Experimental investigation of equilibration conditions during forsterite growth and melt inclusion formation

In order to increase understanding of the mechanisms of melt inclusion formation and the significance of their chemical compositions we performed dynamic forsterite crystallization experiments in the CaO–MgO–Al2O3–SiO2 (CMAS) system at 1 atm. Evolution of melt inclusions as a function of the host crystal morphology was then investigated. The experiments were performed at various cooling rates and degrees of undercooling to study a range of olivine morphology, from polyhedral to skeletal and dendritic crystals, and the compositions of the melt inclusions in the precipitated crystals were compared with the crystallizing melt. Multi-step experiments with different cooling rates were also performed to consider complex morphology such as dendritic polyhedral forsterite. The results show that, depending on the rates of exchange of matter at the advancing interface as compared to the diffusion rates in the melt, two major types of melt inclusions can be distinguished. Melt inclusions formed during diffusion-controlled, rapid skeletal or dendritic forsterite growth will sample small volumes of the compositional boundary layer. Their compositions are thus clearly different from those predicted by differentiation along the liquid line of descent and cannot give direct information about the parental melt of the host crystal. Conversely, when interface attachment processes control the rate of crystal growth, crystals display polyhedral morphology and the trapped liquid is representative of the parental melt whatever the size of the inclusion. For instance, melt inclusions formed during polyhedral forsterite growth from interaction between spiral growth dislocations are in equilibrium with the liquid line of differentiation.