DISSOLUTION KINETICS OF MT. PINATUBO ASH

Dissolution rates of ash collected from the 1991 eruption of Mount Pinatubo were measured to determine the impact of ash weathering on the drawdown of atmospheric CO₂. Ash dissolution experiments were conducted in flow-through and batch reactors over a range of pH. In the flow-through experiments, water or dilute hydrochloric acid (pH ~3, 4, and 5) was added to the top of an ash column periodically over time, and the effluent collected and analyzed to determine total solute flux. Silica and phosphate concentrations in the column effluent depended on the pH and the extent of water-rock interaction. Silica concentrations in the effluents ranged from ~1 to 8 ppm Si, and generally decreased over time. Si concentrations were nearly independent of acidity from pH ~4 to 7, but increased approximately threefold at pH 3. Phosphate concentrations varied substantially with acidity, increasing from ~200 ppb at near neutral pH to ~5000 ppb at pH 3.

In the batch experiments silica dissolution rates were determined from the evolution of solution composition over time, and ranged ~2.3–14X10^-12 mol/m²s. The initial PO₄ flux was two to five-fold greater than that of Si, indicating that apatite dissolution rates may be as high as ~2–3 orders of magnitude faster than silicate dissolution rates. However, Ca release to solution was ~2–50 times greater than for apatite or silicate dissolution due to the dissolution of trace anhydrite phases in the ash.

The reactivity of Ca-bearing accessory minerals such as apatite or anhydrite is not typically considered to contribute significantly to the overall Ca flux from weathering. However, in the dissolution experiments the release of Ca ions to solution is dominated by the reactivity of these accessory mineral phases. This is consistent with measured stream water chemistry in areas draining these ash deposits, where we have observed elevated concentrations of Ca, SO₄^2-, PO₄^3- and Si. In areas that experience rapid physical weathering and erosion, such as high standing islands, trace Ca-bearing phases may contribute significantly to the total weathering flux and drawdown of atmospheric CO₂ as CaCO₃.