Peperite as a record of early-stage phreatomagmatic fragmentation processes: An example from the Hopi Buttes volcanic field, Navajo Nation, Arizona, USA

Peperite is the product of near-surface magma-wet sediment mingling. Some peperites are inferred to record 'frozen' coarse mixing-stage fuel-coolant interactions (FCIs), and hence can be used in attempts to determine specific parameters related to phreatomagmatic fragmentation, such as water/magma mass ratios, confining pressures, and vent-region hydrology. Two lamprophyric dikes in the Miocene-Pliocene Hopi Buttes volcanic field, Arizona, contain ellipsoidal regions of dilation (buds) associated with peperite. These peperite-rimmed zones occur within identifiable stratigraphic horizons, and at known depths beneath the elevation of the local pre-eruptive surface. The presence of both fluidal and blocky juvenile clasts, combined with physical aspects of the sediment-lamprophyre contacts, suggest that coarse mixing-stage FCI was a potential fragmentation mechanism within these systems. Confining pressures calculated for sites of single buds range from 0.6 to 1.0 MPa. Average inferred water-magma ratios within peperite bodies at the time they formed vary from 0.09 to 0.16, consistent with experimental values for water-melt mixing ratios necessary for explosive heat transfer. A number of factors may have inhibited further explosive activity in these peperite-forming mixing events, including anisotropic saturation of the host sediment, low confining pressures and water/melt ratios, and damped explosive interactions resulting from increased density, viscosity, and surface tension of sediment-water coolants. Nevertheless, these deposits record localized thermohydraulic interactions between magma and wet sediment, and provide valuable information about the geometry and mixing ratios present during peperite formation.