Intellectual Merit: In this award, we studied the interaction of silicate melts with aqueous solutions. More specifically:

-Hydrothermal diamond anvil cell (HDAC) experiments were conducted to constrain the role of fluid pH, alkalinity and electrolyte composition on the physical and chemical properties of the coexisting silicate melts. In short, results revealed the dominant presence of carbonate and sulfate groups in silicate melts when coexisting with alkaline aqueous solutions. The melt/fluid immiscibility field appears to be much more extended for melts enriched in carbonate and sulfate. Results reveaL that under alkaline conditions, the immiscibility space for the melt-fluid equilibria was suppressed relative to experiments involving pure H₂O or H₂O-NaCl solutions. This experimental study shows that the interaction between melts and crustal alkaline brines has a profound effect on the physical and chemical heterogeneities between the magmatic systems developed along mid-ocean ridges, shallow continental crust, and subduction zones. Little is known, however, about the interaction of alkaline and oxidized brines with carbonate melts. We demonstrated that fluid pH plays a key role in the speciation and solubility of C-N-S species in silicate and carbonate melts; with important implications for partial melting of the subducted oceanic crust driven by serpentinization-derived alkaline and saline slap fluids.

-Experiments were conducted to constrain the isotope fractionation effects between hydrous silicate melts and H₂O at 1400 °C and 1.5 GPa. Experimental data provide evidence of substantial isotope fractionations between silicate melts, fluids and COH-volatiles driven by molar volume and condensed phase isotope effects.  In detail, hydrogen isotopes in hydrous alkali silicate melts appear to partition preferentially into the silicate network sites that are associated with an alkali ion. This partitioning is stronger for deuterium than it is for protium, which gives rise to an intramolecular hydrogen isotope effect resolvable by NMR spectroscopy. The isotope effect strongly depends on the bulk D/H isotope and water composition of the system. Our experimental observations agree with earlier in-situ HDAC fractionation studies in the Na-(Al)-Si-water system, showing a link between deuterium fractionation processes taking place inside the silicate melt structure and the macroscopic fractionation of hydrogen isotopes in two-liquid systems. Based on the very systematic behavior observed, we develop a model that constrains the intramolecular isotope effect and allows to examine its relevance for variously hydrated sodium silicate melts with deuterium compositions of natural abundance. Our experimental study suggests that the separation of a hydrous fluid phase from a water saturated melt under pressure may be the most effective step in the deep water cycle to separate hydrogen isotopes, and a major driver for the continuous D/H increase of the Earth surface oceans.

- A series of HDAC experiments was conducted to determine the extent of H₂ partitioning into silicate melts in equilibria with aqueous solutions at crustal temperature and pressures conditions. Results show that the extent of H₂ solubility in H₂O-saturated silicate melts can be empirically described at temperatures ranging from 600 to 1500 °C and pressures of 0.5 - 3 GPa. These results enhance our understanding of hydrogen evolution in the interior of Earth and potentially of rocky exoplanets. Planetary evolution models, therefore, could account for the effect of molecular H₂ on establishing redox and density conditions for H₂-bearing hydrous magmas. As described by others, H₂-rich magmas are expected to be more buoyant that H₂-poor but H₂O-saturated magmas. Density-driven differentiation, therefore, can cause the development of planetary reservoirs with distinctively different lithologies and volatile content, driven by the strong decoupling between reducing and oxidizing magmas.

Broader Impacts: This award involved the participation of four undergraduate (George Mason Univ., Howard Univ., Univ. of Virginia), three high school (Stone Ridge HS, Winston Churchill HS), one graduate student (Univ. South Carolina) and one postdoctoral CIW fellow. Students were co-authors in AGU Fall meeting presentations. Opportunities for training and professional development were given to female and minority students. This award supported the Stable Isotope Lab at the Geophysical Lab. Experiments used synthetic CVD (chemical vapor deposition) ultra-pure diamonds. These CVD technological advances have been pioneered at the Earth and Planets Laboratory. Some of the broader scientific implications are related to understanding the introduction, transport, and storage of C-O-H volatiles in the interiors of terrestrial-like planets is profoundly linked to habitability, the origin of life, and the external evidence of conditions favorable for life. Our experimental studies shed light on the mass and heat transfer associated with the deep Earth’s hydrological cycle, wherein water is carried into the mantle at subduction zones and released back to the surface at volcanoes. Supercritical fluids interacted with the solid Earth contribute to the flux of CO₂, CH₄, H₂, H₂S and other volatiles to the Earth’s atmosphere, and thus, affect the modern climate of our planet.

Last Modified: 01/26/2023
Modified by: Dionysios I Foustoukos