16:30 〜 16:45
[SGC37-10] What may modify the 3He/4He signature in subduction settings? Evidence from the Central and South American Volcanic Arc
★Invited Papers
キーワード：3He/4He, subduction, mantle wedge, slab fluids, crustal processes
To draw inferences about the 3He/4He signature of a mantle source, it is essential to constrain the isotopic composition of magmatic fluids degassed from a volcanic system. This is especially crucial in arc volcanism, where changes in the wedge's composition potentially induced by slab sediment fluids must be distinguished from the impacts of magma differentiation, degassing, and crustal contamination. In this respect, the study of fluid inclusions (FI) in mafic minerals of volcanic rocks combined with that of fumarole and spring gases is becoming an increasingly used method.
Here, new data of 3He/4He in FI are combined with a review of existing information available for the Central and Southern American Volcanic Arc, making a data quality check to exclude ratios that cannot be considered representative of the local mantle source signature. These data are integrated with the geochemistry of erupted rocks and subduction parameters, with the aim of constraining the 3He/4He signature of the mantle beneath these two arc fronts extending for more than 1500 km in Central America and ~7000 km in South America, and the processes responsible of its variability.
It emerges that in Central American Volcanic Arc (CAVA) the 3He/4He ratio varies from 7.0 Ra in the Nicaraguan arc segment to about 8 Ra in Costa Rica and Panama arc segments, and up to 9 Ra in Guatemala (Pacaya volcano) (Ra is the atmospheric 3He/4He ratio of 1.39 × 10–6). This variability cannot be explained by radiogenic 4He addition in response to variable crust thickness, considering that the mantle beneath Nicaragua and Panama is at about 35 and 30 km, respectively, while below Guatemala is at about 45 km. Instead, these 3He/4He differences reflect contamination of the underlying wedge by slab sediment fluids and slab melting, as already evidenced by the geochemistry of erupted rocks and changes in subduction conditions.
In the South American Volcanic Arc, the 3He/4He ratios vary from 8.8 Ra (Colombia) to 7.4 Ra (Ecuador) within the NVZ (Northern Volcanic Zone), and down to 6.4 Ra in the CVZ (Central Volcanic Zone). These distinct isotope compositions cannot result from variable radiogenic 4He addition via slab fluid transport of U and Th in the mantle wedge, as observed for CAVA, since both NVZ and CVZ share similar slab sediment inputs (Th/La ≈ 0.08–0.13). Instead, the progressively more radiogenic 3He/4He signatures in Ecuador and Peru reflect 4He addition upon magma ascent/storage in the crust, this being especially thick in Peru (>70 km) and Ecuador (>50 km) relative to Colombia (∼30–45 km). The intermediate compositions in the North (8.0 Ra) and South (7.9 Ra) Chile mostly reflect a more efficient delivery of radiogenic He in the wedge from the subducted (U-Th-rich) sediments.
These results bring strong evidence that most of the 3He/4He variability on Earth mantle wedges is within the MORB-like range (8±1 Ra). Nevertheless, whether the variability of 3He/4He ratios observed in volcanic fluids depends on the mantle source features or crustal processes must be carefully evaluated through a multidisciplinary approach.
Here, new data of 3He/4He in FI are combined with a review of existing information available for the Central and Southern American Volcanic Arc, making a data quality check to exclude ratios that cannot be considered representative of the local mantle source signature. These data are integrated with the geochemistry of erupted rocks and subduction parameters, with the aim of constraining the 3He/4He signature of the mantle beneath these two arc fronts extending for more than 1500 km in Central America and ~7000 km in South America, and the processes responsible of its variability.
It emerges that in Central American Volcanic Arc (CAVA) the 3He/4He ratio varies from 7.0 Ra in the Nicaraguan arc segment to about 8 Ra in Costa Rica and Panama arc segments, and up to 9 Ra in Guatemala (Pacaya volcano) (Ra is the atmospheric 3He/4He ratio of 1.39 × 10–6). This variability cannot be explained by radiogenic 4He addition in response to variable crust thickness, considering that the mantle beneath Nicaragua and Panama is at about 35 and 30 km, respectively, while below Guatemala is at about 45 km. Instead, these 3He/4He differences reflect contamination of the underlying wedge by slab sediment fluids and slab melting, as already evidenced by the geochemistry of erupted rocks and changes in subduction conditions.
In the South American Volcanic Arc, the 3He/4He ratios vary from 8.8 Ra (Colombia) to 7.4 Ra (Ecuador) within the NVZ (Northern Volcanic Zone), and down to 6.4 Ra in the CVZ (Central Volcanic Zone). These distinct isotope compositions cannot result from variable radiogenic 4He addition via slab fluid transport of U and Th in the mantle wedge, as observed for CAVA, since both NVZ and CVZ share similar slab sediment inputs (Th/La ≈ 0.08–0.13). Instead, the progressively more radiogenic 3He/4He signatures in Ecuador and Peru reflect 4He addition upon magma ascent/storage in the crust, this being especially thick in Peru (>70 km) and Ecuador (>50 km) relative to Colombia (∼30–45 km). The intermediate compositions in the North (8.0 Ra) and South (7.9 Ra) Chile mostly reflect a more efficient delivery of radiogenic He in the wedge from the subducted (U-Th-rich) sediments.
These results bring strong evidence that most of the 3He/4He variability on Earth mantle wedges is within the MORB-like range (8±1 Ra). Nevertheless, whether the variability of 3He/4He ratios observed in volcanic fluids depends on the mantle source features or crustal processes must be carefully evaluated through a multidisciplinary approach.