Summary

Cathodoluminescence (CL) imaging can be applied to discriminate quartz types in mudrocks, a determination not readily made from bulk chemical or bulk mineralogical analyses. Matrix-dispersed microcrystalline quartz cement (chert) can have a significant impact on mechanical rock properties and so discrimination of this quartz type from other forms of authigenic quartz and from detrital quartz contributes to our understanding of reservoir quality in fine grained sedimentary systems. This paper reviews CL imaging methodology and illustrates a variety of quartz types as seen in CL.

Introduction

The evolution of mechanical rock properties is a notable effect of diagenesis, as unconsolidated sediments are transformed into hard rocks. The diagenetic processes of compaction and cementation are the main mechanisms that drive the evolution of mechanical rock properties, but cementation is particularly effective in generating rocks capable of brittle behavior because of its capacity to reduce the mobility of grain contacts (e.g., David et al., 1998; Weil et al., 2012; White et al., 2011).

Current understanding of cementation in mudrocks is poor because of the intrinsically small size of crystals constrained to grow within the intergranular pores of fine grained sediments (Milliken and Day Stirrat, 2013). Even scanning electron microscopy (SEM)based imaging techniques provide resolutions that are barely within the range required for observation of mudrock cements that are at most a few microns in diameter. In the common case that cement mineralogy is the same as that of associated detrital grains, imaging techniques sensitive to subtle contrasts in trace elemental composition or crystal defects are required for the discrimination of cements from underlying grains. SEM-based cathodoluminescence (CL) imaging satisfies this criterion and offers the potential for identification of cements in mudrocks as well as for discrimination of different grain types of quartzose composition.

The Nature of Cathodoluminescence and CLimaging

Cathodoluminescence, light emission generated from excitation by accelerated electrons, is one of a wide variety of luminescent phenomena (Leverenz, 1950), several of which are applied to inspection of geological materials (e.g., Waychunas, 1988). Generation and observation of CL can be accomplished by a range of instrumentation, including systems based on light microscopes and also on scanning electron microbeam instruments (Hamilton et al., 1978; Yacobi and Holt, 1990). CL arises from interaction of the energized beam with the outer shell electrons in the luminescing material and for given conditions of excitation, the intensity and wavelengths of CL emission are a complex function of intrinsic CL of the basic crystal lattice, trace elemental substitution, and crystal defects. Time-dependent factors enter into the character of CL because decay of CL emission can be slow relative to the scan rate in electron microbeam instruments (Reed and Milliken, 2003) and because crystal defects can be generated by electron bombardment (e.g., Richter et al., 2003). The excitation volume for CL emission is poorly known, in part because the depth of escape of generated photons relates to the translucency of the crystal.

URTeC 1582467

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