Volatile cycling and oxygen fugacity of subduction zones using stable vanadium isotopes

The outer shell of the Earth that we live on is made up of brittle 'plates'. The migration of these plates across the surface of the planet is directly linked to major geologic events such as earthquakes and volcanic eruptions. In some regions, two plates collide, forcing one beneath the other in a process called 'subduction'. Subduction zones are responsible for much of the explosive volcanism on Earth, including the infamous Pacific 'Ring of Fire'. However, these zones are also critical for the exchange and cycling of chemical elements between the surface and interior of the planet. As an oceanic plate subducts, it is subjected to high pressures and temperatures. During this process, the plate looses chemical components through fluids, degassing, reactions, and sometimes melting. One of the key parameters controlling how much of which elements are lost, is the available oxygen. Geochemists refer to the amount of available oxygen as the 'oxygen fugacity' of a system, which can be simply thought of as the partial pressure of oxygen. Oxygen fugacity has a large affect on the way the carbon (C), hydrogen (H), and other 'volatile' elements behave in a subduction system. Volatiles species (e.g., H2O and CO2) are those that vaporize at low temperatures. Combined with other physical and chemical conditions of subduction, oxygen fugacity controls how much H and C is lost through degassing during explosive volcanism, and how much can be dragged deeper into the Earth. There has been a long-standing debate over how much more oxygenated subduction zones are compared with the rest of the interior of the Earth. The fugacity of samples from subduction zone volcanoes tells us about present day processes and volatile cycling. Furthermore, if subduction zones are significantly more oxygenated than the rest of the interior of the Earth, then they may provide an efficient means of recycling oxygen into the interior of the Earth. Therefore, it is critical to constrain the oxygen fugacity of subduction zones to evaluate the whole Earth cycling of volatile elements and how this may change through time. It is essential to find a robust way of determining oxygen fugacity. Unfortunately, previous studies used methods that can be easily 'reset' by later events, so that they do not give a true indication of the original source. Consequently, there is considerable uncertainty in what the 'real' amount of available oxygen is in subduction systems. My previous research and expertise focused on the novel application of isotopes of chemical elements to solving Earth problems. I have continued in this broad avenue of investigation by working on a precise analytical method for the measurement of vanadium stable isotope variations. The measurements are not trivial, however they are very valuable. The power of vanadium stable isotopes in particular, is that their fractionation should be directly and robustly linked to oxygen fugacity. This fellowship analyses vanadium stable isotope variations in lavas, sediments and deep Earth samples from the Mariana (southwest Pacific), Aleutian (Alaska) and Mexican subduction zones. Through this work, better constraint can be placed on oxygen fugacity and how the Earth system behaves in terms of the fluxes of volatile elements such as carbon and hydrogen between deep and surface reservoirs of the Earth. This will help us tackle far-reaching present day issues related to how the carbon cycle works, and also potentially provide a means of investigating how the amount of oxygen in the Earth has changed over time, its links to the evolution of the atmosphere and ultimately to how our planet became able to sustain life.