Aluminum Complexation and Differential Alteration of Volcaniclastic Minerals: Examples from the Alban Hills, Italy and the Sutter Buttes, California

Studies of the weathering and alteration of volcanic ashes provide important evidence regarding the chemistry of paleo-porewaters in pedogenic and groundwater environments. In two examples of differential alteration, dissolution of an Al-rich phase contrasts with that of an Al-poor phase. The first example is in Pleistocene ashes of the silicic Sutter Buttes Volcano, Northern California. Biotite is commonly more weathered than hornblende, but rarely, hornblende is heavily altered and biotite is nearly pristine. This alteration may be associated with a paleo-crater lake from the volcano's explosive phase. The second example is in Pleistocene ashes of the Alban Hills Volcano, Rome, Italy. These feldspathoid-bearing ashes commonly have weathered leucite and intact clinopyroxene, but in places heavily-altered clinopyroxene is associated with pristine leucite. Stratigraphy suggests alteration occurred in groundwater.

In both of these examples, the Al-rich phase (biotite in California, leucite in Italy) survives alteration, whereas Al-poor phases (hornblende in California, clinopryoxene in Italy) are heavily altered. Thermodynamic modeling using PHREEQC defines intervals of pH in which 1) both phases are undersaturated (and therefore can dissolve), 2) the Al-rich phase is supersaturated and the Al-poor phase is undersaturated, 3) both phases are supersaturated (neither will dissolve), and 4) the Al-rich phase is undersaturated and the Al-poor phase is supersaturated. Model results are sensitive to the water and mineral compositions. Whereas stability of the Al-poor phase has a positive linear relationship with pH, the Al-rich phase has an upside down U-shaped curve. This is because of the U-shape of the Al³⁺ activity curve, which is strongly affected by Al complexation at low and high (but not middle) pH values.

Using these relationships, petrographic observations of differential alteration of Al-rich and Al-poor phases can provide important clues to relative changes in pH of paleo-pore waters, and the environments in which they altered volcaniclastic sediment.