Post on 27-Feb-2022
Western Michigan University Western Michigan University
ScholarWorks at WMU ScholarWorks at WMU
Master's Theses Graduate College
12-2019
Evaluating the Effects of Sodium, Potassium, Magnesium, and Evaluating the Effects of Sodium, Potassium, Magnesium, and
Calcium Concentrations on Dolomite Stoichiometry, Cation Calcium Concentrations on Dolomite Stoichiometry, Cation
Ordering, and Reaction Rate Ordering, and Reaction Rate
Hanna F. Cohen
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Part of the Geology Commons
Recommended Citation Recommended Citation Cohen, Hanna F., "Evaluating the Effects of Sodium, Potassium, Magnesium, and Calcium Concentrations on Dolomite Stoichiometry, Cation Ordering, and Reaction Rate" (2019). Master's Theses. 5096. https://scholarworks.wmich.edu/masters_theses/5096
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EVALUATING THE EFFECTS OF SODIUM, POTASSIUM, MAGNESIUM, AND CALCIUM CONCENTRATIONS ON DOLOMITE STOICHIOMETRY, CATION
ORDERING, AND REACTION RATE
by
Hanna F. Cohen
A thesis submitted to the Graduate College in partial fulfillment of the requirements
for the degree of Master of Science Geosciences
Western Michigan University December 2019
Thesis Committee: Stephen E. Kaczmarek, Ph.D., Chair Peter Voice, Ph.D.
Andrew Caruthers, Ph.D.
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ACKNOWLEDGMENTS
I would like to begin by thanking the Western Michigan University Department of
Geological and Environmental Sciences for providing funding for this project through a teaching
assistantship and the Western Michigan University College of Arts and Sciences for providing
funding through a summer research assistantship. I would also like to thank Dr. Kaczmarek for
his generous support through providing laboratory materials and space for all experiments and
analyses as well as his advising and writing support. I would also like to thank the Society for
Sedimentary Geology (SEPM), Geological Society of America – North Central Section, and
Geological Society of America – Mineralogy, Geochemistry, Petrology, and Volcanology
Division for travel funding to present this research at the annual Geological Society of America
conference in 2017 in Denver, CO and in 2018 in Seattle, WA. Finally, I would like to thank the
members of the Western Michigan University Carbonate Petrology and Characterization Lab for
ongoing collaboration and feedback throughout this project with additional gratitude to Brooks
Ryan for SEM training and feedback on my writing, and to Cameron Manche for XRD support.
Hanna F. Cohen
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EVALUATING THE EFFECTS OF SODIUM, POTASSIUM, MAGNESIUM, AND CALCIUM CONCENTRATIONS ON DOLOMITE STOICHIOMETRY, CATION
ORDERING, AND REACTION RATE
Hanna F. Cohen, M.S.
Western Michigan University, 2019
Numerous environmental factors affect dolomitization. Shallow peritidal and restricted
marine environments, for example, are often associated with more abundant and more
stoichiometric dolomite than deeper marine environments. Higher fluid Mg/Ca ratios resulting
from gypsum precipitation are often invoked to explain this observation, even when evidence of
evaporites is absent. In this study, high-temperature dolomitization experiments show that the
concentrations of major cation concentrations (Na, K, Mg, and Ca) impact dolomite
stoichiometry and reaction rate. Nearly 200 batch dolomitization experiments were run whereby
100 mg of natural aragonite ooids were dolomitized at 215°C in ionic solutions. Fluid [NaCl]
and [KCl] correlate positively with the stoichiometry of the initial protodolomite product (43–48
mol% MgCO3), but negatively with reaction rate. In contrast, the [Mg] and [Ca] of the
dolomitizing fluid correlate positively with both reaction rate and protodolomite stoichiometry
(41–45 mol% MgCO3). The rate at which cation ordering develops is unaffected by [NaCl],
[KCl], [Mg], or [Ca] in the dolomitizing fluid. These findings provide the basis for an alternative
explanation for the observed relationship between restricted, peri-tidal marine carbonate facies
and higher dolomite abundance and stoichiometry without the need to invoke precipitation of
calcium-bearing evaporites. These observations add to our understanding of the fundamental
controls on dolomite stoichiometry and reaction rate, and can help further constrain geological
interpretations based on dolomite stoichiometry.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF EQUATIONS
CHAPTER
