9000-YEAR-OLD ZIRCONS FROM THE LARGE-VOLUME MILLENNIUM ERUPTION OF CHANGBAISHAN VOLCANO: EVIDENCE FOR RAPID MAGMA DEVELOPMENT

Large-volume explosive volcanic eruptions probably present the ultimate natural hazard to mankind, yet our understanding of such catastrophic eruptions is relatively incomplete. The Changbaishan (Baitoushan) volcano extending across the border of China/North Korea erupted ~100 km³ peralkaline rhyolites around 1,000 AD. This great eruption, known as the ‘Millennium eruption’, has a Volcanic Explosivity Index of 7, comparable in magnitude to the great Tambora eruption of 1815 AD. The Millennium eruption mainly produced peralkaline rhyolitic pumice and ash. The rhyolitic pumice formed by pyroclastic flows is up to 70 meters thick near the caldera and extends more than 70 km from the crater. The fallout fan from the Plinian and co-ignimbrite ash extends as far as north Japan, ~1200 km.

We discovered zircon crystals in the rhyolitic pumice and dated the zircons using U-Th disequilibrium method. Eighteen zircons yield a quality U-Th isochron age of 9.2±1.2 ka (2σ) with MSWD of 1.4. Our data thus indicate that the rhyolitic magmas were stored in the crust for only 8.2±1.2 ka prior to eruption. This storage time is very short compared with typical residence times of other 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 ºC). The short magma storage time and low magma temperature may have helped the Changbaishan large-volume rhyolitic magma escape crustal contamination. Although short storage time of 8000 years does not necessarily mean that the next eruption is imminent, our present study does indicate that the Changbaishan volcano is capable of rapidly producing catastrophic, explosive eruptions in the foreseeable future.