Water and the Earth: Using minerals to reconstruct the compositions of fluids deep in the Earth and back to its earliest history.
Vincent van Hinsberg
The abundance of water in and on the Earth has a profound impact on the processes that shape our planet, from allowing plate tectonics to operate, to concentrating elements into ore deposits, to facilitating magmatism. Moreover, water is the likely medium in which life originated, and the early evolution of organisms therefore tightly linked to the compositional evolution of the early hydrosphere. To be able to understand how water impacts these processes it is necessary to know its composition, in particular its trace element content. At present, this information is severely lacking, especially for the deep Earth and for its earliest history, because direct fluid samples are rare and their chemical characterization non-trivial. In this presentation I will propose a different approach in which the composition of the fluid is reconstructed from that of associated minerals, based on the characteristic trace element partitioning between minerals and aqueous fluids. Unlike fluids, minerals with preserved compositions are readily available in the geological record. Moreover, minerals can simultaneously provide us with information on pressure-temperature conditions and age. I will illustrate the potential of this approach with examples on deep fluids in subduction zones and the composition of the ocean in the Archaean.