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The crustal origin of certain mantle heterogeneities finally demonstrated

The composition of the Earth's mantle is partly determined by chemical and isotopic analyses of lavas from different tectonic contexts. While basalts from mid-ocean ridges are chemically and isotopically homogeneous, reflecting mixing in the underlying mantle by convection, basalts from certain oceanic islands retain heterogeneities from a deeper mantle.

The crustal origin of certain mantle heterogeneities finally demonstrated

Ofu island (Samoa) © Adobe Stock

Publication date: 20/09/2021

Press, Research

Related themes : Origins

The latter, whose origin is still debated today, are commonly considered to be either samples of primitive material that escaped mixing, or isotopically different material from the Earth’s surface that was recycled by subduction within the deep mantle.

Mercury is one of the rare chemical elements whose isotopes are fractionated independently of mass by photochemical effects at the Earth’s surface. These effects are immutable and can no longer be modified by processes occurring at depth. Mercury isotopes can therefore be used as tracers to detect the presence of surface material within the source of terrestrial lavas. These photochemical effects create distinct signatures in oceanic and continental materials, making it possible to determine the origin of material recycled in the mantle.

This is what a team led by scientists from the IPGP-Université de Paris has succeeded in demonstrating by identifying this type of isotopic signature in series of lavas from oceanic islands. The researchers have demonstrated for the first time that mercury present deep in the Earth’s mantle was previously found at the Earth’s surface, testifying to the recycling of surface material in the lower mantle.

They also revealed that certain lavas from the island of Tubuai contain so-called ‘continental’ mercury, indicating the incorporation of material from the continental crust into their source mantle. Other lavas analysed on certain Pitcairn and Samoa islands contain so-called ‘oceanic’ mercury, revealing the incorporation of material from the oceanic crust. These results highlight the crustal origin of at least some of the mantle heterogeneities, as well as the continental or oceanic source of the recycled material.

Ref : Moynier, F., Jackson, M. G., Zhang, K., Cai, H., Halldórsson, S. A., Pik, R., et al. (2021). The mercury isotopic composition of Earth’s mantle and the use of mass independently fractionated Hg to test for recycled crust. Geophysical Research Letters, 48, e2021GL094301. DOI : 10.1029/2021GL094301

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