Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

The nature of the deep water cycle, in which water moves from Earth's surface down into the mantle and then back to the surface continues to be debated. It is widely held that only part of water transported from the surface in subduction zones returns to the surface by direct expulsion at oceanic trenches and volcanism, and that a significant amount of water is carried into the mantle transition zone (410–660 kilometres) or even the lower mantle. To test this 'wet lithosphere' model, Harry Green II and colleagues compare patterns of seismicity and the stabilities of potentially relevant hydrous phases determined from experimental and seismic evidence. They conclude that the evidence requires a very dry lithosphere below 400 kilometres, suggesting that subducting slabs do not provide a pathway for significant amounts of water to enter the deep mantle. These authors test whether patterns of seismicity and the stabilities of potentially relevant hydrous phases are consistent with a wet lithosphere. They show that there is nearly a one-to-one correlation between the dehydration of minerals and seismicity at depths less than ∼250 km, but no correlation at greater depths. They conclude that subducting slabs must be essentially dry by 400-km depth and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle. Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10–12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism1. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine’s high-pressure polymorphs, wadsleyite and ringwoodite2, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle3 (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes4,5,6,7) west of and within the subducting Tonga slab8 and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight7). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.