Experimental study of olivine crystallization in basalt under rapid, diffusion-limited growth conditions: tests of chemical equilibrium and re-equilibration timescales

Many olivine phenocrysts in basalt display diffusion-limited rapid growth textures (e.g. hopper, dendritic). In addition, complex P-zoning patterns in olivine phenocrysts have been observed in many basalts, consistent with diffusion-limited rapid growth (e.g., Welsch et al., 2014). In this study, it is hypothesized that olivine phenocrysts with these textures may have crystallized in basalts during rapid ascent to the surface, versus slow crystallization in a crustal reservoir. The fact that some basalts entrain mantle xenoliths is consistent with this scenario. Recently, Pu et al. (2017) proposed that for basalts with olivine displaying diffusion-limited growth textures, it is possible that the most Mg-rich olivine closely approximates the liquidus olivine from a melt represented by the whole rock composition. If so, olivine-melt thermometry can be applied, thus providing the temperature at which olivine first began to crystallize during ascent, assuming chemical equilibrium is preserved. The purpose of this study is to experimentally evaluate if chemical equilibrium is attained during rapid growth of dendritic olivine during ascent. Additionally, we test how long the liquidus olivine composition is preserved before it is lost due to Fe-Mg diffusion after continued crystallization. To address these questions, a wide variety of 1-bar, gas mixing (CO/CO₂) crystallization experiments were performed on a high-MgO (13 wt%) basalt. Two temperature-trajectory experiments were performed on 100% glass. In "bottom-up" experiments, the starting glass was taken directly up to the final equilibration temperature and held for various durations, whereas in "top-down" experiments, the starting glass was held superliquidus for 4 hours and then dropped to a final set of temperatures (either in steps or continuously). Preliminary results show that olivine composition consistently reflects a close approach to equilibrium, irrespective of its crystal growth rate and/or degree of undercooling. Experiments are in progress to document the timescales for diffusive re-equilibration of olivine, as a function of melt composition and temperature, among other parameters. If the most Fo-rich olivine are preserved in natural basalts, timescales for re-equilibration may potentially be used to evaluate geospeedometry.