Thallium isotopic fractionation during adsorption onto natural manganese oxides: a comparison with synthesized analogs

Manganese (Mn) oxides are important hosts of trace metals via adsorption and/or redox processes. Previous adsorption studies on synthesized Mn oxides have provided valuable insights on metal adsorption mechanisms, and yet a comparison of synthesized and natural minerals is still rare. Here we focus on thallium (Tl), a paleo-redox tracer with a major sink via Mn oxide burial, to investigate Tl adsorption and isotope fractionation on various natural minerals that have not been reported so far. We also compare our results on natural Mn oxide phases with previous adsorption experiments on synthetic phases. Adsorption experiments were performed on various natural Mn oxides with different structures and Mn oxidation states. Our results show that Mn 2+ and Mn 3+ -rich oxides (hausmannite, feitknechtite, pyrolusite, and todorokite) have limited Tl affinity and isotopic fractionation upon adsorption. However, our data show unexpected Tl desorption and rapid isotope exchange on vernadite, leading to a much lower Tl affinity compared to synthesized structurally similar δ-MnO 2 . Using an isotope mass balance and X-ray absorption spectroscopy, we suggest that the observed Tl isotope exchange is likely to be non-oxidative with minor oxidative fractionation involved, which could be attributed to occupation of oxidative adsorption sites and competing adsorption by other trace metals (e.g., Co, Ni, Cu, etc). Our vernadite adsorption experiments thus suggest that Tl speciation changes may be prevalent in natural ferromanganese crusts. Should Tl desorption from vernadite occur periodically due to changes in ocean chemistry, the likely release of isotopically heavy 205 Tl could complicate the use of Tl isotopes as an ocean oxygen proxy over geological history. As Mn oxides play an important role in metal biogeochemical cycles, our results highlight the necessity of additional research on desorption to better understand Tl cycling in nature and subsequent implications for interpreting paleo-redox conditions in the rock record.