MORB degassing: Bubble growth and ascent

Abstract

Bubbles grow in upward-moving liquid basalt by diffusion and because of decompression. Transport equations for stationary and non-stationary multicomponent gas bubbles were solved numerically for the diffusion of CO 2, H 2O, He and Ar into bubbles under magmatic conditions at pressures ranging from 20 to 0.2 kbar. In uncontaminated MORB bubbles contain predominantly CO 2, this is due to the fact that of the major gases CO 2 has the smallest solubility. At P > 15 bar MORB is not saturated with water or inert trace gases. However, when it erupts on the sea floor most of the inert trace gases are in the CO 2-rich bubbles, which contain in addition, ∼6% of the total amount of water dissolved initially in the magma, if no bubbles escape. The generation of bubbles has very little effect on the bulk magma density at P > 2 kbar. Strong evidence suggests that MORB are CO 2 saturated when they are generated by partial melting in the upper mantle. Equilibrium between melt-dissolved CO 2 and the bubbles is rapidly attained in spite of the relatively small diffusion coefficient of carbon. As a result the bubbles are in equilibrium with the melt in the magma chamber and before they arrive there. Usually most of the bubbles manage to escape from the magma before and while eruption takes place. During eruption solution equilibrium is not maintained normally.