I. INTRODUCTION…………………………………………………………………….1
Dolomite ……………………………………………………………………..……..1
Research Question …………………………………………………………..........4
II. LITERATURE REVIEW …………………………………………………………….6
Fluid Chemistry ……………………………………………………………..........6
High Temperature Experimental Work ……………………………………..........8
III. METHODS ……………………………………………………………………...…..11
Experimental Procedure ……………………………………………….………..11
Analytical Techniques …………………………………………………………..15
IV. RESULTS …………………………………………………………………………....17
Replacement Reaction ……………………………………………….………….17
Protodolomite Reaction Rate ………………………………………………........20
Protodolomite Stoichiometry …………………………………………….……...22
Dolomite Cation Ordering …………………………………………………........24
V. DISCUSSION AND CONCLUSIONS ……………………………………………..26
Comparison to Previous Work ……………………………………………….…26
Reaction Curve ……………………………………………………….…26
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vii
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Table of Contents–Continued
CHAPTER
Rate of Dolomitization (Protodolomite) ………………………………...27
Ooid Size ……………………………………………………..…..……...28
Calcite Dolomitization………………………………………….………..28
Mechanisms for Salt Influence ………………………………………………….29
Implications ……………………………………………………………………...32
Dolomite as a Proxy for Understanding Fluid Chemistry ........................32
Global Correlations ……………………………………...............…........35
Conclusions ……………………………………………………………...………36
REFERENCES ………………………………………………………………………..………...38
APPENDIX
A. Dataset ………….……………………………...……………………..……….………... 45
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LIST OF FIGURES
1. Model for the structure of well-ordered, stoichiometric dolomite ………...………….….3
2. Illustration of experiment set up.……………………………………………………..... 14
3. Illustration of experimental procedure ……………………….…………………………14
4. X-ray diffraction overlapping diffractograms for a pure aragonite sample and a pure dolomite sample …………………………………..……………………………….16
5. Percent product as a function of reaction time. ……………………………………........19
6. Stoichiometry as a function of percent Ca-Mg-carbonate product. ………………….....19
7. Percent product as a function of reaction time ………………………………………….20
8. Percent product as a function of sodium concentration in the dolomitizing fluid ..…….21
9. Percent product as a function of magnesium and calcium concentrations in the dolomitizing fluid …………..…………………………………………………………...22
10. Protodolomite stoichiometry as a function of sodium concentration in the dolomitizing fluid. ……………..………………………………………………………. 23
11. Protodolomite stoichiometry as a function of magnesium and calcium concentrations in the dolomitizing fluid ...………………..…………………………… 24
12. Percent protodolomite (no ordering reflections) as a function of reaction time …….....27
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LIST OF EQUATIONS
1. Direct Precipitation Reaction…………………………………….………………………..2
2. Dissolution-Precipitation Reaction………………………………………………....……..2
3. Percent Product………………………………………………………………….….……16
4. Stoichiometry……………………………………………………………………….……16
5. Cation Ordering…………………………………………………………………….……16
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1
CHAPTER I INTRODUCTION
Dolomite
Dolomite, [CaMg(CO3)2], is a magnesium-calcium-carbonate mineral named in honor of
the French geologist Déodat Guy de Dolomieu, who was the first to document the occurrence of
a magnesium rich carbonate that did not readily react with hydrochloric acid as do limestones (de
Dolomieu, 1791). While dolomite is abundant in the rock record, modern (i.e. Holocene)
dolomite is extremely rare, and where present, is found only in minor quantities, in specific
settings. This observation, coupled with the failure to synthesize dolomite in the laboratory at
near surface conditions, is the crux of the “dolomite problem” (Van Tuyl, 1916; Sass and Bein,
1980; Land, 1980, 1985, 1998; Sun, 1994; Warren, 2000; Machel, 2004; Gregg et al., 2015;
Kaczmarek et al., 2017). Literature on the subject indicates wide disagreement about the
conditions, processes, and environments responsible for dolomite formation (Land, 1985;
Warren, 2000; Machel, 2004; Kaczmarek et al., 2017). Dolomite is the second most abundant
carbonate mineral in Earth’s crust and is economically significant as a hydrocarbon reservoir
rock (Warren, 2000; Machel, 2004). Due to the economic importance of dolomite and the lack of
modern naturally occurring dolomite, geologists routinely use proxies to infer the factors
controlling dolomitization in ancient rocks (Machel, 2004).
