Fig. 3: Gas compositions and melt inclusions reveal a transition from deep to shallow magmatic carbon-rich sources beneath Ambae during the eruption.

A Fluctuation of gas concentrations recorded on the rim of the new cone developed during the Ambae eruption. The correlations of the measured gases to SO₂ are provided. B Ternary diagrams showing a deep to shallow magmatic carbon-rich source beneath Ambae in comparison to other volcanoes of the Vanuatu arc. The CO₂-SO₂-H₂S diagram is adapted from ref. ^83,84, while the CO₂-H₂O-S_T diagram is adapted ref. ³³. The data sources include ref. ³³ for Gaua, ref. ³⁰ for Ambrym, ref. ³¹ for Yasur, ref. ^34,35 for Merapi and Bromo, and the present study for Ambae. C Comparing the maximum CO₂ versus H₂O content from Ambae inclusions with inclusions in basalt from various subduction zones. The diagram is adapted from ref. ³⁵, with sourced from ref. ^36,85 and other references therein. The pink shaded zone highlights the decreasing trend in CO₂ levels throughout the eruptive event, indicating a progressive reduction in CO₂ contributions from a relatively deep source (>16 km) during EDP1, transitioning to ~10 km in EDP2, and ultimately approaching the surface in EDP3. This gradual decline occurred alongside a relatively modest but consistent H₂O content, which remained stable until the shallower depths of EDP3.