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1. Seismic Structure, Gravity Anomalies and Flexure Along the Emperor Seamount Chain

   https://doi.org/10.1029/2020JB021109  

   Watts, A. B. ; Grevemeyer, I. ; Shillington, D. J. ; Dunn, R. A. ; Boston, B. ; Gómez de la Peña, L. ( March 2021 , Journal of Geophysical Research: Solid Earth)

   The Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s⁻¹, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s⁻¹, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s⁻¹. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating.

    

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2. A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure of the Emperor Seamounts at Jimmu Guyot

   https://doi.org/10.1029/2021JB023241  

   Xu, C. ; Dunn, R. A. ; Watts, A. B. ; Shillington, D. J. ; Grevemeyer, I. ; Gómez de la Peña, L. ; Boston, B. B. ( June 2022 , Journal of Geophysical Research: Solid Earth)

   The intraplate Hawaiian‐Emperor Seamount Chain has long been considered a hotspot track generated by the motion of the Pacific plate over a deep mantle plume, and an ideal feature therefore for studies of volcanic structure, magma supply, plume‐crust interaction, flexural loading, and upper mantle rheology. Despite their importance as a major component of the chain, the Emperor Seamounts have been relatively little studied. In this paper, we present the results of an active‐source wide‐angle reflection and refraction experiment conducted along an ocean‐bottom‐seismograph (OBS) line oriented perpendicular to the seamount chain, crossing Jimmu guyot. The tomographicPwave velocity model, using ∼20,000 travel times from 26 OBSs, suggests that there is a high‐velocity (>6.0 km/s) intrusive core within the edifice, and the extrusive‐to‐intrusive ratio is estimated to be ∼2.5, indicating that Jimmu was built mainly by extrusive processes. The total volume for magmatic material above the top of the oceanic crust is ∼5.3 × 10⁴ km³, and the related volume flux is ∼0.96 m³/s during the formation of Jimmu. Under volcanic loading, the ∼5.3‐km‐thick oceanic crust is depressed by ∼3.8 km over a broad region. Using the standard relationships between Vpand density, the velocity model is verified by gravity modeling, and plate flexure modeling indicates an effective elastic thickness (T_e) of ∼14 km. Finally, we find no evidence for large‐scale magmatic underplating beneath the pre‐existing crust.

    

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3. The multiple depleted mantle components in the Hawaiian-Emperor chain

   https://doi.org/10.1016/j.chemgeo.2019.119324  

   Harrison, L ; Weis, D ; Garcia, M.O. ( January 2020 , Chemical geology)

   null (Ed.)

