Magma plumbing system for the Millennium Eruption at Changbaishan volcano, China: Constraints from whole-rock U–Th disequilibrium

Abstract

A caldera-forming eruption in 946 CE, known as the Millennium Eruption (ME), at Changbaishan (or Baekdusan) volcano, located on the border between China and North Korea, was one of the largest explosive eruptions on Earth in the last 2000 years. In this study, we obtained new whole-rock Sr, Nd, and Pb isotopic data and 238U–230Th disequilibrium data of representative volcanic products of the ME, as well as those of a pre-ME (4–5 ka) eruption, to understand the evolution of the magma plumbing system related to the caldera-forming eruption. The volcanic products of the ME have SiO2 contents ranging from ~54 to ~75 wt%, and the whole-rock (230Th/232Th) ratios tend to decrease systematically from basaltic trachy-andesite through trachyte to comendite. The relatively high (230Th/232Th) ratios of the felsic (trachytic and comenditic) volcanic products suggest that they were formed primarily from fractional crystallization of mafic magmas, rather than from partial melting of crustal materials. The basaltic trachy-andesite sample has a distinctly high (230Th/232Th) ratio and the least radiogenic Pb isotopic ratios among ME volcanic products, suggesting that the mafic magma was recently introduced into the main felsic magma system and is hypothesized to have triggered the eruption. The lower (230Th/232Th) ratios of the comendite samples than those of the trachyte samples, as well as published U-series disequilibrium data, suggest that the comendite magmas were produced by mixing old (> ~ 23 ka) and young (<~6 ka) comenditic magmas. The old comendite magma was the main component of ME volcanic products. Pre-ME pantelleritic magmas with distinctly low (230Th/232Th) and high 87Sr/86Sr ratios from those of ME magmas may have formed a short-lived magma chamber until ~4–5 ka, which was isolated from the long-lived, main comendite magma chamber.