Sulfur Isotopic Evidence on the Age of Recycled Surface Material in the Tristan-Gough Plume Source

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The Earth as we know it today evolved over more than 4.5 billion years through accretion, catastrophic impacts, formation of oceanic crust and continents and establishing plate tectonics. Learning about this evolution is extremely difficult because the memory of this evolution is in rocks but almost all of them have been recycled in our dynamic Earth. An exception are ocean islands (e.g. Hawaii, Galapagos) that are formed by upwellings (mantle plumes) from the core-mantle boundary and represent material that witnessed surface conditions of the Earth a long time ago. But how long ago? This is generally not yet known. Although the age of a volcanic rock can be measured, the age of the source of a volcanic rock cannot. A recently developed tool to address this question looks at sulfur. The atmosphere changed about 2.45 billion years ago when oxygen increased dramatically, the so-called Great Oxygenation Event. The composition of sulfur in contact with the atmosphere before and after this 2.45 billion year mark are very different. Thus it can be determined if sulfur was on the surface of Earth before or after 2.45 billion years ago. Previous studies have found that sulfur from the surface of Earth more than 2.45 billion years ago traveled to the core-mantle boundary some 3000 km deep and back up where it was carried in magmas to the surface. This is an excellent example how the surface of our planet shapes its interior all the way down to the core. In this project sulfur will be used for the first time to investigate one of the major plume systems that have formed flood basalts covering most of Brazil about 130 Ma ago followed by lines of volcanoes marking the passage of the plates over a stationary mantle plume. This project will constrain the age of the source of this major plume system to younger or older than 2.45 billion years. It will also help our understanding how volumetrically significant material older than 2.45 billion year-old material is in the formation of mantle plumes. It has surprised researchers that mantle plume sources can be so old. It is exciting to look at a volcanic eruption and know that what forms lava today had been at the surface of Earth when no higher life forms yet existed, traveled all the way to the core-mantle boundary, and came back up over more than half of Earth's history. The persistence of recycled surface sulfur through subduction and eventual rise in a mantle plume has been demonstrated for two smaller South Pacific plumes (Pitcairn and Mangaia island of Cook-Austral) with basalts including sulfur older than about 2.45 Ga. However, to this point it is not clear whether such old surface material is a common or rare component in plume sources. In this project for the first time sulfur isotopes are measured in rocks related to a major long-lived plume system, the Tristan-Gough plume system in the South Atlantic, with an initial flood basalt province, the Parana, and with a root at the core-mantle boundary related to the African large low shear wave velocity province. This project addresses the following major hypothesis through a detailed analyses of sulfide inclusions in volcanic rocks from Tristan and Gough island: 1. Is the recycled component in the Tristan-Gough plume of Archean age or younger? Previous work showed that the Gough signature persists since at least 100 Ma, thus an Archean age would indicate that a significant volume of a major plume source contains old recycled surface material. 2. How does the absence or presence of Archean sulfur relate to the global endmembers of oceanic basalts in Sr-Nd-Hf-Pb isotopic compositions? Pitcairn and Tristan-Gough both have enriched mantle 1 endmember compositions. Do sulfur isotopes support formation by similar geochemical fractionation events of similar age, or, do different evolution paths lead to similar compositions? 3. What do sulfur isotopes imply regarding contributions from atmospheric, deep ocean hydrothermal or even microbial sediment components?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.