Dr. Elizabeth Ferriss has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the Lamont-Doherty Earth Institute, Columbia University. This study will investigate the diffusion of water in the clinopyroxene mineral group, with particular emphasis on the effects of aluminum, titanium, and iron. The larger goal of the project is to develop a new tool to determine the ascent rate of magma near the surface of the Earth. Accurately quantifying this rate is important because physical models suggest that the ascent rate largely controls the vigor of volcanic eruptions.
Two sets of experiments will be conducted. First, a set of natural clinopyroxenes will be dehydrated, and their diffusion profiles will be measured. This approach allows the investigation of a wide range of natural systems and creates an important link with previous experiments that were conducted in a similar way. Second, aluminum- and water-rich clinopyroxene will be synthesized and equilibrated with a melt at high pressure. This system will then be decompressed, resulting in dehydration of both the melt and the pyroxenes. This experiment will require a substantial amount of preparation, but the increase in difficulty is offset by the value of closely simulating the natural processes experienced by crystals as they grow, incorporate water, and dehydrate upon decompression.
This study will test a novel analytical technique for measuring a parameter that is central to physical models of volcanic eruptions and will also provide data that will be highly useful for investigating the mantle's water budget, which is important at the planetary to tectonic scale. In addition, Dr. Ferriss will co-organize both a geodynamics speaker series and a graduate seminar on diffusion in geological systems.
The overarching goal of this project was to work toward the development of a new tool for measuring the rate at which magma rises, which is a major factor that determines how explosive a given volcanic eruption will be. Specifically, this project experimentally measured the rate at which hydrogen ions travel diffusively through the mineral clinopyroxene. Once this diffusion rate is well understood, then hydrogen diffusion profiles in large clinopyroxene grains (phenocrysts) in volcanic rocks can be used to determine magma ascent times. To measure the hydrogen diffusivity, this project dehydrated a series of natural, gem-quality, homogeneous clinopyroxenes with initially uniform water contents in a gas-mixing furnace and measured the resulting hydrogen diffusion profiles using a combination of Fourier-transform infrared spectroscopy (FTIR) and secondary ion mass spectrometry (SIMS). The project also developed a novel method for performing these measurements that eliminates the need to cut (and thereby destroy) the sample prior to profile measurement by FTIR. This new approach should significantly simplify and improve the results of future hydrogen diffusion rate measurements. A major finding of this study is that hydrogen diffusion rates can vary up to three orders of magnitude, depending on the exact mechanism(s) by which the hydrogen ions are incorporated in the clinopyroxene. In one sample (a diopside from the Kunlun Mountains in China), the measured hydrogen-oxygen stretching bands in the FTIR spectra (which are thought to correspond to different hydrogen incorporation mechanisms and bonding environments) were even observed to change over the course of the dehydration experiments. Such changes and variability significantly complicate our ability to predict the overall hydrogen diffusivity in a given clinopyroxene. The results of this study form the basis of forthcoming quantitative estimates of the hydrogen diffusion rate as a function of bonding environment (which can be determined by FTIR), which should prove useful for improving our understanding of and ability to predict the explosiveness of volcanic eruptions. These findings are also relevant to our understanding of the role of hydrogen in the mantle, which is an important parameter in planetary dynamics and plate tectonics. Further, this project supported the training of an early-career female scientist in an entirely new skill set and a graduate-level seminar on diffusion in geological systems.
