Crystal settling in a vigorously convecting magma chamber

Abstract

There has been much debate concerning the mechanism of fractional crystallization in magma chambers. The traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection. Observations from layered intrusions, however, are equivocal^1–4. Doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by Stokes' law⁵, but there has been no experimental study of crystal settling in such vigorous convection. Here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory. Application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism, at least in basaltic magma chambers, despite large convective velocities.