A new approach to quantification of metamorphism using ultra-small and small angle neutron scattering

Abstract

In this paper we report the results of a study using small angle and ultra-small angle neutron scattering techniques (SANS and USANS) to examine the evolution of carbonates during contact metamorphism. Data were obtained from samples collected along two transects in the metamorphosed Hueco limestone at the Marble Canyon, Texas, contact aureole. These samples were collected from the igneous contact out to ∼1700 m. Scattering curves obtained from these samples show mass fractal behavior at low scattering vectors, and surface fractal behavior at high scattering vectors. Significant changes are observed in the surface and mass fractal dimensions as well as the correlation lengths (pore and grain sizes), surface area to volume ratio and surface Gibbs Free energy as a function of distance, including regions of the aureole outside the range of classic metamorphic petrology. A change from mass-fractal to non-fractal behavior is observed at larger scales near the outer boundary of the aureole that implies significant reorganization of pore distributions early in the metamorphic history. Surface fractal results suggest significant smoothing of grain boundaries, coupled with changes in pore sizes. A section of the scattering curve with a slope less than −4 appears at low-Q in metamorphosed samples, which is not present in unmetamorphosed samples. A strong spike in the surface area to volume ratio is observed in rocks near the mapped metamorphic limit, which is associated with reaction of small amounts of organic material to graphite. It may also represent an increase in pore volume or permeability, suggesting that a high permeability zone forms at the boundary of the aureole and moves outwards as metamorphism progresses. Neutron scattering data also correlate well with transmission electron microscopic (TEM) observations, which show formation of micro- and nanopores and microfractures during metamorphism. The scattering data are, however, quantifiable for a bulk rock in a manner that is difficult to achieve using high-resolution imaging (e.g. TEM). Thus, neutron scattering techniques provide a new approach to the analysis and study of metamorphism.

Introduction

The microstructure of pore space in sedimentary rocks and its evolution during reaction with pore- or fracture-contained fluids is a critically important factor controlling fluid flow properties in geological formations, including the migration and retention of water, gas and hydrocarbons, and the evolution of both contact and regional metamorphic terranes. The size, distribution and connectivity of these confined geometries (pores, fractures, grain boundaries), collectively dictate how fluids of various chemistry migrate into and through these micro- and nano-environments, wet, and ultimately react with the solid surfaces. In order to interpret the time-temperature-pressure history of any geological system, the physical and chemical “fingerprints” of this evolution preserved in the rock must be fully explored over widely different length scales from the nanoscale to the macroscale.

In the case of a sedimentary rock exposed to fluids at extreme temperatures and pressures at the contact with an intrusive magma a fundamental question is how much energy, either in the form of heat or stress, is available to do work, and how is this energy expended. In a contact aureole a significant portion of the energy budget must be expended outside the range of the inner aureole where classic metamorphic reactions occur. Some of this heat will simply be lost, but some will be expended in recrystallization and grain growth in far-field rocks, yielding more or less recognizable marbles or other metamorphic textures but possibly few, if any, of the classic metamorphic mineral assemblages. Because there are few metamorphic assemblages to study, little attention has been paid to these “far field” rocks even though simple observations commonly show that what was initially a limestone is clearly now a marble, and thus has been significantly affected by the metamorphic event.

Mineral growth due to recrystallization can be particularly diagnostic of thermal evolution. It is reasonable to assume that the earliest stages of metamorphism involve lowering the surface free energies of individual mineral grains by growth and/or surface smoothing. Standard techniques for looking at such phenomena, however, such as transmission electron microscopy, suffer from several drawbacks. First, they are, by nature, micro-analytical techniques, and thus may not describe the rock as a whole. Second, the images obtained are difficult to quantify in the same way the thermodynamics of a system is quantified.

In addition, analysis of two of the most critical properties of a contact metamorphic system, permeability and porosity, has proven especially elusive. A summary of this problem was presented by Baumgartner et al. (1997) who noted that “quantitative models of metamorphic fluid flow have largely overlooked the importance of permeability…”. They showed that studies of sedimentary basins and soils have documented both horizontal and vertical permeability variations and variations over spatial scales of many orders of magnitude, and noted that no in-situ permeability measurements were available for high-grade contact metamorphic rocks. Gerdes et al., 1995, Gerdes et al., 1998 showed that stochastic local permeability variations can have very large effects on flow patterns and temperatures in hydrothermal systems. Several authors have noted that large-scale flow in real systems may, in fact, be dominated by flow in multiscale fractures (e.g. Titley, 1990, Ingebritsen and Sanford, 1998), and may be temporally episodic, and enhanced by the effects of excess mixing volumes in H₂O–CO₂ fluids (Labotka et al., 2002).

