D/H isotope exchange between electrolyte-bearing C-O-H magmatic fluids: In-situ experiments involving vapors and brines

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

The properties of H2O at high temperatures and pressures are important to know for both volcanic and magmatic processes, as well as for processes that impact the geothermal energy, nuclear waste, and chemical engineering industries. This research, using newly developed, novel laboratory techniques, such as hydrothermal diamond anvil cells coupled with Raman/Fourier Transform Infrared spectroscopy, will be used to make real time measurements of the solvation of gases and metals in light and heavy H2O at supercritical conditions. This project will bring new experimental techniques to bear to understand the systematics of hydrogen gas and methane dissolved in electrolyte-rich supercritical fluids by looking at the isotopes of hydrogen in the context of the evolution of geothermal fluids in the subsurface and the degassing of magmas in volcanoes. Data collected will be used to augment theoretical models of water cycling in Earth's interior. The experiments and their interpretation will also shed light on the mass and heat transfer associated with Earth's deep hydrological cycle as well as the interaction of geothermal fluids with the solid Earth, a process that contributes to the flux of carbon dioxide, methane, and other gases to the atmosphere. Broader impacts of the work include providing fundamental knowledge about H2O at high temperatures and pressures, which has implications for a broad array of fields in science and engineering, such as chemical engineering (e.g. toxic/radioactive waste remediation, nuclear power reactors), physical chemistry, geophysics, geochemistry, and energy-related research. In addition, a series of lectures and lab demonstrations will be created and presented to undergraduate and graduate students at the George Mason University in Virginia. Undergraduates will have an opportunity to be involved in the project through a 10-week internship program at the Carnegie Institute of Washington that will run during the summer both years that the award is active. Technical aspects of the research involve study of the exchange of deuterium and hydrogen isotopes between various hydrogen bearing volatiles (H2, CH4, H2O). The work will examine these volatiles in the context of understanding the thermal regime of volcanic settings and the source of magmatic fluids. For example, deuterium-depleted brines derived from seawater contribute to the hydrogen-isotope composition of Cl-rich melt inclusions in mid-ocean ridge basalt glasses. However, the composition of these brines is completely unconstrained, hindering efforts to trace the hydrogen-isotope composition of the mantle source of seafloor basalts. Furthermore, at present, there are no experimental data to describe the effect of degassing during magma ascent on the D/H exchange between H2(aq), CH4(aq) and H2O dissolved in Cl-rich, F-rich fluids, or aqueous fluids that contain common ions found in seawater (i.e., Na/K/Mg/Ca/Cl/F). In the latter case, the concentration and electrostatic properties of these ions on the structure of H2O and how they affect the solvation mechanism and solubility of H-D isotopologues of H2 and CH4 in supercritical H2O-D2O mixtures will be examined. Measurements will be made in-situ and in real-time by employing hydrothermal diamond-anvil cells and Raman/FTIR spectroscopy. Results will be complemented by the use of GC-TC/EA-Isotope Mass Ratio Spectrometry to determine bulk D/H ratios and isotope fractionations between species equilibrated at high-P/-T during experiments in solid-media high-pressure apparatus. The proposed study will compliment and support the new frontiers of H-D CH4 isotopologue geochemistry.