CHARACTERIZING BIOTITE AND MICROFRACTURE PROPERTIES TO UNDERSTAND REGOLITH PRODUCTION

During chemical weathering, biotite grains expand and modify the stress field around them. A longstanding hypothesis states that these stresses can be large enough to generate microfractures in the rock matrix, and that this can be an important mechanism for weakening rock and facilitating regolith production in biotite-bearing rocks worldwide. We seek to better understand this process by investigating biotite and microfracture properties in weathered rock. Here we present new measurements of biotite size, shape, and spatial density from fresh bedrock and rindlet thin sections in a Rio Blanco tonalite core (Luquillo Critical Zone Observatory), inferred from a machine learning algorithm for mapping mineralogy in thin sections. We also present microfracture density, length, width, orientation, and mineral association extracted from many high-resolution back-scattered electron image patches from the same thin sections. Combining these observations permits documentation of relationships between mineralogy and microfracturing. Our preliminary data show a higher microfracture density in the rindlet zone, consistent with qualitative descriptions of the rindlet zone in previous studies. Furthermore, the highest microfracture density was seen in a sample with a higher-than-average biotite abundance, consistent with the hypothesis that local variations in biotite abundance could promote the transition from rindlet to saprolite. An advantage our methods is that they can be applied to any granitic lithology. Future work will focus on the Southern Sierra Critical Zone Observatory, where lithologic variations present an opportunity to explore microfracture characteristics in rocks that span large variations in biotite abundance.