Monazite control on Th, U and REE redistribution during partial melting: experiment and application to the deeply subducted crust
Creators: name: Stepanov, Aleksandr
Publisher: name: The Australian National University
Year: 2012
Subjects: geochemistry
petrology
monazite
rare earth elements

description: Rare earth elements (REE), Th and U are elements with similar geochemical properties. In crustal rocks these elements are hosted by the light REE (LREE) phosphate mineral monazite. Piston-cylinder experiments were conducted to constrain monazite solubility and monazite/melt partitioning in hydrous granitic melts at conditions relevant to crustal anatexis and subduction zone melting. Monazite has strong preference for LREE and Th; REE heavier than Nd have decreasing compatibility in monazite, and U is less compatible than LREE and Th. New experimental data and reconciliation with previous studies led to a new formulation of LREE solubility in granitic melts as a function of temperature, pressure, monazite composition and water content in melt. The behaviour of monazite during high-pressure and ultra-high pressure (UHP) metamorphism was studied using a suite of rocks from the Kokchetav massif, Kazakhstan. Detailed petrographic and geochronologic study of the samples from the Kokchetav UHP complex was combined with investigation of trace element geochemistry and mineral inclusions of garnet, monazite and zircon. These data demonstrated that (a) on prograde evolution rocks did not experience a linear increase of pressure and temperature (P and T), but had stages of almost isothermal increase of pressure and heating stage with a small increase of pressure, (b) exhumation produced a close association of UHP gneisses with rocks that experienced metamorphism at lower PT conditions and/or along different paths from typical UHP rocks. The geochemistry of the UHP gneisses of the Kokchetav complex is a perfect target for the application of the new experimental data, because these rocks experienced metamorphism and melting at the highest PT conditions recorded in crustal rocks. Bulk rock geochemistry of the UHP gneisses shows pronounced depletion in LREE, Th and U, and a smaller degree of depletion or enrichment in other elements that are often considered as incompatible. The variation in composition of UHP gneisses is explained by a new petrological model, which takes into account the fact that restites are composed of residual assemblage together with a residual melt. It is demonstrated that together with the residual mineral association, the degree of melting and melt extraction efficiency play an important role in controlling of trace element behaviour. Polyphase inclusions trapped in garnet were found in samples of some UHP gneisses. The original composition of inclusions was obtained by high pressure rehomogenisation experiments. The experiments demonstrated that polyphase inclusions represent former melts of variable compositions, varying from high temperature high-LREE melts formed at peak conditions to low-LREE melts formed during exhumation. These inclusions are the first natural examples of melts formed by melting of sediments at subarc depth. This partial melting led to the complete dissolution of monazite and a strong depletion of LREE, Th and U in the UHP gneisses. Melt inclusions and bulk rock geochemistry provide evidence for the release of high LREE melts from melting of crustal metasediments. Partial melting is thus an important process changing the physical and chemical properties of deeply subducted crustal rocks.