Applications of non-traditional stable MG, FE and CA isotopes to water and minerals : investigations into pyroxenes, brines, river and coastal groundwater systems

Abstract

High precision analysis on non-traditional stable isotopes has been enabled because of the development of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Isotopic fractionation mechanisms and applications of these stable isotopes have been well explored for the high-temperature geochemistry, while limited works on the low-temperature systems are archived. In this thesis, I conducted the stable Mg, Fe and Ca isotopes studies for brine, river, coastal groundwater as well as minerals. In this thesis, systematic Mg isotopic composition for a complete Mg-bearing sulfate (langbeinite, K2Mg2(SO4)3) sequence was firstly reported and the equilibrium Mg isotope fractionation factor between langbeinite and aqueous Mg2+ solution was also calibrated by using quantum first-principle calculation approach. Calculations were carried out at B3LYP/6-311++G(2d,2p) level and simulations were done by the volume variable cluster models (VVCM). The Mg isotope compositions of the Late Permian Salado langbeinite samples are relatively homogeneous and lighter than values of modern seawater (δ26MgDSM3 ranges from -4.12±0.03‰ to -3.81±0.07‰ vs. -0.83‰ for modern seawater). Mg isotope equilibrium fractionation factors between langbeinite and aqueous Mg2+ solutions are 1.096‰, 0.522‰ and 0.303‰ for 10°C, 25°C and 40 °C, respectively. Combining this identical isotopic feature in natural langbeinite with the calculated equilibrium Mg isotope fractionation factors, we proposed δ26Mg of the parent brine of this langbeinite is lighter than -1.2‰ and that a rapid kinetic langbeinite precipitation is accounted for the identical δ26Mg of the langbeinite. Equilibrium fractionation factors of Ca isotopes between two major Ca-bearing mantle minerals, orthopyroxene (opx) and clinopyroxene (cpx), ∆44/40Caopx-cpx, were calibrated also by using the theoretical calculation approach. Basing on the fractionation direction I proposed, isotopically light 40Ca prefers to enrich in cpx relative to opx. Independent of Ca concentration in opx on the ∆44/40Caopx-cpx was clearly suggested. This Ca content effect could successfully explain the difference in Ca isotope fractionations between opx and cpx found in the Kilbourne Hole and San Carlos natural peridotites. It is the first study to illustrate Ca content of mineral would play an effect on the Ca isotopic fractionation. Obviously seasonal elemental Mg and Ca variations have been observed in the Xijiang River. Two times increase in dissolved Mg of Xijiang River water was observed from mild season to flooding season, whereas Ca was elevated by 10 times simultaneously. Input of weathering carbonate was speculated as the cause for this Mg-Ca geochemistry variation. Lighter δ26Mg of Xijiang River is predicted occurring in the flooding season due to the contribution of light δ26Mg in carbonate is massively induced in the high weathering flooding season. Some data on the distribution of δ56Fe of pore water at the subterranean estuary at Tolo Harbour, Hong Kong were obtained. Preliminary results show that δ56Fe decrease with increasing depth in the mixing districts between the groundwater and upwelling seawater. Fe cycles regulated by the oxidation states and contribution of groundwater could affect the distribution of δ56Fe of pore water. Fe isotopes of pore water will thus be useful to trace these Fe geochemical cycles in the coastal ocean area.