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Project Progress

Stable Mg isotope variations, expressed as mass-dependent relations among ²⁶Mg/²⁴Mg and ²⁵Mg/²⁴Mg ratios, are increasingly studied in terrestrial weathering environments, and have great potential for understanding weathering on other planetary bodies such as Mars. Such promise, however, cannot be realized without systematic experimental studies that constrain stable Mg isotope fractionation factors.

We performed Mg synthesis and isotope exchange experiments between brucite and aqueous Mg and brucite and Mg complexed with EDTA. Low temperature (4-55 °C) synthesis and exchange experiments allows us to determine that the fractionation factor between brucite and aqueous Mg (Δ²⁶Mg_brucite-Mg2+) is -0.18±0.07 ‰

Results of MgO hydrolysis experiments in EDTA-bearing solutions suggest that the Δ²⁶Mg_{brucite-Mg-EDTA} fractionation is ≥ 2.0 ‰ at 22 °C. Based on the Δ²⁶Mg_{brucite-Mg-EDTA} and Δ²⁶Mg_brucite-Mg2 fractionations, the Δ²⁶Mg_brucite-Mg2+ -Mg-EDTA fractionation is constrained at ≥ +2.2 ‰, suggesting that light Mg isotopes preferentially partition into Mg-EDTA complex relative to Mg aquo ions.

These findings highlight the importance of ligand bond strength in controlling Mg isotope fractionation. To the extent that brucite is a model for Mg isotope fractionation for trioctahedral smectites we note that our brucite experiments would predict that aqueous solutions should have higher ²⁶Mg/²⁴Mg isotope ratios as compared to the clay and this is the opposite of what is observed in natural terrestrial settings where, for example, on Earth, chemical weathering of rocks results in aqueous solutions that have lower ²⁶Mg/²⁴Mg isotope ratios as compared to its catchment lithology. Moreover, the weathered products of the catchment lithology have complementary high ²⁶Mg/²⁴Mg isotope ratios. The origin of these Mg isotope variations is believed to be a result of weathering and the production of clay minerals. These observations may highlight the importance of ligands in controlling Mg isotopic fractionation during weathering. Indeed, significant Mg isotope excursions in clays from Mars may be a useful biosignature if carbonate precipitation can be eliminated as a possibility. Under such a scenario, highly unusual ²⁶Mg/²⁴Mg isotope ratios in weathering products would likely require a role for organic ligands or biology.