Porosity should be easier to quantify than permeability, and linked to it. Norton and Knapp (1977) describe several types of pores (residual, diffusion, flow), and noted the effects of pore shape as well. They also measured the porosities of a number of different rock samples. Baumgartner et al. (1997), however, showed that paleo-pore structures are often poorly preserved. While they were able to study relict porosity in several environments, these were essentially special cases. In addition, very small pores, which may be quite abundant along grain boundaries, and can play important roles in controlling overall flow patterns, may be hard to observe petrographically, and even harder to quantify in a statistically meaningful manner. Even the porosity of the unmetamorphosed country rocks that form the boundary conditions in time and space for the metamorphic event may be poorly known, as are the “final” conditions of rocks in the aureole. Thus, a technique that could directly quantify rock porosity in a statistically meaningful manner, at least at the hand-sample scale, would be extremely useful.

Elastic scattering techniques provide a potential solution to this dilemma. Neutron scattering in particular is highly penetrating, and looks at thin-section scale samples while providing data on changes in grain surface and grain/pore distribution properties on length scales ranging from approximately 10 Å (depending on the incoherent background scattering level) to ∼3 μm. While the concept of using neutron or other scattering techniques (e.g. light, X-ray) to look at rocks is not new, work to date on scattering has been somewhat limited, although a number of materials have been studied (cf. Schmidt, 1989, Radlinski, 2006). These include coals and hydrocarbon source rocks (Bale and Schmidt, 1984, Reich et al., 1990, Radlinski and Radlinska, 1999, Sastry et al., 2000, Sen et al., 2001, McMahon et al., 2002, Prinz et al., 2004, Radlinski et al., 1996, Radlinski et al., 1999, Radlinski et al., 2000a, Radlinski et al., 2000b, Radlinski et al., 2000c, Radlinski et al., 2004, Connolly et al., 2006), sandstones, shales, and carbonates (Wong et al., 1986, Triolo et al., 2000, Triolo et al., 2006, Sen et al., 2002), clays (Knudsen et al., 2004), and even igneous rocks (Lucido et al., 1985, Lucido et al., 1988, Lucido et al., 1991, Floriano et al., 1994, Kahle et al., 2004, Kahle et al., 2006).

A wealth of structural information can be derived from scattering data. Radlinski et al. (1999) point out that rocks studied in this manner display the most extensive fractal systems observed in nature, with scaling over at least three orders of magnitude and sometimes higher (Bale and Schmidt, 1984 report 7.5 orders on coal). Both surface and mass fractal behavior have been observed, which describe the nature of the grain/pore boundary and the statistical distribution of pores within the sample, respectively. The characteristic lengths associated with these fractal systems describe the average maximum pore size and grain size of the rock. Scattering data also provide information on the pore volume and surface area/volume ratios, and therefore on changes in surface Gibbs Free energy and entropy during the metamorphic process. While it may be true, as noted by Baumgartner et al. (1997), that porosity in many high-grade parts of the aureole no longer reflects that during metamorphism due to pore filling and collapse, insight into their evolution may nonetheless be obtained by analysis of lateral spatial variations – from high-grade to unmetamorphosed – around the aureole. In addition, the extent and importance of stochastic variation within the initial country rock may be assessed. Thus these scattering properties provide an avenue for quantification of metamorphism, and can do so at low-grades, or for rock compositions where the utility of traditional metamorphic techniques is limited.

The purpose of this study is to use neutron scattering to quantitatively investigate changes in a marble during contact metamorphism, including changes in porosity, pore distributions (characterized by mass fractal dimensions), and grain surface roughness (characterized by surface fractal dimensions and derived surface to volume ratios and surface Gibbs Free Energy), and to show how scattering techniques provide a powerful new tool for analysis of metamorphism, as well as recrystallization phenomena in ceramics, metals and other materials. It will clearly be of interest to link these measurable quantities to the thermodynamics of the metamorphic process, as well as the rheology of the rocks involved (cf. Tanaka, 1992, Tanaka, 1993, Streitenberger et al., 1996) and to couple these to experimental investigations. These topics will be addressed in future work.