Permeability and three-dimensional melt distribution of partially molten mantle rocks
Abstract
Current understanding of the melt percolation in mantle rocks remains unsatisfactory, especially at low melt fraction. At mid-ocean ridges, for example, geochemical data and geophysical observations have produced divergent estimates of how much melt is present in the upper mantle and how quickly melt moves. Better constraints of the permeability of partially molten rocks may help reconcile these observations and shed light on one of the most fundamental geological processes, melt transport in the mantle. The permeability of partially molten rocks depends on grain-scale melt distribution and topology. Accurate estimates of permeability require quantitative knowledge of melt fraction, melt interconnectivity, and grain size. We utilized synchrotron X-ray microtomography to quantify the three-dimensional (3-D) microstructure of several olivine-basalt samples with melt fractions ranging from 0.02 to 0.20. Olivine-basalt aggregates were synthesized at 1350°C and 1.5GPa in a solid-medium piston-cylinder apparatus. We obtained high-resolution (~700 nm) microtomography data of these samples and imaged the olivine-basalt interface by taking advantage of a recently developed edge-enhancing technique called diffraction-contrast tomography. Direct measurements of grain size, melt interconnectivity, and curvature were carried out on the reconstructed 3-D microstructure. Analyses of grain size and connectivity distributions in experimental charges with sintering times of 42, 84, 168, and 336 hours reveal that a minimum sintering time of 168 hours is required to reach textural equilibrium. An interconnected melt network along grain edges is present at every melt fraction with an increasing contribution from melt along grain faces at high melt fraction. Fluid flow simulations were performed on virtual rocks, representing the melt distribution imaged by 3-D microtomography. The velocity and pressure fields inside the melt domain were obtained by solving the Stokes Equations using a finite-volume method. This solution leads to an estimate of the permeability of the sample, which ranges from 3.2×10-16 to 6.3×10-14 m2 for samples with 0.02 to 0.2 melt fraction. Applying a power-law relationship between permeability (k) and melt fraction(Φ), i.e. k = Φn d2 / C, where d is grain size, we find that our data fall close to the exponent n = 3 and the geometric factor C ~ 50. These results place important new constraints on rates of melt migration and melt extraction within the partially molten regions of the mantle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T13G2713M
- Keywords:
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- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 5114 PHYSICAL PROPERTIES OF ROCKS / Permeability and porosity;
- 5139 PHYSICAL PROPERTIES OF ROCKS / Transport properties;
- 8105 TECTONOPHYSICS / Continental margins: divergent