In nature, dolomite forms either as (i) an authigenic cement via direct precipitation from
an aqueous solution (Equation 1), or (ii) as a replacement product via a dissolution-precipitation
reaction whereby a CaCO3 precursor is replaced by dolomite in an Mg-bearing solution
(Equation 2). Dolomite cement is common in ancient carbonate rocks but is volumetrically minor
compared to platform-scale replacement dolomites observed throughout the Phanerozoic (Sun,
2
1994; Machel, 2004). The process whereby limestone is replaced by dolomite is referred to as
“dolomitization” (Equation 2).
Ca2+ + Mg2+ + 2CO32- à CaMg(CO3)2 (Equation 1)
Mg2+ + 2CaCO3 à CaMg(CO3)2 + Ca2+ (Equation 2)
Dolomite has a rhombohedral structure characterized by stacked layers of trigonal carbonate
anions separated by layers of calcium and magnesium cations (Figure 1; Lippmann, 1973;
Reeder, 1983; Gregg et al., 2015; Kaczmarek et al., 2017). The major elemental composition and
mineralogical structure of the mineral are described in terms of stoichiometry and cation
ordering, respectively – both of which can be measured quantitatively using x-ray diffraction
(Goldsmith and Graf, 1958; Royse et al., 1971; Lippmann, 1973; Lumsden, 1979; Reeder and
Sheppard, 1984; Zhang et al., 2010; Gregg et al., 2015; Kaczmarek et al., 2017). Dolomite
stoichiometry refers to the major cation composition in terms of the molar ratio of magnesium
relative to calcium in the crystal structure, commonly denoted as %MgCO3 (Lumsden, 1979;
Gregg et al., 2015). Dolomite stoichiometry is routinely measured using the position of the d-104
reflection in x-ray diffraction patterns (Lumsden, 1979; Zhang et al., 2010; Gregg et al., 2015).
Cation ordering is a measure of the degree to which the Mg and Ca cations are positioned in the
appropriate alternating cation layers within the dolomite lattice. Cation ordering is qualified by
the presence of the d-101, d-015, and d-021 ordering reflections in x-ray diffraction patterns
(Goldsmith and Graf, 1958; Gregg et al., 2015). An absence of these reflections indicates that a
Ca-Mg-carbonate mineral is not dolomite, but rather very high-magnesium calcite (Gregg et al.,
2015; Kaczmarek et al., 2017). Cation order is commonly semi-quantified using the intensity
ratio of the d-015 and d-110 x-ray diffraction reflections (Graf and Goldsmith, 1958; Gregg et
al., 2015). Well-ordered stoichiometric dolomite, for example, has equal molar proportions of Ca
3
and Mg segregated into alternating cation layers (Figure 1; Folk and Land, 1975; Warren, 2000;
Machel, 2004; Gregg et al., 2015; Kaczmarek et al., 2017).
Figure 1: Model for the structure of well-ordered, stoichiometric dolomite (Gregg et. al., 2015).
