Kinetic aspects of major and minor elements in olivine from variably cooled basaltic melts

Abstract:

Olivine is an important mineral phase in naturally cooled basaltic rocks. The texture and composition of olivine are controlled by the interplay between the degree of magma undercooling and crystal growth rate. Crystals formed at low undercoolings and growth rates generally show polyhedral-hopper textures and quite homogeneous compositions, while skeletal-dendritic textures and strong crystal zonation occur at high undercoolings and growth rates.

In this context, we have performed disequilibrium crystallization experiments to better understand the effects of crystallization kinetics on the incorporation of major and minor cations in the olivine lattice. Experiments were carried out in a 1 atm vertical tube CO-CO2 gas-mixing furnace using basaltic glass (i.e., OIB) as starting material, imposing different undercooling and cooling rates. We used room pressure and QFM-2 oxygen fugacity. Each experiment started at superliquidus temperature, which was kept constant for 2 h before cooling. After this stage, we linearly decreased the temperature to the final target of 1125 °C and 1175 °C, for a total undercooling (-ΔT) of 85 °C and 35 °C, respectively. Each experiment was performed at different cooling rates: 2 °C/h, 20 °C/h, and 60 °C/h. Samples were then quenched for recovery.

Results show that the olivine texture shifts from euhedral (i.e. polyhedral texture) to anhedral (i.e. dendritic texture) as function of the undercooling during rapid crystal growth. In -ΔT = 35 °C experiments, olivine crystals show a faint zonation. Conversely, a well-developed zonation forms in crystals grown at -ΔT = 85 °C. The compositional gradient in the melt increases with increasing cooling rate and undercooling, forming a diffusive boundary layer that expands towards the far field melt (15 μm wide at low undercooling and more than 30 μm at high undercooling). Due to the effects of crystallization kinetics, skeletal-dendritic olivine incorporates higher proportions of minor elements, which are generally incompatible within the crystal lattice at equilibrium conditions. Al, P, Ti and Cr distribution seems to follow the primary and secondary olivine branches.