X-Ray Microtomography of Olivine-Basalt Partial-Melts
Abstract
Understanding the physical properties of partially molten rocks and developing the ability to infer their existence at depth in the Earth from remote geophysical measurements are of fundamental importance in geology and geophysics. The interconnectivity and tortuosity of the melt phase, in combination with the properties of the individual melt and crystal phases, have bearing on the extractability of the melt, and on the rheology, seismic velocity and attenuation, and electrical conductivity of the bulk material. We have performed synchrotron x-ray computed microtomography to image olivine-basalt partial-melts at the microscale and to evaluate current imaging capability for prediction of transport properties such as permeability and electrical conductivity. Two different types of samples have been synthesized and imaged thus far: olivine-basalt partial-melts (Fo90) (MMP) and olivine-basalt-sulfide partial melts (Fo90) (BMS). All samples were created in the piston cylinder apparatus. The MMP samples had 5 wt % BaO added to the basalt melt phase to increase x-ray attenuation contrast between the olivine and the melt. There are three different BMS samples with a range of compositions and proportions of silicate and sulfide melts. BMS1 contains both silicate and sulfide melt, BMS2 contains a sulfide melt close to a wetting composition and BMS3 contains a non-wetting iron-rich sulfide melt. Imaging was performed at Lawrence Livermore National Laboratory (X-ray energies up to 120 keV, ~10 μm spatial resolution), Advanced Photon Source (20-40 keV, 1-6 μm) and Stanford Synchrotron Radiation Lab (10-30 keV, 3.3 μm). Initial processing and reconstruction suggests that pores greater than ~10 μm in size are easily imaged but smaller pores and the interconnectness of the melt phases are more difficult to detect. Very high contrast between the olivine and melt phases helps, but it is still not possible to clearly delineate individual interconnecting tubules between larger melt pockets. The attenuation contrast between olivine and sulfide is much greater than between olivine and silicate-melt, and hence the sulfide phase is easiest to detect. The shape of the melt pockets is a strong indication of wetting behavior: wispy star-like shapes are indicative of an interconnected melt phase while spherical blebs are likely isolated. 3-D digital images, when achievable in sufficient resolution, will be used to quantify microstructural properties and predict fluid transport. Coupled with measurements of physical properties such as permeability and electrical conductivity, this will enable the evaluation of empirical and theoretical relationships between electrical conductivity, microstructure, and permeability for partially molten systems. This work was supported by the Office of Basic Energy of the U.S. Department of Energy, and was performed by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. Support was also derived from NSF EAR0073987.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFMGP51A0979R
- Keywords:
-
- 3914 Electrical properties;
- 3994 Instruments and techniques;
- 5114 Permeability and porosity;
- 5139 Transport properties