A REAPPRAISAL OF OLIVINE GROWTH RATES

Olivine is a ubiquitous rock-forming mineral that records a wide variety of igneous processes during its lifetime. However, the details of its crystallization from a silicate melt are complex, specifically regarding crystal habits and growth kinetics. Natural olivine can display a range of hybrid textures, with laboratory growth rates spanning 10 orders of magnitude.

Here, we present the results of nine dynamic crystallization experiments using a heating stage microscope to investigate in situ the early stages of olivine crystal growth inside a plagioclase-hosted melt inclusion (see Welsch et al., accepted at J. Petrol.). Olivine overgrowths were developed on a single seed following rapid cooling (130°C/min) from the liquidus to a target undercooling plateau (-ΔT = 19 – 129 °C). Time-resolved image processing and incremental measurements of the overgrowth thicknesses were used to identify up to three stages of crystal growth, and to calculate olivine growth rates at specific crystallographic interfaces {hkl} – i.e., olivine growth velocities. We explored the limits of this method via determinations of spatial and temporal resolution, and we modeled correction factors based on crystal geometry and crystal orientation. However, the largest sources of uncertainty are thermal: the initial liquidus determination and the temperature measurements of each experiment. Accurately determining these temperatures is critical to properly contextualize the related growth rates.

Within the framework of these new observations and methodological developments, we conducted a systematic and comprehensive review of 19 datasets of olivine growth rates published over the last 50 years. Large discrepancies between datasets arise from a variety of methodological inconsistencies, limitations, and outright errors, as well as broad uncertainties regarding the timing and manner of crystal growth. One major finding of this review is that the majority of literature values can be considered lower bounds on the true olivine growth rates.

Reference

Welsch, B., Faure, F. First, E.C. (accepted at Journal of Petrology). Reappraising crystallization kinetics with overgrowth chronometry: An in situ study of olivine growth velocities.