Global Ba/Nb systematics in arc magmas reflect the depths of mineral dehydration in subducted slabs

Abstract

The transfer of material from subducting slabs to the overlying mantle is one of the most important processes regulating Earth’s geochemical cycles. A major part of this material cycling involves slab devolatilization and the release of sediment- and slab-derived fluids to the mantle wedge, triggering melting and subsequent arc volcanism. Previous geodynamic, geophysical, and geochemical studies have revealed many important controls on fluid fluxing to the mantle and its manifestations in arc magmas. However, it remains difficult to identify the specific mineral breakdown reactions that control element fluxes from the subducting slab into the overriding mantle. To address this challenge, we combine global arc whole-rock compositional data with geophysical information (e.g., depths to slab) and thermodynamic data. We observe three peaks in Ba/Nb in global arc magma whole-rock compositions corresponding to depths to slab of 60, 120, and >290 km. Using published thermodynamic and geodynamic models of slab evolution, we show that these peaks can be linked to the progressive breakdown of hydrous minerals (e.g., epidote, actinolite, lawsonite) in subducting slabs.

Limited water outflux from the subducted slab serpentinite via Mariana arc-basin magmatism

“…Classic geochemical studies typically use the relative enrichments of uid-mobile elements such as Ba compared to less mobile elements such as Th, REE and Nb in arc volcanics to identify contributions from slab crust released at different depths [23][24][25][26][27][28][29] . However, slab serpentinites are depleted in these elements 30,31 but strongly enriched in water (up to 13 wt.% in serpentine 5 ), and halogen [11][12][13] , with more than ten times higher Cl/F ratios than the crust layers 12 .…”

The factors controlling along-arc and across-arc variations of primitive arc magma compositions: A global perspective

“…For example, the pressure-sensitive mineral amphibole breaks down at 2.2-2.4 GPa while small amount of water remaining in hydrous phases of AOC (e.g., lawsonite and phengite) might go further down to sub-arc depth (Schmidt and Poli, 2014;Niu and Green, 2018). However, recent studies show that aqueous fluids release completely before reaching ~60 km at hot subduction zones such as Cascadia while lawsonite can still survive to the sub-arc depth in cold subduction zone such as Honshu (Hernández-Uribe and Palin, 2019; Barber et al, 2022). Most of the fluid released from slab in the forearc depth will either serpentinize the cold corner of the mantle wedge, or eventually pass through to the ocean floor in the forearc region [e.g., Mariana arc, (Fryer et al, 1999;Fryer, 2012)].…”