are not simply stacking mistakes but have some associated structural or compositional modification locally. Excess titanium concentrated at APD's in Red Wine Complex omphacites may account for their anomalously low observed APD size.