Investigating Mg Isotope Fractionation During Cation Exchange Processes
Description
Mg stable isotopes are known to fractionate during secondary mineral formation, diffusion, and bio-uptake. Compared to these processes, Mg sorption-desorption and associated isotope fractionations are less well documented. To resolve this, a series of batch experiments were firstly conducted, which have revealed insignificant fractionation of Mg isotopes (< 0.2 ‰) during exchange with clay minerals (kaolinite and montmorillonite), cation resin (an analogue of organic matter) and natural regolith sediment. The lab experiment observation was further corroborated by our field research showing similarly insignificant difference in Mg isotope composition between concomitant dissolve Mg and exchangeable Mg in riparian zone, river channel and marine environments. Our findings suggest that Mg isotope exchange behaves as a simple mixing process, resulting in nearly identical δ26Mg values in both dissolved and exchangeable phases once equilibrium is achieved. One implication of this result is that the potential for exchangeable Mg to alter the δ²⁶Mg of water—or vice versa—depends on the relative masses of Mg in these two phases. Table S1 Mg isotope fractionation during adsorption on cation resin Table S2 Mg isotope fractionation during exchange on natural samples Table S3 Mg concentration and isotope composition of natural water samples and exchangeable fractions of sediments Table S4 Concentration of Exchangeable Mg in Regolith Sediments Along a Weathering Profile in the Mulanxi Catchment Table S5 Concentration and Isotope Composition of Exchangeable Mg in Samples Used for Exchange Experiments Table S6 Data quality control
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All measurements were conducted at the State Key Laboratory of Marine Science at Tongji University, China. Mg concentrations in the filtered supernatant, exchangeable fraction, and water samples were measured using quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7900). Calibration was performed over a range of 10 ppb to 500 ppb for the isotope ²⁴Mg, with Rh and In used as internal standards. Unknown samples were diluted to ensure that Mg concentrations fell within this range. The precision and accuracy of the concentration measurements were evaluated through replicate analyses of standard reference materials (SLRS-6 and in-house standards), with uncertainties in Mg concentrations estimated at approximately 8%. To validate the ICP-MS results, Mg concentrations in 22 samples were also analyzed in parallel using ICP-OES, yielding consistent results with difference less than 10 % (Table S6). For Mg isotope measurements, all solutions, including water samples, extracted solutions, and reaction solutions, were first dried. Organic matter and dissolved Si were removed by reacting the samples with an acidified H₂O₂ solution (a mixture of 1 mL ultrapure 30% H₂O₂, 1 mL 14 N HNO₃, and 1 mL Milli-Q water) and then a HF-HNO₃ acid mixture (two drops of HF in 200 μL concentrated HNO₃) at 110°C for 12 hours. The solutions were then dried again and re-dissolved in 1 N HNO₃ for cation exchange chromatography. As for the column chemistry, 2.8 ml of the resin Bio Rad AG-50W-X12 200-400 mesh was loaded in Spectrum 104704 MiniColumns. 200 μL of solution (containing ~5-10 μg Mg) was loaded onto the resin bed and matrix elements were eluted by 31 ml 1N HNO3 and Mg was then collected in 10 ml 2N HNO3. The pure Mg solutions were evaporated to dryness and re-dissolved in 1 mL 0.3N HNO3. Purity and yield were monitored by ICP-MS (Agilent 7900), and the recovery of Mg generally higher than 98%. Mg isotopes were measured using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS, Neptune Plus, Thermo Scientific), with the SSB method employed to correct for instrumental mass bias. A quartz-glass spray chamber (SIS) and a 50 μL/min self-aspirating PFA nebulizer were used for sample introduction. 30 cycles were measured in 1 block and 3 replicates were run for each sample. Analytical results are converted to relative to DSM3 in delta notation, δxMgsample = [ (xMg/24Mg)sample / (xMg/24Mg)DSM3 ̶ 1] × 103, where x = 26 or 25. Reference materials SLRS-6 (river water), CASS-6 (seawater) are routinely monitored, yielding δ26Mg values of -1.22 ± 0.07 ‰ (n=11), -0.84 ± 0.06 ‰ (n=6), respectively, which agree well with previously published values. One in-house standard (MgCl2 solution) is also routinely measured, consistently yielding results that align with those from the other laboratory (see Table S6 for detailed data quality control result).