Buoyant hydrous mantle plume from the mantle transition zone.

Magmatism at some intraplate volcanoes and large igneous provinces (LIPs) in continental areas may originate from hydrous mantle upwelling (i.e. a plume) from the mantle transition zone (MTZ) at 410-660 km depths in the Earth's deep interior. However, the ultimate origin of the magmatism, i.e. why mantle plumes could have been generated at the MTZ, remains unclear. Here, we study the buoyancy of a plume by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology. Based on carefully determined water contents of the basalts, the potential temperature of the source mantle is estimated to be 1310-1400 °C, which is within the range of the normal upper mantle temperature. This observation suggests that the mantle plume did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ. Such a hydrous mantle plume can account for the formation of extremely hydrous LIP magmatism. The water was originally sourced from a stagnant slab and stored in the MTZ, and then upwelled irrespective of the presence or absence of a deep thermal plume.

Fingerprint of silicic magma degassing visualised through chlorine microscopy.

Volatile-rich silicic magma erupts either explosively as a jet of a mixture of pyroclasts and high-temperature gas, or non-explosively to effuse lava. The bifurcation of the eruption style is widely recognised as being controlled by the efficiency of open-system gas loss from vesiculated magma during ascent. However, the fundamental question of how the gas escapes from highly viscous magma still remains unsolved because the pathways of gas flow are rarely preserved in dense lava. Here we show that such pathways are visualised in groundmass glass using high-resolution chlorine (Cl) mapping analysis on the rhyolitic lava of the Mukaiyama volcano, Japan. The results showed that the glass was highly heterogeneous in Cl content. A spatial distribution of the Cl content in the groundmass glass showed that volatiles diffused towards most bubbles, but the bubbles collapsed into the dense melt rather than growing. All observations, in combination with melt inclusion analysis, indicate that vesiculation, the formation of interconnected bubble channels, open-system gas loss via the channels, and channel collapse repeated within the period of a few days to two weeks during ascent. This cycle repeated individually in centimetre-sized portions of magma with different timing.