Chemical Weathering

Weathering processes are one of the prime mechanisms that control the system of material recycling upon the Earth's surface, playing important roles in a variety of geological issues. These issues include (1) the regulation of long-term climate by the consumption of atmospheric CO2 during the alteration of silicate materials (e.g., Berner and Kothavala, 2001), (2) the release of nutrients into the biosphere as a result of rock dissolution (e.g., Hamdan and Burnham, 1996), (3) the control of primary sediment yield derived from rock decomposition (e.g., McLennan, 1993), and (4) exploration for laterite resources such as bauxite and ferronickel ores (e.g., Gleeson et al., 2004). These fields of research are commonly investigated using geochemical indices that quantitatively reflect compositional changes attributed to weathering. Conventional weathering indices generally fall into two categories on the basis of their underlying approach, that is, they quantify the degree of weathering by either monitoring the decomposition of an unstable mineral or tracing the mass transfer of a labile element.

Despite their merits, the above geochemical indices are founded on restrictive assumptions that in many cases pose a formidable obstacle in practical use. The principal assumption involved in chemical indices that monitor the decomposition of an unstable mineral is that the initial geochemistry of the unweathered parent rock is invariant. For example, the CIA assumes Al2O3, Na2O, CaO and K2O reside exclusively in feldspars. If significant amounts of other minerals (e.g., amphiboles) that contain these elements were present in the rock, the application of the CIA becomes problematic. Consequently, different CIA values may simply reflect variations in the chemistry of the unweathered parent rock rather than the degree of weathering (e.g., Price and Velbel, 2003). Accordingly, the usefulness of these indices is strongly dependent on the geochemistry and petrology of the unweathered parent rock.

Indices that reflect the mass transfer of labile elements rely on the behavior of the specific element used in the ratio and do not consider the effects of other vital elements. During the course of weathering, elements generally behave in an inconsistent and even contradictory manner (e.g., Nesbitt and Young, 1984, Hamdan and Burnham, 1996, Nesbitt and Markovics, 1997 and Price and Velbel, 2003) that is dependent upon the physico-chemical conditions (e.g., Hieronymus et al., 2001 and White et al., 2001). For example, isovolumetric studies reveal that Fe is commonly leached during the early stages of weathering, but accumulates during the later stages (Hamdan and ...