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Abstract The along‐strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian‐Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full‐wave ambient noise tomography, we present a high‐resolution 3‐D shear wave velocity model of the AASZ for the depths of 15–110 km. The velocity model reveals the distinct high‐velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low‐velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average‐velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower‐velocity back‐arc mantle beneath the adjacent Buzzard Creek‐Jumbo Dome volcanoes to the east. The back‐arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction‐driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism.
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Tracking carbon from subduction to outgassing along the Aleutian-Alaska Volcanic Arc
Subduction transports volatiles between Earth’s mantle, crust, and atmosphere, ultimately creating a habitable Earth. We use isotopes to track carbon from subduction to outgassing along the Aleutian-Alaska Arc. We find substantial along-strike variations in the isotopic composition of volcanic gases, explained by different recycling efficiencies of subducting carbon to the atmosphere via arc volcanism and modulated by subduction character. Fast and cool subduction facilitates recycling of ~43 to 61% sediment-derived organic carbon to the atmosphere through degassing of central Aleutian volcanoes, while slow and warm subduction favors forearc sediment removal, leading to recycling of ~6 to 9% altered oceanic crust carbon to the atmosphere through degassing of western Aleutian volcanoes. These results indicate that less carbon is returned to the deep mantle than previously thought and that subducting organic carbon is not a reliable atmospheric carbon sink over subduction time scales.
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- Award ID(s):
- 1551978
- PAR ID:
- 10474373
- Publisher / Repository:
- AAAS
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 9
- Issue:
- 26
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Nitrogen (N) dominates Earth's atmosphere (78% N2) but occurs in trace abundances in silicate minerals, making it a sensitive tracer of recycled surface materials into the mantle. The mechanisms controlling N transfer between terrestrial reservoirs remain uncertain because low N abundances in mineral‐hosted fluid inclusions (FIs) are difficult to measure. Using new techniques, we analyzed N and He isotope compositions and abundances in olivine‐ and pyroxene‐hosted FIs from arc volcanoes in Southern Chile, Cascadia, Central America, and the Southern Marianas. These measurements enable an estimate of the global flux of N outgassing from arcs (4.0 × 1010 mol/yr). This suggests that Earth is currently in a state of net N ingassing, with roughly half of subducted N returned to the mantle. Additionally, the N outgassing flux of individual arcs correlates with the thickness of subducting pelagic sediment, suggesting that N cycling in the modern solid Earth is largely controlled by sediment subduction.more » « less
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New Zealand's Hikurangi margin is known for recurring shallow slow slip, numerous forearc seeps, and a productive volcanic arc. Fluids derived from the subducting slab are implicated in these processes. However, prior studies lacked evidence of basic crustal structure of the slab, or of its water content that would allow an assessment of fluid budgets. We review several recent studies that place bounds on the fluid reservoirs within the subducting Hikurangi Plateau that could be released between the forearc and backarc regions. Subducting sediments are thickest (> 1 km) in the southern Hikurangi margin, where there is a unit of turbidites beneath the regional proto decollement. These subducting sediments begin draining near the deformation front, resulting in a 20-30 % loss of volumetric fluid content. In contrast, the central and northern Hikurangi margins lack a continuous unit of subducting sediment. Here, lenses of poorly drained sediment underthrust the forearc in the wakes of seamount collisions. The Hikurangi Plateau's crustal structure resembles normal oceanic crust with a doubled upper crust of basalt and diabase. Above this upper crust is a ~1.5 km thick unit of hydrated volcaniclastic conglomerates. Seamounts can locally increase the upper crust's thickness by an extra ~1-3 km, raising the amount of porous, altered volcanic material. Finally, P-wave velocity models of the slab's upper mantle show velocity changes that could indicate moderate differences in serpentinization. Active bend-faults that could circulate fluids to the upper mantle are sparse prior to subduction. However, upon subduction the upper mantle seismic velocities of the Hikurangi Plateau are significantly less in the north compared to the south, possibly due to enhanced slab faulting beneath the forearc. Separate thermo-petrologic models for the shallow forearc and deeper subduction system suggests that fluid release from volcaniclastic units and the thickened Hikurangi Plateau upper crust is expected to occur over a range of depths extending from ~12 km to ~130 km, providing fluids for onshore seep systems and hydrous melting of the mantle wedge, whereas dehydration of serpentinite is greatest beyond the arc front. Subducting sediments and volcaniclastic units are the most readily available source of fluids for shallow slow slip.more » « less
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SUMMARY The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.adf3024
