Physiochemical processes in the nuclear cloud: a record from aerodynamic Trinity fallout
Tuesday, 16 December 2014
We present results of the first detailed microanalytical study of aerodynamic glassy fallout from Trinity, the world’s first nuclear device. The aerodynamic morphology of trinitite beads is consistent with their origination inside the nuclear cloud and solidification prior to reaching the ground. Existing conceptual models of aerodynamic fallout formation involve progressive condensation about a nucleus of ground material, followed by diffusive compositional homogenization. Microstructural and chemical composition data from eleven trinitite beads instead suggest that fallout records a multitude of physiochemical processes, including partial melting, agglomeration, volatile exsolution, condensation, melt mingling, and quenching. SEM-based mapping and EPMA data show that beads are composed of variable amounts of four distinct constituents – alkali-rich glass; silica glass (lechatlierite); Ca-Mg-Fe-bearing glass; and unmelted or partially melted mineral aggregates. Smooth, ovoid trinitite beads (2-5 mm diameter) comprise vesicular alkali and silica glass blebs with the compositions of fused feldspar and quartz grains, respectively, embedded within CaMgFe glass. Flattened or elongate beads (4-8 mm long axis) are mineral aggregates encased in a rind of predominantly CaMgFe glass. Increasing vesicularity toward the glass-aggregate interface suggests volatile exsolution as hotter melt contacted cooler mineral debris. CaMgFe glass is compositionally heterogeneous, with SiO2 contents ranging from basaltic to rhyolitic, and compositional banding indicating low-viscosity flow. Contacts between alkali, silica, and CaMgFe glasses are sharp to slightly diffuse, and EPMA traverses across glass contacts confirm limited diffusive exchange of major and minor elements. Autoradiography of trinitite shows that radioisotopes are confined to the CaMgFe glass, where they are patchily distributed, supporting limited diffusive mobility of trace elements as well. Overall, the microscale compositional heterogeneity of CaMgFe glass suggests that its precursor melt formed by condensation from the detonation-generated plasma (~8400 K) and quenched rapidly upon contact with cooler minerals and melts during turbulent mixing within the nuclear cloud.