High-precision Measurement Of Mg Isotopes And Their Geochemical Behaviors During Continental Weathering

Continental weathering is a geochemical link among the lithosphere, hydrosphere, biosphere and atmosphere. Continental weathering can consume the greenhouse gas CO2 and thus regulate the global climate. On long-time scale, silicate weathering in particular is thought to control global CO2 concentration and climate through the consumption of atmospheric CO2 that is eventually stored as carbonates in the oceans. By contrast, chemical weathering of carbonate rocks has not an equivalent impact on CO2 cycle as that of silicates on long time scales, because all Mg2+ and Ca2+ released and CO2 consumed from dissolution of carbonates are again incorporated into the carbonates and returned to the atmosphere, keeping a general balance on CO2. However, for short time scales(<3 ka), the role of carbonate weathering in the CO2 cycle is too important to be ignored due to the much faster kinetics of dissolution and the much greater solubility of carbonates compared to silicates. Thus, it is of significance to understand chemical weathering of silicates and carbonates to track the carbon cycle and climate change.In recent years, Mg isotope for tracing geochemical process has become more attractive given that 1) Mg is a major element on the Earth, 2) Mg is a water-soluble element that is prone to migration in geochemical processes, and 3) the three Mg isotopes has large relative mass difference that can potentially lead to large mass-dependent Mg isotope fractionation in geological processes.Based on above, we firstly established the high-precision measurement of Mg isotope composition, then systemically investigate the Mg isotope fractionation behavior during continental weathering through silicate and carbonate weathering profiles. The main results related to this thesis are given details in the following: 1. Establishment for high-precision measurement of Mg isotopeThis study established a method for high precision measurement of Mg isotope composition, including chemical separation and instrumental analysis. Several parameters that could affect the accuracy and precision of Mg isotope analyses were examined, including temperature effect, matrix effect, and the acidity and concentration match between standards and samples. Analyses over a 2-year period of USGS rock standards(BHVO-2, BCR-2, AGV-2 and GSP-2) yielded consistent d26Mg with published values and obtained long-term external precision better than ±0.06‰(2SD) for d26Mg. We additionally reported eleven new reference materials that would help interlaboratory calibration of Mg isotope measurement in the future. 2. The Mg isotope fractionation behavior during granitoid weatheringThis study systemically measured the Mg isotope composition of granitoid weathering residues. As weathering progresses, Mg concentration decreases(τTi,Mg = 0 to-26%) and Mg isotope composition become heavier(d26Mg =-0.34‰ to-0.16‰). This fractionation could be caused by the preferential dissolution of hornblende than biotite but not the formation of secondary clay minerals with heavy Mg isotope composition. Because Mg in kaolinite is characterized by the form of adsorption and this Mg fraction has the light Mg isotope composition as fluid, then the kaolinization of feldspar would lead to the weathering residues with light Mg isotope composition but not heavy. As weathering intensity increases, the abundance of hornblende decreases while biotite increases. Given that the Mg isotope composition of hornblende is heavier than biotite, the preferential dissolution of hornblende would lead to the weathering residues with heavy Mg isotope composition. In addition, the preferential loss of light Mg during hornblend and biotite dissolution would result in the weathered residues with heavy Mg isotope compositions. Thus, the heavy Mg isotope compositions of weathered residues would be caused by combined action of two processes that are the decrease of hornblende/biotite and the preferential loss of light Mg during hornblend and biotite dissolution. Finally, we discussed the relationship between weathering index of CIA and d26Mg of weathering residues. We speculate that to some degree the Mg isotope composition of weathering residues can reflect the weathering degree. 3. The Mg isotope fractionation behavior during dolostone weatheringThis study systemically measured the Mg isotope composition of a dolostone weathering residues. According to the element and isotope ratios(Mg/Ca, Mg/Al, Ca/Al, δ26Mg and 87Sr/86Sr), the weathering profile can be divided into the weakly and intensely weathered zones. In the weakly weathered zone, Mg/Al(atomic) ratios decrease successively from 4.99 to 1.75 with increasing weathering intensity, while Ca/Al ratios slightly decrease from 13.70 to 9.02. These suggest massive loss of Mg and simultaneous conservation of Ca, implying that the fluids coexisting with the dolostone are saturated with respect to calcite while not saturated with respect to dolomite. δ26Mg values slightly decrease from-1.90‰ to-2.22‰, and this isotope variation is likely caused by the decrease of dolomite/calcite and the preferential incorporation of 24 Mg into secondary calcite. By contrast, as weathering processes, both Mg/Al and Ca/Al ratios of intensely weathered samples display a decreasing trend, ranging from 1.62 to 0.16 and 10.24 to 2.05, respectively. The significant loss of Mg and Ca indicates the considerable dissolution of both dolomite and calcite, suggesting that the fluids are undersaturated with respect to both dolomite and calcite. The dissolution of calcite and dolomite leads to the increase of silicate fractions and thus the increase of δ26Mg(-2.22‰ to-0.41‰) and 87Sr/86Sr(0.71128 to 0.71368). Overall, our study reveals large Mg isotope fractionation during dolostone weathering. The direction and extent of Mg isotope fractionation are mainly governed by the dissolution and re-precipitation of carbonate minerals, which are further controlled by the fluid’s saturation with respect to carbonate minerals(calcite and dolomite).