Magma convection, temperature distribution, and differentiation

Abstract

Normal heat fluxes from the Earth are sufficient to cause magma convection in plutons greater than 15 m for magma viscosities to 10 (super -8) poise. Convection increases markedly with size of the magma chamber and becomes the dominant method of heat transfer within the magma. Large solidifying intrusions have an approximately isothermal temperature distribution during part of the crystallization history. Natural convection also redistributes settling crystals. The population distribution (differentiation) of crystals in the vertical direction depends on the settling and convective back diffusion fluxes. An equation for population distribution is derived, and calculations made for olivine and quartz crystals in ultramafic and granitic magmas are in agreement with observed distribution of these minerals in corresponding solidified intrusions.