Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust, and mineral resources.
Subduction zones, where tectonic plates converge and one plate sinks beneath another, are essential parts of the cycle – with large volumes of water going in and coming out, mainly through volcanic eruptions. Yet, just how (and how much) water is transported via subduction and its effect on natural hazards and the formation of natural resources has historically been poorly understood.
Exactly where and how volatiles is released and how they modify the host rock remains an area of intense research. Most studies have focused on subduction along the Pacific Ring of Fire. However, new research by the University of Bristol has focused on the Atlantic plate, and more specifically, the Lesser Antilles volcanic arc, located at the eastern edge of the Caribbean Sea.
The study offers first conclusive evidence directly linking deep Earth’s water cycle with magmatic productivity and earthquake activity.
Lead author of the study, Dr. George Cooper, Honorary Research Fellow at the University of Bristol’s School of Earth Sciences, said:
“As plates journey from where they are first made at mid-ocean ridges to subduction zones, seawater enters the rocks through cracks, faults, and by binding to minerals. Upon reaching a subduction zone, the sinking plate heats up and gets squeezed, resulting in the gradual release of some or all of its water.”
“As water is released, it lowers the melting point of the surrounding rocks and generates magma. This magma is buoyant and moves upwards, ultimately leading to eruptions in the overlying volcanic arc. These eruptions are potentially explosive because of the volatiles contained in the melt. The same process can trigger earthquakes and may affect key properties such as their magnitude and whether they trigger tsunamis or not.”
For the study, scientists collected data over two marine scientific cruises on the RRS James Cook, temporary deployments of seismic stations that recorded earthquakes beneath the islands, geological fieldwork, chemical and mineral analyses of rock samples, and numerical modeling.
They then traced the influence of water along the length of the subduction zone by studying boron compositions and isotopes of melt inclusions (tiny pockets of trapped magma within volcanic crystals). Boron fingerprints revealed that the water-rich mineral serpentine, contained in the sinking plate, is a dominant supplier of water to the central region of the Lesser Antilles arc.
Prof. Colin Macpherson, Durham University, said, “By studying these micron-scale measurements, it is possible to understand large-scale processes better. Our combined geochemical and geophysical data provide the clearest indication to date that the structure and amount of water of the sinking plate are directly connected to the volcanic evolution of the arc and its associated hazards.”
Prof. Saskia Goes, Imperial College London, said, “The wettest parts of the downgoing plate are where there are major cracks (or fracture zones). By making a numerical model of the history of fracture zone subduction below the islands, we found a direct link to the locations of the highest rates of small earthquakes and low shear wave velocities (which indicate fluids) in the subsurface.”
Dr. Cooper said, “Our study provides conclusive evidence that directly links the water-in and water-out parts of the cycle and its expressions in terms of magmatic productivity and earthquake activity. This may encourage studies at other subduction zones to find such water-bearing fault structures on the subducting plate to help understand patterns in volcanic and earthquake hazards.”
Prof. Colin Macpherson said, “In this research, we found that variations in water correlate with the distribution of smaller earthquakes, but we would like to know how this pattern of water release may affect the potential – and act as a warning system – for larger earthquakes and possible tsunami.”
- Cooper, G.F., Macpherson, C.G., Blundy, J.D., et al. Variable water input controls evolution of the Lesser Antilles volcanic arc. Nature 582, 525–529 (2020). DOI:10.1038/s41586-020-2407-5