The waxing and waning of eruptive activity at giant volcanic systems like the Toba Caldera Complex of Sumatra, Indonesia or at Yellowstone Caldera, USA, raise fundamental questions about the tempo and physical evolution of these systems, and about the phenomena that could trigger the next eruption. This project targets the highly explosive 75 ka Youngest Toba Tuff (YTT) - at >2800 km3 of magma, the largest eruption experienced by the human race - and the subsequent, more quiescent eruptions that could signal renewal of the sub-volcanic system, or its death throes. Crystals contained within volcanic rocks like the YTT are effective recorders of the origin, assembly, and thermal history of the magmatic systems responsible for them. For the YTT, the crystal record is extraordinarily complicated, and existing results are fragmental and sometimes contradictory. Estimates for the interval of near-eruption crystallization, for example, range from a few hundred years to several tens of thousands of years, with drastically different implications for the magnitude and duration of volcanic unrest. This study aims to reconcile the various disparities in crystal-scale records using samples and analyses strategically selected to complement, extend, and more carefully examine previous work. Investigations will focus on long term changes in crystal growth environments, mineral affinities between the YTT and post-YTT eruptions, and the residence time of crystals at liquid-dominated rather than solid-dominated conditions including the duration of system (re)activation from the conditions and duration of near-eruption mineral growth. This study will test the hypothesis that different portions of the YTT and post-YTT eruptions selectively sample distinct portions of the magmatic system in space and time, including a liquid-dominated cap reservoir, remobilized crystal-rich domains (mush), and the background of magmatic inputs into the system, and examine whether a resident silicic reservoir is developing or dying beneath the Toba Caldera Complex. The PI and Dr. Craig Chesner (EIU), the leading expert on the Toba Caldera Complex system, will collaborate on this project, train graduate students at NAU in conjunction with a new Ph.D. program emphasis in Earth and Planetary Systems, and mentor undergraduates at NAU and EIU.
The Toba Caldera Complex is a natural laboratory in which to investigate how highly explosive silicic systems are generated; numerous investigations have also speculated about its impact on major climatic and anthropologic events. The chemical dynamics of magma assembly, the origin of compositional zonation in large volume eruptions, and the timescales of mineral growth, storage, and mush reactivation in high- and low-volume eruptions emitted from the same magmatic system are the main foci of this study. Systematic characterization of representative samples (via serial sectioning of pumices, petrographic studies, and SEM and CL imaging) and strategic in situ chemical and isotopic mineral analyses (via electron and ion microprobe analyses of selected trace elements and 238U-232Th-230Th-207Pb- 206Pb) in the minerals quartz, allanite, and zircon are being undertaken, with complementary analyses of other minerals. A comprehensive approach is needed to distinguish cognate minerals from minerals accidentally incorporated during or before eruption, and to examine the fidelity - to each other and to magmatic conditions - of increasingly-used temperature and chemical proxies. Delimiting the duration of magma remobilization and the frequency of post-caldera magmatism will also contribute to understanding magmatic signals in the lead-up to eruptions, and the potential for future volcanic hazards associated with the Toba Caldera Complex.
