## 4 Discussion  
Light noble gases are powerful geochemical tracers of volatiles' composition and origin, due to their inert physical-chemical features and the different origins of their isotopes. Ne and Ar isotopic ratios more commonly provide information on the extent and origin of the atmosphere-derived contamination that often occurs in natural fluids. In subduction  
Figure 3: \({}^{4}\)He/\({}^{20}\)Ne versus \({}^{3}\)He/\({}^{4}\)He (R/Ra) in FI, fumaroles and springs from Nicaraguan (**A**) and Panamanian (**B**) segments of CAVA. Fumarole and springs data from previous studies are reported for comparison [27; 28; 29; 30; 34; 38; 42; 45; 48; 54; 55; 56; 57; 58; 59; 60; 61]. More details on the source data are reported in Supplementary Table S3. The black and red lines indicate binary mixing between air and two hypothetical magmatic/mantle sources assumed below the respective segments of CAVA. related settings, revealed Ne and Ar isotopes have been useful in recognising the recycling of atmospheric components into the mantle wedge, likely in the form of air-saturated oceanic water trapped in clay minerals and sediments [52; 78; 79; 88; 89; 90; 91; 92; 93; 94]. This is due to the higher concentration of Ne and Ar in atmosphere (Ne-18 ppm, Ar-9300 ppm) [84] than in magmatic, mantle and crustal fluids, where Ne is normally present in a few ppb and Ar in ppm. Instead, helium is used to distinguish the origin of fluids and to recognise contamination or mixing processes occurring at mantle or crustal depths (e.g., [49; 50; 51; 78; 79; 95; 96; 97; 98; 99; 100]). This is due to the distinct origins of its two isotopes, whereby \({}^{3}\)He is cosmogenic, not renewed and stored in the Earth's interior, while \({}^{4}\)He originates from the radioactive decay of U and Th in the crust [84]. In subduction-related settings, the \({}^{3}\)He/\({}^{4}\)He ratio of the mantle wedge, and thus of magmatic/volcanic gases, is normally expected within the MORB range (8 \(\pm\) 1 Ra) [87], due to the fact that the oceanic crust originally has a MORB-like signature and early models of noble gases hypothesised a subduction barrier for their recycling into the mantle [101]. However, this is not always observed in arc volcanoes on Earth, where \({}^{3}\)He/\({}^{4}\)He values lower than 7.0 Ra are often found [70; 102; 103; 104]. Nevertheless, it must be noted that it is only at volcanic arcs, where continental crust is involved in the subduction, that slab fluids are likely to significantly play a game in determining \({}^{3}\)He/\({}^{4}\)He values of \(<\)\(<\)7.0 Ra (e.g., Italy, [47; 105]). Elsewhere, arc volcanoes showing a \({}^{3}\)He/\({}^{4}\)He signature lower than MORB are likely to primarily reflect contamination of magma/magmatic gases in the crust by radiogenic \({}^{4}\)He addition. This is especially the case for datasets based on measurements carried out in spring gases sampled far from the main eruptive centres, and/or in inactive volcanoes (e.g., [52; 53] and references therein [102]). This is the reason why, in recent years, most of studies aimed at constraining the \({}^{3}\)He/\({}^{4}\)He signature of the wedge below arc volcanoes have focused on the study of FI in olivine and pyroxene crystals. It is important to remember that minerals growing and crystallizing during igneous processes are expected to trap fluid and melt inclusions. FI can be either primary or, more frequently, secondary in developing along crystal fractures or growing directions. Other fluids are also trapped and appear as contraction bubbles of melt inclusions (i.e., shrinkage bubbles). These fluids may preserve a more primitive volatile signature than volcanic and hydrothermal gases. Recent studies have put the accent on the possible influence of slab fluids in lowering \({}^{3}\)He/\({}^{4}\)He values in the wedge [38; 44; 53], although the final isotopic signature may remain well within the MORB range.