## 4 Discussion
### Slab Influence on along-CAVA \({}^{3}\)He/\({}^{4}\)He Variations  
CAVA is one of the most studied subduction-related settings on Earth, and this favoured the identification of clear geochemical patterns along the \(\sim\)1500 km arc [3; 4; 20; 21; 22; 23; 107]. Some of the best documented variations include those based on the analysis of volcanic rock geochemistry, and especially the trace element proxies for slab sediment fluids (such as Ba/La, Ba/Th, U/Th ratios; Figures S2 and S3). In detail, Ba/La and U/Th have been found to be the highest in Nicaragua, and lowest in Costa Rica and Panama [4; 11; 12; 13; 108; 109]. It must be mentioned that volcanoes in central-southern Costa Rica (i.e., Platatan, Poas, Barva, and Irazu volcanoes) and Panama (i.e., Tisingal, Baru, La Yeguada and El Valle) show an OIB magma signature that contrasts with that usually observed in the rest of CAVA (Figures S2 and S3). This specific feature has been attributed to the subduction of Galapagos seamounts originating along the hot-spot track [3; 4; 21; 22]. Adakitic magmas found at Turrialba and in Panamanian Quaternary volcanoes are thought to result from the direct melting of the subducting oceanic crust rather than of the wedge [3; 4; 10; 67; 108; 109; 110; 111; 111; 112; 113; 114], although other hypotheses have also been set out to explain this composition [115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 190; 188; 189; 191; 192; 193; 194; 195; 196; 197; 198; 199; 200; 201; 202; 203; 204; 205; 206; 207; 208; 209; 210; 211; 212; 213; 214; 215; 216; 217; 218; 219; 22; 23; 230; 231; 232; 233; 234; 235; 236; 237; 238; 239; 240; 241; 242; 243; 244; 245; 246; 247; 2 It must be recalled that the Panamanian volcanoes (Tsingal, Baru, La Yeguada and El Valle) are considered part of CAVA (Figure 1A), although the erupted rocks are of variable (and in part different) composition, and formed in a different geodynamic context with respect to the neighbouring Costarican segment. Two eruptive periods were identified for the Panamanian volcanoes: (1) from Miocene to about 5 Ma, during which rocks show typical calcalkaline features as in the rest of CAVA [110,112], and (2) Quaternary volcanism, starting after a \(\sim\)2-3 Ma hiatus, and characterised by adakiti magma affinity [67] and a few high-Nb basalts [11,13]. The adakiti signature is characterised by a high Sr/Y and significant depletion in heavy rare earth elements, which is not present in older calcalkaline rocks, while it is observed in some active centers across southern Costa Rica (e.g., Turris alba) [3,4,11,67,113,116,120]. While there is consensus that the old calcalkaline magmatism of Panama resulted from the subduction of oceanic crust, with features similar to the rest of CAVA [11,108,110,121,122], different models have been proposed to explain the Quaternary adakite and high-Nb magmatism. After a relatively abrupt shift in subduction angle beneath Panama, which occurred during the late Miocene [121,122] period, there is evidence of an oblique subduction of the Nazca plate under the so-called Panama Block at a rate of \(\sim\)40 mm/yr [6,13,108,110,123] (Figure 1A). This subduction is ongoing and is aseismic, probably due to a slab made by hot oceanic lithosphere [108]. What is controversial and strongly debated is the origin of the adakite and high-Nb magmatism in Quaternary volcanoes [26], and the role played by the paleo-Galapagos Hotspot track that lies \(>\)1500 m above the surrounding seafloor [124].