Porous Media Flow in Granitoid Magmas: An Assessment

Abstract

Compaction and compositional convection as potential in-situ differentiation mechanisms in granitoid intrusions has been investigated numerically for melt fractions of between 10 and 50 percent. The results show that the major factor controlling fluid movement, and hence chemical and mineralogical variation during late stage crystallisation is the viscosity of the interstitial melt. Thus, for anhydrous melts where viscosity increases with crystallisation, fluid migration rates are trivial over the average lifespan of even the largest silicic magma chambers (ca. 106 years). Alternatively, if the melt viscosity decreases during crystallisation, the relative movement of evolved fluid relative to the solid phase is such that both processes become potentially viable mechanisms of in-situ magma chamber differentiation. At initial porosities in excess of 20% and melt viscosities at or less than 105 pascal seconds, compositional convection is the dominant process of fluid movement in the crystallising pluton. As crystallisation proceeds and porosities drop to values below ~0.2, convective velocities become subcritical and compaction becomes increasingly dominant. A major consequence of both compaction and compositional convection in basic magmas is the production, through superefficient melt extraction, of texturally equilibrated, layered monominerallic rocks. The absence of similar rock types in granitoid plutons suggests that although compaction and compositional convection may go some way to explain chemical and mineralogical variations in zoned granitoids, neither process is capable of producing the extreme mineralogical variations seen in large basic intrusions.