The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth's surface; therefore, variations in mantle fO2 may influence the fO2 at Earth's surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2 We show that low-pressure degassing resulted in reduction of the fO2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.
Keywords: Great Oxidation Event; Hawaii volcanism; ocean island basalts; oxygen fugacity; volcanic degassing.