Particle dynamics and aggregation of crystals in a magma chamber with application to Kilauea Iki olivines

Crystals settling in relative proximity to each other will settle faster than their respective Stokes' velocities (U-S) due to their mutual interactions. Pairs of equant spheres (within 100 diameters of each other) are accelerated to 1.55U(S). Modification of Han and Lawler's [Han, M., Lawler, D.F., 1991. Interactions of two settling spheres: settling rates and collision efficiency. Journal of Hydraulic Engineering 117 (10) 1269-1289] equations to include inertia shows that the velocities of a pair of spheres differ and change during descent, allowing them to come into contact. In quiescent settling experiments with olivine-shaped clay prisms, the pairs of prisms usually settled faster than their U-S. The clay prisms aggregated 80% of the time, with the pairs of prisms rotating and tumbling as they fall, coming into and out of contact. Size variations in Kilauea Iki olivines control aggregation during quiescent settling. The settling velocities of individual crystals are variable, increasing when they are incorporated into aggregates. A suspension of olivine crystals (more than 0.01 vol.%) in a magma will settle slower than the average Stokes' velocities, except when a buoyant phase is present. Addition of a buoyant phase, such as CO2 bubbles, can increase the settling velocity of the olivines when the total concentration (olivine plus bubbles) (epsilon) is < 35%, or carry the olivines in a packed bed when epsilon > 35%. (C) 1999 Elsevier Science B.V. All rights reserved.