Quantitative Simulation of the Hydrothermal Systems of Crystallizing Magmas on the Basis of Transport Theory and Oxygen Isotope Data: An analysis of the Skaergaard Intrusion

Application of the principles of transport theory to studies of magma-hydrothermal systems permits quantitative predictions to be made of the consequences of magma intruding into permeable rocks. Transport processes which redistribute energy, mass, and momentum in these environments can be represented by a set of partial differential equations involving the rate of change of extensive properties in the system. Numerical approximation and computer evaluation of the transport equations effectively simulates the crystallization of magma, cooling of the igneous rocks, advection of chemical components, and chemical and isotopic mass transfer between minerals and aqueous solution. Numerical modeling of the deep portions of the Skaergaard magma-hydrothermal system has produced detailed maps of the temperature, pressure, fluid velocity, integrated fluid flux, δ18O-values in rock and fluid, and extent of nonequilibrium exchange reactions between fluid and rock as a function of time for a two-dimensional cross-section through the pluton. An excellent match was made between calculated δ18O-values and the measured δ18O-values in the three principal rock units, basalt, gabbro, and gneiss, as well as in xenoliths of roof rocks that are now embedded in Layered Series; the latter were evidently depleted in 18O early in the system's cooling history, prior to falling to the bottom of the magma chamber. The best match was realized for a system in which the bulk rock permeabilities were 10−13 cm2 for the intrusion, 10−11 cm2 for basalt, and 10−16 cm2 for gneiss; reaction domain sizes were 0.2 cm in the intrusion and gneiss and 0.01 cm in the basalts, and activation energy for the isotope exchange reaction between fluid and plagioclase was 30 kcal/mole. The calculated thermal history of the Skaergaard system was characterized by extensive fluid circulation that was largely restricted to the permeable basalts and to regions of the pluton stratigraphically above the basalt-gneiss unconformity. Although fluids circulated all around the crystallizing magma, fluid flow paths were deflected around the magma sheet during the initial 130,000 years. At that time, crystallization of the final sheet of magma and fracture of the rock shifted the circulation system toward the center of the intrusion, thereby minimizing the extent of isotope exchange between rocks near the margin of the intrusion at this level. For comparison, similar calculations were also made for pure conductive cooling;