The partial melting of a forsterite-albite mixture under vacuum observed in-situ by high-temperature X-ray diffractometry

Some of the chemical diversity of chondrule compositions may be due to evaporative losses of moderately volatile elements such as Fe and Na. For example, type I chondrules are depleted in both Fe and Na compared to the nearly chondritic composition of type II chondrules. Evaporative losses occur on heating but experimental work that simulates evaporation losses are typically measured on the residues after cooling. The effect of the evaporation of sodium on the residues formed during the partial melting of a forsterite-albite (Fo ₇₅Ab₂₅ wt.%) mixture was observed in-situ using high-temperature X-ray diffractometry. The composition of this mixture corresponds to type A chondrules (chondrules and glass). A mechanical mixture of micron-sized forsterite and albite was heated under vacuum (10−5 to 10−6 torr) in order to enhance the evaporation of Na. After the albite completely melted, pyroxene began to form and the pyroxene/olivine ratio increased until it reached a maximum at 1260°C and then decreased with increasing temperature up to 1340°C. The sample after cooling was a granular mixture of fine-grained particles with some interstitial melt. Another sample quenched after isothermal heating of the mixture at 1400°C was composed of large elongated pyroxene grains, tens of microns in width, embedded in a groundmass of glass and olivines that ranged in size from a few microns to approximately 20 microns. The pyroxenes, olivines, and glass were all depleted in Na compared to the initial composition and plotted on the MgO-Al₂O₃-SiO₂ ternary equilibrium diagram. According to the phase diagram, the pyroxene/olivine ratio should decrease with an increase in temperature above 1400°C. The pyroxene phase is therefore an intermediate phase that appears on heating following the formation of a melt subjected to evaporative loss of Na. The 1400°C experiment produced minerals, glass, and textures similar to a common type of chondrule (IAB). Therefore, evaporation of alkalis (such as sodium), with or without the evaporation of FeO (Cohen et al., 2000), will tend to give pyroxene-bearing chondrules from an olivine-rich precursor (like CI) without significant SiO₂ enrichment.