Mud Volcanoes as windows to subduction zone processes; slab devolatilisation and mantle serpentinisation in the Mariana forearc

Overview

Plate boundaries, are the principal regions on Earth where the greatest and most rapid geological changes occur, including catastrophic earthquakes and volcanic eruptions. Subduction zone plate boundaries control the resurfacing of the ocean floor every ~200-270 million years. During subduction, materials from the Earth’s surface are transported to the mantle, inducing changes in the surrounding mantle rocks, and facilitating melting that drives volcanism in arcs. Volcanism above subduction zones, and where new crust is formed at ocean spreading centres is the main route for material in the mantle to return to the Earth’s surface. These zones fundamentally control the plate tectonic cycle and the composition of the crust, atmosphere, and hydrosphere, and moderate climate over geological timescales.
As the lithosphere subducts, pressures and temperatures increase. Initially sediment compaction releases pore waters to the decollement and forearc mantle wedge, followed by dewatering of clays and altered oceanic crust, carbonate dissolution and decarbonation during prograde prehnite-pumpellyite-greenschist-blueschist metamorphism (Fig. 1). The mantle is far from equilibrium with the Earth’s surface such that the release of these waters drives serpentinisation; serpentinites are less dense than the other mantle rocks and in the Mariana forearc this density discrepancy activates movement of this newly formed rock, driving it to move upwards where it erupts from mud volcanoes on the seafloor. Such mud volcanoes are up to 50 km across and 3 km high (Fryer, 1996) and are located at various distances above the subducting slab (Fig. 2). Mud Volcanoes act as telescopes into the shallow subduction zone as they carry samples of fluid and rock from the subducting slab, forearc mantle and overlying plate to the seafloor. This provides an excellent opportunity to sample a shallow subduction zone across a range of depths to subducting slab to evaluate processes controlling plate dehydration and fluid rock interaction in the mantle wedge.
A suite of unique samples from IODP Expedition 366 that drilled into these mud volcanoes will be the focus of this study. These samples include erupted serpentinite muds and xenoliths from different parts of the subducting slab, forearc mantle and forearc crust, providing a cross section through the mantle wedge at an active subduction zone across a range of depths to slab.
Aims of this project:

a. To determine fluid sources and the controls on slab devolatilization and slab fluid chemistry across a range of depths of subducting slab
b. To determine fluid-rock interactions that controlled fluid chemistry and metasomatism in the mantle wedge
c. To calculate a fluid budget of slab devolatilization and fluid flow in the mantle wedge across the Mariana Forearc

Click on an image to expand

Image Captions

Fig1.png – Figure 1 A summary diagram of the Mariana Forearc devolatilisation and serpentinisation chemistry of mud volcano pore fluids, modified from Fryer et al (1999) and Fryer et al (2017), including data from Hulme et al (2010).
Fig2.jpg – Figure 2 Bathymetry of the Mariana forearc region showing Mud Volcano (MV) summit sites, after Fryer (2017).
Fig3.png – Figure 3 LA-ICP-MS data of Li, B and Sr concentrations in complexly zoned serpentinite veins in a xenolith from a Mariana mud volcano. Brc = brucite; Atg = antigoire; Fo = forsterite; Chr/Liz = chrysotile/lizardite; Di = diopside; En = enstatite. Data: Albers et al., 2019

Methodology

Characterisation of serpentinite muds and slab and mantle xenoliths, and veins within these xenoliths. 1) Detailed petrography and mineralogy of rock and mud samples to characterise differences between mud volcanoes (and therefore depth to subducting slab). 2) Serpentinite muds will be analysed for stable oxygen, hydrogen and zinc isotopes, and radiogenic strontium isotopes to assess fluid sources and the fluid-rock interactions ongoing in the mantle wedge. 3) Xenoliths of ultramafic rock contain abundant cross-cutting serpentinite veins that have variable trace element chemistry (Fig. 3) and these will be targeted by stable O and H and radiogenic 87Sr/86Sr isotopes to build a picture of progressive serpentinisation. 4) Chimney material from three MVs (one brucite and two carbonate) will be analysed to compare with published pore fluid analyses and to investigate the processes occurring during hydrothermal fluid and seawater mixing. 5) Geochemical modelling of data generated to build a model of the controls on progressive devolatilisation of the subducting Pacific Plate in the Maraiana convergent margin.

