Recent work by Jeff Roberts (LLNL) and his coworkers at LBNL’s ALS Beamline 8.3.2 has utilized synchrotron radiation computed tomography to derive highly resolved 3-D images of a variety of geologic materials. The 8.3.2 beamline is dedicated to tomography, and has a high flux and short scan times (700 photons/s/μm²) at 10 keV over a 0.6 × 6 cm area, with an energy range of 3-60 keV. The spatial resolution of this beamline is less than 1.5 μm, thus resulting in the generation of large data volumes for studied samples.

Two types of geologic materials, pumice and quenched partial melts, have been examined to determine their porosity and permeability structure.  Vesicles in pumice are commonly elongated, with irregular geometries and dimensions.  Image brightness resulting from x-ray attenuation is directly related to electron density and the average atomic number of the material, so that there is a strong contrast between void space and the pumice walls. 3-D images were of pumice were discretized into lattices for solution of the Stokes flow equation using a lattice-Boltzmann solver to calculate permeability.  The calculated porosity and permeability values can then be compared with those determined in the laboratory.

3-D images of metallic iron-sulfide partial-melts in an olivine matrix. (left) FeS-olivine (4% melt by volume). (right) FeNiS-olivine melts (6% melt by volume).  Upper images depict melt distribution in cylindrical volumes ~0.7 mm diameter and 1.0 mm length, while  bottom images provide higher magnification to show melt morphology in greater detail (50 μm scale bars). 

This technique was also used to estimate melt permeability under the P-T conditions representative of the Earth’s core. Quenched high pressure and temperature experiments involving olivine and interstitial iron sulfide melts were examined using this technique to determine the porosity and permeability of melts at different melt fractions by taking advantage of the large density contrast between the olivine grains and the quenched partial melts. Observations of a number of partial melt samples were used to construct a relation between porosity and permeability using a Kozeny-Carman approach. This relation can be used to constrain melt percolation models associated with planetary core formation.

For additional details, see:

Wright, H.M.N., Roberts, J.J., and Cashman, K.V., 2006, Permeability of anisotropic tube pumice: Model calculations and measurements. Geophysical Research Letters, 33, L17316, doi:10.1029/2006GL027224.

Roberts, J.J., Kinney, J.H., Siebert, J., and Ryerson, F.J., 2007, Fe-Ni-S melt permeability in olivine: Implications for planetary core formation. Geophysical Research Letters 34, L14306, doi:10.1029/2007GL030497.