Figure 4

Noble gases isotopes ratios obtained by crystal crushing from lavas and compared to volcanic gases from thermal springs at Cilaos. (a) Rc/Ra vs. ⁴He/⁴⁰Ar* modelling the effect of magmatic fluids flushing and melts mixing starting from the highest Rc/Ra vs. ⁴He/⁴⁰Ar* values. Thresholds of the >95%-contribution of either the “primitive” mantle component or the “steady-state” mantle component to Rc/Ra value determined using the maximum-likelihood method⁵⁹. See Methods for supplementary information on the modelled curves and the statistical method. Same legend than for Figs. 2 and 3. Blue field for the range of compositions from volcanic gases from thermal springs. Dark grey field for published data from lavas at PdF. The green arrow represents the chronology of the products from the August 2015 eruption. ⁴He/⁴⁰Ar* variability as a function of the location of magma ponding in the plumbing system¹⁰. (b) Time variations of Rc/Ra from both FI in olivine crystals from PdF lavas and from volcanic gases. During the last century^30,57, major eruptive events (caldera collapse and erupted volume >100 Mm³) occured only in 1931 and 2007. The meaning of σ_error and σ_replicates is detailed in the Methods and the caption of Fig. 2.