EMPG Orléans, 1998
QUENCH TEXTURES : FORSTERITE AND DIOPSIDE INVESTIGATION
F. FAURE, J.-M. MONTEL, C. NICOLLET and G. TROLLIARD
(CNRS, UMR 65-24: Magmas et Volcans, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont - Ferrand cedex)
The process of dynamic crystallization is not well constrained, although it occurs in various rock types: xenoliths, submarine basalts, chondrules... We are studying quench textures of four minerals in the CMAS system. Four bulk compositions are studied, each having the same liquidus température at 1 atm. but différent liquidus phases: forsterite, diopside, anorthite, spinel. Liquidus phase relationships redetermined results agree with previous studies. The charges are first melted above the liquidus and two different temperature programs are subsequently applied: (1) isotherrnal crystallization : temperature is dropped rapidly to température below the liquidus, -DT=50°C to -DT=150°C, -DT is the différence between the liquidus and the run température, (2) dynamic crystallization : the charge is cooled at a constant rate varying from 15°C/h to 1600°C/h. At the end of the experiment, the samples are quenched by dropping them into water. Anorthite and spinel show little variation of morphology for the different cooling rates. On the contrary, diopside and forsterite grow as hopper-shaped, dendritic, swallow-tail... depending on the thermal history. Two important results are demonstrated : (1) forsterite and diopside do not have the same rate-law for growth. The morphology of forsterite seems to be controlled by the degree of supercooling (-DT) whereas morphology of diopside appears to be governed by the cooling rate. (2) the delay of the nucleation is smaller for forsterite than for diopside.
Key-words : dynamic crystallization, cooling rate, forsterite, diopside