CONSTRAINTS ON SEDIMENT-MAGMA MINGLING FROM PEPERITES FORMED IN A SUBAQUEOUS BASALTIC FISSURE ERUPTION

Phreatomagmatic eruptions occur due to the interaction of magma with water and/or wet sediment. One example is 71 Gulch, a shallow (<15 m below the surface), well exposed basaltic fissure structure located in the western Snake River Plain, southwestern Idaho. This field site provides excellent exposure of the intrusive to eruptive interface. Between the Miocene and Pleistocene, Lake Idaho inundated the western Snake River Plain, depositing lacustrine host sediment and subaqueous pyroclastic deposits, such as peperites. Peperites are classified into two different textures: blocky and fluidal, both of which were observed at 71 Gulch, and sometimes in the same outcrop. A detailed characterization of these textures is used to better understand the conditions that yield sediment-magma mingling both in large-scale (m) and in small-scale (mm) peperites.

Basaltic intrusions are widespread throughout the field area and preserve peperite deposits from tens of centimeters to tens of meters vertically. Blocky and fluidal peperites occur in both types of sand-sized host sediment: lacustrine and pyroclastic. The blocky peperite contains angular, fragmented basaltic blocks that measure 8 – 43 cm wide dispersed up to 142 cm into the host sediment from the dike source. The basaltic blocks are all aphanitic, do not show preferential alignment, and have jagged and angular margins. Some blocks contain vesicle bands of fragmented dikes. The fluidal peperite contains irregular, aphanitic basaltic chunks with more continuous margins that measures up to tens of centimeters long. The basaltic peperite at 71 Gulch that occurs in both lacustrine and pyroclastic sediment shows that the type of host material does not control the various sediment-magma mingling textures. Further investigations into the sorting of the host material, confining pressure, and magma flux will help create constraints on mingling and resulting heat transfer rates, thus influencing passive or explosive phreatomagmatic activity.