Unveiling the Distinct Structure of the Upper Mantle Beneath the Canary and Madeira Hotspots, as Depicted by the 660, 410, and X Discontinuities

• Published in: Journal of geophysical research. Solid earth Vol. 129; no. 5
• Main Authors: Bonatto, Luciana, Schlaphorst, David, Silveira, Graça, Mata, João, Civiero, Chiara, Piromallo, Claudia, Schimmel, Martin
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-05-2024
• Subjects: Accumulation; Basalt; Crystal structure; Depth; Discontinuity; Earth mantle; Eclogite; Hot spots; Hot spots (geology); hotspot; Islands; Mantle plumes; P-waves; Phase transitions; Plumes; receiver function; recycled eclogite; S waves; Seismic activity; Seismic discontinuities; Seismic waves; Silica; Temperature variations; thermochemical plume; Tomography; Transition zone; Upper mantle; Volcanic activity; Volcanism; Waveforms; X‐discontinuity
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2023JB028195

The Canary and Madeira Islands are two distinct hotspots in the Central‐East Atlantic that are close to each other. Their volcanism is generally attributed to underlying mantle plumes, but the detailed structure of these plumes is still not well understood. The thermal and compositional structure of the plume introduces complexities in the phase transitions of the mantle, which impact the depth and magnitude of seismic discontinuities. We use 1,268 high‐quality receiver functions from stations located at the two hotspots to detect P‐to‐s converted phases through a common‐conversion point stacking approach and conduct a detailed analysis of mantle seismic discontinuities. The results show that both hotspots are characterized by a thin mantle transition zone (MTZ), with sharp 410 and 660 discontinuities at depths of 428–421 km and 647–664 km, beneath the Canaries and Madeira respectively. The results indicate that the Canary plume crosses the MTZ, whereas the Madeira plume mainly influences the upper portion of the MTZ. Furthermore, we find reliable detections of a sharp X discontinuity beneath the Canaries at 287 km. Its presence suggests the accumulation of silica‐rich recycled eclogite at these depths. We also use the amplitudes of P410s and PXs to derive velocity jumps at corresponding discontinuities. Based on these measurements, we estimate that the basalt proportion is 60%–80%, with accumulation being more significant in the Canaries than in Madeira. The MTZ thickness, the presence of the X discontinuity, and the high basalt proportion provide compelling evidence for a deep‐rooted thermochemical plume beneath the study area.
Plain Language Summary
We examine the Canary and Madeira Islands, two separate groups of islands in the Atlantic Ocean, close to each other. While it is generally accepted that volcanic activity in these regions is driven by underlying mantle plumes, the precise characteristics of these plumes remain relatively unknown. Tomography studies have revealed distinct structures of the plumes, with the Canary plume extending vertically throughout the upper mantle, while the Madeira plume is visible up to a depth of 300 km, beyond which the resolution of the images deteriorates. We use receiver functions, derived from the analysis of seismic waveforms, particularly the conversion of compressional waves to shear waves at seismic discontinuities within the Earth. Receiver functions offer a higher resolution compared to tomographic images, primarily due to their ability to directly measure seismic wave conversions at specific depths, reflecting where mantle minerals change their crystal structure. This analysis provides insights into the variations in temperature and mineral chemistry in the mantle, as both factors influence the depths of crystal rearrangements. Our research provides evidence supporting the existence of a deep‐seated thermochemical plume beneath the study area. However, beneath Madeira, the plume may be less vigorous or presently disconnected from its deep source.
Key Points
We investigate the upper mantle discontinuities below Canary and Madeira using P receiver functions
Deeper 410 in both, shallower 660 in Canaries, normal in Madeira: suggests Canary plume crosses MTZ, Madeira plume only affects upper MTZ
Detection of X discontinuity suggests a deep‐rooted thermochemical plume