Subduction zones occur where tectonic plates converge, and underthrust lower plate materials are recycled back into the Earth’s mantle. Modern subduction zones are associated with distinctive magmatic activity that produces volcanic arcs (e.g., the Pacific “Ring of Fire”) and associated intrusive complexes. Subduction-related magmatic processes commonly are viewed as contributing to the formation of continental crust over geologic time. Thus, knowledge of how subduction zones work is fundamental to understanding important aspects of Earth’s evolution, including how and to what extent recycling of subducted materials (e.g., oceanic crust and sediments) over time has affected the composition of the Earth’s mantle. An important direct source of such information comes from investigations of 1) magmatic and fluid outputs in volcanic arcs, and 2) exposed sections of uplifted subduction terranes, including relict fragments of ancient slabs.

Extensive literature on modern volcanic arcs supports the contention that involvement of subducted H₂O plays a dominant role in determining the compositions of most arc magmatic suites. However, the diversity of magma types found in volcanic arcs suggests that other factors may be important (cf. Leeman, 2020). For example, to what extent is the nature of arc magmatism controlled by competing tectonic processes (e.g., subduction erosion), versus variations in compositions of slab-derived contributions to arc magma sources (cf. Agostini et al., 2008; Marschall and Schumacher, 2012; Codillo et al., 2018; Barnes et al., 2019; Straub et al., 2020; Li et al., 2021)?

This special issue focuses on physical and chemical processes associated with subduction, with emphasis upon the record that can be deduced directly from magmatic products in modern volcanic arcs, and from subduction-related rocks and associated fluids, that may be reflective of the fluid and material contributors to arc magma source domains. Such information provides an essential foundation upon which realistic petrogenetic and physical models can be further developed and evaluated. Specifically, how are magmatic sources established, and what processes are involved? What tools effectively track the role of H₂O and other volatiles through the arc magmatic cycle, from source to eruption? In detail, what is the interplay between subducted fluids and volatiles (directly, via hydrous minerals, or via melts) and other material components that contribute to the production and evolution of arc magmas? Importantly, how and to what extent are these components extracted from subducted materials and/or adjacent mantle domains (cf. Morris et al., 1990; Ryan and Chauval, 2014; Sugden et al., 2019; Straub et al., 2020)?

Considering the spectrum of magma types seen overall in volcanic arcs (cf. Schmidt and Jagoutz, 2017), a first-order question regards whether they represent melts of compositionally distinct source domains, or are they to some extent products of evolutionary processes (e.g., differentiation, contamination, etc.). And, if the latter, to what extent are arc magmas modified during their ascent and/or storage? Papers in this collection address aspects of this issue.

Wei et al. focus on relatively evolved high-K calcalkalic type dacites from Meiji Atoll, that are interpreted to represent partial melts of lower continental crust of the South China block in response to Paleo-Pacific subduction during Triassic time. Crustal melting is attributed to intrusion of and underplating by subduction-related mafic magmas. This is a common theme in many modern arcs.

Cooper and Inglis utilize Fe isotopic data (δ⁵⁶Fe) to evaluate formation of magmas from two Lesser Antilles volcanic centers (Martinique and Statia) in terms of both sub-arc mantle source heterogeneity and potential modifying effects of crustal processes (differentiation, mixing, contamination, etc.). In this case, plutonic xenoliths were investigated to more directly ascertain processes occurring in the magmatic plumbing systems as well as crustal heterogeneities. It is concluded that magmatic processes in the sub-arc crust can overprint and/or distort source variability in δ⁵⁶Fe inherited from sub-arc mantle sources. Decoupling between Fe isotopes and radiogenic isotopes suggests that the former are not sensitive to crustal and/or sediment assimilation.

Bouvier et al. focus on the utility of Cl isotope data for melt inclusions (MIs) from Stromboli volcano. δ³⁷Cl values are found to correlate with bulk compositional variations of the MIs, and suggest mixing between two distinct magmatic compositions: 1) high-K alkali basalt attributed to melting of an amphibole-bearing mantle source, and 2) evolved shoshonitic melt enriched in a sediment-derived component. This approach potentially can help address sources of heterogeneity in arc magmas in general.

Atlas et al. look in detail at compositional variability of sediment packages along the Lesser Antilles arc and the extent to which spatial differences may be reflected along the volcanic chain. Trace element compositions of the arc magmas appear to correlate significantly with spatial variations in the nature of sediments on the subducting plate. Thus, they conclude that varied enrichments of fluid-mobile elements (FMEs) and large-ion lithophile elements (LILE) in the arc lavas reflect lateral differences in relative contributions of subduction components from north to south.

Selles et al. describe temporal evolution of lavas of Nevado de Longavi (NLV) volcano (S. Chile Andes). Here, the Mocha Fracture Zone (MFZ)—a significant oceanic transform fault—potentially supplies an anomalously high fluid flux. Obliquity of the MFZ to the plate convergence vector results in its southward migration beneath NLV during the past 1.2 Ma. Secular variations at NLV in concentrations of FMEs, H₂O content, and oxygen fugacity reflect the waxing and waning proximity of the MFZ—with involvement of serpentine-derived fluids and sediment-derived melts, presumably products of H₂O-flux melting. This work indicates that influx of slab-derived fluids can be effective over relatively short timeframes.

Ribeiro et al. address potential relations between temporal arc magmatic activity and climate in the Izu arc. Assuming that dehydration and/or melting of subducting slabs controls decarbonation and carbonate dissolution, they use trace element slab-fluid markers (e.g., Ba/Th, Cs/Th, etc.) as proxies for variation in slab-derived CO₂ captured by arc magmas. For Izu basaltic lavas, such proxy ratios suggest relative steadiness in outgassed CO₂ outputs over the past 40 Ma. These results indicate that the contribution of island arcs to long-term climate changes are minimal given that steady state CO₂ arc outputs are likely balanced by chemical weathering and tectonic erosion.