DISSOLUTION KINETICS OF ANDESITIC VOLCANICS

Our recent work on weathering on high standing islands (HSIs) of New Zealand and the Republic of Dominica in the Lesser Antilles shows that weathering rates based on stream water chemistry for areas draining andesitic terrains are comparable to weathering rates determined for basalt. These results suggest that weathering of andesitic terrains, in particular recent volcanics, could be much more important in the long-term sequestration of atmospheric CO₂ than previously recognized. However, there are many knowledge gaps in our understanding of these systems including the reactivity of materials with different compositions, crystallinity, and age. Batch dissolution experiments have been conducted with andesitic tephra samples collected from the island of Dominica to determine weathering rates. To determine the effect of crystallinity and composition on reaction rates, experiments were conducted on the bulk material, on an Fe-rich crystalline (predominately pyroxene) fraction and on an Fe-poor glassy fraction. The concentrations of Na, K, Ca, Mg, Si, and PO₄^3- increased over time as the material dissolved. Reaction rates based on the sum of positive charge ranged from ~0.1 to 0.4 μmole g^-1 day^-1. However, total ion concentrations were approximately five-fold higher for the crystalline fraction compared to the glassy fraction, and this difference is largely attributed to the dissolution of accessory apatite.

These results have important implications for carbon flux and sequestration due to weathering on HSIs. Typically, the reactivity of Ca-bearing accessory minerals such as apatite has not been considered to contribute significantly to the overall Ca flux from weathering and subsequent CaCO₃ precipitation, but in these experiments the release of Ca ions to solution is dominated by the apatite reactivity.