Survival of the Fattest: Titanite Growth During Thermal Cycling Experiments

Abstract:

Titanite, an important accessory mineral in plutonic rocks, is commonly interpreted as crystallizing early owing to euhedral habit and large size (up to 1 cm across), but this form can also result from temperature cycling. We performed experiments in the synthetic system titanite-albite (ttn-ab) to observe how ttn growth responds to thermal cycling. Thermal cycling causes crystal coarsening through precipitation-dissolution where crystals with larger surface areas outlive smaller or newly nucleated crystals during up-temperature cycle segments.

Experiments were carried out using a 1-atmosphere furnace at a mean temperature of 1280°C, 30°C below the experimentally determined liquidus for bulk composition ttn₅₀ab₅₀. Temperature cycles were pseudo-sinusoidal, with amplitudes up to 20°C, a 24-minute period, and run times up to 100 hours. Heterogeneous nucleation on starting material oxides and carbonates occurred in all experiments. Median sizes of the largest 50 crystals show that growth rates increase from 2.7 μm²hr^-1 to 15.3 μm²hr^-1 from static to high amplitude. Static runs generated crystal number densities of ~8000 mm^-2, whereas large-amplitude runs were ~400 mm^-2. 20°C amplitude at ~70 hours run time created the largest crystal size of 4610 μm².

Growth rate is a strong function of cycle amplitude. Crystals grown during cycling conditions are more euhedral and significantly larger than those grown under static run conditions. Cycled runs compared to static show reduced crystal number densities by one order of magnitude and growth rates increased by a factor of 5. Thermal cycling coarsens crystal populations and is an important process to consider when interpreting plutonic rock textures.