Paper No. 7
Presentation Time: 10:30 AM
AN ALTERNATIVE PROCESS FOR LARGE SILICIC CALDERA ERUPTIONS
MANAOIS, Alexander1, LUNDSTROM, Craig C.
2, CHAKRABORTY, Pinaki
3, FINLAYSON, Valerie
4 and LI, Xiaoxiao
1, (1)Dept of Geology, University of Illinois, 1301 W Green St, Urbana, IL 61801, (2)Dept of Geology, Univ of Illinois, 1301 W Green St, Urbana, IL 61801, (3)Fluid Mechanics Unit, Okinawa Institute of Science & Technology, 1919-1 Tancha, Onna-son, Okinawa, 904-0495, Japan, (4)Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building, 1301 W. Green St, Urbana, IL 61801, manaois2@illinois.edu
The origin of large silicic caldera eruptions remains poorly understood. Despite debate about whether silicic volcanic and plutonic rocks reflect a similar differentiation process, examination of NAVDAT data shows no difference between the compositional trends of these different igneous rocks. The two most accepted views of silicic caldera volcanism, extraction of melt from a crystal mush in the shallow upper crust or partial melting in the lower crust and eruption upon arrival in the upper crust, have troublesome inconsistencies with existing observations. Above all, geophysically imaged blobs of mostly melt have yet to be observed in Earth’s upper crust. Here we advocate a third possibility—that the magmas reflect 100% melting of a previously emplaced silicic mush. Although previously proposed, this model is not well accepted because of the excessive amount of heat required for wholesale melting.
Here, we propose a two-step model for silicic magma body formation and subsequent ignimbrite eruption. In the first step, a zoned silicic mush forms by a top down thermal migration zone refining process similar to that proposed for granitoids (Lundstrom, 2009). As suggested by thermal migration experiments, the mush includes an interstitial hydrous peralkaline melt, which has the unusual property of retrograde immiscibility with water. This melt is key to the second step of eruption triggering. The eruption process starts when underplating melts reach a P-T condition depth where the immiscibility boundary is crossed. A water rich vapor phase forms at the base of the mush and then carries heat to the overlying mush causing it to also cross the immiscibility boundary. This provides a self-sustaining process that leads to a run-away process causing wholesale melting of the silicic mush. This work will present natural isotopic observations, hydrothermal diamond anvil cell experiments and numerical modeling providing the basis for this alternative model for silicic caldera eruptions.
