Volcanic controls on ash iron solubility: New insights from high-temperature gas–ash interaction modeling G. Hoshyaripour a, ⁎, M. Hort a , B. Langmann a , P. Delmelle b a Institute of Geophysics, University of Hamburg/CEN, Bundesstr. 55, 20146 Hamburg, Germany b Earth and Life Institute, Universitè catholique de Louvain, Croix du Sud, 2 bte L7.05.10, B-1348 Louvain-la-Neuve, Belgium abstract article info Article history: Received 6 May 2014 Accepted 5 September 2014 Available online 19 September 2014 Keywords: Volcanic ash Thermodynamic equilibrium Iron release Eruption plume 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 in- teraction within the hot core (T N 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 H 2 and H 2 S content of the magmatic gas to the amount of entrained O 2 into the hot plume (X mix ). The model results indicate that most of the iron in the ash surface layer is oxidized to ferric iron (Fe(III)) when log X mix drops below -3.5 in the hot core. Such conditions may be encountered at convergent plate volcanoes, which release H 2 O-rich magmatic gases. In contrast, high tem- perature gas–ash interaction at divergent plate and hot spot volcanoes, which tend to be associated with CO 2 -rich and SO 2 -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 X mix does not fall below -3.5 and N 80% of the iron in the ash surface remains ferrous (Fe(II)), which is more soluble in water than Fe(III). © 2014 Elsevier B.V. All rights reserved. 1. Introduction After the 1991 eruption of Mt. Pinatubo, Philippines, it was hypoth- esized that ash in contact with seawater releases iron and other nutri- ents in sufficient amounts to the surface ocean to stimulate marine primary productivity (MPP) and in turn, global atmospheric CO 2 draw- down (Sarmiento, 1993; Watson, 1997). Frogner et al. (2001) found that the ash from the eruption of Hekla in 2000, Iceland, released signif- icant amounts of dissolved iron, silicon, and manganese together with sulfate, chloride and fluoride upon exposure to seawater. Subsequent studies have confirmed that volcanic ash affects MPP through rapid sol- uble iron release upon contact with seawater (Duggen et al., 2007). The first direct evidence of a phytoplankton bloom following fertilization by volcanic ash deposition was reported by Langmann et al. (2010) and later, Hamme et al. (2010) in the wake of the 2008 eruption of Kasatochi volcano in the Aleutian Islands. Achterberg et al. (2013) also reported a significant perturbation in the biogeochemistry of the Iceland Basin of the North Atlantic through the dissolved iron release from the ash erupted from Eyjafjallajökull, Iceland, in 2010. Volcanic ash refers to tephra with a diameter of b 2 mm (Rose and Durant, 2009) and is typically composed of silicate glass and crystalline materials generated during an explosive eruption through magma frag- mentation and to some extent, through erosion of the conduit wall rock (Heiken and Wohletz, 1992). Iron in volcanic ash produced through magma fragmentation is essentially found in non-soluble forms, i.e., in silicate glass and in primary Fe-bearing silicates and Fe-oxide minerals (Heiken and Wohletz, 1992; Schmincke, 2004). However, the source of bio-available iron involved in the alteration of the surface ocean's bio- geochemistry is believed to be soluble iron species on the ash surface (Duggen et al., 2010; Hamme et al., 2010; Achterberg et al., 2013). Bio-availability of iron is suggested to be strongly linked to its solubility, which is influenced by chemical speciation (Fe(II) is more soluble), min- eralogy (amorphous phases are more soluble), and Al substitution in Fe(III) oxides (Al-rich Fe phases are less soluble) (von der Heyden et al., 2012). Volcanic and atmospheric processes that modulate these properties and consequently, ash iron solubility are poorly constrained so far. Ayris and Delmelle (2012) emphasized that both high and low temperature reactions within the eruption plume can significantly alter the ash surface composition, and hence iron mineralogy and speci- ation. These reactions are expected to modify the surface reactivity of the ash, thus potentially influencing further (photo)chemical reactions during transport of the ash in the atmosphere. Journal of Volcanology and Geothermal Research 286 (2014) 67–77 ⁎ Corresponding author. Tel.: +49 40 42838 5053. E-mail address: gholamali.hoshyaripour@zmaw.de (G. Hoshyaripour). http://dx.doi.org/10.1016/j.jvolgeores.2014.09.005 0377-0273/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores