Constraining magma storage conditions of the Toba magmatic system: a plagioclase and amphibole perspective

Silicic magma reservoirs are responsible for producing the largest explosive eruptions in the geologic record. Petrologic and geochronological data provide evidence for these systems spending substantial periods of time (10⁴–10⁵ yrs) within the upper crust prior to eruption; however, the long-term thermochemical evolution of these systems is not fully understood, as existing petrologic data make it challenging to quantify the time interval a magmatic system has spent at certain temperatures, or its “thermal history”. Here, we investigate the 74 ka Youngest Toba Tuff (YTT), one of the largest explosive eruptions in the geologic record, to better constrain the long-term thermal evolution of its magmatic system. We combine forward models of Sr diffusion in plagioclase and hornblende, mineral thermometry, and pre-existing trace-element evolution models to quantify the thermochemical evolution of the YTT magmatic system. We find that plagioclase crystals record decades to centuries of storage at temperatures \(>\sim\)750 \(^\circ\)C, while hornblende records up to 6200 years at the same temperatures. Hornblende crystallizes at temperatures around 820 \(^\circ\)C and adjusting our diffusion modeling to this temperature results in no more than 900 years at initial crystallization conditions. Combined with previous trace-element modeling work, these results indicate that although there was chemical diversity for long durations in the YTT magma system sufficient to produce unique composition eruptive products, the entire system was experiencing a relatively similar thermal history that did not allow for large bodies of eruptible magma to be present for long periods (\(>>\) 10²–10³ years). Rather, we suggest that magmas within the YTT magmatic system were stored for long durations at thermal conditions where they were uneruptible and only remobilized within a few centuries prior to eruption.