Evidence for the reversal of gradients in the uppermost parts of silicic magma reservoirs

Evidence from large-volume ignimbrites indicates that the source-magma reservoirs for most of these voluminous silicic pyroclastic deposits contained monotonic vertical chemical gradients at the time of eruption. However, gradients from a large-volume magma reservoir that produced a group of penecontemporaneous silicic lava domes, but no ignimbrite, show a reversal of the usual ignimbrite pattern. This reversal originated by modification of the usual pattern through minor assimilation of partially melted roof rocks. Eruptions that produced these domes apparently just tapped the uppermost part of their source reservoir. They thereby provide a high-resolution instantaneous view of this variably contaminated part of the magma system. Although not yet reported in the literature, similar reversals should be recorded in other groups of penecontemporaneous silicic domes and perhaps also in the lowermost parts of ignimbrites or in their underlying coeruptive Plinian deposits. The long-standing paradigm for monotonic zoning in large-volume reservoirs of silicic magma may require modification.