ISOTOPIC AND ELEMENTAL ZONING IN GARNET REVEALS MULTIPLE MECHANISMS OF CHEMICAL TRANSFER IN SUBDUCTION ZONES (Invited Presentation)

Subduction facilitates volatile exchange between Earth’s surface and interior. The efficiency of this planetary-scale exchange is controlled by rock-scale processes. Garnet, a refractory mineral that is stable over a wide range of pressure–temperature (P–T) conditions, is witness to these chemical transfer processes, providing a time-resolved record in its growth zoning. We report on oxygen isotope and element zoning data for a suite of garnets from five subduction complexes.

Along core–rim transects, small-wavelength (< 300 µm-wide) vacillations in elemental and oxygen isotopic composition are superimposed on grain-scale elemental zonation. Across garnet rims, little to no net variation in δ¹⁸O is typically observed. However, garnets from Oman display a distinct break between a low-Al, high-Eu core and high-Al, low-Eu mantle–rim zone, and δ¹⁸O values that decrease continuously (rimward) across the mantle–rim zone of the garnets. This grain-scale zonation is interpreted to record large-scale influx of reducing, low-δ¹⁸O fluids.

Small wavelength elemental vacillations most clearly observed in Mn are axisymmetric for a given garnet and comparable among garnets at the rock scale. Conversely, small wavelength δ¹⁸O vacillations are not axisymmetric nor comparable among garnets from a single thin section. Principle component analyses suggest no covariance among the short-wavelength elemental and δ¹⁸O zoning patterns, meaning they likely have independent origins. The δ¹⁸O vacillations may record disconnected, isotopically distinct pore fluid reservoirs resulting from heterogeneous dehydration processes. The elemental vacillations may be explained by pulses of rock-wide element supply and/or solubility increase that coincide with high grain boundary permeability. The data from this study support a view of chemical transfer in subduction zones that involves: (1) long-distance, transient and channelized fluid transfer events not ‘seen’ by most rocks in the subduction zone; (2) an ambient state of highly disconnected grain boundary porosity; and (3) episodic, rock-scale high grain boundary permeability events that may involve P and/or T spikes. Seismic events may play a significant role in enabling periodic rock-scale chemical transfer/equilibration.