## 4 Discussion
### Slab Influence on along-CAVA \({}^{3}\)He/\({}^{4}\)He Variations
The most recurrent models invoke: (i) melting of the subducting oceanic crust of young (\(<\)25 Ma) age [3,4,10,11,67,108,113,114]; (ii) formation of a slab window that opened 8 Ma ago [115,116]; (iii) subduction erosion of the overlying plate [117]; (iv) high pressure crystal fractionation [13,14,15,114,118]; (v) formation of a slab  
Figure 4: Crust thickness versus the maximum \({}^{3}\)He/\({}^{4}\)He corrected for atmospheric contamination (Rc/Ra) measured in a selection of volcanoes from different segments of CAVA. Data selection is based on the criteria discussed in the text. Data source for the volcanoes, other than Telica and Baru, is reported in Figures 2 and 3 captions and in Tables S3 and S4. Crust thickness values below the selected volcanoes are from [16,17,18,19,106]. tear arising from subduction of the Panama Fracture Zone [119]; (vi) infiltration of Galapagos plume-like mantle \"wind\" through the hypothesised slab window [61] (Figure 1A). We compare the available Rc/Ra signatures of CAVA volcanoes with their corresponding Ba/La and U/Th ratios in Figure 5. We find a systematic negative tendency, in which the lowest \({}^{3}\)He/\({}^{4}\)He values in the Nicaraguan volcanoes are matched by the highest Ba/La and U/Th ratios; while the highest \({}^{3}\)He/\({}^{4}\)He values are found associated with MORB-OIB-like [95], low Ba/La and U/Th ratios (in both Guatem and Panama). This trend was first brought to light by [38; 44] in their studies focussing on the Turrialba and San Cristobal volcanoes, respectively, and was interpreted as being due to a common slab fluid control on He and trace element signatures. Here, we provide additional validation and support to these ealier studies thanks to the availability of new \({}^{3}\)He/\({}^{4}\)He results, especially for previously unstudied volcanoes from Nicaragua and Panama. We conclude from Figure 5 that slab fluids do play a central role in determining the mantle/magmatic \({}^{3}\)He/\({}^{4}\)He signature at CAVA. If this interpretation is correct, then the distinct Telica vs. Baru signatures found here may simply result from a higher presence of subducted slab sediment fluids (rich in U and Th, from which radiogenic He is produced) in the Nicaraguan mantle wedge. Figure 5: Ba/La (**A**) and U/Th (**B**) mean values (taken from GeoRoc pre-compiled files, considering only samples with SiO\({}_{2}\) < 60 wt.%; [125; 126] versus the \({}^{3}\)He/\({}^{4}\)He corrected for atmospheric contamination (Rc/Ra), for the selection of volcanoes from different segments of CAVA already mentioned in Figure 4 caption. Trace elements ratios for MORB, OIB and Cocos plate sediments are taken from [127; 128; 129]. Our conclusion of a dominant slab control on the He signature at Baru is not inconsistent with the results of [PERSON] et al. (2021). The authors interpreted the highest \({}^{3}\)He/\({}^{4}\)He (8.9 Ra), measured in a \"cold\" spring (T = 50 \({}^{\circ}\)C) from central Panama, as a mixture between crustal \({}^{4}\)He and a mantle source with \({}^{3}\)He/\({}^{4}\)He as high as 10.3 Ra, and potentially up to 26 Ra. This higher than MORB \({}^{3}\)He/\({}^{4}\)He signature would derive from the infiltration of Galapagos plume-like mantle \"wind\" through a slab window ([61]; Figure 1A). Geophysical and mantle flow modelling evidence suggests that the influx of Galapagos mantle plume materials would concentrate behind the volcanic front (where alkaline magmatism prevails); in contrast, adakitic volcanoes, such as Baru, would be located at the edge of the slab window, where magmatism would result from the partial melting of the subducting slab during the influx of hot astenosphere [61]. As such, a dominant slab control for Baru volcanism is implied in (and thus fully consistent with) the model of [PERSON] et al. (2021). With our results alone, we can neither support nor dismiss the hypothesis of a Galapagos mantle plume inflow in the Panama back-arc.