Get to know: The Rock and Mineral Physics Laboratory

The Rock and Mineral Physics Laboratory at the University of Minnesota uses laboratory-based experiments to investigate the physical properties of geological materials. We primarily focus on the mechanics and deformation of rocks at high temperatures and pressures (Figure 1). Our research interests address a wide range of problems that relate the mechanisms of deformation at the atomic scale to the dynamics of the solid Earth at the scale of lithospheric plates. Much of our research focuses on the interactions among crystal defects, microstructure, and the rheological properties of mantle and crustal rocks.

RAMP group
Figure 1: Rock and Mineral Physics Group along with our two Paterson apparatuses. These apparatuses are designed to measure the mechanical properties of rocks at extreme temperatures and pressures. The key feature of their design that the sample and measurement equipment are housed inside (rather than outside) an Argon gas pressure vessel, which allows high precision measurements to be made. These apparasuses allow simulation of conditions relevant to Earth’s crust and upper mantle.

Rheological behavior of rocks and minerals

Our primary research target is elucidating the small-scale physics that control the rheological behavior of geological materials. We conduct deformation experiments to determine how the motion and evolution of crystal defects, such as dislocations, influence macroscopic behavior. We work at a range of conditions to examine plasticity and viscoelasticity in materials characteristic of Earth's upper mantle and crust. These experiments include uniaxial compression; triaxial compression (Figure 2), extension, and torsion; and micromechanical testing including nanoindentation and micropillar compression. You can find an overview lecture of some of these topics from 2019 here.

D-DIA experiemnt
Figure 2: An example of an experiment conducted with a D-DIA at the Advanced Photon Source (6-BM-B). Two samples with different average grain size are stacked on top of each other and subjected to oscillating deformation at room temperature. The samples can be imaged within the apparatus, even though the pressure is equivalent to hundreds of km depth, using a very bright X-ray source. The resulting behavior reveals plastic yield, strain hardening, the Hall-Petch effect, and the Bauschinger effect.

The dynamics of partially molten rocks

Partially molten rock (melt fraction < 0.2), mush (melt fraction from 0.3 to 0.5) and magma (melt fraction > 0.5) are found throughout Earth’s mantle, asthenosphere and crust. The seismic, conductive, and transport properties of these materials are significantly influenced by melt fraction and melt distribution, both of which can change during deformation and as a result of chemical reactions. We conduct experiments to determine the rheological properties and microstructural evolution of partially molten rocks and mushes as a result of deformation, reactive flow, and exposure to pore-pressure gradients (Figure 3). Experimental results are also used to identify the processes and conditions that facilitate melt migration and the formation of melt-rich features. Specific applications of our work include chemical exchange between Earth’s crust, mantle, and atmosphere; mantle flow and melt extraction on Jupiter’s moon Io; and magma ascent in volcanic plumbing systems.

Viscous Finger of melt
Figure 3: Backscattered-electron image of a viscous finger of melt intruding into a mush from Amy Ryan's work as part of an NSF postdoctoral fellowship. A soda-lime melt (bright) is intruding into a mush composed of quartz (medium gray) and borosilicate melt (dark gray). The sample is subjected to a 200 MPa difference in pore pressure (decreasing from bottom to top), and an instability forms due to the difference in viscosity between the soda-lime melt and the mush. This mechanism of intrusion is potentially critical in controlling the migration of melt within and the stability of volcanic systems.

Texture development and anisotropy

The deformation of crystalline materials at high temperatures often leads to the formation of crystallographic textures, in which individual crystals rotate into preferred orientations. Because individual crystals exhibit anisotropy in their physical properties, textured materials also exhibit some anisotropy. Physical properties that can be anisotropic include elasticity (and therefore seismic properties), viscoplasticity, and electrical conductivity. We conduct experiments to investigate both texture formation and the magnitude of anisotropic properties of textured rocks. Many of these experiments involve extreme amounts of deformation, which is imparted on samples by twisting them in torsion (Figure 4). The results of these experiments are used to calibrate models of texture formation to predict anisotropy in geodynamic simulations and to interpret textures preserved in rocks now exposed at Earth's surface.

Deformation experiment
Figure 4: Example of a large-deformation experiment from Nicole Wagner’s MS work. A jacketed sample olivine has been simultaneously deformed in torsion and extension. The sample is approximately 13 mm in diameter. Experiments conducted in this geometry are allowing us to determine how complex loading geometries associetd with the formation of tectonic plate boundaries influence the seismological properties of the upper mantle.

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