Submillennial timescales of eruptible rhyolite magma: Converging evidence from Aira Caldera, Japan

Recent studies debate whether caldera-forming silicic magma is primarily stored in a crystal-poor, eruptible state for >100,000 years (kyr) (“warm storage”) or exists as a crystal-rich “mush” with defrosting events causing brief periods (<1 kyr) of eruptibility (“cold storage”). To address this, we examined Aira Caldera (Japan), where the 400–cubic kilometer Aira-Tanzawa (AT) eruption (30,000 years ago) was preceded by three smaller eruptions over 3.5 kyr. All eruptions consist of roughly similar high-silica rhyolite stored at comparable depths (75 to 175 megapascals). Diffusion modeling in quartz and plagioclase is consistent and gives short residence times (<1 to 2.5 kyr) for all eruptions with no progressive increase up-section. The AT eruption also shows signs of hotter, less evolved magma and higher lithium concentrations, possibly indicating greater mush rejuvenation by a hotter fluid-rich recharge. We suggest that the 400–cubic kilometer AT magma was eruptible for less than 1 kyr, consistent with the hypothesis that silicic magmas are primarily held in cold storage with brief periods of eruptibility.