The motions of the Earth's interior control and organize much of what goes on at the surface. From earthquakes and volcanoes, to oil and mineral deposits, even the evolution of life itself; the world humans live in is largely the product of the deep processes within our planet's interior. Because one cannot directly sample the deep Earth, one has to use indirect physical and chemical measurements to understand what is happening below our feet. The proposed study seeks to further our understanding of the inner motions and composition of the Earth's interior using the concentrations of noble gases (He, Ne, Ar, Kr, Xe) and their isotopes in lavas as probes of the high pressure processes that have such a large effect on our lives.
Due to their lack of charge, it has generally been assumed that noble gases are inert and are not retained in mantle minerals. A number of paradigms about the Earth use this as a key assumption: from the understanding structure of mantle convection to quantifying the volatile content of the planet. However, recent high-pressure experimental studies suggest that in fact noble gases have surprisingly high solubilities in the mantle, potentially requiring a wide range of theories to be revamped. In fact, there are few experimental measurements of noble gas behavior at mantle conditions. Utilizing new analytical and experimental advances, this study will measure the partitioning and diffusivities of noble gases in a range of mantle minerals, many for the first time, including orthopyroxene, garnet, wadsleyite, ringwoodite and majorite. A range of high-pressure devices (piston-cylinder, multi-anvil, internally-heated gas pressure) will be employed to produce pressures over 20 GPa and temperatures up to 2000 oC, simulating conditions in the upper and lower mantle. Samples will be measured by new generation laser-ablation mass spectrometry. The unprecedented data set will be used to reevaluate current ideas about the structure and composition of the Earth.
Overview Our study set out to measure the partitioning of noble gases (He, Ne, Ar, Kr and Xe) between minerals and melts in the mantle. These elements, and particularly their isotope ratios, are key to understanding the geochemical evolution and structure of the Earth's interior. Prior to our study, despite their importance, only two minerals (olivine and clinopyroxene) had any noble gas partitioning data at pressures and temperature relevant to the mantle. Our project was highly successful in its goals and we have measured noble gas partitioning into 14 minerals, including the minerals involved in mantle melting (olivine, orthopyroxene, clinopyroxene and spinel) and in minerals important for recycling of noble gases back into the mantle by subduction (amphibole, serpentine). The data have fundamental implications for the noble gas composition of basaltic lavas, the noble gas isotopic evolution of the mantle and the degassing of volatiles into the atmosphere. Noble gas behavior during mantle melting The study produced the first comprehensive and self-consistent data set of partitioning data for He, Ne and Ar for all minerals involved mid-ocean ridge melting: olivine, orthopyroxene, clinopyroxene and spinel (Figure 1). The results show that noble gases have similar partitioning into different mantle minerals, quite different than most trace elements, which are highly concentrated in clinopyroxene. This has major implications for fractionation of noble gases from the parent isotopes. As melting proceeds, noble gases should become increasingly compatible compared to their parents. Thus at high degrees of melting, perhaps relevant for the early Earth or in plumes, noble gases are likely to be more compatible than their parent isotopes, and so depleted mantle could preserve unradiogenic isotope signatures, which are typically interpreted to be evidence of undegassed mantle. This work was published in an EPSL paper (Jackson et al, 2013) and in numerous presentations given at international conferences. Noble gas solubility in ring-structured minerals Partitioning during mantle melting is a main control on how noble gases are extracted from the mantle. But are they returned to the mantle? Subduction of oceanic lithosphere recycles large amounts of H2O and CO2. But until recently, most models assumed that noble gases were not recycled by subduction, because they have no charge and so were thought not to be bound into minerals as H2O and CO2 are. We measured partitioning of noble gases into amphibole, a common mineral in hydrated oceanic crust. We found that, contrary to previous assumptions, noble gases are highly soluble in amphibole, more than 3 orders of magnitude higher than in olivine or other mantle minerals. This means that hydrated oceanic crust could hold substantial amounts of noble gases, which would require existing models of noble gas evolution of the mantle to be reworked to include a recycling pathway. This could explain the similarity of Ar, Kr and Xe isotopic composition between the atmosphere and mantle. Fractionation by minerals may also explain why He and Ne have not been strongly recycled. One of the most interesting aspects of the amphibole experiments is that they allowed us to identify where the noble gases were located in the mineral structure, for the first time to our knowledge. We varied the composition of the amphibole such that the number of vacant ring(A)-sites in the structure varied. The solubility of noble gases showed a clear correlation with the number of vacant ring-sites. This is a key observation, as many other minerals have ring-structures, such as serpentine. Based on this we have made preliminary measurements of noble gas solubility in a range of ring-structured minerals and used the results as motivation for a successful grant proposal to continue working on subduction of noble gases. The results of the amphibole study were published in Nature Geoscience (Jackson et al, 2013) and in numerous conference presentations. A second paper has just been accepted by American Mineralogist (Jackson et al, 2015). Broader Impacts In addition to the fundamental scientific importance of the research to geoscience, the solubility of noble gases in solids is important in a number of materials science fields. For example, nuclear reactors produce large amounts of 4He during nuclear fission reactions. Implantation of He into the reactor walls is a major source of weakening of the materials. Thus understanding what types of materials can accept noble gases without breaking down are important to nuclear reactor design. The study funded the training and education of Colin Jackson in experimental petrology and geochemistry. He now has a unique set of skills in that he is an expert on running noble gas partitioning and diffusion experiments as well as being adept at analyzing the experimental materials using laser-ablation ICPMS. This allowed rapid progress and numerous improvements in methodology. Colin is now a post-doctoral fellow at the Geophysical Laboratory in Washington, D.C., and has a very promising academic career.
