Effect of Superheating on Olivine Nucleation and Growth in a Silica-Undersaturated Melt: An Experimental Study

Nucleation rate strongly affects the texture of an igneous rock, controlling the number of crystals present, and influencing crystal growth rate and morphology. In experimental petrology, homogenization of a starting material is typically achieved by heating the material above its liquidus temperature (superheating). A common assertion is that melt structure relaxes so rapidly that the degree and duration of superheating are inconsequential to subsequent nucleation and growth of minerals. We investigated this hypothesis, exploring melt structural changes via their effects on the nucleation and growth of olivine in a silica-undersaturated melt composition. A rejuvenation-stage Hawaiian olivine melilitite with olivine on the liquidus was used as a starting material to eliminate homogenization problems associated with synthesizing melts from reagent mixtures. Dynamic crystallization experiments were conducted in a 1 atm gas-mixing furnace at moderately reducing conditions following thermal treatments of 0, 3, and 12 hours at temperatures of -5, +10, +25, +50, and +100 °C relative to the experimentally determined liquidus temperature (1395 °C). All cooling experiments were performed at constant rate (25°C/hr), and were rapidly quenched 200°C below the liquidus. Olivine morphologies were characterized by quantifying the ratio of the crystal perimeter to surface area; the ratio increases as crystals become more anhedral. We found that the ratio is correlated with experimental superheat degree and duration, a result that is not consistent with a uniform nucleation response to subliquidus cooling. We infer that superheating produces a delay in olivine nucleation that causes crystal growth to proceed rapidly and produce skeletal morphologies. A delay in nucleation is a potential consequence of the temperature-dependence of melt structure, and suggests inability of the melt to relax on the experimental time scale.