Despite stoichiometric dolomite being more stable at near-surface temperatures, most natural
dolomites have excess Ca (up to 12 mol%) in their lattice and thus some degree of cation
disorder (Goldsmith and Graf, 1958; Shinn et al., 1965; Illing et al., 1965; Lumsden, 1979;
Lumsden and Chimahusky, 1980; Sperber et al., 1984; Searl, 1994; Budd, 1997; Reeder, 2000;
Jones et al., 2001; Jones and Luth, 2002; Kaczmarek and Sibley, 2007; Turpin et al., 2012; Zhao
and Jones, 2012; Kaczmarek and Thornton, 2017; Ryan et al., 2019). Several factors have been
shown to affect the rate of dolomitization and dolomite stoichiometry. These include temperature
(Gaines, 1974; Usdowski, 1994; Thornton and Kaczmarek, 2015; Kaczmarek and Thornton,
2017), fluid Mg/Ca ratio (Land, 1967; Glover and Sippel, 1967; Sibley et al., 1987; Sibley, 1990;
Kaczmarek, 2005; Kaczmarek and Sibley, 2007, 2011), reactant mineralogy (Sibley et al., 1987;
Rose and Kaczmarek, 2019), reactant surface area (Sibley et al., 1987; Rose and Kaczmarek,
4
2019), organic matter (Zhang et al., 2012), sulfate reduction (Baker and Kastner, 1981; Morrow
and Rickets, 1988), pCO2 of the dolomitizing fluids (Sibley, 1990), and recrystallization
(Kaczmarek and Sibley, 2014; Ryan et al., 2019). Various case studies have presented evidence
suggesting that dolomite composition directly reflects the geochemistry of the local environment
of formation (Sass and Bein, 1988; Folk and Land, 1975; Lumsden and Chimahusky, 1980;
Kaczmarek and Sibley, 2011; Ren and Jones, 2017; Manche and Kaczmarek, 2019a).
Research Question
This study focuses on the question of how do the concentrations of common cations in
dolomitizing fluids, namely sodium, potassium, magnesium, and calcium, which all increase
during evaporation, influence dolomite stoichiometry and reaction rate. Based on observations
from natural environments, it is hypothesized that increasing salinity ([NaCl], [KCl], [Mg], and
[Ca]) of the fluid will increase the rate of dolomitization as well as the stoichiometry and cation
ordering of the resulting dolomite. High-temperature dolomitization experiments are commonly
used to understand the effects of various parameters on the dolomitization reaction (e.g., Glover
and Sippel, 1967; Liebermann, 1967; Land, 1967; Baker and Kastner, 1981; Sibley et al., 1987;
Sibley, 1990; Zempolich and Baker, 1993; Malone et al., 1996; Kaczmarek and Sibley, 2007,
2011, 2014; Zhang et al., 2012; Kaczmarek and Thornton, 2017). With high-temperature
experiments, individual factors influencing dolomitization can be controlled rather than inferred.
To date, only two studies have set out to directly evaluate how common environmental
conditions impact dolomite stoichiometry (Kaczmarek and Sibley, 2011; Kaczmarek and
Thornton, 2017). The results from the experiments presented in this thesis will provide a new
5
way to explain the higher abundance of stoichiometric dolomite in evaporitic depositional
settings.
6
CHAPTER II
LITERATURE REVIEW
Fluid Chemistry
In nature, stoichiometric dolomite is commonly associated with evaporitic depositional
environments (e.g., Goldsmith and Graf, 1958; Folk and Land, 1975; Sass and Bein, 1988).
Evaporitic settings also tend to contain more abundant dolomite than their subtidal counterparts
(Goldsmith and Graf, 1958; Folk and Land, 1975; Machel, 2004). Such observations are
routinely explained by invoking elevated fluid Mg/Ca ratios caused by sedimentary gypsum
precipitation in restricted peritidal settings (Carpenter, 1980; Machel, 2004). This explanation is
geologically reasonable as it fits with our understanding of the conditions that promote dolomite
formation. More specifically, gypsum (CaSO4 • 2H2O) precipitation should drive fluid Mg/Ca
ratios higher, which is more thermodynamically favorable for dolomite (Carpenter, 1980).
However, the main limitations of this model are such that (i) in addition to a Mg/Ca ratio
increase, other chemical changes take place during evaporation of marine waters, and (ii) many
restricted marine settings lack evidence of evaporite precipitation. Folk and Land (1975) studied
natural dolomites and inferred that with increasing concentrations of Mg, Ca, and Na in the
solution, a significant increase in the Mg/Ca ratio from the precipitation of evaporites would also
be required for well-ordered, stoichiometric dolomite to form. They hypothesized that the Mg/Ca
ratio in the fluid was a determining factor on the dolomitization reaction. Sass and Bein (1988)
studied natural Permian to Neogene dolomites from Israel and Texas (U.S.) with varying sodium
contents and concluded that the chemical and mineralogical makeup of natural dolomites directly
reflected the geochemistry of their depositional and diagenetic environments. They inferred
water chemistry (i.e. the degree of evaporation) from the [Na] in the dolomite and measured
7
stoichiometry using XRD. Despite very contrasting sodium concentrations in the dolomitizing
fluids, they showed that the dolomite stoichiometries from marine and gypsum saturated fluids
evenly spanned the same range of 44-50 %MgCO3. Sass and Bein (1988) also reported, however,
that dolomite associated with halite saturated brines was more stoichiometric (49-50 %MgCO3).
