Variation of Mg isotopes during mineral dissolution at pH = 1 and T = 25 °C: Part 1. Hornblende and chlorite

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BackgroundSilicate mineral weathering, particularly involving Mg-bearing minerals like hornblende and chlorite, plays a crucial role in the global geochemical cycle of magnesium. This process is intrinsically linked to carbon sequestration and climate regulation. Mg isotopes offer a valuable tool to investigate the complex interplay between climate and silicate weathering. In this study, we aimed to elucidate the mechanisms and extent of Mg isotope fractionation during the dissolution of hornblende and chlorite under far–from–equilibrium conditions in a plug-flow reactor. By understanding these processes, we can gain insights into the behavior of Mg isotopes in natural environments and their potential as a paleoclimate proxy.MethodsHornblende and chlorite minerals were mechanically disaggregated, sieved to a particle size range of 63–245 µm, and subsequently used in dissolution experiments. These experiments were conducted in a plug–flow reactor under far-from-equilibrium conditions. A 0.1M HCl solution was continuously pumped through columns containing the minerals. Effluent solutions were collected over a duration of 2061 h. Elemental concentrations and Mg isotopic compositions of these solutions were determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP–OES) and Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC–ICP–MS), respectively.ResultsElemental concentrations exhibited distinct temporal trends for both minerals. During hornblende dissolution, concentrations peaked at 4 h and subsequently decreased to near-zero levels by the end of experiment. In contrast, chlorite dissolution resulted in a rapid decline in elemental concentrations until 20 h, followed by a steady–state phase for the remainder of the experiment. Hornblende exhibited a δ26Mg value of − 0.24‰, while its leachates (exchangeable, weakly bound, and structural Mg) displayed δ26Mg values of − 0.62‰, − 0.72‰, and − 0.26‰, respectively. Similarly, chlorite possessed a δ26Mg value of − 0.24‰, but its leachates yielded δ26Mg values of − 0.36‰, − 0.62‰, and − 0.18‰, respectively. Solution δ26Mg values ranged from − 0.65‰ at 4 h to − 0.20‰ at 45 h, ultimately stabilizing at − 0.35‰ during hornblende dissolution, while solution δ26Mg values varied from − 0.66‰ at 4 h to − 0.23‰ at 20 h, eventually reaching a steady–state value of − 0.40‰ during chlorite dissolution.ConclusionsThe output solutions displayed initial periods of rapid, incongruent mineral dissolution, followed by a transition to a quasi–steady–state dissolution regime. The δ26Mg values of the solutions during hornblende and chlorite dissolution fell within the range of δ26Mg values for “labile” and “structural” Mg of each mineral, suggesting that the solution δ26Mg reflects a mixture of different Mg pools with distinct isotopic compositions. This study indicates that δ26Mg variations during mineral dissolution are comparable to Mg isotope fractionation during secondary mineral formation, implying that labile Mg can exert a significant influence on the Mg isotope geochemistry of rivers draining silicate-rich regions.