Synchrotron X-ray Tomography: A New Technology for Studying the Earth's Mantle

When one thinks of the standard tools geologists use to study the Earth, a particle accelerator is unlikely to come into mind. Nevertheless, Dr. Wenlu Zhu of the University of Maryland is utilizing the X-ray radiation given off by a synchrotron, which is a type of particle accelerator, to image the melt structures of synthetic partially molten mantle rocks. Zhu proposes that by imaging the melt distribution in three-dimensions, one can more completely quantify the transport properties of the partially molten region beneath ocean ridges, where plates are created. Zhu is the lead researcher on the project, which is a collaborative effort, involving scientists from the University of Maryland, Woods Hole Oceanographic Institute, and Argonne National Labs.

Experiments have already been conducted on similar synthetic systems. However, these studies rely on 2D measurements to infer the transport properties and therefore, paint an incomplete picture of the melt structure. At the Advanced Photon Source, Zhu et al. rotate their cylindrical, partially molten samples 180° through an x-ray beam in .12° increments to build digital 3D representations of their partially molten samples. Using the digital melt data, one can perform a so-called numerical experiment to simulate magma flowing through their samples. By doing this, Zhu et al. are able to quantify flow properties such as permeability. Permeability is defined as the capacity for the material to allow fluid, in this case magma, to pass through it. It is a fundamental property of the rock.

Figure 1: Diagram of imaging technique.

Figure 2: 3D image of melt distribution of olivine-melt from Zhu et al. (2011).

Preliminary results look promising. However, extrapolating these to the Earth may be challenging. So far, the only experiments that have been conducted have used the simplest system, which is olivine-melt. However, in the mantle, there is an assortment of minerals. Adding different mineral types may significantly alter the flow properties of the rock. In addition to having a compositional variety, the Earth’s mantle is moving at a rate of about 10 cm yr^-1. Although slow, this motion causes deformation that may give way to drastically different melt structures. The experiments currently being conducted by Zhu and her team are performed in isostatic equilibrium, and therefore, do not take in account the motion of the mantle. However, Zhu et al. have made plans to investigate the role of composition as well as deformation on the flow properties of partially molten mantle rocks.

Synchrotron radiation is a useful tool for studying magmatic processes in the Earth. So far, research performed by Zhu et al. has led to the first direct measurements of permeability for synthetic, partially molten mantle rocks. If their results help better constrain the melt transport properties at ocean ridges, they may help geologists better understand the mantle melting elsewhere on the Earth, such as at subduction zones where volcanoes usually form.

Zhu, W., Gaetani, G.A., Fusseis, F., Montési, L.G.J., De Carlo, F. (2011), Microtomography of Partially Molten Rocks: Three-Dimensional Melt Distribution in Mantle Peridotite, Science: 332 (6025) 88-91, [DOI:10.1126/science.1202221].