Complex storage of rhyolite in the central Snake River Plain

Analysis of hundreds of co-existing individual clinopyroxene crystals and crystal aggregates from rhyolitic ignimbrites of the central Snake River Plain has revealed that magma storage prior to eruption was complex. Crystal aggregates are dominated by plagioclase, augite, pigeonite, ilmenite, magnetite and zircon, and commonly lack sanidine and quartz which may be present as single crystals. In the ignimbrites, multiple compositions of pigeonite and augite may occur as single crystals, yet within the crystal aggregates, only single compositions of augite and pigeonite are recorded. Multiple crystal aggregates may be present within a deposit containing the whole spectrum of clinopyroxene compositions within the unit, yet only one pair of clinopyroxene compositions occur in a single aggregate. Evidence of compositional zonation is rare to absent both throughout the deposit as a whole and within individual pyroxene grains. The restricted compositions of the crystal aggregates are interpreted as reflecting a magmatic system in which the magma was physically segregated into a number of separate bodies evolving in parallel prior to eruption. One case was recorded whereby the composition of a crystal aggregate did not agree with the compositions of single crystals in that ignimbrite. This aggregate represents a silicic magma not currently recognised in surface outcrops and reflects entrainment of surrounding crystal mush. Although multiple compositions of pigeonite and augite appear to be present only in ignimbrites, no relationship exists between the volume of erupted material and the number of clinopyroxene compositions present within the deposit.