Trace elements speciation under supercritical conditions: A diagnostic tool for understanding deep Earth carbon cycle

Trace elements are diagnostic tools of redox conditions of their environment. Their mobility and isotopic fractionation are of great interest in the study of the evolution of the crust and mantle. A recent revelation that "arc volcanoes" have different trace elements fingerprint compared to the "hotspot volcanoes" (Edmonds et al., 2018) opens up new frontiers of research needed to understand the deep Earth carbon cycle.

The arc magmatism is the result of subduction, whereas hotspot magmatism is due to the generation of magma in the mantle. The subduction fluids are mostly hydrous. Since the dielectric constant of water is different under these extreme conditions, the ionic interactions are more complicated. Such ionic interactions change the "activity coefficient" of the ions. As the subduction slab moves further and further down, the water content becomes less and less, and the dielectric constant also undergoes significant changes. The application of  Debye–Hückel theory to such fluids poses operation problems as "surface charges" and the "electric field strength" of the ions may add additional components in the description of ionic interactions. Further, the dissolution of carbonates and prevailing P-T conditions can make supercritical subduction fluids rich in CO₂. Laboratory experiments designed to unravel trace elements speciation and their isotopic fractionation under these extreme conditions can provide vital information necessary to understand the deep Earth carbon cycle - because redox conditions in the mantle can affect it.