Magmatic plumbing system of the 3400 BC caldera-forming eruption (Numazawako eruption) of Numazawa volcano as deduced by componentry and whole-rock and mineral compositions of the pyroclastic deposits

Abstract

Magmatic plumbing system of the 3400 BC caldera-forming eruption (Numazawako eruption) of Numazawa volcano as deduced by componentry and whole-rock and mineral compositions of the pyroclastic deposits BC AbstractThe BC caldera-forming eruption of Numazawa volcano (the Numazawako eruption), NE Japan, began with a cataclysmic pyroclastic flow (phase I) followed by a Plinian eruption, phreatomagmatic eruptions, and a second Plinian eruption (phases II-IV, respectively). Petrological examinations revealed that a variety of juvenile pyroclasts were ejected during the eruption. White pumice (WP) rich in euhedral phenocrysts ( . -. wt.% SiO ) dominated the juvenile material expelled during the two early eruption phases, suggesting that WP-forming dacitic magma (~ -°C) constituted the main and upper portions of the pre-eruptive magma chamber. Gray pumice, rich in crystal fragments ( . -. wt.% SiO ), was a minor component of this eruption, produced by the mechanical breakdown of phenocrysts as the WP magma ascended in the conduit. Wholerock compositions of scoria suggest that two different mafic magmas, a low-Ba type (LBa; < wt.% SiO , > °C) and a high-Ba (HBa) type (< . wt.% SiO , > °C), were injected separately into the magma chamber. Diffusion profiles of dacite-derived magnetite phenocrysts in black scoria (BS; -. wt.% SiO ) show that the Numazawako eruption was triggered by the injection of LBa magma. LBa magma mixed with dacite magma in the chamber and erupted as BS during the two early eruption phases. In contrast, gray scoria (GS; . -. wt.% SiO ) has a distinct chemical composition found only during the two later eruption phases. The abundance and whole-rock composition of GS suggest that injection of HBa magma occurred immediately before or during phase III; the HBa magma mixed with BS-forming magma in the chamber to produce the GS magma. This second magma injection probably resulted in over-pressurization of the chamber, thereby triggering the second Plinian eruption.