Ancient volcanic rocks in Western Australia are providing evidence that Earth may have been recycling surface water into its mantle about 3.1 billion years ago, potentially through a primitive geological process that existed before modern plate tectonics developed.
An international research team led by Adelaide University geochemist Eric Vandenburg studied the exceptionally well-preserved Whundo Group in the Pilbara Craton. The volcanic rocks formed between 3.13 and 3.10 billion years ago and preserve evidence of ancient volcanic activity despite having survived billions of years of geological change.
Researchers sampled a roughly 10-kilometre thick sequence of volcanic rocks and identified three major types: tholeiites, calc-alkaline basalts and boninites. The boninites are particularly important because these rare magnesium rich volcanic rocks are strongly associated with water rich mantle conditions. Today, boninites are generally produced in environments where one tectonic plate sinks beneath another.
Chemical analysis of the ancient rocks indicates that the mantle source that produced the Whundo boninites contained an estimated 0.8 to 1.5 per cent water by weight. That is substantially higher than estimates for primitive and depleted mantle and is within the range found beneath some modern volcanic arcs.
Water can significantly lower the melting temperature of mantle rocks. This means that the presence of substantial amounts of water deep underground can promote melting and volcanic activity. The findings therefore suggest that surface water was somehow transported into Earth mantle during the Archean eon which lasted from about 4 billion to 2.5 billion years ago.
The discovery raises a major geological question: how could water have reached such depths before conventional plate tectonics became established?
The researchers suggest a process known as “dripduction”. Under this model, sections of water-rich crust on the young Earth could have become dense enough to sag and sink into the hotter mantle. Unlike modern subduction, where large tectonic plates descend beneath one another this process would have involved localized pieces of crust sinking downward under gravity.
According to the study calculations, fluids released from the sinking material may have supplied as much as 93 per cent of certain trace elements found in the mantle source of the boninites. Similar chemical signatures occur throughout the volcanic sequence, suggesting that the process may have continued for tens of millions of years.
The findings could also help explain why relatively little Archean crust survives today. Researchers estimate that a large proportion of the evolved crust produced during Earth’s early history may have disappeared through geological recycling. Ancient continental cores, known as cratons, now represent only a small fraction of the continents.
However, the study does not establish that modern plate tectonics was already operating in the Pilbara 3.1 billion years ago. Evidence for subduction during the Archean remains debated and the researchers acknowledge that alternative geological processes could potentially explain some of the chemical characteristics observed in the rocks.
The evidence instead points to a more specific conclusion at least in the Whundo region, surface derived water was reaching mantle melting depths and influencing volcanic activity billions of years ago.
The exceptionally preserved rocks therefore provide a rare window into Earth’s early geological history. They suggest that some of the planet’s fundamental processes of water circulation crustal recycling and mantle melting may have begun long before the modern plate tectonic system took shape.