Micro-analytical perspectives on the trans-crustal magma plumbing system feeding the Millennium Eruption of the Tianchi volcano in the Changbaishan volcanic field, northeast China

Abstract The 946 to 947 AD Millennium Eruption (ME) of the Tianchi volcano with an erupted volume of ~23 km3 dense rock equivalent (DRE) is the most voluminous eruption in the Changbaishan volcanic field (CHVF). However, the pre-eruptive storage conditions and magma dynamics of Tianchi’s ME-feeding reservoirs, which are fundamental to unraveling magma generation, migration, and evacuation throughout the prolonged (>5 Myr) eruptive histories of the Tianchi volcano, remain elusive. In this study, we investigate mineral, glass, and whole-rock chemistry of the various ME products, including comenditic white pumices, trachytic grey pumices, and mafic enclaves. Olivine and clinopyroxene phenocrysts in the comenditic white pumices have Mg# values (Mg# = 100×MgO/(MgO+FeOt) on a molar basis) significantly lower than those in the trachytic grey pumices. The alkali feldspar phenocrysts in the two pumices span a similar compositional range (Ab51-66An0-5Or32-46), likely resulting from pre-eruptive mingling of the comenditic and trachytic magmas. The phenocrysts in the mafic enclaves share similar compositions with those of the post-shield (~1 to 0.01 Ma) trachybasalts of Tianchi volcano and are in equilibrium with the trachybasaltic magma represented by its whole-rock compositions, indicating that the mafic enclaves are trachybasaltic in composition. Thermometric calculations based on matrix glass and mineral equilibria yielded distinct temperature estimates for the ME comenditic and trachytic magmas. For the comenditic magma, estimated temperatures include 749 ± 20 °C (Fe-Ti oxides), 822 ± 3 °C (clinopyroxene-melt), 777 ± 3 °C (alkali feldspar-melt). For the trachytic magma, the corresponding estimates are 815 ± 25 °C, 917 ± 22 °C, and 840 ± 9 °C. Barometry results show that comenditic and trachytic magmas are both stored at pressures of 0.8 to 1.2 kbar. These lines of evidence indicate the presence of a single thermally and compositionally stratified upper crustal silicic magma reservoir. In contrast, the trachybasaltic magmas exhibit higher temperatures (1097 ± 18 °C) and a wide range of storage pressures (2.0 to 7.7 kbar), likely imply multiple trachybasaltic lenses located at mid- to lower-crustal depth. Reinforced by Rhyolite-MELTS modelling results, we propose that a two-stage trans-crustal magma fractional crystallization process of the trachybasaltic melts produced the erupted silicic melts. Recharge and mingling of the trachybasaltic magma with the silicic magma result in tumescence and rejuvenation of the upper crustal magma reservoir, likely triggering the explosive ME. These findings provide crucial insights into the mechanisms driving explosive silicic eruptions and have important implications for hazard assessment and monitoring at Tianchi volcano.