The timing and nature of fluid release from ocean crust during subduction

Overview

Subduction of oceanic lithosphere involves substantial chemical and mass transfer from the Earth’s surface to the mantle, playing a key role in the global water and carbon cycles, and exerting a fundamental control on arc magmatism. During subduction water and other volatiles are released due to the breakdown of hydrous minerals, including lawsonite, chlorite, chloritoid, phengite and serpentine (e.g. Schmidt & Poli, 1998; Magni et al., 2014). In the oceanic crust, lawsonite, CaAl2Si2O7(OH)2·H2O, is a common hydrous mineral containing ~12 wt% water, making the metamorphic reaction (breakdown) of lawsonite an important fluid-producing reaction within subduction zones. Over recent years, it has been suggested that the liberation of oxidizing slab-derived fluids to the overlying sub-arc mantle may, at least in part, account for the oxidized and volatile-rich signatures seen in arc magmas (e.g. Kelley & Cottrell, 2009). In parallel, it has been suggested that the release of oxidizing sulfur species, such as sulfate (SOx) species, occurs during the breakdown of hydrous phases such as amphibole and lawsonite (e.g. Walters et al., 2020). Recent work combining mineral oxybarometry and Fe isotope measurements on garnet from metabasalts from Sifnos (Greece) indicates that that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions (Gerrits et al., 2019). These garnets grew during lawsonite dehydration consistent with the hypothesis that release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones.

The Western Alps are a classic subduction-related collisional orogen with well-preserved, deeply subducted remnants of oceanic lithosphere. Amongst these, the Zermatt-Saas ophiolite forms a coherent 30-km-long tectonic body that experienced P-T conditions of 550–600°C and 25–30 kbar. Zermatt-Saas ‘classical’ metabasalts (with typical N-MORB geochemical signatures) contain relict eclogite facies assemblages with mm- to cm-large garnet porphyroblasts hosting inclusions of other high-pressure phases (mainly epidote, omphacite, paragonite and quartz). Whereas, ‘Magnesian’ metabasalts, mainly found in the southern Aosta valley, preserve unusual high-pressure lithologies, including garnet glaucophanites (see Angiboust et al., 2009).

This project aims to determine the timing of garnet growth accompanying dehydration (using high precision Sm-Nd chronometry) and the redox conditions accompanying garnet growth (using mineral oxybarometry and Fe stable isotopes). With the overall aims of (i) documenting the redox nature of the fluids released during dehydration and garnet growth, and (ii) determining the timing of that release relative to the timing of high-pressure metamorphism and subduction.

Methodology

The goal of this project is to determine the nature of the fluids released by metamorphic dehydration of oceanic mafic rocks during subduction, and the timing of that volatile release recorded in the growth of garnet. This will be achieved through: (i) fieldwork in the Swiss and Italian Alps to sample mafic rocks and associated metasediments from key localities in the Zermatt-Saas ophiolite (detailed above). Followed by (ii) petrological study to determine the P-T-fO2 and fluid conditions experienced by each sample, coupled with (iii) High-precision Nd isotope measurements to determine the 147Sm-143Nd ages of garnet growth, and (iv) precise Fe isotope measurements on the same garnets, which coupled with mineral oxybarometry (above) will be used to determine any changes in redox conditions accompanying dehydration. These results will show whether the release of oxidizing species, such as sulfate, accompanying mineral dehydration plays a widespread and measurable role in the redox budget of fluids released from down-going ocean crust and how that might contribute to sub-arc mantle oxidation in subduction zones.

Project Timeline

Year 1

Training in the measurement of Nd radiogenic isotopes and Fe stable isotopes, sample petrography; Preliminary isotope measurements of mafic samples already in house, fieldwork in the West European Alps; Completion of Year 1 Research Proposal and review. Attendance of national conference.

Year 2
Selection and characterisation of Alpine samples; continued petrographic and isotope analysis of all samples; Further field sampling. Prepare research for presentation/publication. Attend national geochemistry research in progress conferences.
Year 3

Completion of isotope work and interpretation and modelling of data. Presentation at national and international conferences. Work on publication and thesis writing.

Year 3.5

Complete and submit thesis, finalise manuscripts for publication.

Training
& Skills

Training in the measurement of Fe stable and Nd radiogenic isotopes using high precision MC-ICP-MS and TIMS techniques at Durham, respectively, as well as geochemical sample characterization (trace element analysis) using laser ablation techniques.

Petrographic and mineralogical characterisation using the electron microprobe at St Andrews.

Fieldwork in the Italian and Swiss Alps.

Interpretation and modelling of petrographic and isotope data to place new constraints on the nature of the fluid, redox conditions and timing of fluid release during dehydration in subduction zones, and the implications for arc magmatism, the water and carbon cycle.

Presentation of research at both national and international geochemistry conferences.

References & further reading

Angiboust S., Agard P., Jolivet L. & Beyssac O. The Zermatt-Saas ophiolite: the largest (60-km wide) and deepest (c. 70–80 km) continuous slice of oceanic lithosphere detached from a subduction zone? Terra Nova, 21,171–180 (2009).

Gerrits A.R., Inglis E.C., Dragovic B., Starr P.G, Baxter E.F. & Burton K.W., Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet. Nature Geoscience 12, 1029–1033 (2019).

Kelley, K. A. & Cottrell, E. Water and the oxidation state of subduction zone magmas. Science 325, 605–607 (2009).

Magni, V., Bouilhol, P. & van Hunen, J. Deep water recycling through time. Geochem. Geophys. Geosyst. 15, 4203–4216 (2014).

Schmidt, M. W. & Poli, S. Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth Planet. Sci. Lett. 163, 361–379 (1998).

Walters J.B., Cruz-Uribe A.M., Marschall H.R., Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation. Geochem. Persp. Let. 13, 36-41 (2020).

Further Information

For further information please contact Kevin Burton (kevin.burton@durham.ac.uk)

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