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.