Assessing volumetric strains and mass balance relationships resulting from biotite-controlled weathering: Implications for the isovolumetric weathering of the Boulder Creek Granodiorite, Boulder County, Colorado, USA

•We investigate volumetric strains associated with saprock development at two sites.•Biotite in saprock displays splitting parallel to {001} cleavage planes.•Clay minerals include vermiculite ± biotite/vermiculite, kaolinite ± illite.•CIA values and weathering intensity factors suggest negligible to weak weathering.•Positive volumetric strains are consistent with theoretical values.

An important process often cited in the development of saprock and saprolite is the isovolumetric weathering of granitic bedrock. Evidence for isovolumetric weathering includes the observation that the texture of the granitic protolith often remains recognizable in its weathered products. In contrast to the above often cited view, we present results from a study of two sites located on the 1.65 Ga Boulder Creek Granodiorite near Boulder, Colorado, USA, in which weakly chemically altered saprock derived from tonalitic and granodioritic–tonalitic bedrock records volumetric strains on the order of 7.6%–8.5% and 7.9%–8.2% respectively. We show that dilation is likely due to the alteration of biotite to vermiculite during the early stages of weathering, and is associated with an expansion normal to the {001} cleavage of ~ 40%. Theoretical calculations of volumetric strains based on the modal biotite present in the protolith support such an interpretation.