Minor Phases as Carriers of Trace Elements in Non-Modal Crystal-Liquid Separation Processes I: Basic Relationships

Some trace elements have the property that, although they areincompatible with most mineral phases in magmatic systems, theyare strongly concentrated in certain minor mineral phases. Theseminor phases, termed here ‘carrier-phases’, andtheir associated trace elements include platinum group elementsin base metal sulphide and chromite; chromium and vanadium inmagnetite; uranium group metals in zircon and monazite; andrare earth elements in monazite and xenotime. Carrier-phasesmay form only a small fraction of a source rock undergoing partialmelting and tend to be eliminated from the residue at an intermediatepoint in the partial melting history; conversely, those sameminor carrier-phases tend to precipitate late during fractionalcrystallization of a liquid produced in the above manner, butmay constitute a high proportion of the cumulate then forming.This paper explores the phase equilibria aspects of such processesin a simple system, outlining a nomenclature which is then usedin a mathematical treatment applicable to non-modal meltingand crystallization processes involving several crystal species.The treatment at this stage assumes constant individual crystal–liquiddistribution coefficients. Equations are developed, which areapplied in a companion paper to illustrate the behaviour thatcan be anticipated when carrier-phases play a significant rolein trace element location during melting and crystallization. KEY WORDS: uranium; thorium; platinum group elements; carrier-phase; trace element