What processes control the chemical compositions of arc front stratovolcanoes?

Abstract

Arc front stratovolcanoes have global chemical systematics that constrain processes at convergent margins. Positive correlations exist for arc averages among ''fluid mobile,'' ''high field strength,'' and ''large ion lithophile'' elements. 143 Nd/ 144 Nd and 87 Sr/ 86 Sr from rear-arc lavas lacking subduction signature align with the oceanic ''mantle array,'' and correlate with arc front 143 Nd/ 144 Nd. Most chemical parameters (but not isotopes) also correlate well with crustal thickness and slightly less well with the slab thermal parameter, but not with the depth of the slab nor model slab surface temperatures. Successful models of arc volcanism should account for these global regularities. Two distinct models can quantitatively account for the observationsdifferent extents of melting of the mantle wedge caused by variations in wedge thermal structure, or varying contributions from the subducting slab owing to variations in the slab thermal structure. Both successful model scenarios require a significant flux of melted ocean crust to the mantle source of all volcanic arcs. The wedge melting model has constant contributions from ocean crust, sediment, and mantle wedge to lavas globally, while the slab model varies slab contributions with slab temperature. The wedge melting model fit improves by incorporating convergence rate and slab dip, which should affect the wedge thermal structure; the slab model is not supported by a similar analysis. The wedge model also more easily accommodates the isotope data. The two models predict different primary H 2 O contents, with large variations in H 2 O for the wedge model, and relatively constant H 2 O for the slab model. An evaluation of the effects of varying sediment compositions on arc lavas will benefit from considering the very different consequences of the two models.

Effects of aseismic ridge subduction on the geochemistry of frontal arc magmas

“…One could try to disentangle these two processes by considering only the geochemical signatures of the least evolved rocks of each volcano. This has been tested by previous studies based on a global compilation (Turner and Langmuir, 2015;Turner et al, 2016) and on Ecuador samples as well (Ancellin et al, 2017).…”

Transition From Low‐K to High‐K Calc‐Alkaline Magmatism at Approximately 84 Ma in the Eastern Pontides (NE Turkey): Magmatic Response to Slab Rollback of the Black Sea

“…Systematic geological investigations (e.g., Haschke et al, ; Lipman et al, ) and numerical modeling (e.g., Menant et al, ; Sternai et al, ) showed that with oceanic subduction continued, the subducted oceanic lithosphere progressively becomes colder and denser, and thus, the subduction angle is getting steeper, resulting in the rollback of the descending slab and a lateral retreat of the trench due to its negative buoyancy (e.g., Elsasser, ; Garfunkel, ; Gorczyk et al, ; Niu, ). An increasing subduction angle will cause the hot isotherms to be shallower, and increase the effective temperature, and lower the pressure of the mantle wedge (Karlstrom et al, ; Turner & Langmuir, for review). As a result, (1) the mantle wedge flow can convect vast amounts of heat beneath the overriding continental crust and, consequently, produce large volumes of magma that results in an enhanced underplating of the basaltic magma (e.g., Best et al, ; Ferrari et al, ; Gvirtzman & Nur, , ; Keith, ), and (2) the overriding continental crust is under an extensional setting, allowing the development of back‐arc basins behind the magmatic arc (e.g., Dvorkin et al, ; Garfunkel et al, ; Heuret & Lallemand, ; Molnar & Atwater, ; Nakakuki & Mura, ; Niu, ).…”

Slab melting and magma formation beneath the southern Cascade arc

“…The pattern of variable Sr/Nd in the forearc and decreasing and consistently low values of Sr/Nd in the back-arc has been interpreted to indicate the waning addition of a subduction component with distance from the trench (Borg et al, 1997). The 87 Sr/ 86 Sr ratios display an opposite pattern and generally increase toward the back-arc, indicating that the subduction component has a less radiogenic Sr isotope signature than the sub-arc mantle, which is unusual for arc volcanoes (e.g., Turner and Langmuir, 2015). Variability in wholerock Nb/Zr and mineral cheMItry (olivine and spinel) suggests that the Lassen subarc mantle is heterogeneous before any slab addition ( Supplementary Fig.…”