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Water stored in the mantle for millions of years may be linked to continental volcanism

The mantle transition zone (MTZ), which occurs 410–670 kilometers below Earth's surface, may store several oceans' worth of water. This water, which is carried to such depths by subducting tectonic slabs, is stored in minerals like ringwoodite and wadsleyite.

The distribution of water in the MTZ, both today and in the past, is not fully understood or mapped. However, because hydrated slabs enter the mantle in different locations and with different speeds, shapes, and sizes, researchers expect that water isn't distributed evenly throughout the zone.

Intraplate volcanism, or volcanism occurring away from plate boundaries, can provide clues about which areas of the MTZ are the most hydrated. Some of this volcanism occurs when water-rich mantle upwellings trigger the melting of mantle rocks, forming magma that can erupt.

Helene Wang and colleagues used plate reconstructions from the past 400 million years to estimate where subducting slabs may have helped transport water into the MTZ. The researchers then compared these maps of mantle water to locations where intraplate volcanism occurred over the past 250 million years. The study is in the journal Geochemistry, Geophysics, Geosystems.

Their findings showed a strong correlation between wet areas of the MTZ and continental intraplate volcanism, with 42%–68% of intraplate volcanism occurring over more-water-saturated MTZ locations around the globe. The correlation is stronger in locations where water has remained in the MTZ for 30–100 million years; thus, the researchers suggest, a long timescale with multiple subduction events is needed to hydrate the MTZ and possibly trigger intraplate volcanic activity.

The link between water storage in the MTZ and continental intraplate volcanism could provide an explanation for minor volcanic activity across eastern Asia, western North America, and eastern Australia, as well as global intraplate volcanism patterns over the past 200 million years. By contrast, the Indian Ocean, southeast Africa, and the south Atlantic Ocean sit above swaths of the MTZ that have been dry for the past 400 million years, which may have contributed to the lack of volcanic activity in these regions, the researchers say.