Stacked Magma Lenses Beneath Mid‐Ocean Ridges: Insights From New Seismic Observations and Synthesis With Prior Geophysical and Geologic Findings

Abstract

Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events.

Thermal Regime of Slow and Ultraslow Spreading Ridges Controlled by Melt Supply and Modes of Emplacement

“…The periodicity of melt supply and the fluctuation of the thermal regime documented at fast spreading ridges (Marjanović et al., 2014; Soule et al., 2009) are much lower than at slow and ultraslow ridges (Chen et al., 2021; Klischies et al., 2019). The predictions of our simulations are, however, consistent with the dynamics of magmatic systems documented at the gabbro/sheeted dikes transition zones of the Oman ophiolite and the fast spreading East Pacific Rise (Carbotte et al., 2021; Gillis, 2008; Nicolas et al., 2008). The AMLs migrate upward during waxing phases, corresponding to the reheating of the AML roof and hence the recrystallization of hydrothermally altered sheeted dikes into pyroxene and hornblende hornfels with the formation of xenoliths, while during waning phases, AMLs migrate downward, causing crystallization at the AML roof and/or sides to form isotropic gabbros that are crosscut by later dikes (France et al., 2009; Gillis, 2008; Nicolas et al., 2008, 2009).…”

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Petrography and Petrology

“…However, most scientists believe that the magmatic processes responsible for magmatic accretion at fast‐spreading oceanic crust are too complex to be explained with such simple end‐member models, and that the truth probably lies somewhere in between these models (Boudier et al., 1996; Maclennan et al., 2004; Natland & Dick, 2009). Such hybrid models are also supported by the estimation of crystallization depths of MORB (Wanless & Shaw, 2012) and from recent multi‐channel seismic studies indicating the presence of deep melt sills under recent fast/intermediate‐spreading ridges (e.g., Canales et al., 2009; Carbotte et al., 2012, 2021; Marjanovic et al., 2014; Nedimovic et al., 2005).…”

“…So the initial thickness of the melt lens remains unconstrained as of today. For stacked deep magma lenses in the plutonic crust at the Juan de Fuca Ridge which were investigated using seismic experiments, Carbotte et al (2021) recently estimated a maximum thickness of the observed melt reservoirs of 140-170 m.…”

780 Thousand Years of Upper‐Crustal Construction at a Melt‐Rich Segment of the Ultraslow Spreading Southwest Indian Ridge 50°28′E

“…We tentatively draw the contours of this hot and melt‐bearing domain down to Moho depth, where we propose that it roots into the melt‐bearing upper mantle (Figure 9c). Inspired by observations of stacked melt sills at fast and intermediate spreading ridges (Carbotte et al., 2020, 2021; Marjanović et al., 2014), and by a conceptual model of the transcrustal magma plumbing system beneath subaerial volcanos (Cashman et al., 2017), we also propose that some of the melt in this hot lower crustal region resides in vertically stacked melt lenses (Figure 9c), and that these lenses play a role in transferring melt from the mantle to the upper crust to feed eruptions. In Figure 9c, we also infer that most hydrothermal cooling occurs in a domain that is, limited both by temperature (ca 600°C) and pressure (corresponding to depths <6 km) as proposed by Morgan and Chen (1993).…”