Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Abstract

Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Archaean continental crust formed from mafic cumulates

“…For instance, high La/Yb and Sr/Y values, which were typically attributed to deep garnet-stable and plagioclase-absent melting (≥2.0 GPa 7 ), were shown to be equally possible for TTGs formed at significantly lower pressure (1.4 GPa) 27 . Plagioclase-absent melting, as also indicated by high Eu N /Eu* values, neither requires nor implies large melting depth; pressures as low as 1.0 GPa may suffice depending on source composition, redox conditions, and water content 28 . Closed-system, continuous melting and crystallisation to produce TTGs have also been questioned, with alternative models highlighting the importance of mineral segregation 29 , loss of interstitial liquids 26 , and hybridisation of melts formed at progressively increasing depth 30 .…”

Continental Crust Rejuvenation Across the Paleo‐Mesoarchean Transition Resulted From Elevated Mantle Geotherms

“…This potentially indicates that early Mesoarchean zircon crystallized from a hotter magma compared to zircon from prior and subsequent periods. Furthermore, Eu/Eu* in zircon has been argued to directly correlate to crustal thickness (Tang et al., 2020), although this correlation has also been questioned (Roberts et al., 2024; Triantafyllou et al., 2022; Yakymchuk et al., 2023). Using the same zircon trace element compilation (Table ), we demonstrate that mean Eu/Eu* ratios peak during this Paleoarchean to Mesoarchean transition, with a secular trend resembling the MgO contents in global komatiites (Figure 3d).…”

“…Using the same zircon trace element compilation (Table S8), we demonstrate that mean Eu/Eu* ratios peak during this Paleoarchean to Mesoarchean transition, with a secular trend resembling the MgO contents in global komatiites (Figure 3d). This may indicate that zircon from this transitional age crystallized from melts within the garnet stability field (i.e., thick crust) (e.g., Triantafyllou et al, 2022).…”