Forming Olivine Phenocrysts in Basalt: A 3D Characterization of Growth Rates in Laboratory Experiments

Abstract

Our current knowledge of the genesis, ascent, storage, and eruption of mafic magmas is intimately linked with olivine, its primary crystal cargo. Recent claims that phenocrystsize crystals can grow rapidly and non-concentrically are challenging our perception of what olivine zoning represents (e.g., controlled by growth and/or diffusion), and whether accurate magma crystallization and diffusion timescales can be retrieved. A series of cooling experiments using a dry basalt was carried out in order to quantify the kinetics of olivine growth as a function of the degree of undercooling, and to characterize morphological changes occurring with time. After a 24 h equilibration step at 1290 • C (10 • C above the olivine liquidus), experiments were rapidly cooled to final temperatures T f = 1270, 1255, 1240, or 1220 • C (imposing undercoolings of − T = 10, 25, 40, and 60 • C, respectively). Growth rates estimated via 3D microtomography renderings of experimental crystals attain 10 −7 m/s, and are found to be almost an order of magnitude higher than those calculated using 2D sections of the same experiments. We show that mm-sized crystals similar to those found in natural Kīlauea samples can be produced after a few hours under moderate undercooling conditions (25-60 • C). Growth rates decrease faintly with time, accompanying transitions between skeletal/hopper and more polyhedral morphologies. Growth rates generally increase until − T = 40 • C, and decrease slightly at − T = 60 • C as rates of nucleation likely increase. The − T = 40 • C vicinity may therefore represent a thermal "sweet spot" for the formation of phenocrysts in dry basalt. Olivine overgrowths on crystals that survived initial dissolution grow slower than homogeneously nucleated crystals, illustrating how new and old crystals in natural magmas likely respond differently to a thermal perturbation. We suggest that the main growth direction of natural olivine (a-or c-axis) may be a sensitive function of undercooling and the presence of a pre-existing growth substrate. Olivine grows faster along the a-axis under moderate to high undercooling conditions, while preferred development along the c-axis likely occurs under lower undercooling conditions and/or as rims grow around existing crystals. The early history of skeletal olivine crystals is controlled by diffusion in the melt (diffusion-controlled growth regime), while their long-term compositional zoning history is mainly controlled by diffusive re-equilibration.

Out-of-sequence skeletal growth causing oscillatory zoning in arc olivines

“…Using the findings in these natural crystals we propose an updated olivine growth model that is consistent with our textural observations and chemical zoning features. This model complements existing interpretation for zoning and textural parameters retrieved from natural and experimental samples observed elsewhere 11 – 15 , 39 . This type of olivine growth needs to be considered when interpreting olivine zoning together with the conventional interpretation of sequential core-to-rim growth; the latter being most effective under conditions of small undercooling 15 .…”

“…Two-dimensional (tabular) morphologies (e.g., ref. 43 ), as well as preferential initial growth along the plane ac, has been also observed in experiments 15 . However, a distinctive elongation along the a axis described in these experiments was not observed in our natural samples.…”

“…The particular morphology along this section is the first evidence that growth develops along preferential directions (anisotropic growth) at the expense of the whole plane of a given face, consistent with the so called Berg effect 40 . Experimental growth rate constraints 15 , 39 , 41 suggest that dendritic extensions like those described here grow on the order of minutes (~1.5 min for a ~100 microns dendrite). The crystal shape that emerges during this stage is similar to that previously described for high cooling rate experiments (1890 °C/h; −∆T = 118 °C) 42 (see the photomicrography B2 of Fig.…”

“…The presence of distinct dendritic zoning patterns in slow diffusing elements (e.g., P, Al), as well as the rare preservation of their morphologies in some olivine crystals from ocean island basalts, argue that an early dendritic olivine growth may be followed by an episode of maturation that fills the dendritic network and potentially explains the common occurrence of olivine-hosted melt inclusions. These findings have been tested experimentally on a Hawaiian basalt composition that shows that phenocryst-size (>100 µm) olivines may form within such two-stage growth for undercooling of 25–40 °C, with the crystal experiencing textural maturation as undercooling diminishes 15 .…”

Defects in olivine

“…The surface tension of the solid-liquid surface is then less dependent on the crystallographic orientation of the surface plane, yielding a diffuse surface which promotes rapid crystallization and thus enhances the possibility of incorporating atomic impurities (Sutton and Balluffi, 1995;King et al, 2014). This also leads to the competitive formation of euhedral (tablet surfaces), skeletal (hopper cavities or concentrated terranes), and/or dendritic (swallowtail) shapes during dynamic growth as a function of temperature and pressure (see Faure et al, 2003;Welsch et al, 2013;Mourey and Shea, 2019).…”