The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

“…The second possibility is that equilibrium fractionation was obtained in our experiments. Schott et al (2016) reported that mineral-fluid Mg equilibrium fractionation factors can vary by more than 4‰ depending on the fluid composition. For example, according to Schott et al (2016), the equilibrium fractionation factor for magnesium between a mineral its co-existing aqueous fluid Mgmineral-fluid is 4‰…”

“…These studies suggest that at equilibrium Mg in these hydromagnesite phases is ~1.0‰ lighter than their co-existing aqueous fluid phases. It should be emphasized, however, that the equilibrium distribution of Mg isotopes among minerals and their co-existing aqueous fluids depends strongly on the fluid composition (Schott et al, 2016); the presence of substantial carbonate or aqueous organic species can alter these equilibrium fraction factors by up to several per mil. Due to these non-zero fractionation factors, the congruent dissolution of hydromagnesite could lead to an isotopic disequilibrium between the remaining hydromagnesite and an originally Mg-free fluid phase.…”

Equilibrium Fractionation of Non-traditional Isotopes: a Molecular Modeling Perspective

“…-Magnesium: Black et al (2007) studied the equilibrium Mg isotope fractionation in chlorophylls. This and several later studies made efforts to improve methods to determine isotopic fractionation in liquids, with a particular focus on the fractionation between aqueous Mg 2+ and Mg-bearing carbonate minerals Schauble 2011;Pinilla et al 2015, Schott et al 2016. Mg isotopes in mantle silicates were treated in Schauble 2011 -Silicon: Méheut et al (2007Méheut et al ( , 2009Méheut et al ( , 2014 computed the equilibrium Si isotope fractionation factors in various silicate minerals, including phyllosilicates.…”

Using Mg Isotopes to Estimate Natural Calcite Compositions and Precipitation Rates During the 2010 Eyjafjallajökull Eruption

“…This may imply that more experimental work is required to understand any changing α values with precipitation rates, and are therefore not used in our modeling. There is also a postulated speciation effect on Mg isotope fractionation (Schott et al, 2016), implying that a pH increase will drive carbonate δ 26 Mg lower. We have factored this effect into our model, but for the Eyjafjallajökull samples the effect on the final calcite Mg/Ca ratios is small (<5%) and within uncertainty (discussed below).…”

“…The balance of carbonate to silicate in a catchment plays a significant role, with carbonates being isotopically lighter than silicates (Tipper et al, 2008;Pogge von Strandmann et al, 2014). The amount of Mg isotope fractionation during carbonate precipitation also appears to be dependent upon mineralogy (Immenhauser et al, 2010;Wombacher et al, 2011;Geske et al, 2012;Saulnier et al, 2012;Geske et al, 2015), organic vs. inorganic precipitation (Chang et al, 2004;Pogge von Strandmann, 2008;Saenger and Wang, 2014), precipitation rate (Mavromatis et al, 2013), fractionation mechanism (Buhl et al, 2007), and potentially speciation (Schott et al, 2016), resulting in a wide range in isotope ratios. In addition, Mg isotopic fractionation occurs due to the silicate weathering process itself, owing to both preferential incorporation and adsorption of Mg isotopes by secondary minerals (Pogge von Strandmann et al, 2008;Opfergelt et al, 2010Opfergelt et al, , 2011Huang et al, 2012;Pogge von Strandmann et al, 2012;Tipper et al, 2012a;Liu et al, 2014).…”