   Oceanic island basalts are targeted for geochemical study because they provide a direct window into mantle composition and a wealth of information on the dynamics and timescales associated with Earth mixing. Previous studies mainly focused on the shield volcanic stage of oceanic islands and the more fusible, enriched mantle components that are easily distinguished in those basalts. Mantle depleted compositions are typically more difficult to resolve unless large amounts of this material participated in mantle melting (e.g., mid-ocean ridges), or unique processes allow for their compositions to be erupted undiluted, such as very small degrees of melting of a source with minimal fusible enriched components (e.g., rejuvenated basalts) or as xenoliths (e.g., abyssal peridotites). Mantle depleted components, defined here as material with low time-integrated Rb/Sr (low 87Sr/86Sr) and high time-integrated Sm/Nd and Lu/Hf ratios (high 143Nd/144Nd and 176Hf/177Hf) relative to primitive mantle, derive from a potentially very large volume reservoir (up to 80% of the mantle), and therefore need adequate characterization in order estimate the composition of the Earth and mantle-derived melts. This review focuses on mantle depleted compositions in oceanic island basalts using the Hawaiian-Emperor chain as a case study. The Hawaiian-Emperor chain is the ∼6000 km long geological record of the deeply sourced Hawaiian mantle plume, active for>81 Myr. Hawaiian volcanism evolves through four volcanic stages as a volcano traverses the Hawaiian plume: alkalic preshield, tholeiitic shield (80–90% volcano volume), alkalic postshield (∼1%), and silica undersaturated rejuvenated (< 0.1%). We report Pb-Sr-Nd-Hf isotope compositions and trace element concentrations of three rejuvenated Northwest Hawaiian Ridge basalts and compare them to an exhaustive compiled dataset of basalts from the Hawaiian Islands to the Emperor Seamounts. The Northwest Hawaiian Ridge (NWHR) includes 51 volcanoes spanning ∼42 m.y. between the bend in the Hawaiian-Emperor chain and the Hawaiian Islands where there is no high-precision isotopic data published on the rejuvenated-stage over ∼47% of the chain. NWHR and Hawaiian Island rejuvenated basalts are geochemically similar, indicating a consistent source for rejuvenated volcanism over ∼12.5 million years. In contrast, shield-stage basalts from the oldest Emperor Seamounts are more depleted in isotopic composition (i.e., higher 176Hf/177Hf, and 143Nd/144Nd with lower 87Sr/86Sr and 208Pb*/206Pb*) and trace element concentrations (i.e., much lower concentrations of highly incompatible elements) than all other depleted Hawaiian basalts younger than the bend, including NWHR rejuvenated basalts. The strongly depleted source for the oldest Emperor Seamounts (> 70 Ma) was likely related to interaction with the Kula-Pacific-Izanagi mid-ocean ridge spreading system active near the Hawaiian plume in the Late Cretaceous. In contrast, the incompatible trace element ratios of NWHR rejuvenated basalts require a distinct source in the Hawaiian mantle plume that was imprinted by ancient (> 1 Ga) partial melting, likely ancient recycled oceanic lithosphere. This review of the geochemistry of Hawaiian depleted components documents the need for the sampling of multiple distinctive depleted compositions, each preferentially melted during specific periods of Hawaiian plume activity. This suggests that the composition of depleted components can evolve during the lifetime of the mantle plume, as observed for enriched components in the Hawaiian mantle plume. Changes in the composition of depleted components are dominantly controlled by the upper mantle tectonic configurations at the time of eruption (i.e., proximity to a mid-ocean ridge), as this effect overwhelms the signal imparted by potentially sampling different lower mantle components through time. 

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4. Mariana Convergent Margin and South Chamorro Seamount

   https://doi.org/10.14379/iodp.proc.366.2018  

   Fryer, P. ; Wheat, C.G. ; Williams, T. ( February 2018 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 19 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access materials from this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at the Mariana forearc and for billions of years on Earth. At least 19 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the farthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of up to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the volcanic arc and backarc areas, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 focused on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes to compare with results from drilling at the two previously cored serpentinite mud volcanoes and with previously collected gravity, piston, and remotely operated vehicle push cores across the trench-proximal forearc. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the crust of the Pacific plate, most notably subducted seamounts (even corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through distal serpentinite mudflows and into the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinized ultramafic rocks and mudflow matrix materials are largely uniform in major element composition, spanning a limited range in SiO2, MgO, and Fe2O3 compositions. However, variation in trace element composition reflects interstitial water composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2, CH4, and C2H6 are highest at the site farthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and gamma ray attenuation density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that slow compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-Lite. 

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5. Expedition 366 Preliminary Report: Mariana Convergent Margin and South Chamorro Seamount

   https://doi.org/10.14379/iodp.pr.366.2017  

   Fryer, P. ; Wheat, G. ; Williams, T. ( November 2017 , Preliminary report)

   null (Ed.)

   Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program (IODP) Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 18 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at this site and for billions of years on Earth. At least 10 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the furthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program (ODP) Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of about 18 to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the forearc and volcanic arc, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 centered on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes defined by the two previously cored serpentinite mud volcanoes and the trench. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the lithosphere of the Pacific plate and from subducted seamounts (including corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. Cores from each of the three seamounts drilled during Expedition 366, as well as those from Legs 125 and 195, include material from the underlying Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through serpentinite flows to the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinites are largely uniform in major element composition, with serpentinized ultramafic rocks and serpentinite muds spanning a limited range in SiO2 , MgO, and Fe2 O3 compositions. However, variation in trace element composition reflects pore fluid composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2 , CH4 , and C2 H6 are highest at the site furthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and GRA density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that minimize compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly in dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-lite. An active education and outreach program connected with many classrooms on shore and with the general public through social media. 

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