A Laboratory Investigation of Assimilation at the Top of a Basaltic Magma Chamber

Abstract

The fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments. An aqueous solution of $$Na_{2}CO_{3}$$ was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition. The tank was then drained and refilled with warmer solution, either $$Na_{2}CO_{3}$$ (of the same or different composition) or, in two experiments, $$KNO_{3}$$. The metal top in all cases was held below the eutectic temperature for the system $$Na_{2}CO_{3}-H_{2}O$$. The experiments were continued until substantial crystallization, and in some cases complete solidification, were achieved. Starting with a dense solid roof layer of $$Na_{2}CO_{3}10H_{2}O$$ and ice on the $$Na_{2}CO_{3}$$ side of the eutectic, and a warmer eutectic $$Na_{2}CO_{3}$$ solution below, there was rapid convective cooling of the liquid layer accompanied by melting of the roof, producing a compositionally dense liquid which convected downwards. When the roof was a solid of eutectic composition and the warm input fluid was much denser, a light stable liquid layer formed. The two liquid layers were separated by a double-diffusive interface through which heat was transported rapidly, but through which there was little mass transfer. As the lower layer cooled and crystallized at the bottom, its $$Na_{2}CO_{3}$$ concentration decreased, and the latent heat released by crystallization was transported upwards through the interface and contributed to the heat required for melting at the roof. In an experiment in which the mean compositions of the roof and the input solution were comparable, melting also led to the development of a stable upper liquid layer, as denser $$Na_{2}CO_{3}10H_{2}O$$ crystals fell off, leaving a lighter melt of eutectic composition behind. With a different denser solute ($$KNO_{3}$$) below, the system at first behaved similarly to the $$Na_{2}CO_{3}$$ experiment and produced a light roof layer, but later the densities of the two liquid layers evolved due to cooling and crystallization until they became equal, resulting in overturning and thorough mixing. The implication for a basaltic magma chamber, in which the magma produced by melting of the roof rocks is lighter than the magma below, is that the melted material will remain at the top of the chamber and be chemically isolated from the basaltic magma at the bottom. The process of AFC (assimilation with fractional crystallization) implies that the heat required for assimilation comes from the latent heat of crystallization, and that these two processes occur simultaneously in a single body of well-mixed magma. The melt released forms a light roof layer which may initially mix with some of the underlying basaltic magma, but then mixes very little. As a consequence, assimilation is separated from the crystallization in space and time. The interface between the upper and lower layers still allows rapid thermal transport vertically, so that crystallization at the floor supplies heat to assimilate the roof, but little roof material is incorporated in the lower layer as it crystallizes. At a later stage the contaminated magma at the roof crystallizes so that fractional crystallization follows assimilation, rather than the two processes occurring simultaneously from the same body of magma. True AFC is confined to the lower basaltic layer but is limited by the amount of material which can be transported across the double-diffusive interface which divides the basaltic layer from the melted roof layer. AFC does become important if large numbers of blocks of the roof fall through the upper layer and melt in the lower layer.