Experimental Studies in Petrologic and Geochemical Kinetics

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

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.