1/12/2016 - Variations in magmatic redox caused by source and process


12:00 PM - 1:00 PM
Slichter 3853

Presented By:
Maryjo Brounce - Caltech


Variations in magmatic redox caused by source and process

There is considerable interest in the variability of the oxygen fugacity (fO2) of the mantle sources of erupted magmas in both space and time. This is because fO2 controls the speciation of multi-valent elements and thus material transfer from the interior to the exterior of Earth. Spatial variations in the fO2 of the mantle are debated, in part because Fe in arc basalts is more oxidized than in mid-ocean ridge basalts, but trace element proxies for fO2 suggest that the mantle is uniform across tectonic regimes. Temporal variations in the fO2 of the mantle may have been transmitted to Earth’s atmosphere and oceans by volcanic degassing. However, this is also unclear because it is not certain how the redox states of volatiles relate to their source magmas because degassing and assimilation can impact magmatic fO2 before or during eruption. Here, I present measurements of the oxidation states of Fe and S in a variety of natural basaltic glasses from the global mid-ocean ridge, Mariana arc, Mariana back-arc, and Hawaiian volcanism in order to elucidate the relative influence of mantle sources and volcanic processes on the redox state of basaltic magmas erupted to the surface. I offer a dynamic explanation for the oxidized nature of Mariana arc basalts, where oxidized fluids and melts from the subducting slab generate melt production in the mantle wedge and produce oxidized primary arc basalts. An imbalance between oxidized inputs to the Mariana subduction system and the output of oxidized basalts along the arc and back-arc suggests that there may be a reservoir of oxidized materials that can be stored and transported in the deep mantle on geologic timescales. Undegassed Hawaiian magmas are more oxidized than mid-ocean ridge magmas, which may reflect the incorporation of ancient, oxidized surface materials in the Hawaiian plume. Hawaiian magmas become reduced as the result of volcanic degassing, which serves to limit the effect of depressurization in emitting oxidizing volcanic gases to Earth’s atmosphere.

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