Fractionation of Mg isotopes between brucite and aqueous Mg
Abstract
Fractionation of Mg isotopes between brucite (magnesium hydroxide, or Mg(OH)2) and aqueous Mg bears on the behavior of Mg isotopes during weathering of silicates, because Mg in the octahedral layer of smectite group clays is coordinated like Mg in brucite. Brucite is also a secondary mineral that is commonly produced during serpentinization of ultramafic rocks at mid-ocean ridges where Mg in seawater is scavenged. Therefore, knowledge of fractionation factors between brucite and aqueous Mg is important in studies of weathering and Mg global cycling. The Mg isotope fractionation between brucite and aqueous Mg was investigated using a recrystallization method and via a synthesis process using MgO hydrolysis to brucite. For the recrystallization experiments, synthesized nanometer-sized brucite was recrystallized to coarser sized brucite in a brucite saturated Mg solution. The recrystallization method utilized the "three-isotope method" using a 25Mg-enriched isotope tracer to monitor the degree of isotope exchange between brucite and aqueous Mg, in which complete isotope exchange likely represents the equilibrium Mg isotope fractionation factor. In the recrystallization experiments at 22°C, about 34%-37% of Mg in brucite underwent isotope exchange with the aqueous Mg after 198 days. Using the "three-isotope method", we extrapolated to the Mg isotope fractionation factor at complete isotope exchange (Δ26Mgbrucite-aq) to be -0.22±0.17‰ and -0.37±0.31‰ in two recrystallization experiments. This is consistent with Δ26Mgbrucite-aq fractionation factors of -0.13±0.23 ‰ to -0.36±0.25 ‰ as obtained from MgO hydrolysis experiments, where MgO was hydrolyzed in a weak HCl solution to form Mg(OH)2 at 22°C. The slightly negative Δ26Mgbrucite-aq fractionation indicates that light Mg isotopes preferentially partition into the solid phase in a brucite-aqueous Mg system. In contrast, the Mg isotope fractionation factors between weathered materials and aqueous solutions inferred from field studies, generally indicate that heavy Mg isotopes are preferentially partitioned into the weathered materials such as clay minerals. The difference in Mg isotope fractionation between brucite-aqueous Mg system and clay-aqueous Mg system implies the effect of lattice configuration of minerals on Mg isotope fractionation, or that natural systems are complicated by Mg speciation in natural aqueous solutions or Mg adsorption to clay minerals.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V13G..04L
- Keywords:
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- 1000 GEOCHEMISTRY;
- 1039 GEOCHEMISTRY / Alteration and weathering processes;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry