This proposal requests funding to continue a program of experimental research targeting the rates and mechanisms of atom migrations in geologic materials at high temperatures and pressures. Two specific foci are proposed: 1) fractionation of isotopes by diffusion in molten silicates (with emphasis on magmatic volatiles); and 2) diffusion and solubility of noble-gas atoms in rock-forming minerals.
Intellectual Merit. Isotope ratios are used by geochemists for many purposes, including but not limited to: estimation of the temperatures of geologic systems, 'sourcing' of fluids and melts, quantifying element cycling, and documenting biological activity. In all these applications, the use of isotope ratios relies upon a full accounting of the processes capable of changing them. Recent studies have revealed that the simple process of chemical diffusion in molten silicates can induce significant isotopic fractionation because the isotopes diffuse at slightly different rates. The differences in diffusivity are subtle for the elements characterized to date (Li, Ca, Mg), but geochemists specialize in measuring and exploiting very small differences in isotope ratios. The proposed project would focus mainly upon diffusive fractionation of the isotopes of volatile magmatic constituents (S, Cl, C, H) during isothermal chemical diffusion and in the presence of a temperature gradient. Simple models reveal that anticipated differences in the diffusivities of the isotopes of these elements would lead to substantial fractionations during, for example, nonequilibrium magmatic degassing. If the project is funded, the relevant diffusive fractionation factors will be measured and applied to quantitative models of magmatic systems. Knowledge of the mobility and partitioning behavior of noble gas atoms in geologic materials is central to understanding how, and to what extent, the Earth has lost its volatiles. As products of radioactive decay, 40Ar and 4He also see widespread use in thermochronology-i.e., the extraction of time-temperature histories from rocks. As a complement to information obtained in other laboratories through thermal degassing studies of rock-forming minerals, this proposal offers a systematic experimental program in which minerals will be exposed to noble-gas atmospheres at high temperatures and the gas uptake characterized directly by ion-beam profiling techniques. The goal will be to quantify the rate at which the noble-gas atoms diffuse into the mineral lattices, as well as their solubilities as gauged by their concentrations at the mineral surfaces. Studies of Ar behavior at RPI using this approach have been very successful, so with continued funding the effort would be directed mainly toward other noble gases, beginning with He and Ne. The resulting data will be used to evaluate the storage, dispersal and interphase partitioning of noble gases in the Earth using quantitative models. The overall intellectual merit of the proposed activities lies in the acquisition of basic information about diffusion-related phenomena whose role in solid-Earth geochemistry is not fully understood. In most instances, the planned experiments also target specific geochemical problems, such as isotopic fractionation during magma degassing and retention of 40Ar and other noble gases in the solid Earth.
Broader Impacts. In addition to providing basic, strategic data on properties of Earth materials, a first-order role of the RPI experimental geochemistry lab is to serve as an educational environment where doctoral students learn a variety of skills and strategies, maturing into resourceful, versatile scientists who are prepared for independent research in other settings. Undergraduates participate regularly in lab activities in a mode that encourages application of their classroom learning, helps them develop responsibility in a team environment, and promotes attention to detail. This project would provide major support for the RPI experimental geochemistry lab, allowing the facility to continue in its role as a resource for scientific visitors from both inside and outside the Rensselaer community (recent 'outside' visitors have come from Harvard, Boston University, and the Federal University of Brazil). Several members of RPI's Department of Earth and Environmental Sciences have open access to the lab that has benefited their independent NSF-funded research projects. The laboratory has also served the needs of researchers from General Electric working on the behavior of GaN semi-conductor materials at elevated P-T conditions. The RPI experimental geochemistry lab has both created and benefited from synergistic activities with the particle-accelerator facility at the State University of New York at Albany (UAlbany). The pairing of experimental techniques developed at RPI with analytical techniques available at the UAlbany facility has enabled a great deal of science to be done at a modest cost of $25/h of beam time. The broader impact here is the introduction and availability of new analytical approaches to the geochemical community.
