Several previous active and passive-source seismic studies have observed weak or absent Moho arrivals at other locations within the Cascadia forearc region14,16,18 (Fig. 1), including a reversal of the Moho velocity contrast in central Oregon13. The inferred reduction of mantle velocities has been attributed to the formation of low-velocity serpentine in the mantle wedge11,12,13,14. Serpentinite also has a relatively low-density and high-magnetic susceptibility, which is consistent with regional magnetic and gravity anomalies15. The presence of ∼50–75% serpentine is enough to reduce upper mantle P-wave velocities to 7.1–7.5 km s−1 (ref. 12), similar to the observed lower crustal velocities beneath Mount St Helens9,17. However, we note that this is only a rough estimate of the amount of serpentine required to extinguish the Moho discontinuity and is on high end of reported estimates for Cascadia which range 15–60% (refs 13, 19).

The high pressure serpentine phase (antigorite) is stable in the mantle wedge at relatively low temperatures (<∼700 °C (ref. 20)) and can form by hydrous alteration of peridotite11,12. From a global perspective, Cascadia is a relatively warm arc due to the young age of the subducting Juan de Fuca plate21. However, modelling suggests that the down-going slab dewaters primarily beneath the forearc region in warm subduction zones22 and these fluids may therefore contribute to the serpentinization of the forearc mantle wedge12. Thermal modelling of the Cascadia subduction system21 (Fig. 6) suggests that the eastern limit of the antigorite stability field could extend to near Mount St Helens, therefore, we interpret the lack of PmP arrivals west of Mount St Helens as evidence for a serpentinized upper mantle. The observed transition in Moho reflectivity is abrupt, occurring over a distance of about 5–10 km and is consistent with imaging the antigorite dehydration front whose rapid reaction kinetics23 should produce a thermally well-defined boundary.

Figure 6: Two dimensional thermal model of the Cascadia subduction system in Central Oregon and geologic interpretation. The surface locations of Mount St Helens and Mount Adams relative to the thermal model21 are determined by using their distance from the subduction trench. The top of the subducting plate and the crust-mantle boundary of the overriding plate are plotted as white lines. The hachured area denotes the region of the mantle wedge where serpentine is stable (<∼700 °C (ref. 20)). Locations of deep long period (DLP) earthquakes are plotted as white dots whose size scales with magnitude. The low velocity anomaly observed in the lower crust from iMUSH tomography9 is shown. Arrows show two possible pathways for the lateral migration of melt towards Mount St Helens; These include a path through the mantle wedge (B) and one through the mid-crust (A). Full size image

Areas where serpentinite has been inferred in the Cascadia forearc mantle are typically offset from the main axis of arc volcanism by ∼50 km trenchward and are not usually associated with significant volcanism (Fig. 1). This observation is consistent with the view that the location of the volcanic arc relative to the trench is predominately controlled by the thermal structure of the mantle wedge6,8. Mount St Helens therefore presents a thermal paradox because it lies directly adjacent to the cold mantle wedge and yet still produces arc derived magmatism which requires elevated temperatures. For example, Mount St Helens primarily erupts dacites that are produced in the lower crust by either partial melting of a mafic source24 and/or partial crystallization of mantle derived basalts25. Analysis of melt inclusions from the 1980 and 2008 eruption cycles indicate that magma temperatures were 860–900 °C (ref. 26), consistent with thermal models of arc lower crust (800–1,000 °C)5. Additionally, analysis of young (post-Miocene) mantle derived basalts from southern Washington suggest melt segregation temperatures that range ∼1,200–1,450 °C (ref. 27). A hot mantle wedge directly beneath Mount St Helens is incongruent with the presence of a cold (<∼700 °C (ref. 20)) serpentinized uppermost mantle just west of the volcano (Fig. 6).

One way this dilemma can be resolved is if the lower crustal source region resides east of the volcano, towards the hotter axial region of the volcanic arc, and the ascending melts migrate laterally in the crust towards the forearc region. Magnetotelluric results indicate that Mount St Helens is electrically connected to a region of high conductivity in the mid-crust28, which is located near the southern terminus of the Southern Washington Cascades Conductor29 (Fig. 1). This feature extends east to Mount Adams and has been interpreted as a layer of partial melt28, which could thus provide a pathway for the westward migration of melts derived from the lower crust towards the cold forearc (see path A in Fig. 6). However, dacites erupted at Mount St Helens have notably different geochemistry than those found at Mount Adams which suggests that the melt source regions for these two volcanos are not the same30.

Alternatively, recent iMUSH active-source tomography resolves a vertical column of low seismic velocities southeast of the volcano that extends from Moho to mid-crust depths and is interpreted as partial melt9. This feature could therefore represent a localized lower crustal source region for the volcanism at Mount St Helens. The mantle derived basalts that ultimately drive the magmatic system25,31 require a hot source region (>∼1,200 °C (ref. 27)) suggesting that these melts are formed further to the east and migrate west into the cold forearc mantle wedge (path B in Fig. 6). The true thermal state of the mantle wedge beneath Mount St Helens is undoubtedly complicated by effects not accounted for in the two-dimensional (2D) model (Fig. 6), such as three-dimensional (3D) topography of the slab surface32 (Fig. 1) and heat transfer caused by the migration of melts33. Future geophysical constraints from the iMUSH project and increasingly realistic 3D modelling efforts are required to better constrain the deep melt pathways beneath Mount St Helens.

A cluster of deep long period (DLP) earthquakes has been identified just southeast of Mount St Helens in the lower crust (23–44 km depth, Fig. 6). DLP events are commonly observed in volcanically active regions, including beneath many of the volcanoes in the Cascade Range34, and these events are thought to be caused by the movement or cooling of magmatic fluids35,36. The DLP at Mount St Helens are located near the edge of the low velocity column observed in the iMUSH tomography (Fig. 6) and have been interpreted as the result of magma injection9. Our PmP results suggest the DLP earthquake cluster is located above the antigorite stability boundary in the mantle wedge (Fig. 5) and therefore these features may be related. Buoyant ascent of water produced by serpentine dehydration has been suggested to explain non-volcanic DLP events observed in the forearc region of Central Oregon37. A local heat source is required to drive dehydration37 and this could be caused by the advection of heat associated with the arrival of mantle melts near the Moho beneath Mount St Helens (path B, Fig. 6).