By Craig M. Schiffries, Geophysical Laboratory, Carnegie Institution of Washington
Under the auspices of the Deep Carbon Observatory, Dimitri Sverjensky (Johns Hopkins University, Baltimore, USA) and an international group of collaborators have published a series of papers that is revolutionizing our understanding of the chemistry of water-rock interactions occurring under the extreme pressure and temperature conditions of Earth’s interior.
As quoted in Science, Sverjensky said, “I’m inventing a new kind of chemistry of water-rock interactions at high pressures. It simply hasn’t been possible to model any of these processes in the deep Earth” (Hand, 2015). The cornerstone of this endeavor is the Deep Earth Water model (Sverjensky, Harrison, and Azzolini, 2014), which can be downloaded at no cost from the DCO website (deepcarbon.net). To date this new model has produced a broad range of startling results:
• Diamond can form as a result of water-rock interactions in Earth’s mantle without changes in oxidation state. Fluctuations in the pH of an aqueous fluid can cause growth and dissolution of diamond (Sverjensky and Huang, 2015).
• Carbonate solubility under mantle conditions enables Earth’s deep carbon to be recycled on a large scale through aqueous transport in subduction zones (Pan et al., 2013; Facq, Daniel, and Sverjensky, 2014).
• Organic compounds (which may be building blocks of life) and hydrocarbons (which may provide sustenance for deep life) can form as a result of water-rock interactions in subduction zones (Sverjensky, Stagno , and Huang, 2014; Ague 2014; Huang et al., 2015).
• Earth’s nitrogen-rich atmosphere may have formed as result of water-rock interactions in subduction zones (Mikhail and Sverjensky, 2014).
Many previous calculations of fluid rock interactions were largely limited to crustal conditions because the models depended on the dielectric constant of water, which was poorly constrained at pressures above 5 kbar or depths greater than approximately 15 km. To eliminate this constraint, Pan et al. (2013) used ab initio molecular dynamics to compute the dielectric constant of water under conditions of Earth’s upper mantle to a pressure of 100 kbar, which corresponds to a depth of approximately 300 km. This knowledge opened the door for the Deep Earth Water model to explore water-rock interactions in critically important regions of Earth’s deep interior.
The amazing success of the Deep Earth Water model involves the integration of theory and modeling with experimental research as well as with field and analytical studies of natural samples. For example:
• Theory and modeling. Ab initio molecular dynamics calculations of the dielectric constant of water (Pan et al., 2013); Deep Earth Water model (Sverjensky, Harrison, and Azzolini, 2014); thermodynamic models of dissolution of carbonates under deep Earth conditions (Pan et al., 2013; Facq), formation of hydrocarbons and organic molecules as a result of fluid-rock interactions in subduction zones (Sverjensky, Stagno, and Huang, 2014), formation of Earth’s nitrogen-rich atmosphere due to fluid-rock interactions in subduction zones (Mikhail and Sverjensky, 2014), and formation of diamond due to decreases in pH (Sverjensky and Huang, 2015).
• Experimental research. In situ Raman spectroscopy study of carbonate speciation in a diamond anvil cell at subduction zone conditions (Facq et al., 2014); experimental study in the diamond anvil cell using Raman spectroscopy results in the formation of immiscible hydrocarbon and aqueous fluids under subduction zone conditions and implications for the deep carbon cycle (Huang et al., 2015). A new high pressure NMR probe has been designed and is being implemented for aqueous solution speciation at high pressures (Pautler et al., 2014; Ochoa et al., 2015) based on the DCO Physics and Chemistry of Carbon Workshop held at UC Davis on 29-31 March 2012.
• Field and analytical research. Analyses of fluid inclusions in natural diamonds (Weiss et al., 2015) provide important constraints on models of fluid-rock interactions and diamond formation in Earth’s deep interior (Sverjensky and Huang, 2015). Hydrous ringwoodite inclusions in natural diamond provide evidence of large quantities of water in the mantle transition zone (Pearson et al., 2014). Diamonds and their mineral inclusions (Walter et al., 2011; Shirey et al., 2013) provide important constraints on the geology of mantle carbon and the Deep Earth Water model (Sverjensky, Harrison, and Azzolini, 2014).
The Deep Earth Water model addresses the goals of all four DCO Science Communities and serves as a tool for integrating science and scientists across communities. This connection is best illustrated by a series of research collaborations and papers:
• Extreme Physics and Chemistry. Dielectric constant of water (Pan et al, 2013); Deep Earth Water model (Sverjensky et al. 2014); carbonate solubility under mantle conditions (Pan et al 2013; Facq et al., 2014); diamond formation as a result of fluid-rock interactions under mantle conditions (Sverjensky and Fang, 2015).
• Reservoirs and Fluxes. Carbonate solubility and transport under mantle conditions (Pan et al., 2013; Facq et al., 2014); diamond growth and resorption as a result of pH changes of aqueous fluids (Sverjensky and Fang, 2015); formation of Earth’s nitrogen-rich atmosphere as a result of fluid-rock interactions in subduction zones.
• Deep Energy. Formation of hydrocarbons and organic molecules as a result of fluid-rock interactions in subduction zones (Sverjensky, Stagno, and Huang 2014); formation of abiogenic immiscible hydrocarbon and aqueous fluids under subduction zone conditions, with implications for the deep carbon cycle (Huang et al., 2015).
• Deep Life Formation of organic molecules under subduction zone conditions can provide building blocks of life and may contribute to the origins of life (Sverjensky, Stagno, and Huang, 2014); hydrocarbon produced by fluid-rock interactions in the deep Earth can provide deep “food” for a deep biosphere (Sverjensky, Stagno, and Huang, 2014 (Huang et al., 2015).
The Deep Earth Water Model was developed with partial support from the Deep Carbon Observatory, and DCO facilitated its continued development and application. Recently, Dimitri Sverjensky and Isabelle Daniel (Université Claude Bernard 1, Lyon, France), were awarded a small DCO grant, “Building an International Deep Earth Water Group,” to train early career scientists who conduct experimental or fundamental theoretical research in the applied theoretical modeling of the role of water in the deep Earth. The development of an international network of early career scientists who conduct experiment research and use and advance the Deep Earth Water model has the potential to be among the DCO’s crowning achievements.
The Deep Earth Water model was highlighted in a recent news article in Science, “How buried water makes diamonds and oil” (Hand, 2015). This column is intended to complement Hand (2015) by documenting its connections to the Deep Carbon Observatory and providing complete references to the papers under discussion.
Schematic representation of the types of nitrogen molecules that are stabilized within the interiors of Earth, Mars, and Venus, where all three planets have similar nitrogen in their interiors with the sole exception being induced by of subduction zones on Earth. Credit: Sami Mikhail
Diamond with a red garnet inclusion. Credit: Stephen H. Richardson
Dimitri Sverjensky presents the Deep Earth Water model at the 2015 DCO International Science Meeting in Munich, Germany. Credit: Katie Pratt