Subduction of marine carbonates at the origin of carbonatitic magmatism for over 3 billion years

By showing that the calcium carbonate in these carbonatites has an isotopic signature typical of marine sediments, the team has shed light on a phenomenon whereby marine carbonates have been recycled into the Earth’s mantle by subduction for at least 3 billion years.

Some volcanoes emit, or have emitted in the past, a particular kind of magma, rich in carbonate minerals. This is currently the case with the Ol Doinyo Lengai volcano in Tanzania, but there are many ancient volcanoes (some dating back more than 3 billion years) that have produced this type of carbonatitic lava. While the majority of magmatic rocks are rich in silicate minerals, carbonatites are made up of more than 50% carbonate minerals, including calcium carbonate.

The origin of these rocks, which have existed for several billion years, remains a key issue in the Earth sciences, particularly in the study of the composition of the Earth’s mantle. This is because magmatic rocks, produced by the partial melting of the mantle and emitted at the surface, offer one of the rare opportunities to study the chemical composition of this vast and deep reservoir, which is difficult to sample.

Until now, the hypotheses put forward to explain this high carbonatite content have suggested either a primitive source in the mantle, rich in carbonate elements, or the recycling of surface compounds via the subduction phenomenon.
A team from the IPGP has developed a new isotopic method for testing the origin of calcium, the most abundant element in carbonatites. In a study published in Science Advances on 3 June 2020 in collaboration with researchers from the École Normale Supérieure de Lyon, the University of Lisbon, the University of Johannesburg and the University of California at San Diego, they have shown that the calcium isotopic composition of many carbonatites is typical of marine carbonates. This signature is different from all other magmatic rocks and therefore represents a particularity of carbonatites. The presence of this signature can only be explained by the melting of a mantle rich in carbonates recycled by the subduction of the oceanic lithosphere.

This study therefore links carbonatite formation, terrestrial subduction and plate tectonics. The oldest carbonatites analysed in this study date back 3 billion years, so these results confirm the existence of terrestrial subduction since at least that time.

Ref: E. Amsellem, F. Moynier, H. Bertrand, A. Bouyon, J. Mata, S. Tappe, J. M. D. Day, Calcium isotopic evidence for the mantle sources of carbonatites. Sci. Adv. 6, eaba3269 (2020).