Diapirism of carbonate platforms subducted into the upper mantle

Subduction of lithospheric plates at convergent margins leads to transport of materials once close to or at the surface of Earth to great depths. Some of them later return to the surface by magmatism or degassing, whereas others end up being stored in the mantle for long periods of time. The fate of carbon-bearing minerals in subduction is of particular interest because they can arbitrate the long-term availability of CO₂ at the surface. However, there are major gaps in the understanding of even the most fundamental processes that modulate carbon pathways at mantle depths. We use geodynamic models to understand carbonate pathways upon subduction in the form of large carbonate platforms, which were common in the Tethys realm of Europe. We conducted a series of geodynamic forward models for a 1-km-thick carbonate platform entering subduction. We show that most of the carbonate load detaches from the subducting slab and rises up diapirically through the mantle wedge and eventually mixes with the mantle lithosphere. A smaller fraction gets accreted under the forearc, whereas an even smaller fraction descends deeper into the mantle. The cold diapiric plume has a significant role in retarding silicate mantle melting above these subduction zones and promoting the formation of small-volume carbonate-rich melts and, in some cases, alkaline silica-undersaturated silicate melts. We propose that large amounts of CO₂ can be stored as carbonate in the shallow uppermost lithospheric mantle.