MAGMA SEDIMENT MINGLING AND POSSIBLE SEDIMENT MELTING IN A BASALTIC PHREATOMAGMATIC FISSURE ERUPTION

Phreatomagmatic explosions resulting from the interaction of magma and water/water-saturated sediments occur in all volcanic environments. These interactions are recorded in intrusive and pyroclastic deposits. The sub-lacustrine 71 Gulch basaltic fissure structure in southwestern Idaho has been eroded to expose the shallow (<15 m below surface) volcanic plumbing system while preserving some of the eruptive products. These deposits were investigated to establish the subsurface conditions that contributed to explosive and effusive activity including sediment magma mingling and possible sediment melting.

Of particular interest at 71 Gulch are anomalous mottled light grey to red to black colored vesicular lapilli and bombs that occur in two horizons of the preserved eruptive deposits. Less than half of these clasts have a mm to cm thick basalt coating. These clasts occur within poorly sorted and weakly bedded lapilli tuff and tuff breccias with less vesicular basaltic lapilli and bombs near the main vent structure. Within the vent complex, lapilli sized mottled clasts occur in dikes and vent-fill pyroclastics. The mottled pyroclasts display centimeter to millimeter mingling of weakly vesicular (30-50%) basaltic material with grey to red more vesicular (70-90%) material, and when present, outer coatings are variably (20-40%) vesicular. Mottled pyroclasts commonly have 2 mm sized white to grey crystals (<5%).

Bulk rock analysis of mottled clasts revealed variable silica contents (44-71 wt %), but consistent enrichment in Ca, P and less of Rb, Ba, and Th relative to the basalt. X-ray diffraction analysis of grey and red vesicular groundmass material and white crystals were used to identify which components contributed to Ca and P enrichment. Fluorapatite was found within all groundmass samples. The white crystals were composed of crystobalite and quartz. These all strongly suggest variable chemical contamination of the basalt by more than one external source 1) heated and disaggregated quartz clasts from the sandstone host, 2) heated to melted diagenetic phosphate host layers. Both of these materials are available in the (2 km thick) Glenns Ferry host sediments.