How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot.

Kaj Hoernle,Dieter Garbe-Schönberg,Jason P Morgan,Joana Rohde,Reinhard Werner,Stephan Homrighausen,Folkmar Hauff

doi:10.1038/ncomms8799

Abstract

Increasingly, spatial geochemical zonation, present as geographically distinct, subparallel trends, is observed along hotspot tracks, such as Hawaii and the Galapagos. The origin of this zonation is currently unclear. Recently zonation was found along the last ∼70 Myr of the Tristan-Gough hotspot track. Here we present new Sr–Nd–Pb–Hf isotope data from the older parts of this hotspot track (Walvis Ridge and Rio Grande Rise) and re-evaluate published data from the Etendeka and Parana flood basalts erupted at the initiation of the hotspot track. We show that only the enriched Gough, but not the less-enriched Tristan, component is present in the earlier (70–132 Ma) history of the hotspot. Here we present a model that can explain the temporal evolution and origin of plume zonation for both the Tristan-Gough and Hawaiian hotspots, two end member types of zoned plumes, through processes taking place in the plume sources at the base of the lower mantle.

Highlights

Spatial geochemical zonation, present as geographically distinct, subparallel trends, is observed along hotspot tracks, such as Hawaii and the Galapagos
To address questions concerning the longevity of plume zonation and how it originates, we investigate the entire history of the Tristan-Gough hotspot here and compare it with what is known about the history of the Hawaiian hotspot
The 143Nd/144Nd of the mafic (MgO Z11 wt.%) flood basalts is similar to the oceanic Tristan-Gough track rocks (Fig. 4a), the 87Sr/86Sr in some mafic samples extends to higher ratios, which can be explained by small amounts of assimilation of crust with very radiogenic 87Sr/86Sr and/or through melting of enriched portions of the lithospheric mantle with radiogenic Sr and Pb isotope ratios[27]

Summary

33 Ma 27 Ma

Even though they are completely separated on the uranogenic Pb isotope diagram, the Tristan and Gough fields completely overlap on the (b) 206Pb/204Pb versus 208Pb/204Pb (thorogenic Pb) isotope diagram, indicating that the Tristan compositions cannot be explained by mixing of Gough compositions with Atlantic MORB. Changing the assumed concentrations for Gough and MORB end members will shift the mixing percentages, but mixing of Gough and Atlantic N-MORB will still require Tristan to be shifted towards N-MORB compared with Gough on the thorogenic Pb isotope diagram. Evaluate published data from the Etendeka and Parana flood basalts to assess the composition of the plume during its earliest history. These results are used to establish a model for the entire temporal (last 132 Myr) geochemical evolution of the TristanGough hotspot. We compare the evolution of the Tristan and Hawaiian plumes and evaluate the origin of geochemical zonation in both plumes

Results

Discussion

Methods