Plume-Plateau Interaction

Age progressive volcanic trends, known as hotspot tracks, are thought to be produced by partial melting of buoyant mantle plumes rising from the deep mantle (Morgan, 1971; Wilson, 1963). Hotspot tracks record the relative motion between plates and mantle plumes, and they are used to reconstruct the history of plate motion and to constrain the geochemical heterogeneity within the mantle, which are important to our understanding of mantle dynamics (e.g., Koppers et al., 2021; Weis et al., 2023).

Through a careful examination of isotopic, geochronological, and plate motion reconstruction data, Jackson et al. (2024) argued that certain Cretaceous (87–106 Ma) Magellan seamounts north of the Ontong-Java Plateau (OJP) may have been produced by the Samoan plume. This finding places the Samoan hotspot track among the longest-lived ones. However, there is a significant gap in volcanic activity from 24 to 87 Ma, excluding the 44 Ma Malaita volcanism. Raising the question, what mechanism could produce a 63 Ma gap in an otherwise enduring hotspot track?

It has long been observed that most hotspot tracks manifest as discrete volcanoes, exemplified by the long-lived Hawaii-Emperor Volcanic Chain, rather than continuous ridges. It is suggested that the locations of these volcanoes are controlled by fractures within the lithosphere, facilitating the migration of plume-generated magma (e.g., Hieronymus & Bercovici, 1999). Consequently, discrete volcanoes are anticipated along hotspot tracks.

To explain the bilaterally zoned hotspot tracks (e.g., Abouchami et al., 2005; Huang et al., 2011; Weis et al., 2011), Rohde et al. (2013) argued that mantle plumes originating from the lower mantle may bifurcate at the mantle transition zone (Figure 1a). Because of the different mantle viscosities in the upper and lower mantle, a plume might rise much slower in the lower mantle compared to in the upper mantle. To maintain the same plume flux, a plume would become thinner in the upper mantle, which may lead to plume bifurcation at the mantle transition zone (Rohde et al., 2013). Alternatively, it is also possible that after entering the upper mantle, a plume fragments into discrete upwelling diapirs rather than maintaining a continuous flow (Figure 1a), resulting in volcanic activity gaps along hotspot tracks.

However, neither of these theories explains the prolonged absence of volcanism within a significant period (24–87 Ma) of the Samoan hotspot track. Jackson et al. (2024) noted that during this particular period of time, the Samoan plume was under the thick OJP. Mantle plumes ascend adiabatically, with a steeper pressure-temperature slope compared to that of the mantle solidus. As such, plumes start to melt and produce magma when reaching shallow depths (low pressure). The upwelling stops at the base of rigid lithosphere, halting partial melting. Jackson et al. (2024) argued that the OJP's lithosphere is sufficiently thick to inhibit the plume's ascent to a shallow enough depth for melting, thus precluding volcanic activity and creating a seamount-free corridor (Figure 1b).

However, if the lithosphere is not thick enough to completely prevent a plume from melting and if a plume contains both an enriched lithology with a lower melting point and a refractory lithology with a higher melting point, the enriched lithology will melt preferentially (e.g., Phipps Morgan, 2001; Stracke & Bourdon, 2009). This results in magmas produced under thicker lithosphere having more enriched geochemical signatures (Figure 1b), as observed at the Emperor Seamount Chain (Frey et al., 2005; Regelous et al., 2003) and the Magellan seamounts, the Cretaceous Samoan volcanoes (Jackson et al., 2024).

Key geochemical signatures linking Magellan seamount lavas to Samoa include their distinctively high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd ratios, characteristic of the Enriched Mantle 2 (EM-2) mantle endmember, which is indicative of recycled ancient continental material in their mantle source (Jackson et al., 2007). However, it remains to be better elucidated where the Samoan plume, and mantle plumes in general, originates from. Are mantle endmembers, such as EM-2, inferred based on geochemical data of plume-derived lavas related to large mantle structures imaged by seismic waves, such as Large Low-Shear-Velocity Provinces (LLSVPs) in the deep mantle (e.g., Huang et al., 2011; Koppers et al., 2021; Weis et al., 2011, 2023)?

The findings of Jackson et al. (2024) suggest that the global plume flux might be underestimated if based solely on hotspot volcanic flux, as plume productivity can be suppressed under thick lithosphere. Furthermore, the isotopic compositions of erupted hotspot lavas may not be representative of their mantle source characteristics, as they are biased toward the enriched endmembers. If the enriched mantle endmembers contain recycled ancient surface materials, such as sediments, continental and oceanic crusts, their proportions in mantle plumes may be overestimated. They all hold significant implications for advancing our understanding of mantle dynamics.

The authors declare no conflicts of interest relevant to this study.