The Black Hills Tomahawk obsidian deposit remains enigmatic; two independent geochron studies reveal significant age differences: 55 Ma (Kirchner, Science 1977) vs 10 Ma (Redden et al., Science 1983); both are problematic for preserving unaltered glass. The deposit consists of a dense network of anastomosing perlitic veins (0.1-3mm) encapsulating regions of unaltered glass (10 µm) chemically homogenous border zone separating the unaltered glass from the hydrous veins. This behavior resembles chemical zoning preserved in high-temperature phenocrysts, with distinct core, mantle, and rim compositions. Major elements show opposite mobility when comparing the unaltered glass to the veins and border zone: the behavior of Al, Na, and K (+3/+1 cations) is inverse to Si, Ca, and Ba (+4/+2 cations) with a 1:1 anticorrelation. This behavior differs from multiple studies on cation diffusion in silicate melts and high temperature glasses, which document smooth concentration gradients. Our results suggest cations in glasses at temperatures below the glass transition temperature are frozen into fixed sites, and subsequent mobilization requires coupled multi-variate substitutions to accommodate size and charge constraints.
