Processes of chemical weathering of selected Cainozoic Eastern Australian basalts

Abstract

The processes of chemical weathering of basalt have been observed across a range of time and climate space, and studies from isolated areas in eastern Australia integrated into a single extensive work, with a view to understanding the variations in processes and products of basalt weathering. In particular geochemical and mineralogical variations are characterised, and an attempt to relate these to physicochemical variations in environment has been made. For the first time the graphical isocon technique has been used to interpret the geochemical data of weathered basalts, and was found to be an appropriate technique to use in evaluating element enrichment and depletion, and mass change as an indication of degree of weathering. However, this technique must be used in close association with careful petrographic work and where possible X-ray diffraction. The graphical isocon technique is superior to other weathering indices because there is an internal check on the degree to which immobile elements have remained immobile during weathering. The observed order of dissolution of the principal basalt phases is: basaltic glass, olivine, plagioclase, pyroxene and opaque oxides, but pyroxene weathers more rapidly than plagioclase feldspar, and is wholly weathered before plagioclase feldspar. In general, basalts weather through the stages: trioctahedral/dioctahedral smectites; dioctahedral smectites ± minor kaolinite; dioctahedral smectite, randomly interstratified kaolinsmectites, halloysite and minor kaolinite; halloysite and kaolinite; halloysite, kaolinite and gibbsite. The trioctahedral smectites are mostly saponite and Fe-saponite, while the dioctahedral phases are dominantly nontronite and beidellite with some montmorillonite. The smectite phase in the randomly interstratified kaolin-smectite clays is nontronite and/or Febeidellite. Randomly interstratified kaolin-smectite clays, intermediate in the break down of smectite to kaolinite, are widespread in the products of basalt weathering in eastern Australia, but are difficult to distinguish using cation saturation, X-ray diffraction and cation exchange capacity alone. Halloysite is more widespread in the products of basalt weathering than previously documented and appears to be the kaolin group clay preferentially formed during the break down of smectite clays, although kaolinite is also produced. A comprehensive compilation of major and trace element mobility for the Eastern Australian basalts, evaluated using the isocon technique, is presented. Many elements behave as previously documented for weathering of well drained basalt. For example: the rapid removal of alkali and alkaline earth cations from the weathering profiles; progressive weathering of silica from the profiles; the oxidation of iron during weathering resulting in the fixation of iron, largely as goethite and hematite; similar oxidation of manganese to form coatings on weathered surfaces; the variable but limited mobility of zinc and vanadium reflecting the partitioning of these elements into resistant primary minerals, the relatively high mobility of copper once copper-sulphides were oxidised to sulphate; the limited mobility of the actinides; and, the residual concentration of chromium and nickel in weathering profiles. For major cations and closely related trace elements, the general order in which elements are lost is: Na>Sr>Ca>K>Rb>Mg>Al>Si, and this may be quantified using the slope of the elemental loss curve. If these data are normalised to silica, sodium is lost approximately three times faster than silica; rubidium, potassium, calcium and strontium are lost approximately 2.3 to 2.6 times more rapidly than silica; and, aluminium may be lost at a rate less than or similar to silica. Some incipiently to moderately weathered basalts show significant enrichment in the rare earth elements, cerium and lanthanum, as well as yttrium, barium and lead. This may be attributed to the formation of secondary phosphate, carbonate and sulphate minerals, in particular secondary phosphates of the plumbogummite group. Other elements that show unusual behaviour are those that are generally considered immobile during weathering (niobium, zirconium, yttrium, aluminium, titanium, iron), but are apparently mobile under certain weathering conditions. Niobium and zirconium appear to be mobile in alkaline fluids. Titanium may be concentrated in smectite rich alkaline weathering profiles, and more depleted in kaolin rich acid profiles. Aluminium mobility varies with extremes of pH, and iron is known to be mobile under weathering conditions. Once clay mineralogy and element mobility for a number of profiles is compiled, evaluations can be made in light of paleoclimate and contemporary climate and it is possible to relate the development of weathering profiles through time to the evolution of climate environment in Eastern Australia. A study of aqueous geochemistry of fluids issuing from actively weathering basalt in the Monaro Volcanic Province has enabled explanation of the unusual juxtaposition of smectite and gibbsite in a cool climate weathering regime; has shown that zeolites may form in weathering in equilibrium with the percolating fluids; and, has allowed some evaluations with respect to tectonic stability of the Monaro Volcanic Province throughout the Tertiary. A calculated lowering rate, due to chemical dissolution, for the Monaro basalt is in the order of 0.7 mm/1000 years. Show more