During subduction, in which one tectonic plate dives beneath another, carbon is transferred from the surface into deep Earth. This massive geological process creates unique conditions, with aqueous fluids and subducting rocks encountering each other as temperature and pressure increase. In a new paper published in Nature Communications, DCO’s Dimitri Sverjensky and Fang Huang of Johns Hopkins University, USA use the Deep Earth Water (DEW) model to probe diamond formation during subduction .
The DEW model was developed by Sverjensky, building on the groundbreaking elucidation of the dielectric constant of water up to 10 GPa by DCO colleague Giulia Galli (University of Chicago, USA) , and direct measurements of carbon speciation in water at high pressures by DCO colleague Isabelle Daniel (Université Claude Bernard 1 and ENS, Lyon, France). By using this model, Sverjensky and colleagues addressed a series of questions pertaining to the behavior of carbon in aqueous solutions in deep Earth. Such questions were impossible to answer previously.
In this most recent contribution, Sverjensky and Huang showed that diamond formation is possible in subducting fluids as a direct consequence of a drop in fluid pH. The model also shows the potential for repetitive diamond formation and dissolution. Furthermore, their data suggest that organic carbon species within subducting fluids may play important roles in these reactions. Sverjensky et al  only recently suggested the idea of organic carbon species contributing to diamond formation.
As a first step to check the model against real world samples, Sverjensky and Huang compared their predictions with fluid inclusions from diamonds found at different locations. Their data broadly align with inclusion characteristics, however some discrepancies remain.
“That diamond can form or dissolve by pH changes without a change in redox conditions is an exciting development for the Deep Carbon Observatory,” said Sverjensky. “The DEW model is a powerful tool for future modeling of the amount of carbon in Earth, and how that carbon is transported from surface to mantle reservoirs and back again.”
The DEW model is available for download here.
Related news: How buried water makes diamonds and oil by Eric Hand, Science Magazine
Image: Visualization by Ding Pan.