Why Is Crustal Underplating Beneath Many Hot Spot Islands Anisotropic?

Abstract

High‐frequency harmonic regression (2.0–4.0‐Hz cutoff) of receiver functions at two long‐running seismic observatories at mid‐Pacific hot spot islands confirms earlier detections of underplated material with seismic velocities intermediate to crust and mantle, and reveals it to be multilayered and anisotropic within ~30 km of the surface. Magmatic underplating beneath the oceanic Moho has been proposed to accompany basaltic melt that erupts at the seafloor and (eventually) atop a subaerial volcano. An alternate hypothesis is “metasomatic underplating” whereby crustal fractures developed during magma ascent allow seawater to infiltrate and to serpentinize the sub‐Moho mantle partially. Metasomatic underplating would lower seismic wave speeds, promote the buoyancy of the hot spot swell, and induce textural anisotropy as metamorphic expansion of olivine‐rich peridotite promotes a crack network along which serpentinization spreads. Differential expansion of mantle peridotite and crustal gabbro promotes cracks in the crust that offer new pathways for seawater to descend to the Moho, allowing metasomatic underplating to expand laterally and to contribute anisotropy to the underplated layer. Rare serpentinized mantle xenoliths confirm that crack textures can develop during serpentinization at depth. The discovery of iron‐oxidizing microbial mats on the seafloor flank of the Loihi volcano, and many locations of diffuse low‐temperature venting worldwide, is consistent with the circulation of metasomatic fluids with reducing chemistry, sourced from serpentinization at depth. Posteruptive uplift of Santa Maria Island (Azores), and asymmetry of the Hawaiian swell, suggests that underplating requires 2–4 Myr to complete, suggesting that fluid infiltration is slow, subject to cycles of blockage and fresh fracturing.

Broader Impacts of the Metasomatic Underplating Hypothesis

“…During metasomatic underplating the carbonate deposition likely occurs within the oceanic crust. In fact, calcite deposition is likely to be a major impediment to the free descent of seawater to the Moho, rate limiting the 2-to 4-Ma progression of metasomatic underplating inferred by Park and Rye (2019) from massif uplift in the Lost City seafloor vent fields (Blackman et al, 2002), inferred island uplifts at Santa Maria Island in the Azores (Ramalho et al, 2017) and postemplacement uplift at other midocean islands (Anguita & Hernan, 2000;Konter & Jackson, 2012;Madeira et al, 2010;Ramalho et al, 2015). Dasgupta et al (2004) modeled the influence of carbonation on melts derived from subducted oceanic crust, suggesting 5 wt.% CaCO 3 as a benchmark of petrologic interest in partial melting.…”

“…This geometry restricts sharply the area of Moho in a mature oceanic plate that is too hot for serpentinization to proceed, allowing metasomatic underplating to occur underneath most of a hot spot island and its associated bathymetric swell (Leahy et al, 2010;ten Brink & Brocher, 1987). Park and Rye (2019) argue that the ascent and eruption of plume lavas will induce stresses and small fractures in the crust, surrounding the magma conduit. Korenaga (2017) argues that a thermal crack network would be a better choice for seawater infiltration than earthquake faults, which are thought to offer seawater access to the mantle in the outer rises of subduction zones (e.g., Faccenda et al, 2008).…”

“…Alternate explanations for these sub-Moho layers include "magmatic underplating," implying a plutonic origin (Richards et al, 2013), and "diapiric underplating," effected by a buoyant lowviscosity solid (Koch & Manga, 1996), for example, a thermal plume with partial melt. Park and Rye (2019) argued against magmatic and diapiric underplating by analogy to topographic features that develop at the edges of shallow diapiric bodies above the Yellowstone hot spot (Anders et al, 1989;Farrell et al, 2014) and Venus coronae (Jurdy & Stoddard, 2007;Koch, 1994). Stresses at the boundaries of laterally spreading magma or diapirs induce stresses that would subject the oceanic crust to sharp concentric stress gradients (Koch & Koch, 1995) that would generate bathymetric rings in the seafloor surrounding hot spot islands.…”

“…Park and Rye (2019) suggest that low-temperature hydrothermal circulations near the deep flanks of hot spot volcanic systems (Edwards et al, 2011;Johannessen et al, 2017) are fed by the reaction products, particularly the Fe 2+ cation, of deeper serpentinization reactions, leading to extensive seafloor ecosystems based on Fe-oxidizing bacteria (Bach, 2016). Several estimates of hot spot-island uplift history (Konter & Jackson, 2012;Ramalho et al, 2017) were suggested by Park and Rye (2019) to reflect the slow infiltration of seawater and gradual serpentinization, on time scales of 2-4 Myr. Finally, a serpentinized sub-Moho layer would facilitate partial melting from any later thermal event, such as the abundant small-scale non-hot spot seafloor volcanic features reported in French Polynesia by Hirano et al (2016).…”

“…Park and Rye () proposed “metasomatic underplating” to explain the common observations of a sub‐Moho layer of intermediate seismic velocity beneath hot spot islands (Caress et al, ; Contreras‐Reyes et al, ; Leahy et al, ; Leahy & Park, ; Lodge & Helffrich, ; Wolfe et al, ). Metasomatic underplating hydrates a layer of mantle rock via seawater infiltration to form a mineral assemblage of lower density and lower seismic wavespeed, primarily via the conversion of olivine to serpentine (e.g., Bach et al, ).…”