Minor phases as Carriers of trace elements in non-modal crystal-liquid separation processes I: Basic relationships

Abstract

Some trace elements have the property that, although they are incompatible with most mineral phases in magmatic systems, they are strongly concentrated in certain minor mineral phases. These minor phases, termed here 'carrier-phases', and their associated trace elements include platinum group elements in base metal sulphide and chromite; chromium and vanadium in magnetite; uranium group metals in zircon and monazite; and rare earth elements in monazite and xenotime. Carrier-phases may form only a small fraction of a source rock undergoing partial melting and tend to be eliminated from the residue at an intermediate point in the partial melting history; conversely, those same minor carrier-phases tend to precipitate late during fractional crystallization of a liquid produced in the above manner, but may constitute a high proportion of the cumulate then forming. This paper explores the phase equilibria aspects of such processes in a simple system, outlining a nomenclature which is then used in a mathematical treatment applicable to non-modal melting and crystallization processes involving several crystal species. The treatment at this stage assumes constant individual crystal-liquid distribution coefficients. Equations are developed, which are applied in a companion paper to illustrate the behaviour that can be anticipated when carrier-phases play a significant role in trace element location during melting and crystallization.