Fig. 2: Diagram of major species’ composition for CF fumaroles.
From: Escalation of caldera unrest indicated by increasing emission of isotopically light sulfur
The H2O, CO2 and H2S concentrations are scaled by factors 1, 3 and 300, respectively, for presentation purposes. The 1983–2023 fumarole results (Supplementary Table 1) are chronologically distinguished (by colour tones) to emphasize temporal evolution. The 1970 and 1979–1983 results for other CF fumaroles are also shown (Supplementary Information). Fumarole composition is contrasted against (1) the modelled composition of magmatic gases formed during decompressional degassing of trachybasaltic CF melts (Fig. 5); (2) the modelled composition of hydrothermal gases in which H2S fugacity is controlled by S-bearing hydrothermal minerals (pyrite, native sulfur and anhydrite) (Methods); (3) the observed fumarole compositions at eight global calderas/hydrothermal volcanoes that have recently been in a state of unrest (Supplementary Information; YWS, Yellowstone; LVC, Long Valley Caldera; NYS, Nisyros; KU, Kusatsu–Shirane; HA, Hakone; SNG, Sierra Negra; PPR, Planchón–Peteroa; KRB, Kuchinoerabujima); (4) the composition of high-temperature magmatic gases (for data provenance, see Supplementary Information). Note the distinct CF fumarole evolutions in 1983–1987 (towards the hydrothermal pole, H) and 2018–present (towards the magmatic pole, M). The almost linear post-2018 CF fumarole progression can be interpreted to reflect a combination of (1) pure sulfur addition from native S/anhydrite breakdown, caused by exposure of new mineral surface due to fracturing/seismicity (2) mixing with sulfur-rich magmatic gas supplied by magma degassing (by decompression) within the mid-crustal (≥6 km) magmatic plumbing system (Fig. 4).
