Damaged tectonic plates underlying large igneous provinces heal themselves during cooling new study shows

Researchers at the Department of Earth Sciences, University of Oxford, together with Imperial College London and Colorado State University, have revealed how mantle plumes that result in very large magmatic eruptions damage the tectonic plate but that it will heal again over time.

The Earth’s tectonic plates are composed of crust and a denser and generally thicker lower layer called the lithospheric mantle.  The thickness, structure, and evolution of this lower layer is critical for determining where and when mineral resources develop over time and the composition of volcanic rocks erupted at the surface.  It can even indirectly influence the climate because it stores climate altering elements that can be disturbed by magmatic eruptions.

This study, published in Science Advances, addresses a paradox.  Scientists have long known that a thin tectonic plate is a pre-requisite for large amounts of magma to be erupted.  A bit like water boiling at lower temperatures at the top of a mountain where air pressure is lower, the hot, solid mantle melts when it is brought closer to the surface and to lower pressures.  However, while young volcanic provinces overlie thin plate, old provinces more often than not sit on top of a tectonic plate that is far too thick to allow the mantle to melt.

Lead author Dr. Simon Stephenson said ‘The tectonic plate acts as a lid that stops the mantle melting, it is only when it is thinner than 100 km that melting occurs.  It makes sense then, that beneath modern volcanoes the tectonic plate is very thin, however when we look at ancient volcanoes – including these unimaginably large eruptions called large igneous provinces – the tectonic plate is very thick. Why should the plate be thin beneath young volcanoes but not old ones?’

The study, which uses both geochemical and seismological observations, has found that when there is a big magmatic event, a hot mantle plume thins the lithospheric mantle from below.  When this plume fades or the plate moves away, the lithospheric mantle cools and thickens again over time.  This process hides the fact that the plate was ever thin.

The authors investigated the location and timing of giant magmatic eruptions called large igneous provinces.  They explored the thickness of the tectonic plate beneath these provinces and found that it gets progressively thicker the longer ago the large igneous province was erupted.  They showed that this pattern can be explained by a model that simulates mantle cooling over time.

‘The study is actually quite simple’ said Stephenson, ‘but it has quite important implications for how we understand the stability of the lithospheric mantle.  It has been a widely held view that when a  plate is damaged by a hot mantle plume, the lithospheric mantle becomes permanently scarred.  We show that simply by cooling, the damage actually heals over time leaving very little evidence or scarring.  It was only by looking at a global dataset of all of these large igneous provinces together that we spotted this systematic thickening after eruption stops’.

The work may help in locating mineral resources since predicting how the thermal structure of the tectonic plate – and in particular the lithospheric mantle – evolves through time is an important step in determining prospectivity.  ‘The exciting thing is that this provides a new tool to predict and model the thermal evolution of the tectonic plate in regions where this was not possible before’ Stephenson said.  ‘Old, cold lithospheric mantle also contains a lot of nasty elements like chlorine that damage the climate.  If the lithospheric mantle is damaged during these rare, but catastrophic, eruptions then it might go some way to explaining why mass extinction events often occur at the same time as these massive outpourings of magma’

Simon Stephenson is funded by Geoscience Australia’s Exploring for the Future program and Fred Richards is funded by an Imperial College Research Fellowship.

You can read the full paper here

For more information contact Simon Stephenson