Ancient Pilbara volcanic rocks push Earth's water recycling back to 3.1 billion years

The geochemistry and the proposed process are two different claims, and they do not carry the same weight. Rocks can record whether the magma that fed a volcano came from a source that already held water. The Adelaide-led team says it did, and that the resulting volcanoes resemble the arcs of the Pacific Ring of Fire today ^[3]. How the water got down there is a separate question. Dripduction is the answer the team proposes: dense, water-rich pieces of the cooler outer crust sagged down into hotter mantle and released their water as they sank ^[5].

Eric Vandenburg, the Adelaide University geochemist who led the work, keeps those apart. "What surprised us was finding evidence that large amounts of water had already made their way deep into the Earth's interior and influenced the formation of volcanic rocks," he said ^[7]. On the tectonic setting he was more careful. "The early Earth was too hot for plates to behave that way, so until now it has been unclear whether surface water could have made that journey more than three billion years ago, and if so, how," he said ^[6].

The team had to work with what survives. Rocks of this age are extremely rare, and the Pilbara holds some of the best-preserved early Earth material ^[11]. Chemical signatures locked inside them let the team reconstruct events back to roughly 3.1 billion years ^[4]. There is no second Earth to act as a control, so the comparison is with modern subduction, where one plate sinks beneath another and carries ocean water toward the mantle to help generate magma ^[10]. Which chemical signatures produced the result is not in the Adelaide University summary of the work ^[1].

The evidence sets a floor. Water was in the magma source at or before about 3.1 billion years ago, which makes that age a minimum for surface-to-interior recycling ^[2]. It does not date the onset of plate tectonics; Vandenburg's claim there is relative, and it rests on early Earth being too hot for plates to behave as they do now ^[6]. Dripduction is also episodic by the team's own description, and a plate sinking steadily beneath another one is a different picture ^[5].

The rocks show a wet source. How much water, and how often the drips happened, are not in the result.

The team says it matters because this recycling affects volcanic activity, the growth of continents, and the movement of ingredients important for life ^[13]. Ingredients important for life is a statement about chemistry. The study is about where water went, and the summary says Earth may have been recycling it between surface and interior much earlier than researchers had realised ^[14]. "The Earth wasn't operating exactly as it does now, but it appears some of the key processes were already in place," Vandenburg said ^[8]. The paper is in Nature Communications ^[2], with contributors from Monash University, the Geological Survey of Western Australia and Curtin University ^[12].