『Abstract
Positive correlation between Sr/Ca and Mg/Ca in cave dripwaters
and speleothem is often attributed to ‘prior calcite precipitation’
(PCP), where exclusion of Mg and Sr from calcite precipitated
upstream of the dripwater/speleothem site results in enrichment
in the Mg/Ca and Sr/Ca ratios of residual effluent. Modelling
predicts that PCP will produce a straight line correlation in
graphs of ln(Sr/Ca) vs. ln(Mg/Ca) with a theoretical slope that
is given by (KdSr - 1)/(KdMg
- 1) (calculated to be 〜0.88±0.13) regardless of rock or absolute
dripwater composition (Sinclair et al., submitted for publication).
While this makes the correlation slope potentially diagnostic
for PCP, Mg and Sr are also preferentially released from calcite
during incongruent calcite dissolution (ICD), and it is possible
that ICD produces a similar correlation between ln(Sr/Ca) and
ln(Mg/Ca). To examine this possibility, mathematical formulations
for two different mechanisms of ICD are presented. Model 1 simulates
net dissolution as a dynamic kinetic process where forward and
reverse reactions compete: dissolution is congruent but ions are
fractionated by a simultaneous reverse reaction where new calcite
forms with Sr/Ca and Mg/Ca ratios that are determined by the solution
composition and KdSr and KdMg.
Model 2 simulates leaching of Mg and Sr from a fresh calcite surface
by tracking the dissolution of a thin layer where high ion mobility
allows the calcite to maintain a homogeneous composition controlled
by KdSr and KdMg. In
both models, generalised kinetic equations are used in the derivations
meaning that the results are not specific to a particular kinetic
model.
In both models of ICD, the mathematics predict that under conditions
that may prevail during dissolution of limestone, the correlation
between ln(Sr/Ca) and ln(Mg/Ca) is approximately linear with a
slope given by (KdSr - 1)/(KdMg
- 1). Thus, this property is not specific to just PCP. However,
for both PCP and ICD, this slope is universal - expected to occur
regardless of kinetics and the absolute composition of solution
or host rock. A correlation slope of 〜0.88 in either karst waters
or speleothem is therefore diagnostic for calcite-water interaction.
Keywords: Incongruent calcite dissolution; Speleothem; Cave drip
water; Mathematical model; Magnesium; Strontium』
1. Introduction
2. Mathematical formulations
2.1. Model 1 - dissolution/re-precipitation of calcite
2.1.1. Overview
2.1.2. Summary of equations
2.2. Model 2 - ion leaching from fresh surfaces
2.2.1. Overview
2.2.2. Summary of equations
3. Model summary and discussion
3.1. Model 1
3.1.1. Description of the model behaviour
3.1.2. Model predictions
3.2. Model 2
3.2.1. Description of the model behaviour
3.2.2. Model predictions
4. Conclusions
Acknowledgements
Appendix A. Model 1 - proving the general case
Appendix B. Model 1 - deriving equations for a simple kinetic
model
Appendix C. Model 2 - ion leaching from fresh surfaces
References