Study finds seawater is deeply circulated throughout the Earth's mantle

Tuesday 28 February 2017

Our findings make alternative theories for the origin of the atmosphere and oceans equally plausible, such as icy comets or meteorites bringing water to the Earth.

Dr Mark Kendrick with a sample of volcanic glass. Image: Stuart Hay, ANU.

A new study led by The Australian National University (ANU) has found seawater cycles throughout the Earth's mantle down to 2900 km, much more deeply than previously thought possible.

Seawater is introduced into the Earth's interior when two tectonic plates converge and one plate is pushed underneath the other into the mantle.

The study has overturned the notion that seawater only makes it about 100km into the mantle before it is returned to the Earth's surface through volcanic arcs, such as those forming the Pacific Ring of Fire that runs through the western Americas, Japan and Tonga.

The team analysed samples of volcanic glass from mid ocean ridges and oceanic islands in the Atlantic, Pacific and Indian oceans, and showed they contained traces of seawater that had been deeply cycled through the Earth's interior.

The transfer of seawater into the deeper mantle is made possible by cold serpentinised lithosphere in the base of subducting slabs, which bypasses volcanic arcs before complete dehydration.

"The involvement of serpentinites is revealed by the high water and halogen contents of parts of the mantle with trace element signatures otherwise characteristic of dehydrated ocean crust," said Dr Kendrick from the ANU Research School of Earth Sciences.

The work provides a new model for the origin of the HIMU mantle reservoir and reopens questions about the origins of water and gases in Earth’s mantle and surface reservoirs.

ANU collaborated on the study with the University of Tasmania, Institut Universitaire Europeen de la Mer in France, the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany, and the University of California Santa Barbara and the University of Florida in the United States.

The study is published in Nature Geoscience.

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