U-Th zircon dating of the great Millennium eruption of Changbaishan volcano: Evidence for rapid development of a catastrophic eruption

The Changbaishan volcano extending across the border of northeast China and North Korea erupted about 100 km3 peralkaline rhyolites around 1000 AD. This Millennium eruption is one of the two largest explosive eruptions in the past 2000 years. We conducted uranium-thorium dating of zircons from the Changbaishan volcanic rocks. Zircon isochron ages are 9.2±1.2 ka. The rhyolitic magma chamber beneath Changbaishan was formed at 9.2 ka BP (before present) by closed-system fractionation of parental trachytic magmas, and explosively erupted at 1 ka BP. The magma storage time is about 8 ka, which is significantly short compared with typical residence times of large volume explosive eruptions (50-135 ka). This work demonstrates that peralkaline rhyolitic magmas from the Changbaishan volcano can develop into a catastrophic eruptive phase quite quickly. Based on titanium-in-zircon geothermometer and alkali feldspar-glass geothermometer, the rhyolitic magmas were formed at a relatively low temperature (~ 740±40 °C). The short magma storage time and low magma temperature may have helped the Changbaishan large volume rhyolitic magma escape crustal contamination. Changbaishan volcano is still an active volcano. There is a low seismic velocity zone below Changbaishan volcano extending from 10 to over 65 km depth. An electrical conductivity anomaly exists at 20 km depth below the volcano. Numerous hot springs and fumaroles are present on the volcano. Although short storage time of 8000 years does not necessarily mean that the next eruption is imminent, our present study does indicate that the still dangerous Changbaishan volcano is capable of rapidly producing catastrophic, explosive eruptions in the foreseeable future.