Europium anomalies in zircon: A signal of crustal depth?

Abstract

Trace element concentrations and ratios in zircon provide important indicators of the petrological processes that operate in igneous and metamorphic systems. In granitoids, the compositions of zircon have been linked to the behaviour of garnet and plagioclase—pressure-sensitive minerals—in the source during partial melting. This has led to the proposal that Europium anomalies in detrital zircon are linked to the depth of crustal melting or magmatic differentiation and are a proxy for average crustal thickness. In addition to the mineral assemblage present during partial melting, Eu anomalies in zircon are also sensitive to redox conditions as well as magma evolution during extraction, ascent, and emplacement. Here we quantitatively model how rock type, mineral assemblages, redox changes, and reaction sequences influence Eu anomalies of zircon in equilibrium with silicate melt. Partial melting of metasedimentary rocks and metabasites yields felsic to intermediate melts with a large range of Eu anomalies, which do not correlate simply with pressure (i.e. depth) of melting. Europium anomalies of zircon associated with partial melting of metasedimentary rocks are most sensitive to temperature whereas Eu anomalies associated with metabasite melting are controlled by plagioclase proportion—a function of pressure, temperature, and rock composition—as well as changes in oxygen fugacity. Furthermore, magmatic crystallization of granitoids can increase or decrease Eu anomalies in zircon from those of the initial (anatectic) melt. Therefore, Eu anomalies in zircon should not be used as a proxy for the crustal thickness or depth of melting but can be used to track the complex processes of metamorphism, partial melting, and magmatic differentiation in modern and ancient systems. Secular changes of Eu/Eu* from the zircon archive may reflect a change in thermal gradients of crustal melting or an increase in the reworking of sedimentary rocks over time.