ALBERT

Description: The origin of hotspot trails is controversial. Explanations range from deep mantle plumes rising from the core-mantle boundary (CMB) to shallow plate cracking. However, these mechanisms cannot explain uniquely the scattered hotspot trails distributed across a 2,000-km-wide swell in the sea floor of the southeast Atlantic Ocean. This swell projects down to one of the two largest and deepest distinct regions at the CMB, the Africa Low Shear Wave Velocity Province. Here we use 40 Ar/ 39 Ar isotopic analyses to date lava samples erupted at several hotspot trails across the Atlantic swell. We combine the eruption ages with an analysis of the structure and age of the sea floor, and find that the trails formed synchronously, in a pattern consistent with movement of the African Plate over plumes rising from the edge of the Africa Low Shear Wave Velocity Province. However, we also find that the seamounts initially formed only at the edge of the swell, where the oceanic crust was spreading apart. Later, about 44 million years ago, the hotspot trails began to cross the swell, but only in locations where the lithosphere was sufficiently young and thin that magma could reach the surface. We conclude that the distribution of hotspot trails in the southeast Atlantic Ocean is controlled by the interplay between deep-sourced mantle plumes and the motion and structure of the African Plate.

Description: Establishing if and when South Atlantic hotspots interacted with surface processes during rifting and continental breakup is important for understanding the mechanisms that control the evolution of passive margins and their adjacent continents. One approach is to reconstruct the volcanic history of hotspot trails located on the African Superswell in order to find the locations of hotspots during rifting and breakup to determine if, for example, they caused extreme fluxes of magma and post-rift uplift along the continental margin. However, because hotspot trails located south of the classical Tristan-Gough are virtually un-sampled we don’t know how many hotspots might have existed or for how long, and whether they originated from the core-mantle boundary or much shallower depths. In 2006 we dredge sampled hotspot trails located on the African Superswell using the RV Polarstern, an icebreaker capable of working in the poor weather conditions in the Southern Ocean. Combining new and existing Ar/Ar isotopic ages shows that volcanism migrated synchronously along co-parallel hotspot trails consistent with northeastern African plate motion relative to the leading edges of the African Superswell and an underlying stable Superplume (large low-shear-velocity province) extending from the core-mantle boundary. Between roughly 132 and 100 million years ago only the Tristan-Gough hotspot trail developed where rifting and breakup facilitated the rise of hotspot melts to the surface, while along rest of the leading edge hotspot volcanism was suppressed by the African continent. Such a notion implies that the African passive continental margin was migrating relative to the leading edge of the African Superplume for as long as 30 million years after continental rifting and breakup had facilitated the 132 Ma Parana-Etendeka continental flood basalts and initiation of the Tristan-Gough hotspot trail. This provides a mechanism for extended late stage interplay between deep mantle processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of postrift uplift.

Description: The origin of hotspot trails ranges controversially1 from deep mantle plumes rising from the core-mantle boundary2 to shallow plate cracking. But these mechanisms cannot explain uniquely the scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell3, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province, which marks a massive thermo-chemical ‘pile’ at the core-mantle boundary4,5,6. Here we use 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – to show that age progressive hotspot trails formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province. We show also that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We conclude that if plume melts formed synchronous age progressive hotspot trails wherever and whenever they could penetrate the swell lithosphere then hotspot trails in the South Atlantic are controlled by an interplay between deep plumes and the motion and structure of the African plate.

Description: Discovering if hotspots observed on the Earth’s surface are explained by underlying plumes rising from the deep mantle or by a shallow plate-cracking mechanism continues to be an essential goal in Earth Science. Key evidence underpinning the mantle plume concept is the existence of narrow, age-progressive volcanic trails recording past plate motion relative to surface hotspots and their deep causal plumes. Using the icebreaker RV Polarstern we sampled scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province – caused by a massive thermo-chemical ‘pile’ on the core-mantle boundary. We showed recently using 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O’Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. Our observations reveal a plate tectonic-controlled cycle from the creation of deep thermo-chemical piles (LLSVP) and initiation of deep mantle plumes at the CMB to the shallow formation of the resulting hotspot trails. Moreover, suppression of plume melts from venting to the plate surface for tens of millions of years implies that the plumes responsible for the southeast Atlantic hotspot swell and hotspot trails transported more material and heat from the core mantle boundary than measured by hotspot volcanism.

Description: Discovering if hotspots observed on the Earth’s surface are explained by underlying plumes rising from the deep mantle or by shallow plate-driven processes continues to be an essential goal in Earth Science. Key evidence underpinning the mantle plume concept is the existence of age-progressive volcanic trails recording past plate motion relative to surface hotspots and their causal plumes. Using the icebreaker RV Polarstern, we sampled scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province – caused by a massive thermo-chemical ‘pile’ on the core-mantle boundary.We showed recently using Ar/Ar isotopic ages – and crustal structure and seafloor ages – that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O’Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. If the distribution of hotspot trails reflects where plume melts could or could not penetrate the continental or oceanic lithosphere then plumes could have been active for significantly longer than indicated by their volcanic chains. This provides a mechanism for extended late stage interplay between deep mantle processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of post-rift uplift on continental margins and nearby continents.