Earth's Unstable Crust in the Archean Eon Dripped into the Mantle

About 4 billion years ago during the Earth's Archean eon, the planet's mantle temperatures were far higher. Now, scientists have learned a little bit more about this time period, revealing that the crust that formed at that time was so dense that large portions of it were recycled back into the mantle.

Because mantle temperatures were higher during the Archean eon, the Earth's primary crust must have been very thick and very rich in magnesium. Yet it seems that very little of this original crust is preserved, which means that a lot of it must have been recycled back into Earth's mantle.

In order to learn a bit more about this time period, the scientists created model calculations, including thermodynamic calculations. This allowed them to establish that the mineral assemblages that formed at the base of the 45-kilometer-thick magnesium-rich crust at this time were denser than the underlying mantle layer. The scientists then created new computer models to explore the physics when the Earth was still relatively young.

So what did they find? It turns out that the base of a magmatically over-thickened and magnesium-rich crust would have been gravitationally unstable at mantle temperatures greater than 1,500 to 1,550 degrees Celsius. This would have caused it to sink in a process called "delamination." The dense crust would have dripped down into the mantle, triggering a return flow of mantle material from the asthenosphere that would have melted to create new primary crust. In fact, continued melting of over-thickened and dripping magnesium-rich crust may have produced the hydrated magnesium-poor basalts necessary to provide a source of the tonalite-trondhjemite-granodiorite complexes. The dense residues of these processes probably now reside in the mantle.

The findings reveal a bit more about Earth's history and could tell researchers about what lies beneath our planet's surface today.

The findings are published in the journal Nature Geoscience.