Positive correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin challenges the interpretation of apparent isotopic fractionation during weathering

• Published in: Earth and Planetary Science Letters
• Main Authors: Tipper, E. T., Calmels, D., Gaillardet, J., Louvat, P., Capmas, F., Dubacq, B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: 02 - Geodynamics; Geophysics and Tectonics; Canada; Mackenzie Basin
• Online Access: http://eprints.esc.cam.ac.uk/2550/; http://eprints.esc.cam.ac.uk/2550/1/Tipper_et_al._-_2012_-_Positive_correlation_between_Li_and_Mg_isotope_rat.pdf; http://www.sciencedirect.com/science/article/pii/S0012821X12001902; https://doi.org/10.1016/j.epsl.2012.04.023

During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In the river waters of the Mackenzie Basin (Canada), the Mg content is high and Mg isotope ratios (26Mg/24Mg expressed as δ Mg 26 ) show in excess of one per mil variability. Part of this variability is attributed to the 3‰ range in the carbonate and silicate rocks drained. Despite this inherent lithological control on river water δ Mg 26 values, there is also evidence for a fractionation control. A linear positive covariation between lithium (7Li/6Li, expressed as δ Li 7 ) and Mg isotope ratios in the river waters of the Mackenzie Basin is reported. This covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with Mg isotope ratios.