Discovery of Axial Seamount Deep Melt-Mush Feeder Conduit

Suzanne M. Carbotte, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA, and Adrien Arnulf, Institute for Geophysics, University of Texas at Austin, TX, USA. Extracted from OOI Science Plan, 2021.

Recent geophysical observations at Axial Seamount provide new seismic images of the deep magma plumbing system at this submarine volcano and reveal a stacked sill complex extending beneath the main magma reservoir that underlies the Axial summit caldera (Figure to right). This pipe-like zone of stacked sills is interpreted to be the primary locus of magma replenishment from the mantle beneath Axial and indicates localized melt accumulations are present at multiple levels in the crust (Carbotte et al., 2020). How and where melt accumulations form, how melt is transported through the lower crust to feed shallower reservoirs, and how eruptions are triggered are fundamental questions in volcanology about which little is known. The discovery of this deep melt-mush conduit at Axial, where long- term monitoring observations supported by the OOI are available, is providing new insights into these questions that are broadly relevant for understanding magmatic systems on Earth.

Background: The new observations are derived from previously acquired multi-channel seismic data reprocessed using modern techniques. The data reveal a 3-5 km wide conduit of vertically stacked quasi- horizontal melt lenses, with near-regular spacing of 300-450 m, extending to depths of ~ 4.5 km below seafloor into the mush zone of the mid-to-lower crust. The stacked sill conduit is roughly centered beneath the southern shallowest and melt-rich portion of the broad upper crustal melt reservoir called the Main Magma Reservoir or MMR (Arnulf et al., 2014) that, based on previous studies, is interpreted to be the source initiation region for the three documented seafloor eruptions at Axial that occurred in 1998, 2011, and 2015. We conclude that magma flux within the deep pipe is linked to the initiation of all three eruptions. This melt-mush conduit also underlies the International District hydrothermal vent field at Axial Seamount and likely plays a critical role in maintaining the robust hydrothermal system at this location.

Long-term monitoring arrays of geodetic sensors and seismometers deployed at Axial Seamount as part of the OOI provide constraints on the history of seamount inflation and deformation and the nature of magma transport during pre- and syn-eruption phases at this volcano. Seafloor geodetic studies conducted since the late 1990’s document a history of steady seamount inflation during inter-eruption periods and rapid deflation associated with the three eruptions (Nooner and Chadwick, 2016; Hefner et al., 2020). From modeling of the OOI geodetic records prior to and during the 2015 event, these studies obtain a best fit pressure source that corresponds to a steeply dipping prolate spheroid centered at 3.8 km below seafloor, extending well beneath the MMR. The pressure source derived from the geodetic modeling is similar in geometry and depth extent to the quasi- vertical conduit of stacked lenses imaged in our study. Likewise, continuous seafloor compliance data derived from two OOI broadband seismometers also suggest a narrow lower-crustal conduit beneath the summit caldera (Doran and Crawford, 2020). We interpret the deep melt lens column revealed in the seismic reflection images as the inflation/deflation source for the recent eruptions, with the MCS data defining its location and revealing an internal structure composed of a series of melt lenses embedded within a more crystalline mush. Magma replenishment from the lower crust and upper mantle is interpreted to be focused within this conduit region with magma transport by steady porous flow inferred from the record of uniform rates of inflation prior to the recent eruptions.

Magma replenishment sourced from the deep melt sill column may also explain the spatial patterns of microseismicity detected using the OOI prior to and during the 2015 eruption (e.g. Wilcock et al., 2016; 2018). The detected seismicity is largely confined to the shallow crust, above the MMR and is concentrated on outward facing ring faults along the south-central portion of both east and west caldera walls, as well as along a two diffuse bands of seismicity that crosses the caldera floor one of which coincides well with the interpreted northern edge of the deep melt column (Fig. to the right). We interpret this distribution of inflation-related seismicity to fracturing of the shallow crust linked to inflation centered within the imaged melt column.

The origin of the conduit of quasi-horizontal melt lenses, in a region where magma replenishment via steady porous flow is documented, is attributed in our study to processes of melt segregation from a compacting mush (Carbotte et al., 2020). This interpretation is supported by results from 1D viscoelastic modeling which, for plausible melt fractions, viscosities, and permeabilities, predict a series of porosity waves with similar quasi-regular spacings and over a similar depth range as the observed melt lenses. Other processes can contribute to melt sill formation, such as dike intrusion and formation of sills at permeability boundaries or through conversion of mush to magma with arrival of hotter magmas from depth, but the available data are inadequate to further constrain processes within this deep conduit.

Research Opportunities: At Axial Seamount, the OOI infrastructure combined with constraints on the architecture of the magma plumbing system obtained using marine active source seismic, provides the opportunity to tie dynamic volcano processes of magma recharge and eruption directly to individual magmatic structures imaged within the volcano interior. Our findings of a localized deep stacked sill-mush conduit beneath the shallow broad MMR at Axial raises important questions of how melt accumulations form at these levels, whether they are sources of erupted magmas requiring rapid magma transport from depth during eruptions, and whether there may be deep magma movements in other parts of the volcano away from the conduit region. While the detected seismicity at Axial is largely confined to the upper crust above the MMR, the aperture of the existing seismometer array is narrow and insufficient to detect deeper seismicity. Future studies of the deep magma plumbing system would require wider aperture seismometer and geodetic arrays and could be conducted at Axial leveraging the OOI. Such studies of the deep magma plumbing, conducted within the framework of the even higher-resolution 3D multi-channel seismic imaging data recently acquired at Axial Seamount (Arnulf et al., 2019), would be unprecedented for at any volcano on Earth.