Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Abstract

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

“…These issues are the prime reason why later metasomatism can compromise the reliability of thermodynamic models based on the bulk rock composition (Evans, 2004). In our samples, garnet textures in mafic schists (Figure 4a-d) and Cld-schist (Figure 5), as well as oxygen isotope variations in garnet (Bovay et al, 2021), suggest strong fluid-rock interaction in the post-peak stage. It is thus possible that, of the two garnet generations observed, only the latest stage (Grt MS,2 , Grt MS,3 , Grt CldS,2 , and Grt CldS,3 ) represents an equilibrium with the bulk rock as measured.…”

Greenstone burial–exhumation cycles at the late Archean transition to plate tectonics

“…5c), imply a distinctly sub-mantle whole-rock δ 18 O value of <3.3‰; at the time of garnet formation. Such an 18 O-depleted composition is consistent with intense, near-surface high-temperature (hydrothermal) interaction with an isotopically light fluid, such as seawater or meteoric fluids 45 . We suggest that this garnet δ 18 O preserves the seafloor alteration signal in the unenriched amphibolites, whereas the slightly heavier O isotope composition of the enriched amphibolites reflects later, lower temperature fluid-rock interactions that also concentrated the REE in these rocks.…”

“…5c). Garnet from typical EA rocks (sample 155899) has average δ 18 O values (vs. Vienna Standard Mean Ocean Water, VSMOW) of 4.2‰; ±0.6‰; (2σ), while those in typical UA examples (sample 214208) have distinctly lower δ 18 O values (average of 1.3‰; ±0.5‰;, 2σ), which are well below those of mantle rocks (~5.3‰; ±0.6‰; 44 ) and among the lowest recorded δ 18 O values from garnet in mafic rocks 45 . Such low δ 18 O values in mafic rocks are typical of hightemperature seafloor alteration 46 and are commonly preserved during metamorphism 47 .…”

A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high‐pressure rocks

“…Likewise, garnet from some Jenner blocks have rims zoned in δ 18 O, whereas others do not (Errico et al, 2013). This may reveal varying scales of pervasive fluid infiltration versus channelized flow within the subducting slab (e.g., Bovay et al, 2021). It seems likely that the flow of high-grade blocks within mélange during later stages of exhumation leads to different histories as blocks potentially disaggregate, are exposed to different channelized fluid pathways and move (physically, and perhaps chemically) into and out of different mineral stability fields.…”

“…The record of fluids preserved in exhumed high‐pressure metamorphic rocks is providing new and important information in the study of fluid‐mediated mass transfer from the subducting slab to the sub‐arc mantle (e.g., Bebout & Penniston‐Dorland, 2016). One of a variety of tools that has been applied to this problem is petrographically‐guided in situ analysis of oxygen isotopes in garnet (e.g., Bovay et al, 2021; Cruz‐Uribe et al, 2021; Errico et al, 2013; Martin et al, 2014; Russell et al, 2013; Vho et al, 2020) and garnet and zircon (Page et al, 2014, 2019; Rubatto & Angiboust, 2015). This approach has been used to tie fluid infiltration events, where external fluids have modified a rock's oxygen isotope ratio to pressure, temperature, and time histories preserved in garnet and zircon.…”