The Effect of Off‐Axis Hydrothermal Systems on the Oceanic Mg Budget as Revealed by Mg Isotopes: A Review

This chapter synthesizes our current understanding of how off-axis hydrothermal systems influence the oceanic magnesium (Mg) budget, focusing on stable Mg isotopes ( δ 26 Mg). Past variations in the oceanic Mg budget, driven by continental weathering, oceanic crust alteration, and marine sedimentation, can be reconstructed using Mg elemental and isotope mass balances. We summarize the current knowledge of oceanic Mg fluxes and their δ 26 Mg values, showing that although Mg fluxes remain poorly constrained, they can be inferred from such budget calculations. This approach relies on understanding Mg dynamics within hydrothermal systems. However, knowledge of low-temperature hydrothermal (LTH) systems at mid-ocean ridge flanks remains limited due to their heterogeneity and sampling challenges. Available concentration and isotope data mostly indicate that LTH systems act as a Mg sink, although some data suggest additional Mg leaching from basaltic crust (i.e., Mg exchange rather than a net sink). Recent isotope studies show that during LTH alteration, heavier Mg isotopes are preferentially incorporated into the crust. We simulate the oceanic Mg budget over the past 20 million years for these two LTH scenarios (Mg sink and Mg exchange), showing that they yield different outcomes, particularly for LTH, dolomite, and groundwater fluxes.