From single batholith to global detrital zircon archive: Earth dynamics as seen from zircon Eu anomalies

Due to the continuous reworking of the continental crust and the limited rock record associated with the early Earth, the long-term evolution of the Earth's continental crust is mostly studied using the physically- and chemically-resistant mineral zircon. In particular, the europium anomaly [Eu/Eu* = Eu_N/(Sm_N x Gd_N)^0.5; where the subscript N denotes chondrite-normalized] of detrital zircon populations has been proposed as a robust proxy for tracing the evolution of crustal thickness. However, recent studies have made the use of the zircon Eu anomaly for geodynamic reconstructions controversial. To provide new insights into the petrological controls on zircon Eu/Eu*, we first review the zircon archive and the evolution of the Adamello batholith, the largest Tertiary intrusion in the Alps and one of the best-studied examples for elucidating the genesis and evolution of continental arc magmas. From the existing extensive zircon archive, the Eu/Eu* in the different intrusive super-units decreases with decreasing age, fO₂ and εHf_(t), and increasing δ¹⁸O. When the Eu/Eu* of zircon is converted to crustal thickness, the values of ≈70 km obtained for the most juvenile granitoid suites are inconsistent with the rock record and with the well-known geodynamic evolution of the Alps. A similar mismatch is also obtained when the whole-rock (La/Yb)_N of the most juvenile felsic magmas is converted to crustal thickness, because it does not take into account the effect of slab-derived metasomatic agents affecting the mantle wedge source. The marked decrease in zircon Eu/Eu* (i.e., the marked increase in the Eu anomaly) is primarily influenced by the increasing assimilation of reduced metasedimentary rocks in the crustal hot zone, before significant differentiation and without any significant pressure drop between the different intrusive super-units. This is consistent with graphitic metapelites being an important component of the mid to lower continental crust in the Southern Alps domain. Based on these results, we then review the detrital zircon archive throughout the Earth's history and interpret changes in zircon Eu/Eu* as mainly controlled by the pressure-independent redox state of magmas. We interpret the marked decrease in Eu/Eu* from the Neoarchean to the Mesoproterozoic as reflecting the increase in reduced metasedimentary material as an important component of the mid to lower crust, which was increasingly available for crustal reworking and magma contamination. This evolution is consistent with the Paleoproterozoic being a period of anomalously high burial of organic carbon and with the highest T/P ratios of metamorphism and the abundant mantle- and crustally-derived magmatism that characterized the Mesoproterozoic to early Neoproterozoic. In our opinion, the minimum Eu/Eu* at ca. 1 Ga cannot be used to support the view of orogenic quiescence in the mid-Proterozoic. The marked increase in zircon Eu/Eu* throughout the Neoproterozoic and Phanerozoic, accompanied by a concomitant general decrease in the T/P ratios of metamorphism, is indicative of the emergence of modern plate tectonics, characterized by deep, prolonged and cold subduction zones and the formation of more oxidized magmas.