## 4 Discussion
### Crust Thickness vs. \({}^{3}\)He/\({}^{4}\)He Variations along CAVA  
We now consider the \({}^{3}\)He/\({}^{4}\)He signatures constrained above for Telica and Baru volcanoes in the context of previous results for the Nicaraguan and Panama arc segments, respectively. To do this, we start evaluating the \({}^{4}\)He/\({}^{20}\)Ne vs. R/Ra plot that allows us to report all the available data from FI, fumarole and spring gases together (Figure 3). Among Nicaraguan volcanoes (Figure 3A), data for fumarole gases (\(n\) = 29) are available for Mombacho, Masaya, Momodombo, Cerro Negro and San Cristobal volcanoes ([42; 57; 58; 60]), spring gases and waters (\(n\) = 46) for Mombacho, Masaya, Momodombo, Telica and Conseguina [42; 56; 57; 58], FI in ol and/or cpx (\(n\) = 36) for Momodombo, Cerro Negro and San Cristobal ([34; 44; 62; 63] (Tables S3 and S4). We filtered this dataset by only considering data from fumarole gases and FI that, being typically less affected by shallow secondary processes, are therefore more representative of the \({}^{3}\)He/\({}^{4}\)He signature of the magmatic/mantle source [38; 46; 48; 49; 50; 51; 52; 53; 54; 53; 52; 56; 57; 59; 63; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 73; 75; 74; 79; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94;  
it was proposed that the crust may contribute up to 14-66% of He discharged by Panama gas/spring manifestations which, if correct, would implicate a pristine (pre-mixing with crustal fluids) mantle He signature of \(\geq\)10.2 to \(\sim\)26 Ra. This \({}^{3}\)He-rich signature would be caused by the infiltration of a Galapagos plume-like mantle through a slab window that opened \(\sim\)8 Ma underneath Panama [61]. Recent work has demonstrated that crustal processing (\({}^{4}\)He addition) can indeed play a role in lowering the \({}^{3}\)He/\({}^{4}\)He signature of mantle-derived magmatic/volcanic gases of arc volcanoes that developed on continental crust [52; 53]. We therefore attempt to evaluate this effect for Nicaragua/Panama by looking at the relationship between crustal thickness and He isotopes (Figure 4). We note (Figures 4 and S2) that the Panamanian segment volcanoes developed above a \(\sim\)30 km of continental crust [19; 61; 106], while in Nicaragua the crust is \(\sim\)34-35 km thick. Although the crust is, thus, slightly thicker below Nicaragua than underneath Panama, it remains that the two areas have the lowest crustal thicknesses reported for CAVA (Figures 4 and S2, [16]). We therefore extend the crustal thickness vs. magmatic \({}^{3}\)He/\({}^{4}\)He analysis to all CAVA volcanoes (including other segments, such as Guatem, El Salvador and Costa Rica) for which data are available [27; 28; 29; 30; 34; 38; 42; 44; 45; 46; 48; 54; 55; 56; 57; 58; 59; 60; 61]. In this exercise, we adopted the same data selection criteria discussed above, by considering only the maximum \({}^{3}\)He/\({}^{4}\)He ratios among fumarole gases and FI, and spring gases with Rc/Ra values within MORB range. Based on this data selection, we obtained the following Rc/Ra signatures for the volcanoes: Pacaya (Guatemala) 9.0 Ra, Santa Ana (El Salvador) 7.6 Ra, Miravalles (Costa Rica) 7.4 Ra, Poas (Costa Rica) 7.6 Ra, Telica (Costa Rica) 7.2 Ra, Turrialba (Costa Rica) 8.3 Ra. The results, illustrated in Figure 4, demonstrate that there is no obvious relationship between magmatic \({}^{3}\)He/\({}^{4}\)He and crustal thickness. In particular, we note that Rc/Ra \(>\) 7.6 are not restricted to Panama (Baru, La Yeguada, El Valle), but are indeed rather common to Costa Rica (Turrialba) and Guatema (Pacaya), where crust thickness is far higher (\(\sim\)45 km) than underneath Baru. Based on all this evidence, we conclude that crustal processing of magmas/gases is unlikely to be a primary control factor on the distinct \({}^{3}\)He/\({}^{4}\)He signatures of the Panamanian and Nicaraguan arc segments, as well as between the other arc volcanoes of CAVA.