A new study led by an international team of scientists, including Professors Jon Blundy and Mike Kendall, who recently joined Oxford Earth Sciences, has provided the first conclusive evidence directly linking deep Earth’s water cycle with magmatic productivity and earthquake activity.
Water, sulphur and carbon dioxide, which are cycled through the deep Earth, play a key role in the evolution of our planet – including in the formation of continents, the emergence of life, the concentration of mineral resources, and the distribution of volcanoes and earthquakes.
Subduction zones, where tectonic plates converge and one plate sinks beneath another, are the most important parts of the cycle – with large volumes of water entering the Earth’s mantle 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.
The study, published last week in Nature, was a result of the NERC-funded Volatile Recycling in the Lesser Antilles (VoiLA) consortium project, on which Professors Blundy and Kendall were leading collaborators. VoiLA brings together a large multidisciplinary team of researchers including geophysicists, geochemists and geodynamicists from the Universities of Bristol, Durham, Imperial College London and many other institutions.
The research team studied the Lesser Antilles volcanic arc at the eastern edge of the Caribbean Sea. Collecting data over two marine scientific cruises, they deployed seismic stations that recorded earthquakes beneath the seafloor and the islands, and undertook geological fieldwork, chemical and mineral analyses of rock samples, and numerical modelling.
To trace the influence of water along the length of the subduction zone, the scientists studied compositions of the element boron 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.
The ‘wettest’ parts of the plate (with the most serpentine) are where there are major cracks (or fracture zones). By making a numerical model of the history of fracture zone subduction below the islands, the researchers found a direct link to the locations of the highest rates of small earthquakes and the presence of fluids in the subsurface.
The history of subduction of water-rich fracture zones can also explain why the central islands of the arc are the largest and why, over geologic history, they have produced the most magma.
Prof. Jon Blundy, co-author on the study said: ‘Subduction zone volcanism, mineralisation and seismicity are profoundly influenced by the presence of water delivered by the subducting lithospheric plate. Understanding how the water supply varies from arc to arc and along individual arcs has long been a topic of controversy. Our new study shows that the amount and distribution of the hydrous magnesium silicate serpentine in the subducting plate exerts an overarching influence on water release, with consequences for volcanic activity, earthquakes and ore formation.’
This discovery will 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.
This news story is adapted from the University of Bristol press release here.
The Nature publication can be accessed here:
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). https://doi.org/10.1038/s41586-020-2407-5
Top Photograph: The Quill on the Caribbean island of Saint Eustatius, one of several volcanoes in the Lesser Antilles arc fuelled by fluids delivered by the underlying subducted plate. Photo credit: Jon Blundy.