Evidence of a Pre-eruptive Fluid Phase for the Millennium Eruption, Paektu Volcano, North Korea

Abstract:

We present initial results of a study of comenditic to trachytic melt inclusions from the Millennium Eruption (ME) of Paektu volcano (AD 946; VEI≥7; 25 km³ DRE). Paektu volcano (aka Changbaishan) is a remote and poorly studied intraplate stratovolcano whose 37 km² caldera is bisected by the political border between North Korea and China, limiting studies of its proximal deposits. ME magmas are predominantly phenocryst-poor (≤3 vol%) comendites plus a volumetrically minor late-stage, more phenocryst-rich (10-20 vol%) trachyte. Sizeable (100-500 µm diameter) glassy but bubble-bearing melt inclusions are widespread in anorthoclase and hedenbergite phenocrysts, as well as in rarer quartz and fayalite phenocrysts. Comparing the relative enrichments of incompatible volatile and non-volatile elements in melt inclusions along a liquid line of descent shows decreasing volatile/Zr ratios suggesting the partitioning of volatiles into a fluid phase. This suggests that current gas-yield estimates (Horn & Schminke, 2000) for the ME, based on the petrologic method (difference in volatiles between melt inclusions and matrix glass), could be severe underestimates. Establishing the composition and quantity of a pre-eruptive fluid phase is the primary goal of this study and has implications for eruption triggering and for modeling the climatic effects of one of the largest eruptions in the last 10,000 years. Including results from Horne and Schminke (2000), melt inclusions from within a single pumice fall unit show a wide range in dissolved volatile contents and magma chemistries. Concentrations of H₂O are moderate (2–3.5 wt% via FTIR), with Cl and F ranging from 500–4600 ppm and 1100–4700 ppm (via EPMA). CO₂ is below the detection limit of 2 ppm (FTIR with N₂ purge) in bubble-bearing melt inclusions, but is detectable (≤56 ppm) in melt inclusions homogenized at 100 MPa and 850–900 °C for ~30 min (conditions also leading to reduction of dissolved H₂O to 0.6–2 wt%). Characterization of a co-existing fluid phase in ME magmas is being investigated with further melt inclusion analyses, mineral-melt-fluid equilibrium calculations, and phase equilibrium experiments on fluid-saturated liquid lines of descent and on volatile solubilities of the ME trachyite-comendite suite.