Investigating the Redox Conditions and Timescales of Fluid Release from the Subducting Slab: Insights from coupled Fe and Zn Isotopes and Sm/Nd Geochronology within Garnet

Abstract:

The release of metamorphic fluids from the subducting slab exerts an important control upon the redox conditions of the slab and overlying sub-arc mantle and may explain the relatively oxidized nature of volcanic arc magmas [1,2,3,4]. In particular, redox-sensitive elements (e.g. S, C, Fe) contained within these fluids play an important role in impacting the redox state of the slab and sub-arc mantle [1,2,3,4]. Whilst tracing the movement of such elements within fluids presents many challenges, recent studies involving stable Fe isotope measurements in whole rock samples [3] and growth zones within garnet [4] have shown that they represent a sensitive recorder of changing redox conditions and the release of oxidizing species (e.g. SO_X, CO_X) from the slab.

In this study, we make the first coupled measurements of Fe and Zn isotopes in growth zones in garnet from mafic eclogites from two localities in the Zermatt Saas Ophiolite, Western Alps. The different isotopic fractionation mechanisms for Fe and Zn are utilized to investigate the potential release of oxidizing species within slab-derived fluids. Analysis of five growth zones within garnet from the St Marcel Valley reveal that the garnet cores record lower iron isotope values (δ⁵⁶Fe between -0.30 and -0.35), whilst garnet rims have higher δ⁵⁶Fe (-0.05). By contrast, core and rim zones of garnet from a mafic eclogite from Lago di Cignana reveal no resolvable difference in iron isotope values. These redox-sensitive isotope measurements will be combined with thermodynamic modelling to assess whether the change (or absence of change) in Fe and Zn isotope compositions from core to rim corresponds to the breakdown of specific hydrous minerals (e.g. lawsonite, amphibole) during subduction. To test whether metamorphic devolatilization may occur over very short timescales (<1Myr) [5], the Fe and Zn isotope analyses will be combined with high resolution Sm/Nd geochronology applied to the same garnet growth zones to provide a novel record of the rates and timescales of redox-controlling fluid release.

[1] Kelley and Cottrell (2009) Science; [2] Debret and Sverjensky (2017) Scientific Reports; [3] Debret et al. 2016 Geology; [4] Gerrits et al. (2019) Nature Geoscience; [5] Dragovic et al. (2012) Chemical Geology