Introduction The phenomenon of atomic migration in materials (diffusion) has been documented in many ways and by many types of scientists motivated by highly diverse interests. Even so, our knowledge of diffusion is not sufficient to predict the details of atomic migrations in complex Earth materials, which are crucial to understanding the cycling of elements through Earth systems, as well as to age determinations — both absolute ages from radioactive decay and residence times at the surface of the Earth (based on exposure to cosmic rays). Challenging questions that come to mind include: Is diffusion the same for all atoms, or does it depend on atom identity? Does the diffusion rate of a given atom depend on the atomic structure and composition of its "host" mineral? How do diffusion rates change with temperature and pressure? Goals of the research The overarching goal of this 5-year NSF project was to learn how fast atoms and their isotopes move around in the Earth's crust and upper mantle at pressure-temperature (P-T) conditions ranging from those at or near the surface (depths less than 1 kilometer) to those prevailing in the upper mantle (~100 km deep). We used mainly experimental approaches to address the following general questions: 1) Do the isotopes of a given element all diffuse at the same rate, or do there exist subtle differences attributable to isotope mass? Established theory says that the heavier isotope should diffuse more slowly, but the theory is based upon simple materials, not complex geomaterials. 2) How fast do inert gas atoms (for example, helium and neon) diffuse in minerals of the crust and upper mantle? This is an important question because inert ("noble") gases are excellent tracers of terrestrial outgassing — they are the quintessential volatile elements. Noble gases are also vital to age estimations because they are produced by radioactive decay and/or by cosmic rays impinging on Earth's surface. Findings and significance Intellectual merit Isotope diffusion. We characterized differences in diffusion rates of the isotopes of lithium, potassium, magnesium, calcium and iron in molten silicates (magmas) and of lithium in pyroxene — a common mineral of Earth's crust and upper mantle). Our findings indicate that diffusion is capable of altering isotope ratios of natural materials. Isotope ratios are used by geochemists for many purposes, including but not limited to: estimation of the temperatures of past environments, determining the sources of fluids and melts, quantifying element cycling, and documenting biological activity. In all these applications, the use of isotope ratios relies upon a full accounting of the processes capable of changing them. Our studies establish that the simple process of diffusion in molten silicates and minerals can produce significant changes in isotope ratios. Our data will help geoscientists to "reverse engineer" the chemical systems of our planet. Inert gases. We measured the diffusion rates of helium and neon in olivine (abundant in Earth's upper mantle) and of neon in quartz (an important crustal mineral). Our new data indicate that at upper mantle temperatures (1200-1500°C), helium diffusion in olivine is fast in one sense: individual olivine crystals can be expected to exchange helium on timescales of a year or less. Our data are not the first to establish this fact, but they are the first to demonstrate that there is no measurable effect of pressure on helium diffusion up to ~30,000 atmospheres. Viewed on the much larger planetary scale, helium diffusion in olivine is quite slow in the sense that helium atoms in the mantle have migrated only about 1 km in the age of the Earth — which means that diffusion alone is an inefficient means for degassing helium from the bulk Earth, or for homogenizing large reservoirs in the mantle. Another key outcome of our noble gas diffusion studies is that olivine retains helium (and neon) indefinitely on the surface of the Earth, so accumulated "cosmogenic" noble gases produced by impinging cosmic rays will preserve an accurate record of the time a given sample has spent on Earth's surface. On the quartz front, previous results have suggested that neon is fleetingly retained in quartz even at surface-ambient conditions, which would make this key mineral effectively useless for exposure-age determinations. Our study paints a more optimistic picture: neon diffusion in quartz is sluggish enough at surface conditions to preserve a complete record of cosmic-ray exposure. Broader impacts The main broader impacts of the project involved human resource development and implementation of new experimental protocols. Doctoral students and post-doctoral researchers gained valuable experience in the conduct and analysis of high P-T experiments that is transferable to any discipline of science and technology. In terms of technique innovation, we expanded the previously existing scope of methods applied to noble-gas diffusion measurements on minerals by coupling ion implantation with depth-profiling by nuclear reaction analysis (NRA).