Project Timeline

Year 1

Thorough literature review; training in elemental and isotopic (Zn, 87Sr/86Sr, O, H, C) measurements, sample petrography and preliminary elemental and isotope measurements. Training in Geochemist’s Workbench and Thermocalc modelling software.

Year 2

Selection and characterisation of key samples based on preliminary data; further isotopic measurements and method development. Start of build a geochemical model based on preliminary data. Prepare preliminary findings for presentation at an International Conference. Visit Prof. Patty Fryer at the University of Hawaii to discuss results and take samples from her career-long sample collection from the Mariana convergent margin.

Year 3

Complete isotope analyses. Build a geochemical model to explore the controls on devolatilisation in the Mariana convergent margin. Presentation of data at an international conference.

Year 3.5

Finalise geochemical model; writing up; preparation of manuscripts for publication

Training
& Skills

1. Training in the preparation of geological materials for geochemical analyses and measurement of novel stable isotopes using high precision MC-ICP-MS and TIMS at Durham.
2. Training in analyses of H and O stable isotopes in silicates at SUERC.
3. Geochemical modelling using Geochemist’s Workbench and Thermocalc modelling software
4. Presentation of research at national and international conferences
5. Training in writing skills through detailed feedback on manuscripts and thesis drafts

References & further reading

Albers, E., Bach, W., Klein, F., Menzies, C. D., Lucassen, F., and Teagle, D. A. H., 2019, Fluid-rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions: Solid Earth Discuss., v. 2019, p. 1-41.
Fryer, P., Wheat, C. G., and Mottl, M. J., 1999, Mariana blueschist mud volcanism: Implications for conditions within the subduction zone: Geology, v. 27, no. 2, p. 103-106.
Fryer, P., Wheat, C.G., Williams, T., Albers, E. Bekins, B., Debret, B.P.R., Deng, J., Dong, Y., Eickenbusch, P., Frery, E.A., Ichiyama, Y., Johnson, K., Johnston, R.M., Kevorkian, R.T., Kurz, W., Magalhaes, V., Mantovanelli, S.S., Menapace, W., Menzies, C.D., Michibayashi, K., Moyer, C.L., Mullane, K.K., Park, J.-W., Price, R.E., Ryan, J.G., Shervais, J.W., Sissmann, O.J., Suzuki, S., Takai, K., Walter, B., and Zhang, R., 2018. Expedition 366 summary. In Fryer, P., Wheat, C.G., Williams, T., and the Expedition 366 Scientists, Mariana Convergent Margin and South Chamorro Seamount. Proceedings of the International Ocean Discovery Program, 366: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.366.101.2018
Hulme, S. M., Wheat, C. G., Fryer, P., and Mottl, M. J., 2010, Pore water chemistry of the Mariana serpentinite mud volcanoes: A window to the seismogenic zone: Geochemistry, Geophysics, Geosystems, v. 11, no. 1.
Maekawa, H., Shozul, M., Ishll, T., Fryer, P., and Pearce, J. A., 1993, Blueschist metamorphism in an active subduction zone: Nature, v. 364, no. 6437, p. 520-523.
Mottl, M. J., Wheat, C. G., Fryer, P., Gharib, J., and Martin, J. B., 2004, Chemistry of springs across the Mariana forearc shows progressive devolatilization of the subducting plate: Geochimica et Cosmochimica Acta, v. 68, no. 23, p. 4915-4933.

Further Information

Contact Dr Catriona Menzies (catriona.d.menzies@durham.ac.uk; tel:+44 (0)191 334 4603) for further details
https://iodp.tamu.edu/scienceops/expeditions/mariana_convergent_margin.html

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