Stacked sills forming a deep melt-mush feeder conduit beneath Axial Seamount

Abstract

Magmatic systems are composed of melt accumulations and crystal mush that evolve with melt transport, contributing to igneous processes, volcano dynamics, and eruption triggering. Geophysical studies of active volcanoes have revealed details of shallow-level melt reservoirs, but little is known about fine-scale melt distribution at deeper levels dominated by crystal mush. Here, we present new seismic reflection images from Axial Seamount, northeastern Pacific Ocean, revealing a 3–5-km-wide conduit of vertically stacked melt lenses, with near-regular spacing of 300–450 m extending into the inferred mush zone of the mid-to-lower crust. This column of lenses underlies the shallowest melt-rich portion of the upper-crustal magma reservoir, where three dike intrusion and eruption events initiated. The pipe-like zone is similar in geometry and depth extent to the volcano inflation source modeled from geodetic records, and we infer that melt ascent by porous flow focused within the melt lens conduit led to the inflation-triggered eruptions. The multiple near-horizontal lenses are interpreted as melt-rich layers formed via mush compaction, an interpretation supported by one-dimensional numerical models of porous flow in a viscoelastic matrix.

Revised Magmatic Source Models for the 2015 Eruption at Axial Seamount Including Estimates of Fault‐Induced Deformation

“…This conduit is consistent with the geometry and depth extent to the deformation source of Nooner and Chadwick (2016) as well as the new model results presented in this paper. However, while the source of Nooner and Chadwick (2016) is located just outside the extent of the 3–5 km wide, deep melt mush conduit of Carbotte et al (2019), our revised deformation source location is located within its extent, so it is perhaps in better agreement. The vertically stacked melt lenses also underlie the melt‐rich part of the shallow summit reservoir interpreted as the source of the 1998, 2011, and 2015 eruptions and dike intrusions (Carbotte et al, 2019).…”

“…However, while the source of Nooner and Chadwick (2016) is located just outside the extent of the 3–5 km wide, deep melt mush conduit of Carbotte et al (2019), our revised deformation source location is located within its extent, so it is perhaps in better agreement. The vertically stacked melt lenses also underlie the melt‐rich part of the shallow summit reservoir interpreted as the source of the 1998, 2011, and 2015 eruptions and dike intrusions (Carbotte et al, 2019). Despite some differences in details, these independent lines of geophysical evidence (deformation modeling with and without considering faulting, and multichannel seismic imaging of both the shallow and deep crust) all agree that the core of the high‐melt magma supply and storage system at Axial Seamount is beneath the southern part of the summit caldera.…”

“…New analysis of seismic data from the Juan de Fuca Ridge and Axial Seamount has revealed the presence of deeper reflections interpreted as magma sills beneath the previously imaged axial magma lens (AML) (Carbotte et al, 2018; Carbotte et al, 2019). The deeper reflections are interpreted as vertically stacked melt lenses extending to depths of 6 km beneath the southern portion of the caldera over a 3–5 km wide region and suggest a vertical conduit.…”

Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

“…Altogether, the Lower unit most likely formed from emplacement of sills (∼6 m thick in average) at the bottom of the intrusion forming a thick crystal mush and potentially crosscutting one another during emplacement, as indicated by the presence of intrusive contacts and the strong variability in compositions recorded. A sill-like geometry of the intrusions is favored according to seismic observations of such features in the oceanic crust (e.g., Marjanović et al, 2014;Canales et al, 2017;Carbotte et al, 2020), field observations and previous drilled section observations (e.g., Bédard and Hébert, 1996;Boudier et al, 1996;Kelemen et al, 1997;Lissenberg et al, 2004;Sanfilippo and Tribuzio, 2011;Coogan, 2014). In that case, the magmatic fabric would result mainly from crystal orientation due to local flow within each sill (Deans and Yoshinobu, 2019), hence explaining the lack of orientation systematics for the magmatic fabrics in the section.…”

In situ lower crustal accretion by melt sill injection revealed by seismic layering

“…Multiple magma lenses in the lower crust have been recently discovered through modern seismic surveys at the fast [22][23][24] and intermediate spreading ridges 25,26 , supporting the idea of in-situ magma intrusion and crystallisation in the lower crust. However, no such secondary lower crustal melt sills have been observed beneath slow spreading ridges, although low velocity anomalies have been reported in the lower crust, suggestive of the presence of partial melt (mush) 8,27 .…”