Dallas Geological Luncheon

Cathodoluminescence Imaging for Understanding Mechanisms of

Rock Property Evolution in Mudrocks

Kitty L. Milliken

Bureau of Economic Geology, University of Texas, Austin, TX, USA

Kinetic barriers inhibit the low-temperature nucleation and growth of quartz. Thus, the formation of authigenic quartz in fine-grained systems is preceded inevitably by the early stages of compaction. The availability of nucleation sites and the abundance and sizes of pores into which quartz cement can be emplaced are limited by the compactional state at the time of precipitation initiation. The two main types of grain alteration that are proposed to yield authigenic quartz, dissolution of biogenic opal and illitization of smectite, occur in different temperature ranges, and thus, contrasting compactional regimes. The size (1-2 µm) and abundance of authigenic microquartz (up to 10s of volume percent) that arises from the biogenic opal pathway is consistent with the compactional state of muds at the temperatures of the opal-A to opal-CT transition. In the absence of early cementation that inhibits compaction, mudrocks at the temperatures of illitization have few available nucleation surfaces for direct quartz precipitation and little available pore space (mostly nm-scale) to accommodate cementation. Nucleation at the nanometer scale may encounter further kinetic barriers as a consequence of the large surface to volume ratios that characterize very small crystals.

The composition of the detrital grain assemblage is an important control on the relative timing of compaction and cementation in fine-grained systems. Carls and sarls, containing, respectively, abundant calcareous and siliceous biogenic grains, are prone to develop authigenic microquartz by the opal reaction pathway. In contrast, tarls, with grains dominated by extrabasinal quartz, feldspar, and smectitic clays, feature diagenesis dominated by compaction that dramatically reduces pore space prior to the onset of volumetrically significant grain reactions such as albitization and illitization. The contrasting diagenetic pathways followed by different mudrock grain assemblages have significant implications for the evolution of bulk rock properties such as porosity and brittleness. As a consequence of early cementation carls and sarls are far more likely to have significant preservation of larger intergranular pores and to manifest brittle behavior.

Cathodoluminescence (CL) imaging has provided vital data that underpin the concepts described here. The discrimination of the many petrographic forms of quartz (detrital and authigenic) depends heavily on the integration of elemental X-ray mapping and CL imaging.

Location: Brookhaven College Geotechnology Institute
3939 Valley View Lane
Farmers Branch , Texas 75244

Date: March 20, 2018, 11:30 a.m. - March 20, 2018, 1 p.m.