Researchers at the Geodynamics Research Center (GRC) at Ehime University have discovered a significant influence of aluminum on the sound speeds of superhydrous phase B, a dense hydrous magnesium silicate that may act as a reservoir for water deep within the Earth. Their findings indicate that this aluminum-rich phase B could provide an explanation for seismic velocity discrepancies observed in the Earth’s mantle transition zone and the uppermost part of the lower mantle.
The research team from Ehime University’s Geodynamics Research Center (GRC) uncovered a significant impact of aluminum on the acoustical velocities of superhydrous phase B—a type of dense hydrous magnesium silicate that could store water deep in the Earth. Their study utilized X-ray synchrotron radiation alongside ultrasonic measurements conducted in a large volume press apparatus, leading to the insight that aluminous phase B might clarify what causes seismic velocity anomalies in the mantle transition zone and the uppermost lower mantle.
Dense Hydrous Magnesium Silicates (DHMSs) are typically viewed as the main transporters of water from the shallow lithosphere to the mantle transition region (MTR), which spans depths of 410 to 660 kilometers. Superhydrous phase B (referred to as SuB) has the chemical formula Mg10Si3H3O18 and is thought to accommodate a significant amount of water. This attribute may play a crucial role in the water retention capabilities of the MTR and the movement of water deeper into the Earth’s interior. However, due to its relative instability under the extremely high temperatures present in the mantle, SuB is often linked with colder zones, such as the deeper sections of subducted tectonic plates. A recent experimental study from Ehime University revealed that incorporating aluminum into SuB greatly enhances its thermal stability (Kakizawa et al., AmMin 2018), allowing it to withstand the pressure and temperatures typical of the lower mantle.
In 2022, the same research team at Ehime University reported on the longitudinal (VP) and shear (VS) wave velocities of SuB (Xu et al., GRL 2022) by utilizing X-ray and ultrasonic techniques within a multi-anvil apparatus at the SPring-8 synchrotron facility in Japan. Their earlier findings indicated a correlation between the presence of SuB and the reduced seismic velocities found in regions with subducted slabs. In their most recent study, they conducted similar high-pressure and high-temperature assessments on SuB samples that had been infused with aluminum. The new results indicate that aluminum incorporation causes notable changes in velocity, particularly as water content increases, compared to SuB without aluminum.
This latest discovery, alongside evidence that aluminum improves the temperature stability and water storage capacity of SuB, implies that the aluminum-containing version of SuB may explain visible seismic anomalies detected at the base of the MTR and beneath subduction zones in the uppermost lower mantle. These findings are expected to significantly aid in tracking the existence and recycling of previously hydrated crustal materials within the Earth’s lower mantle, as well as in understanding seismic velocities in relation to mantle composition and estimating the amount of water potentially transported into the deep mantle.
