『Abstract
This study presents lithium (Li) and magnesium (Mg) isotope data
from experiments designed to assess the effects of dissolution
of primary phases and the formation of secondary minerals during
the weathering of basalt. Basalt glass and olivine dissolution
experiments were performed in mixed through-flow reactors under
controlled equilibrium conditions, at low pH (2-4) in order to
keep solutions undersaturated (i.e. far-from equilibrium) and
inhibit the formation of secondary minerals. Combined dissolution-precipitation
experiments were performed at high pH (10 and 11) increasing the
saturation state of the solutions (moving the system closer to
equilibrium) and thereby promoting the formation of secondary
minerals
At conditions far from equilibrium saturation state modelling
and solution stoichiometry suggest that little secondary mineral
formation has occurred. This is supported by the similarity of
the dissolution rates of basalt glass and olivine obtained here
compared to those of previous experiments. The δ7Li
isotope composition of the experimental solution is indistinguishable
from that of the initial basalt glass or olivine indicating that
little fractionation has occurred. In contrast, the same experimental
solutions have light Mg isotope compositions relative to the primary
phases, and the solution becomes progressively lighter with time.
In the absence of any evidence for secondary mineral formation
the most likely explanation for these light Mg isotope compositions
is that there has been preferential loss of light Mg during primary
phase dissolution.
For the experiments undertaken at close to equilibrium conditions
the results of saturation state modelling and changes in solution
chemistry suggest that secondary mineral formation has occurred.
X-ray diffraction (XRD) measurements of the reacted mineral products
from these experiments confirm that the principal secondary phase
that has formed is chrysotile. Lithium isotope ratios of the experimental
fluid become increasingly heavy with time, consistent with previous
experimental work and natural data indicating that 6Li
is preferentially incorporated into secondary minerals, leaving
the solution enriched in 7Li. The behaviour of Mg isotopes
is different from that anticipated or observed in natural systems.
Similar to the far from equilibrium experiments initially light
Mg is lost during olivine dissolution, but with time the δ26Mg
value of the solution becomes increasingly heavy. This suggests
either preferential loss of light, and then heavy Mg from olivine,
or that the secondary phase preferentially incorporates light
Mg from solution. Assuming that the secondary phase is chrysotile,
a Mg-silicate, the sense of Mg fractionation is opposite to that
previously associated with silicate soils and implies that the
fractionation of Mg isotopes during silicate precipitation may
be mineral specific. If secondary silicates do preferentially
remove light Mg from solution then this could be a possible mechanism
for the relatively heavy δ26Mg value of seawater. This
study highlights the utility of experimental studies to quantify
the effects of natural weathering reactions on the Li and Mg geochemical
cycles.』
1. Introduction
2. Methods
2.1. Experimental methods
2.2. Analytical methods
2.2.1. Concentration measurements
2.2.2. Isotope chemistry
2.2.3. Isotope analyses
2.2.3.1. Lithium
2.2.3.2. Magnesium
3. Results
3.1. Mineral phases
3.2. Dissolution experiments
3.2.1. Dissolution rates
3.2.2. Lithium and magnesium isotopes
3.3. Secondary phase precipitation experiments
3.3.1. Dissolution rates
3.3.2. Lithium and magnesium isotopes
4. Discussion
4.1. Dissolution experiments
4.1.1. Stoichiometry
4.1.2. Dissolution rate
4.1.3. Mineral saturation states
4.1.4. Lithium isotope behaviour
4.1.5. Magnesium isotope behaviour
4.2. Dissolution/precipitation experiments at high pH
4.2.1. Stoichiometry
4.2.2. Mineral saturation states
4.2.3. Dissolution rates
4.2.4. Secondary phase identification
4.2.5. Lithium isotope behaviour
4.2.6. Magnesium isotope behaviour
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References