Plate tectonics: back to the drawing board
Searching for gold and then finding an even bigger treasure still
Central New Guinea hosts huge, world class quantities of gold and copper. Thanks to research into the origin of these metals, previously puzzling subterranean phenomena can now be explained, say geoscientists from Utrecht University this week in the prestigious journal Geophysical Research Letters. Besides benefitting practical applications, the research also has far-reaching consequences for one of the pillars of the geosciences: the mechanism of plate tectonics and how this has been taught for decades. ‘We now see that the Earth’s mantle under the plates does not swirl and quickly convect, but is instead more or less passive and stationary while the plates move over it and sink into it.’
The copper and gold ores in question are located in volcanoes several million years old. Chemical analyses of these ores and the rocks in which they occur reveal that they formed by the melting of mantle rock above a plate that glided under another plate during a process called subduction. ‘However, the geology of central New Guinea reveals that no subduction has occurred there in the past 50 million years. We therefore observed chemical traces of subduction in volcanoes that formed long after subduction’, says Douwe van Hinsbergen, Professor of Global Tectonics and Paleogeography at Utrecht University. He coordinated the research into the New Guinea volcanoes in collaboration with the University of Nevada, Las Vegas.
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Remains of an old plate
So van Hinsbergen and his team had to look for a different explanation. What exactly caused that melting? And where did that subduction influence come from? ‘During subduction water enters the mantle, causing a chemical change in, and the melting of, the mantle above the subducting plate. That gives rise to magmatism and the formation of a volcanic arc, a series of volcanic islands or mountains, such as the Ring of Fire around the Pacific Ocean.’ The researchers demonstrated that subduction definitely took place at the location where the volcanoes with their ores now lie, but that New Guinea was then situated 1500 kilometres farther south and had nothing to do with this. That was 25 million years ago. After that, the volcanic arc was transported with the moving plates to where it now lies in the Philippines. New Guinea, meanwhile, moved with Australia to the north and came to lie directly above the location where 25 million years ago the Earth’s mantle was “enriched” - as geoscientists call it – by the ancient subduction. Van Hinsbergen: ‘The remains of that old plate that sank away 25 million years ago can now be found using seismological techniques. Those remains now lie in the mantle, 600 km below New Guinea.’
Why did the previously enriched mantle melt again, millions of years later, as New Guinea moved over it, even though no subduction took place? ‘New Guinea lies on a continental edge and that ploughs, as it were, through that piece of mantle. So the old subduction-related liquids, which were still present in the mantle, were stirred up and initiated that melting process.’ It is therefore a different process than melting due to subduction. ‘We were somewhat fooled by the chemical composition’, says co-author Michiel van Dongen, who more than ten years ago laid the foundations of the current research when he did his PhD at Monash University (Australia) into the ores of New Guinea.
The discovery that the mantle under the present New Guinea has a different geological history than the Australian plate above it led to a striking subsequent step in the research. ‘We critically re-examined the relationship between plate movement and the movement of the underlying mantle. That proved to be different than previously thought. After all, the upper mantle under New Guinea has moved far more slowly than the plates above it.’ That observation could not be correlated with the traditional idea of the “conveyor belt movement”, in which the flow in the Earth’s mantle drags along the plates lying above it. This idea that mantle convection, the deep flow, drives plate tectonics originated almost 100 years ago. More recent calculations have revealed that the sinking away of cold, heavy plates in the hot, less dense Earth’s mantle must also play an important and perhaps even dominant role. However, even as part of this idea, it is still commonly assumed that the flow in the upper mantle under the plates could be about just as strong as movements of the actual plates, for example because the mantle is dragged along with the plate movement. ‘What our study reveals, however, is actually a surprising additional aspect, namely that the upper mantle seems to convect far slower than the plates which move over it.’ Plate tectonics with a mantle that does not convect or scarcely convects? ‘That seems to be the case. At the very least, it means that we need to go back to the drawing table with our ideas about the nature of mantle convection and even the basics of plate tectonics.’
‘That the mantle under moving plates seems to stay more or less still explains why the geochemical signature of the mantle remains the same for tens of millions of years’, adds Van Hinsbergen. Deviations in the composition of the Earth’s mantle, for example as a consequence of subduction, can therefore “survive” far longer because they do not become mixed here with surrounding materials in a strongly flowing mantle. ‘Thanks to this insight we can now also explain the subduction-related minerals found on the mid-Atlantic Ridge: they must already have been there for 200 million years. In the deep mantle, there might still be remains of materials from the earliest period in the Earth’s history, from billions of years ago, an idea that has always appealed to geochemists.’ The “New Guinea recipe” also appears to work for other areas, such as the Fiji Islands, the western edge of North America and the north-west of South America. ‘There, you can observe exactly the same processes at work.’
The latest insights could not only cause a fundamental scientific landslide within the geosciences. ‘What we have learned from New Guinea also says something about the presence of copper and gold reserves and reserves of other ores that we need for the energy transition. The realisation that the Earth’s mantle directly under the moving plates has a different geological history, with its own chemical signature that is relevant for ore formation, coupled with our ability to predict this via plate reconstructions and seismic data, could mean an enormous step forwards in the search for these ores. A search that should be conducted in a sustainable way, of course.’
The research is published in: Douwe J.J. van Hinsbergen (Utrecht University), Wim Spakman (Utrecht University), Hugo de Boorder (Utrecht University), Michiel van Dongen (Monash University, Australia), Simon M. Jowitt (University of Nevada, Las Vegas, USA), Paul R.D. Mason (Utrecht University), ‘Arc-type magmatism due to continental-edge plowing through ancient subduction-enriched mantle’, Geophysical Research Letters Vol. 47, Issue 8 (28 april 2020), https://doi.org/10.1029/2020GL087484.