Garnet crystallization does not drive oxidation at arcs

Abstract

Arc magmas, the building blocks of continental crust, are depleted in total iron (Fe), have higher ratios of oxidized Fe to total Fe (Fe 3+ /∑Fe), and record higher oxygen fugacities ( f O 2 ’s) compared with magmas erupted at mid-ocean ridges. Garnet crystallization could explain these observations if garnet removes substantial amounts of Fe 2+ , but not Fe 3+ , from magma, yet this model for continental crust generation has never been tested experimentally. Analysis of garnets and melts in laboratory experiments show that the compatibilities of Fe 2+ and Fe 3+ in garnet are of similar magnitudes. Our results indicate that fractional crystallization of garnet-bearing cumulates will remove 20% of total Fe from primary arc basalts but negligibly alter the Fe 3+ /∑Fe ratio and f O 2 of the melt. Garnet crystallization is unlikely to be responsible for the relatively oxidized nature of basaltic arc magmas or the Fe-depletion trend observed in continental crust.

Porphyry Copper Deposit Formation: Identifying Garnet and Amphibole Fractionation With REE Pattern Curvature Modeling

“…On the other hand, garnet most likely participated in the magmatic differentiation process that led to the formation of the NeAn rocks. Since all provinces contain PCDs, we conclude that garnet fractionation is not a prerequisite for PCD formation, consistent with the results of Holycross and Cottrell (2023) and Barber et al. (2021).…”

“…As heavy rare earth elements (REE) partition strongly into garnet relative to Sr and the lighter REE, garnet fractionation was suggested to cause relatively high Sr/Y, and La/Yb in PCD‐forming magmas, a chemical signature commonly referred to as “adakitic.” Additionally, Tang et al. (2019) suggested that Fe 2+ partitions into garnet preferentially to Fe 3+ , potentially leading to the high f O 2 recorded in these rocks, a claim disputed by Holycross and Cottrell (2023). Recently, the link between garnet phenocrysts and PCD‐forming dacitic intrusions in Colombia were reported by Bissig et al.…”