Replenished magma chambers

Abstract

Several recent publications have reported investigations of the dynamical effects and geological consequences of the replenishment of a homogeneous magma chamber by the rapid injection of hot, relatively dense, fresh magma. The new magma forms a separate layer at the base of the chamber where it cools and crystallizes until the density of the residual liquid in the lower layer becomes equal to that of the upper layer, at which time the fluid in the two layers becomes intimately mixed. The fluid dynamical aspects of this model were confirmed by laboratory experiments using aqueous solutions. This paper reports three experimental variations on the basic theme which examine the effects of the rate of injection into homogeneous and compositionally stratified chambers. First, the rapid injection of a hot, dense layer beneath a fluid with a lower temperature and a vertical compositional gradient is considered. A series of layers gradually builds up from the base of the upper gradient region, while the lower layer cools, crystallizes and finally overturns. In this situation, however, the rise of the lower-layer fluid is restrained by the density gradient and mixing takes place with only part of the upper fluid. Second, the slow influx of a hot dense liquid beneath a cold, less dense homogeneous layer is considered. A considerable quantity of the input fluid cools and crystallizes immediately, releasing some lighter fluid to mix with the fluid in the homogenous layer. With time, some dense input spreads along the bottom so that locally the situation is the same as the one considered in the previous studies, with a local overturning occurring when the density of the lower fluid has decreased sufficiently. The deposition of crystals continues and some rather fine, chimney-like structures can develop which bear a resemblance to the "black smokers" recently discovered on the sea floor. Third, a combination of the first two variations is studied by the consideration of the slow influx of hot, dense liquid beneath a uniformly cooler fluid with a vertical compositional gradient. The input fluid is constrained in its rise by the density gradient. As the influx continues, the additional thermal energy is also confined and causes the crystals to redissolve and build up a lower layer, as was observed in the case of a rapid injection beneath a compositinal gradient. Show more