Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)
The Rio Grande Rise in the South Atlantic off of the coast of Brazil is a volcanic oceanic plateau that formed on, or close to the Mid-Atlantic Ridge spreading center. The Walvis Ridge, which is now located off of Africa, also formed near the Mid-Atlantic Ridge close to the same time. The Rio Grande Rise is about twice the volume of Walvis Ridge and together they record ~130 Myr of intra-plate volcanism and are hypothesized to represent the products of a deep mantle plume. Despite its extremely large size and prolonged volcanic history, Rio Grande Rise truly is terra incognita. Based on the few rock samples available, it appears that Rio Grande Rise formed between 70 and 90 million years ago at the same time as the older part of Walvis Ridge. To understand the history of these large volcanic features, this project will carry out a seagoing cruise to the Rio Grande Rise to survey and collect rock samples from 40 seamounts, rift zone valleys and steep escarpments. These new data will allow a thorough investigation of the formation of the Rio Grande Rise and its relationship to the Walvis Ridge where rock samples have already been collected. The new data will also provide a unique opportunity to address a wide range of questions relating to plate tectonics, including how the Rio Grande Rise is related to the deep dense mantle zone of the African large low shear wave velocity province (LLSVP) which is thought to be a place where mantle plumes are generated. This project is in collaboration with Brazilian scientists and students. The project supports the training of U.S. graduate and undergraduate students. The newly collected geochronological, geochemical and geophysical data will contribute significantly to the following two major science questions: (1) Is Rio Grande Rise shaped by plume dynamics, shallow tectonics or both and what are the consequences for understanding hotspot evolution? (2) What are the mantle sources related to the deep dense mantle zone of the African large low shear wave velocity province (LLSVP)? Determining when Rio Grande Rise and Walvis Ridge volcanism became associated with intra-plate hotspot volcanism alone is fundamental as these volcanic traces are essential for calculating African absolute plate motion and global plate circuit models back to 130 million years. Moreover, if the large volume of the Rio Grande Rise-Walvis Ridge system and its source link back to the edge of the LLSVP, this will provide a unique possibility to investigate whether their extreme enriched composition represents a major component in the lower mantle.