Variation of Mg isotopes during mineral dissolution at pH = 1 and T = 25 °C: Part 2. Olivine and pyroxene

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BackgroundThe high reactivity of mafic minerals, such as olivine and pyroxene, plays a crucial role in controlling the global magnesium (Mg) flux and its isotopic composition. This study investigates the mechanisms and extent of Mg isotope fractionation during the dissolution of these minerals under far–from–equilibrium conditions. These findings provide valuable insights into the behavior of Mg isotopes in natural environments and their potential as a geochemical proxy for understanding Earth's carbon cycle.MethodsMafic minerals (olivine, enstatite, and diopside) with particle sizes ranging from 63 to 245 µm were subjected to dissolution experiments in a plug–flow reactor under far–from–equilibrium conditions. Effluent solutions were collected over a period of 1872 h. Elemental concentrations were determined using a Inductively Coupled Plasma Optical Emission Spectroscopy (ICP–OES), while Mg isotopic compositions were analyzed using Multi–Collector Inductively Coupled Plasma Mass Spectrometry (MC–ICP–MS).ResultsElemental concentrations displayed distinct temporal trends for olivine and pyroxene. Olivine dissolution demonstrated a gradual increase in elemental concentrations during the initial 20 h, followed by a subsequent decline towards baseline levels. In contrast, pyroxene dissolution exhibited a rapid initial release of elements within the first 4 h, followed by an abrupt decrease towards near-zero concentrations. Olivine dissolution exhibited pronounced oscillations in δ2⁶Mg values, ranging from − 0.53‰ to − 0.21‰. Conversely, pyroxene dissolution (enstatite and diopside) displayed more gradual trends, with δ2⁶Mg values ranging from − 0.41‰ to − 0.26‰ for enstatite and from − 0.43‰ to − 0.21‰ for diopside.ConclusionsThe output solutions exhibited an initial period of rapid, incongruent mineral dissolution, followed by a sharp decline towards near–zero rates. The observed δ2⁶Mg variations between solution and mineral, ranging from − 0.43‰ to + 0.18‰, suggest that these shifts are likely influenced by either differences in their crystal structures or the formation of secondary phases. This finding highlights the significant influence of mafic minerals, with their high reactivity compared to other silicate minerals, on the Mg flux and isotopic compositions in terrestrial waters draining basalts and the ocean.