Secular and recent trends in the geochemistry of the Réunion hotspot and other global hotspots
The origins of volcanic activity at intraplate “hotspot” localities cannot be easily explained by fundamental principles of plate tectonics. Decades of geochemical and geophysical research have produced controversial hypotheses that attempt to explain the physical mechanism of magma generation and the origin of the magmas in the deep Earth. The mantle plume hypothesis conjectures that hotspot volcanism results from upwelling of buoyant mantle material from the deep Earth, which may carry recycled crust and lithosphere that resided on Earth’s surface billions of years ago. One requirement of the
plume hypothesis is that initial, intense stages of hotspot volcanism, which exist only in the geologic record, are genetically linked to later, waning stages of volcanism observed on Earth today. Physiographic evidence exists in several global localities, including the Réunion hotspot, that appears to link large igneous province volcanism to oceanic volcanism, however geochemical evidence linking the two is tenuous. Here, I demonstrate that certain trace element signatures of global hotspots are unlikely to reflect their mantle source in the same way as isotope systems. Next, I show that igneous rocks on Réunion possess a unique geochemical and isotopic signature that make them distinct from igneous rocks from other global hotspots. Finally, I interrogate the composition of the Deccan Traps mantle source in two ways. First, I use early-formed mineral phases to evaluate the presence of recycled material in the Deccan Traps mantle source. Second, I use lava compositions to re-construct a lithospheric and crustal assimilation history for Deccan Traps parental magmas and attempt to relate the composition of this parental magma to that of Réunion. Considering all chapters together, I show that these two volcanic provinces likely share a mantle source with a common composition, and that this source contains a small amount of recycled material. In addition, I provide new evidence that the Réunion mantle source was formed early in Earth’s history and remained isolated from other mantle domains for long periods of geologic time.