Intellectual Merit: Extraction of melt deep in the Earth from mantle rocks composed of solid grains plus only 1 or 2% melt ultimately results in eruption of magma from volcanoes at Earth’s surface.  This phenomenon controls the chemical and physical evolution of our planet.  Since processes occurring at great depths in the Earth are not directly observable, much of our understanding of the dynamics of partially molten regions of Earth’s interior relies on numerical/computer models of the behavior of partially molten rocks.  The starting point for models of a mechanically weak melt in a strong but deformable rock is known as two-phase flow theory.  Application of this theory to large-scale processes occurring far below Earth’s surface requires equations describing the viscosity (strength) of rocks and the rate at which melt flows through them. A critical test of the validity of models used to describe the interactions of deformation, melt distribution, and melt migration in Earth’s mantle is their ability to explain phenomena observed in laboratory experiments on partially molten rocks. 

 A fundamental breakthrough in the theory and numerical (computer) models describing the dynamics of partially molten rocks occurred ten years ago with the recognition that the viscosity (i.e., the ability of rock to flow) depends on direction.  That is, viscosity is not isotropic, rather it is highly anisotropic.  This anisotropic behavior results from the anisotropic distribution of melt that develops during deformation of the rock.  It was hypothesized that viscosity should be anisotropic because pockets of melt become aligned during deformation.  Basically, if melt pockets align east-west, then the viscosity will be smaller in that direction than in the north-south direction.  Furthermore, this analysis predicted that melt should segregate from regions within the rock that are experience a low stress to regions under a high stress. 

 We tested this hypothesis, and thus a fundamental prediction of two-phase flow theory, with newly designed experiments.  Our recent high-temperature, high-pressure laboratory experiments demonstrated this exact behavior in partially molten samples deformed in torsion and in partially molten samples extruded through a narrow pipe.  While ultimately, agreement between theory and experiment is clearly important, discrepancies represent an avenue for progress in refining theory.  Discrepancies between theoretical prediction and experimental observation represent an opportunity to refine our understanding of the grain-scale mechanics of partially molten rocks.  Thus, well-designed experiments with detailed analysis of melt distribution, melt segregation, and mechanical properties are essential.

 To this end, we carried out both torsion and extrusion experiment on partially molten rocks.  As predicted, melt segregates from the outside toward the middle of a torsion sample. In pipe extrusion experiments, melt segregates from the middle to the edge of the pipe.  In both cases, melt segregation only occurs if the viscosity is anisotropic.  Thus, two-phase flow theory passed a critical test, provided that anisotropic (directionally dependent) viscosity is included.

 Broader Impacts: A unique aspect of this research is our involvement in STEM outreach in Mexico though “Clubes de ciencia México”, an organization of science outreach and mentoring for high school and undergraduate students in Mexico. The science clubs are one-week long, very interactive workshops designed to initiate students to science and research; special focus is given to scientific reasoning and ethics.  Dr. Alejandra Quintanilla Terminel helped organize and lead a recent workshop on “How do rocks flow?”.   With her expertise in experimental studies of rock deformation research, Alejandra illustrated the necessity of experimental studies for understanding our planet’s evolution.  This step is very important because, for many students, it is difficult to understand how experiments carried out in a laboratory on a small sample over a relatively short (geologically speaking) time can have anything to say about the behavior of a huge planet with high mountains, deep oceans, and erupting volcanoes.  Thus, it is critical to use results from our experimental investigations to explain the implications of experimental research for understanding large-scale dynamical and chemical behavior of our planet.

Another part of our Broader Impacts is the important training of the next generation of scientists and citizens.  In specific, one postdoctoral researcher, Dr. Alejandra Quintanilla Terminel, developed skills that helped position her for her current position in outreach at the Massachusetts Institute of Technology.  Two undergraduates also worked on this research project.  Although their assignments were supportive in nature (e.g., preparing samples for experiments), they were immersed in the day-to-day activities in a research lab.  Such experiences not only build skills but also expose students to scientific thinking and hypothesis testing.  For some students, the result is graduate school, while for others, it makes clear that working for a consulting company or a government agency is a better choice.  In both cases, students leave as informed citizens with a deeper insight into the importance of scientific research.

Last Modified: 10/18/2018
Modified by: David L Kohlstedt