Abstract
Exsolution textures of pyroxene minerals are regarded as effective indicators for the sake of deducing the thermal histories. With the view of elucidating quantitative relations of texture to cooling rate (C.R.) and chemical composition in pyroxenes, the size variations of “spinodally-decomposed (001) lamellae” in synthetic augite were examined with transmission electron microscope (TEM). The textural differences were investigated through cooling rate experiments (cooled over the range 1°C/h to 120°C/h) using homogeneous starting glasses with composition of Wo
32.3En
58.2Fs
9.5 and Wo
31.0En
42.7Fs
26.3. All experiments were performed under the condition of regulated partial oxygen pressures with a programmed cooling temperature system. Chemical compositions of these samples were determined with electron probe micro-analyzer (EPMA) and analytical electron microscope (AEM). Quantitative relations between lamella wavelength (λ) and C.R. are reported, which were found in the two sets of augites well defined in their composition. The present results include the recalculated data from McCallister (1978) and Grove (1982). The relations between λ and C.R. could be described at the value of Wo/(Wo+En+Fs) fixed to 0.3 as follows:
| Fs/(Fs+En)=0 | : | λ=110+400×(C.R.)−1⁄3 |
| Fs/(Fs+En)=0.14 | : | λ=100+350×(C.R.)−1⁄3 |
| Fs/(Fs+En)=0.40 | : | λ=70+250×(C.R.)−1⁄3 |
| (λ in Å and C.R. in °C/h) |
The relation of wavelength to cooling rate and composition can be discussed quantitatively and the average cooling rate in natural augite could be appraised by applying these equations.
