Constitutional Zone Refining of Layered Intrusions

Abstract

Much of the extreme fractionation observed in large layered intrusions may be the result of reactions between early-formed crystals and a late-stage liquid that migrates through the zone of crystallization. The crystal-liquid relations in a thick zone of in-situ crystallization are chemically and physically unstable. Steep gradients of chemical potential and density must cause a residual water-rich liquid to move up the temperature gradient and alter crystal-liquid relations. It can cause an horizon of increased liquid which will pass through the zone of crystallization with an effect similar to that of the metallurgical process of zone refining, but in this case remelting does not result from reheating but from nearly isothermal fluxing and, as a consequence, fractionation is more efficient.

Calculations show that a flux-melted zone could grow through many zone lengths, but the most efficient fractionation occurs when the amount of melt increases rapidly and the system passes through frequent cycles. Trace elements are fractionated at different rates than they would be by most other mechanises of differentiation. If the remelted zone periodically becomes unstable and mixes with the main body, modally layered rocks could result when the system begins another cycle.