Mechanisms controlling the Mg isotope composition of hydromagnesite-magnesite playas near Atlin, British Columbia, Canada

Abstract

The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation of hydromagnesite and magnesite occurs at temperatures that do not exceed 15 °C. This environment offers a unique opportunity to examine the impact of different formation mechanisms on Mg isotope compositions of Mg-carbonate minerals at low temperature. In this study, we report the Mg isotope composition of ultramafic bedrock, Mg-carbonate sediments, and both surface and ground waters in this geological setting. The composition of hydromagnesite suggests a Rayleigh-type distillation effect on the fluid Mg isotope ratios in unsaturated sediment above the water table. Through this mechanism of formation, hydromagnesite is progressively depleted in 24 Mg obtaining δ 26 Mg values as high as +1.14‰ near the sediment surface. In contrast, magnesite formation is characterized by enrichment of the solid phase in 24 Mg. The apparent Mg isotope fractionation factor during magnesite formation at ~ 10 °C ranges between ~ 0.7±0.1‰ and 1.8±0.1‰. The distinct Mg isotope composition of hydromagnesite in comparison to magnesite supports magnesite formation occurring by precipitation from the fluid, or dissolution-reprecipitation, rather than solid-phase transformation from a hydrous Mg-carbonate precursor. Overall, the results provide insights on low temperature Mgcarbonate mineral formation that has implications for long-term storage of CO2.

Magnesium Isotope Constraints on the Holocene Hydromagnesite Formation in Alkaline Lake Dujiali, Central Qinghai‐Tibetan Plateau

“…The $$\delta {}^{26}\mathsf{M}\mathsf{g}$$ values of the hydromagnesite samples (a mixture of aragonite and hydromagnesite) are homogenous (mean = −1.35‰ ± 0.14‰, 2SD, Table S2 in Supporting Information S2) and lower than $$\delta {}^{26}\mathsf{M}\mathsf{g}$$ values of modern lake waters (mean = −0.16‰, see below). A similar offset between contemporary hydromagnesite and associated waters has been observed in other lakes, playas, and synthetic hydromagnesite (Mavromatis et al., 2021; Oelkers et al., 2018; Shirokova et al., 2013). The modern lake sediments are comprised of clay minerals, quartz, K‐feldspar, calcite, and dolomite with no hydromagnesite (Table S1 in Supporting Information S2).…”

“…Of the model solutions that predict modal hydromagnesite >85%, even fewer (275 out of 3,750) converged to the observed range of 𝐴𝐴 𝐴𝐴 26 𝖬𝖬𝖬𝖬 values in the hydromagnesite deposit (dark shaded area on Figure 8, Figure S3 in Supporting Information S1). These model solutions impose the constraint that 𝐴𝐴 𝖲𝖲𝖲𝖲𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁𝗁 𝗁𝗁𝗁𝗁 < 2 and a degree of evaporation >3.5, and Δ hydromagnesite <−0.6‰, a reasonable estimate compared to literature values (Mavromatis et al, 2021;Oelkers et al, 2018;Shirokova et al, 2013). The high evaporation is consistent with carbon and oxygen isotope data from the hydromagnesite deposit (Lin et al, 2017), and their positive values are associated with evaporative and restricted environments.…”

“…Similar to the Mg/Ca ratio, the $$\delta {}^{26}\mathsf{M}\mathsf{g}$$ values of lake waters are higher than input waters, ranging from −0.07‰ to +0.46‰ (mean = −0.16‰, Figure 4). $${}^{26}\mathsf{M}\mathsf{g}$$ enrichment is observed in other alkaline lakes and playas, compared to their input waters (Mavromatis et al., 2021; Shirokova et al., 2013; P. Zhang et al., 2021).…”

Mineralisation of atmospheric CO2 in hydromagnesite in ultramafic mine tailings – Insights from Mg isotopes

“…The inverse relationship of brucite and hydromagnesite abundance with depth in the Woodsreef tailings further points to brucite being a source of Mg for hydromagnesite formation, whereas the homogeneous distribution of magnetite is consistent with its lower reactivity (Turvey et al, 2018). Consequently, brucite is often considered as the primary source of Mg during natural carbonation of mine tailings and it has been linked to increases in pore water pH, carbonation rate and yield, as well as the CO2 storage capacity of tailings in laboratory carbonation experiments (Pronost et al, 2011;Harrison et al, 2013;Assima et al, 2014;Boschi et al, 2017) and in Canada (δ 26 Mg = -2.50 ‰, ± 0.69 ‰ (2σ), n=4, magnesite rich sediments in Mavromatis et al, 2021). The extreme depletion of 26 Mg in this type of magnesite has been ascribed to magnesite dissolution/re-precipitation and the associated Mg isotope fractionation between Mg species in alkaline solutions and to concurrent precipitation of serpentine veins with high δ 26 Mg (Oskierski et al, 2019;De Obeso et al, 2020).…”

“…Furthermore, fluid volumes passing through the tailings in the semi-arid climate at Woodsreef are low and, consequently, the tailings are rock-buffered, as opposed to fluid-buffered conditions in laboratory experiments (Harrison et al, 2021). Mg exchange between fluid and solid, and thus the resetting of isotope compositions of hydrate Mg-carbonates, has been shown to be limited in similar mineral-buffered systems (Mavromatis et al, 2021).…”