Chemical processes affecting alkalis and alkaline earths during continental weathering

Victoria. Australia
Abstract
Байду номын сангаас
The alkali and alkalme earth concentrations in the Toorongo Granodlorite weathcrmg profile are controlled by two competing processes; leaching of cations from primary minerals durmg their degradation to clays; fixation, by exchange and adsorption, of the same cations onto the secondar! cla! minerals. Degradation and leaching dominate the early weathering stages whereas during the advanced stages. exchange and adsorption onto clays are of most Intlucnce. The alkali and alkaline earth compositional changes III the Toorongo Granodiorite weathermg profile are typical of changes occurring during weathering of the continents. consequently the following gcneralvations apply to continental weathering. Ca. Sr and Na are most rapidly and most strongly removed (as dissolved species) during weathering of fresh continental rocks. Although large quantities of Mg arc transported to the marine environment as dissolved species. appreciable amounts remain (fixed in secondary claq minerals) at the weathering Gte to be removed during mass wasting of continental wjcathering profiles. Large quantities of Rb, Cs and Ba. fixed in continental weathering profiles hq exchange and adsorption onto secondary clays. are transported from the continent> onI1 during ma,\ wasting of the continents.
Chemical processes affecting alkalis and alkaline earths during continental weathering
H. WAYNE NESAITT.*G. MARK~VICS and R. C. P~rcr Department of Geology. Latrobe University. Bundoora.
THE TOORONGO GRANODIORITE WEATHERING PROFILE
Samples of fresh Toorongo Granodlorite and Its weathering profile (cast central Victoria. Australia) were collected (Fig. II and analyzed by methods discussed above. Sampling details and the geology of the site have been given elsewhere (N~SRITT. 1979). Plagioclase. quartz and K-feldspar are the most abundant minerals in the granodiorite. Biotite is the dominant mafic mineral: hornblende is minor and ilmenite is the
constituents. Ratios of elements are not subject to such changes unless one of the constituents of the ratio is altered; consequently changes in ratios are particularly useful when applied to genetically related suites of materials such as weathering profiles. Here. in all ratios K,O has been used as the denominator because, compared to some alkalis and alkaline earths, its geochemical cycle is well known IWEAVIXR. 1967). The percentage changes to ratios. relative to the fresh parent rock can be calculated according to (NESRITT, 1979). “I’,change = [(R, - Rp)/Rp] x 100 (I)
where Rs is the ratio in a sample and R, is the ratio of the same elements in the parent material. Mg, Ca, St-. Ba and Rb were analyzed by X-ray fluorescence methods (NORRISH and CHAPPELL. 1967: NORRISH and HUTTON, 1969) whereas Na and K were determined by flame photometr). Cs was measured instrumentally by neutron activation as were the concentrations of sodium and potassium, The sodium and potassium values obtained by the two methods were coincident within their analytical errors.
INTRODUCTION
THE HEHAVIOUR elements in the sedimentary cycle, of which includes the processes of weathering. transportation, deposition and diagenesis. is generally complicated and not well understood as is evidenced by the divergence of opinion concerning the important controls affecting sediment compositions (MILLOT. 1949; GRIM and JOHNS, 1954; ZES, 1959; PETERSOS, 1962; MCNAMARA, 1966; LEMCKE rt al., 1953; POTTER and GLASS, 1958; GRIFFIN. 1962; BISCAYE, 1965; KELLER, 1970: SILLEN. 1967; WEARER. 1967; TUREKIAN, 1977). The geochemical cycles of the alkalis and the alkaline earths are particularly complicated because they are influenced by most processes operating in sedimentary environments, processes such as; dissolution of primary minerals of igneous and metamorphic rocks: authigenic formation of secondary phases: cation exchange on different minerals; diagenesis. However, the behaviour of these elements both in the weathering and in the marine environments must be documented and well understood if the geochemistry of the sedimentary cycle is to be fully known. Here. we attempt to document the behaviour of the alkalis and the alkaline earths during the weathering of the Toorongo Granodiorite and to relate their behaviour to the weathering of the continents. OF DATA