DISSOLUTION KINETICS OF ANDESITIC VOLCANIC ASH

Dissolution rates and stoichiometry of volcanic ash from five volcanic eruptions (1980 Mount St. Helens (MSH), USA; 1991 Mt. Pinatubo, Philippines (PIN); 2010 Eyjafjallajökull, Iceland (ICE); 2010 Pacaya, Guatemala (PAC); 2010 Tungurahua, Ecuador (TUN) were investigated to determine the impact of ash weathering on the potential drawdown of atmospheric CO₂. Ash dissolution experiments were conducted in batch reactors with water or dilute HCl at pH 3, 4, 5 and 7 for six months. Ash was characterized by XRF, XRD, BET and SEM for chemistry, mineralogy and surface properties. Reaction rates were determined from evolution of solution composition over time.

Ash reaction rates depended on ash composition, mineralogy, surface area and pH. Dissolved Si release varied by less than a factor of three for any pH treatment but the surface area normalized rates varied by up to two orders of magnitude, due to the large variation in surface area and particle size. Ash reaction rates were nearly independent of pH at pH 5 and 7, though rates increased with decreasing pH. The pH dependence of the reaction rate (n from r=k(H+)^n) ranged -0.5 to -0.9. Ash reactivity was more pH dependent with increasing Ca-Mg-Fe content of the ash.

Analysis of the ash samples with SEM and EDX showed the ash samples comprised a complex assemblage of euhedral crystals of feldspars, olivine, pyroxene, amphibole, apatite and iron-titanium oxides in a glassy matrix. EDX analysis of the glassy phase from the MSH, PIN, ICE, and TUN ash samples showed variable composition, primarily of Si-Al-Na, consistent with an alkali feldspar composition but the glassy matrix of the PAC ash sample had consistently higher iron content than the other samples. SEM analysis of the MSH, PIN, ICE, TUN ashes reacted at pH 3 showed extensive evidence for crystallographically controlled etch pits in the mafic crystal phases (olivine, pyroxene and amphibole) but the feldspar and glassy matrix showed little evidence of reaction. In contrast, the more Fe-rich glassy phase in the PAC ash is more extensively dissolved than the euhedral crystals.

These experiments suggest that although the mafic crystalline phases compose only a small fraction of the total ash sample they react more rapidly than the glassy matrix and contribute disproportionately to the initial weathering flux.