Pore-space distribution and transport properties of an andesitic intrusion
Introduction
The porosity and pore space distribution in rocks provide first order controls on several petrophysical properties, including elastic moduli, diffusivity and permeability (e.g. Guéguen and Palciauskas, 1994). A broad pore size distribution may also drive internal redistribution of mass due to curvature effects on chemical potentials (Emmanuel and Berkowitz, 2007). This may furthermore control the extent to which crystallization processes in the pores of rocks or porous building materials cause fracturing due to growth related stress generation (Scherer, 2004). It is likely that the rate and progress of any fluid infiltration driven volatilization process in porous rocks, including weathering of igneous and metamorphic rocks, serpentinization of oceanic lithosphere, and carbon sequestration by in situ mineral carbonation, is to a large extent affected by the spatial distribution of the pore space as well as by the total porosity.
Recent advances in technology that enable 3D pore structure characterization over a large range of length scales (X-ray tomography, focused ion beam – scanning electron microscopy, neutron scattering, etc.) have dramatically increased our ability to study rock porosity and its effects on rock properties (cf. Zhu et al., 2011, Baker et al., 2012a, Renard, 2012, Keller et al., 2013).
In igneous rocks, porosity is generated when a low-density fluid exsolves from the host magma upon cooling and/or decompression. Bubble formation in rising magmas is a key element in many volcanic processes and has been studied extensively – both experimentally and theoretically (e.g. Sparks et al., 1994, Blower et al., 2002, Yamada et al., 2005, Gonnermann and Manga, 2007, Baker et al., 2012b). Porosity formation during the comparatively slow cooling and crystallization of intrusive rocks has received less attention (see, however Simakin et al., 1999), although it is likely to have a major effect on the petrophysical properties of the magmatic rocks as well as on post magmatic metamorphic or weathering related alteration process.
Here we present a 3D pore space characterization of an andesitic sill intrusion from the Neuquén Basin in Argentina obtained by computed X-ray microtomography. We then analyze the transport properties of a system in which diffusion takes place through a non-percolating pore space, and a matrix in which diffusion is controlled by 2D planar objects such as microfractures and grain boundaries. Our analysis shows that large pores have a higher probability of being part of the transport network than small pores, and thus that large pores will be preferential sites of reaction during post magmatic alteration. This provides a new explanation of why weathering reactions is localized onto only a fraction of the porosity, and why the pore space does not get clogged after incipient reaction.
Section snippets
The rock
The 8–10 m thick sill intrusion investigated here is andesitic to dacitic in composition (62–65 wt% SiO2) and it was sampled near Cuesta del Chihuido in southern Mendoza, Argentina. The intrusion is hosted in the Vaca Muerta Formation of the Medoza Group, a marine limestone-shale unit of Upper Jurassic to Lower Cretaceous age (Leanza and Hugo, 1978). Field relations were described by Jamtveit et al. (2011).
Mineral analysis was performed using the Cameca SX100 electron microprobe at the
The pore space
Two samples of unweathered homogeneous andesite were characterized by multiscale X-ray computed micro-tomography (μCT). Three datasets were obtained at different spatial resolutions. Low resolution data were obtained using a Nikon Metrology model XT H 225 LC industrial type μCT scanner at the Norwegian Geotechnical Institute with a voxel size of 6.6 μm for a 6 mm sized cubic sample. Two additional data sets from a single sample were obtained at spatial resolutions of 0.56 and 2.8 μm on beamline
Discussion
Most studies of pore size distribution in magmatic rocks have been conducted using vesicular volcanic rocks, and both exponential and power law size distributions have been reported (cf. Baker et al., 2006, and references therein). Models that include nucleation and diffusion controlled growth of vesicles lead to an exponential size distribution (Mangan and Cashman, 1996) unless multiple nucleation events are invoked (Blower et al., 2002). Power law distributions on the other hand are often
Conclusions
This study shows that degassing of partly crystalline intrusions may give rise to a power law distribution of pore volumes over a volume range exceeding five orders of magnitude. This scaling behavior may reflect coalescence of pores at the crystal–melt boundaries. As a consequence, large pores tend to be localized near grain boundaries.
Weathering and, by inference, other fluid infiltration driven alteration processes in andesite is associated with preferential mineral growth in the largest
Acknowledgments
We thank Paul Meakin who made several improvements to the manuscript and Elodie Boller at ESRF for the technical help during the tomography scans. We also thank Marcin Dabrowski and Dani Schmid for insightful comments regarding modelling, and Don Baker and an anonymous reviewer for valuable comments that helped us to clarify the paper. This study was supported by a Center of Excellence grant to PGP (grant no. 146031) from the Norwegian Research Council. BJ also benefited from support from the
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