Accessory Mineral Eu Anomalies in Suprasolidus Rocks: Beyond Feldspar

Abstract

Accessory mineral Eu anomalies (Eu/Eu*) are routinely measured to infer changes in the amount of feldspar over time, allowing accessory mineral U‐Pb dates to be linked to the progressive crystallization of igneous and metamorphic rocks and, by extension, geodynamic processes. However, changes in Eu/Eu* can reflect any process that changes the relative availability of Eu2+ and Eu3+. We constructed partitioning budgets for Sm, Eu2+, Eu3+, and Gd in suprasolidus metasedimentary rocks to investigate processes that can influence accessory mineral Eu anomalies. We modeled three scenarios: (1) closed‐system, equilibrium crystallization; (2) fractionation of Eu by feldspar growth during melt crystallization; and (3) removal of Eu by melt extraction. In the closed‐system equilibrium model, accessory mineral Eu/Eu* changes as a function of fO2 and monazite stability; Eu/Eu* changes up to 0.3 over a pressure‐temperature range of 4–12 kbar and 700–950°C. Fractionation of Eu by feldspar growth is modeled to decrease accessory mineral Eu/Eu* by ~0.05–0.15 per 10 wt% feldspar crystallized. Melt extraction has a smaller effect; removal of 10% melt decreases accessory mineral Eu/Eu* in the residue by ≤0.05. Although these models demonstrate that fractionation of Eu by feldspar growth can be a dominant control on a rocks u budget, they also show that the common interpretation that Eu/Eu* only records feldspar growth and breakdown is an oversimplification that could lead to incorrect interpretation about the duration and rates of tectonic processes. Consideration of other processes that influence Eu anomalies will allow for a broader range of geological processes to be investigated by petrochronology.

Timescales of Partial Melting and Melt Crystallization in the Eastern Himalayan Orogen: Insights From Zircon Petrochronology

“…Thus, decreasing Eu/Eu* indicates increasing abundance of plagioclase, but not K‐feldspar. Changes to redox state (Eu 2+ /Eu 3+ ) also depend on pressure (Holder et al., 2020), but predicted changes to Eu/Eu* for very large changes to pressure are only ∼20%–30% (Holder et al., 2020), whereas we observe changes of a factor of ∼2–3 (Figure 10f)…”

“…Thus, decreasing Eu/Eu* indicates increasing abundance of plagioclase, but not K‐feldspar. Changes to redox state (Eu 2+ /Eu 3+ ) also depend on pressure (Holder et al., 2020), but predicted changes to Eu/Eu* for very large changes to pressure are only ∼20%–30% (Holder et al., 2020), whereas we observe changes of a factor of ∼2–3 (Figure 10f) Garnet, allanite, and xenotime take up substantial Y and HREE (Engi, 2017; Hermann, 2002; Spear & Pyle, 2002), but there is no petrographic evidence for prograde allanite or xenotime in the studied rocks. So, lower Y and HREE of zircon and a flatter slope to HREE trends mainly reflect garnet growth.…”

“… Melting dissolves monazite preferentially compared to zircon (Kelsey et al., 2008; Yakymchuk & Brown, 2014), and because monazite preferentially incorporates Th relative to U, increased melting should produce higher Th/U in zircon (Yakymchuk & Brown, 2019; Yakymchuk et al., 2018). Conversely, melt crystallization should result in lower Th/U in zircon Plagioclase strongly partitions Eu 2+ compared to other phases, including melt (the partition coefficient, D , is approximately 2), whereas K‐feldspar moderately partitions Eu 2+ ( D ∼ 0.75), and other phases take very little (Holder et al., 2020). In our models, K‐feldspar is never modally abundant, and any growth is more than offset by dissolution of plagioclase (Figures 11a and 11b).…”

“…Plagioclase strongly partitions Eu 2+ compared to other phases, including melt (the partition coefficient, D , is approximately 2), whereas K‐feldspar moderately partitions Eu 2+ ( D ∼ 0.75), and other phases take very little (Holder et al., 2020). In our models, K‐feldspar is never modally abundant, and any growth is more than offset by dissolution of plagioclase (Figures 11a and 11b).…”

A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

“…Apatite within garnet lacks a distinct Eu anomaly and is characterized by flat HREE signatures, whilst matrix apatite exhibits a strongly negative Eu anomaly and comparative enrichment in HREEs (Figure 12b). Negative Eu anomalies are commonly assumed to be the product of co‐crystallization with feldspar; however, they may also occur due to the sensitivity of Eu 2+ /Eu 3+ to low f O2 (Holder et al, 2020). Apatite inclusions within garnet are connected to the matrix via fractures, preventing them from being armoured in a scenario similar to that described for rutile.…”