Volcanic controls on ash iron solubility: New insights from high-temperature gas-ash interaction modeling

- We model the high temperature volcanic controls on ash iron solubility.
- The hot core of the volcanic plume does not solubilize the ash iron directly.
- The hot core controls the Fe mineralogy and oxidation state at the ash surface.
- The ratio of H2 and H2S to the entrained O2 controls the iron oxidation state.
- Favorable source conditions for soluble iron formation in the ash are identified.

Recent studies strongly suggest that volcanic ash can fertilize the surface ocean by releasing soluble iron. However, the volcanic and atmospheric processes that solubilize ash iron during its transport from the volcano to the ocean are poorly understood. Using thermodynamic equilibrium calculations, we investigate the influence of gas-ash interaction within the hot core (T > 600 °C) of the volcanic plume and the consequences of this for ash iron solubility. Simulations are performed by considering the plume hot core as a box model in which 1000 °C magmatic gas, ash and 25 °C ambient air are mixed together. We show that mixing and the resulting cooling of the gas-ash-air mixture affect the mineralogy and oxidation state of iron in the ash surface rim. Iron mineralogy in the ash surface layer after high-temperature plume processing is primarily governed by the ratio of the H2 and H2S content of the magmatic gas to the amount of entrained O2 into the hot plume (Xmix). The model results indicate that most of the iron in the ash surface layer is oxidized to ferric iron (Fe(III)) when log Xmix drops below − 3.5 in the hot core. Such conditions may be encountered at convergent plate volcanoes, which release H2O-rich magmatic gases. In contrast, high temperature gas-ash interaction at divergent plate and hot spot volcanoes, which tend to be associated with CO2-rich and SO2-rich magmatic gases, respectively, may produce ash surfaces where iron mostly occurs as ferrous (Fe(II)). These volcanoes seem to be more favorable for iron fertilization because log Xmix does not fall below − 3.5 and > 80% of the iron in the ash surface remains ferrous (Fe(II)), which is more soluble in water than Fe(III).