Mapping the chemistry of the Earth’s mantle

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Publication of the LGL-TPE in the journal PNAS on November 21, 2022. CNRS-INSU communication on December 7, 2022.

In the 1980s, geochemical observations led researchers to propose a geodynamic model of the mantle’s marbled structure. A team of scientists, some of whom are attached to the Laboratory of Geology of Lyon: Earth, Planets, Environment (LGL-TPE, CNRS/ENS de Lyon/Claude Bernard Lyon 1 University), has been working on this model using a seismological perspective.

There is no direct access to the deep Earth mantle. Therefore, insights into the deep mantle composition rely on indirect observations from geophysics. We exploit seismic datasets of compressional and shear waves reflected at mineralogical phase transitions globally and reveal the existence of anomalous geochemical reservoirs in the mantle transition zone (MTZ). This first-order compositional pattern is linked to the 100 My old history of tectonic subduction. The existence of basaltic reservoirs and their estimated volumes require mechanisms that allow segregation of the subducted crust from the harzburgitic mantle at mid-mantle depths, regardless of the age of the subducted oceanic lithosphere. Our new constraints on compositional patterns in the MTZ help to understand heat and material flow through the mantle.

The formation and preservation of compositional heterogeneities inside the Earth affect mantle convection patterns globally and control the long-term evolution of geochemical reservoirs. However, the distribution, nature, and size of reservoirs in the Earth’s mantle are poorly constrained. Here, we invert measurements of travel times and amplitudes of seismic waves interacting with mineralogical phase transitions at 400’700 -km depth to obtain global probabilistic maps of temperature and bulk composition. We find large basalt-rich pools (up to 60% basalt fraction) surrounding the Pacific Ocean, which we relate to the segregation of oceanic crust from slabs that have been subducted since the Mesozoic. Segregation of oceanic crust from initially cold and stiff slabs may be facilitated by the presence of a weak hydrated layer in the slab or by weakening upon mineralogical transition due to grain-size reduction.

Reference : Basaltic reservoirs in the Earth’s mantle transition zone . Benoit Tauzin, Lauren Waszek, Maxim D. Ballmer and Thomas Bodin. PNAS, November 21, 2022.
DOI: 10.1073/pnas.2209399119