Oxygen Fugacity of Global Ocean Island Basalts

Mantle plumes contain heterogenous chemical components and sample variable depths of the mantle, enabling glimpses into the compositional structure of Earth's interior. In this study, we evaluated ocean island basalts (OIB) from nine plume locations to provide a global and systematic assessment of the relationship between fO2 and He-Sr-Nd-Pb-W-Os isotopic compositions. Ocean island basalts from the Pacific (Austral Islands, Hawaii, Mangaia, Samoa, Pitcairn), Atlantic (Azores, Canary Islands, St. Helena), and Indian Oceans (La Reunion) reveal that fO2 in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenous fO2 (average = +0.5 increment FMQ; 2SD = 1.5) relative to prior estimates of global mid-ocean ridge basalts (MORB; average = -0.1 increment FMQ; 2SD = 0.6), though many individual OIB overlap MORB. Specific mantle components, such as HIMU and enriched mantle 2 (EM2), defined by radiogenic Pb and Sr isotopic compositions compared to other OIB, respectively, have distinctly high fO2 based on statistical analysis. Elevated fO2 in OIB samples of these components is associated with higher whole-rock CaO/Al2O3 and olivine CaO content, which may be linked to recycled carbonated oceanic crust. EM1-type and geochemically depleted OIB are generally not as oxidized, possibly due to limited oxidizing potential of the recycled material in the enriched mantle 1 (EM1) component (e.g., sediment) or lack of recycled materials in geochemically depleted OIB. Despite systematic offset of the fO2 among EM1-, EM2-, and HIMU-type OIB, geochemical indices of lithospheric recycling, such as Sr-Nd-Pb-Os isotopic systems, generally do not correlate with fO2. Rocks from Earth's surface are mixed back into the interior during crustal recycling as a result of plate tectonics and subduction. For example, plate tectonics results in subduction of oceanic crust back into the mantle. Recycling of surface materials might oxidize the interior of the Earth. Mantle plumes, which are buoyantly rising portions of the mantle that create ocean islands such as Hawaii, Iceland, and Samoa, have the chemical and isotopic characteristics associated with recycled materials in their sources. Here we investigate rocks from mantle plumes that have heterogeneous isotopic compositions as a result of incorporating different types of recycled material to test whether their oxygen fugacity varies systematically with the type of recycled crust in their source. We show that some types of mantle plume-derived rocks, called HIMU and enriched mantle 2 characterized by their extreme isotopic compositions, are more oxidized than the enriched mantle 1 or typical geochemically depleted rocks from mantle plumes and from spreading centers in the oceans. These results link certain recycled materials to oxidation of Earth's mantle. Oxygen fugacity generally does not correlate with radiogenic isotopic compositions that trace recycled material in mantle-derived rocksHIMU and enriched mantle 2 ocean island basalts are more oxidized than enriched mantle 1 or geochemically depleted ocean island basalts and mid-ocean ridge basalts