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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

Dian-Bing Wang 1, Ping-Ping Liu1, Haiquan Wei2, Cheng Xu1, Wenliang Xu3

  • Affiliations: 1School of Earth and Space Sciences, Peking University, Beijing, China; 2Jilin Changbaishan Volcano National Observation and Research Station, Institute of Geology, China Earthquake Administration, Beijing, China; 3College of Earth Sciences, Jilin University, Changchun, China 

  • Presentation type: Talk

  • Presentation time: Monday 16:00 - 16:15, Room R280

  • Programme No: 1.7.10

  • Theme 1 > Session 7

Abstract

The Millennium Eruption (ME) of the Tianchi volcano is the most voluminous eruption in the Changbaishan volcanic field. However, the pre-eruptive storage conditions and magma dynamics of the Tianchi magma reservoirs feeding the ME remain elusive. In this study, we investigate the whole-rock, mineral, and glass chemistry of the various ME products, including comenditic white pumices, trachytic grey pumices, and the mafic enclaves. Olivine and clinopyroxene in the white pumices have lower Mg# values than those in the grey pumices, indicating that the comenditic phase is more evolved than the trachytic phase. However, alkali feldspar phenocrysts in both pumices have similar compositions, suggesting pre-eruptive mingling of the comenditic and trachytic magmas. The phenocrysts in the mafic enclaves are equilibrated with the whole-rock composition of Tianchi trachybasalts, indicating mafic enclaves are also trachybasaltic. Thermobarometric calculations show that the ME comenditic and trachytic magmas have temperatures ranging from ~700 to 800 °C and ~800 to 900 °C, respectively, and both are stored at pressures of ~1 kbar, suggesting a single thermally and compositionally stratified upper crustal silicic magma reservoir. The wide pressure range (~2 to 8 kbar) of trachybasaltic magma likely implies multiple trachybasaltic lenses located at mid- to lower-crustal depth. Together with Rhyolite-MELTS modelling results, we propose that a two-stage magma fractional crystallization process of trachybasaltic melts results in the ME silicic melts. The recharge and mingling of the trachybasaltic magma with the silicic magma result in tumescence and rejuvenation of the upper crustal magma reservoir, finally triggering the explosive ME.