The influence of olivine settling on the formation of basaltic cumulates revealed by micro-tomography and numerical simulations

Abstract

The speed at which crystals settle in magmatic reservoirs affects the solidification rate of magmas and their differentiation. Despite extensive prior work on the subject, most of our quantitative understanding of the process is still restricted to treating crystals as spherical particles and does not address the geometric complexities of natural crystals. Here, we use three-dimensional (3D) X-ray microcomputed tomography (X-μCT) observations on olivine crystals from Kīlauea Volcano (Hawai‘i) to document their highly intricate and variable geometries, and the textural growth relationships between the olivine crystals and their inclusions (melt, spinel, and fluid/vapor bubbles). Olivine crystals generally have clustered polyhedral or skeletal shapes, which reflect variable magmatic conditions (or growth rates) during their formation. The cumulative presence of spinel, melt, fluid, and vapor inclusions affects the density of the host crystals by up to 6% relative, and thus plays a limited role on modifying crystal settling rate. In contrast, the overall crystal shape plays a major role. We performed numerical simulations employing a finite element method to investigate the effect of crystal morphology on settling rate and the evolution of the particle volume fraction in a magmatic convective layer. We show that for all olivine geometries investigated, the settling velocity is highest when the long axis of the crystal is aligned with the flow direction of the melt. Increasing the aspect ratio of the olivine tends to decrease its settling velocity and results in an increase in the influence of its orientation on the terminal velocity (UT). Extrapolation of the simulation results to variable particle volume fractions (Φ = 0–0.5) indicates high crystal settling rates (UT = ∼9.7 × 10−6–1.4 × 10−5 m/s) that are used to estimate the timescales for the formation of olivine cumulates in natural melt-dominated basaltic systems. The formation of olivine cumulates is therefore rapid, potentially leading to the accumulation of a crystal layer at the bottom, where the frictional contacts between the crystals exert a rheological lock-up acting against further convection. Crystal accumulation in the locked layer (parametrized with the solid/liquid volume ratio in the reservoir) is a function of the reservoir size and crystal fraction, and takes a few years in small reservoirs (<1 km thick) and a few decades in larger reservoirs (several kms thick). We propose a calibration of olivine suspension timescales for mafic magma reservoirs (based on the knowledge of the particle volume fraction, reservoir height, and olivine morphology). This calibration is used to estimate the rate of cumulate build-up, and can help interpret crystal size distributions in the framework of crystal suspension times.