Understanding the Earth's deep mantle sulfur reservoir on billion year timescales

Sulfur is essential to all life on Earth. Sediments preserve a record of sulfur chemistry that spans billions of years. Changes in this record provide important insights into the role sulfur has played in shaping Earth’s oceans, atmosphere, and biology. Sulfur-bearing rocks on the seafloor can be carried deep into Earth’s interior, and some of this material may return to Earth’s surface at volcanoes. It is clear that sulfur chemistry in sediments records environmental changes over Earth history, but it is unclear whether samples from Earth’s interior show similar changes. This project will measure sulfur chemistry in volcanic samples ranging in age from more than 2.7 billion years old to the present. The results will help determine whether the interior sulfur record reflects changes at Earth’s surface. If changes in the volcanic record track changes in sedimentary sulfur, this work will provide new insight into the connections between Earth’s surface and interior. The project also supports public engagement by hosting events about volcanoes at a university museum. The overarching objective of this project is to generate a record of sulfur isotopes in plume-derived magmas over billions of years. This record will be used to test hypotheses for how sulfur isotopes in Earth’s deep mantle has changed over time and whether those changes echo changes observed in the surface sulfur reservoirs. Extending this record billions of years into Earth’s past requires exceptional samples that have escaped alteration. The project will analyze melt inclusions, which are tiny droplets of magma encased in crystals and are often shielded from modification by their crystal hosts. Analyses will be conducted on melt inclusions in unusual, primitive magmas known as komatiites. In rare cases, these melt inclusions are naturally glassy, making them suitable for direct measurement of sulfur isotopes via secondary ion mass spectrometry (SIMS). Crystalline melt inclusions will be rehomogenized using high-temperature experimental equipment prior to SIMS analysis. The results will lay the foundation for a long-term record of plume-derived magmas that can be compared with the sedimentary record. This comparison will test the hypothesis that changes in the sedimentary sulfur isotopes record are echoed, with a delay, in plume-derived magmas. If confirmed, the results will identify a new link between Earth’s surface and interior and establish igneous samples as a tool for investigating Earth’s long-term evolution. 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.