They interpreted, as did others previously, that the observed differences likely reflected different
Mg/Ca ratios in the dolomitizing fluids. More specifically, that more stoichiometric dolomite
was formed in restricted marine to evaporitic diagenetic settings because of higher fluid Mg/Ca
associated with the precipitation of gypsum (Goldsmith and Graf, 1958; Folk and Land, 1975;
Sass and Bein, 1988).
Past laboratory experiments focused on how fluid chemistry influences the rate of
dolomitization. Land (1967), for example, performed a series of high-temperature dolomitization
experiments and reported an increase in rate of dolomitization with increased concentrations of
magnesium and calcium. Stoichiometry and cation ordering data were not reported in this study,
however. In other laboratory experiments, Liebermann (1967) observed that the rate of
dolomitization of a CaCO3 precursor decreased as concentrations of NaCl, MgCl2, MgSO4,
CaSO4, and KCl in artificial seawater increased. Liebermann (1967) interpreted his observations
to indicate that the additives caused the solubility of dolomite to decrease and the solubility of
CaCO3 to increase. No data on stoichiometry or cation ordering were reported. Glover and Sippel
(1967) synthesized aragonite and high-magnesium calcite via direct precipitation from fluids in
high-temperature experiments using aqueous solutions containing [Mg], [Ca], and [Na] like
modern seawater and reported that a lower pH (i.e. 6–7 pH) might contribute to the higher total
carbonate concentration in the formation of primary dolomite. Glover and Sippel (1967) did not
disqualify a higher pH and thus a lower total carbonate concentration from forming primary
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dolomite, but did recognize it would require significantly more time. Sibley et al. (1994) reported
no change in dolomite stoichiometry in experiments spiked with NaCl, although reaction rate
data were not reported. More recently, Martin et al. (2013) and Kaczmarek et al. (2015), reported
that increased concentrations of NaCl in the dolomitizing fluid correlated with a decreased rate
of dolomitization of a calcite precursor and an increase in the stoichiometry of the initial
products. Considering contradictory results in the literature, the objective of this study is to more
thoroughly and systematically evaluate the effects of [NaCl], [KCl], [Mg], and [Ca], major
chemical components in modern seawater, on dolomite stoichiometry and cation ordering.
High Temperature Experimental Work
In general, it is known that there are many geochemical parameters that affect dolomitization
including temperature, fluid Mg/Ca ratio, molarity of the fluid with respect to various cations
(Na, K, Mg, Ca), SO42-, microbial activity, and reactant composition and texture (Liebermann,
1967; Land, 1967; Lippmann, 1973; Gaines, 1974; Folk and Land, 1975; Baker and Kastner,
1981; Sibley et al., 1987; Machel, 2004; Kaczmarek and Sibley, 2007, 2011, 2014; Kaczmarek
and Thornton, 2017; Rose and Kaczmarek, 2019). In most diagenetic environments, these
geochemical parameters evolve concurrently, making it difficult to evaluate their individual
effects. Laboratory experiments, in contrast, provide an approach for constraining individual
variables and directly examining their effects. The impact of reaction temperature on the rate of
dolomitization, for example, has been studied extensively (e.g., Gaines, 1974; Usdowski, 1994;
Kaczmarek and Thornton, 2017), but only one such study has examined how temperature
impacts dolomite stoichiometry (Kaczmarek and Thornton, 2017). Similarly, the effect of fluid
Mg/Ca was studied by Land (1967), Sibley et al., (1987), and Kaczmarek and Sibley (2011).
9
These studies all showed that an increase in the Mg/Ca ratio results in an increase in reaction
rate. Only two of these studies examined dolomite stoichiometry (Sibley et al, 1987; Kaczmarek
and Sibley, 2011). Kaczmarek and Sibley (2011) evaluated the effects of Mg/Ca and they
observed an increase in rate of dolomitization with increased Mg/Ca. Sulfate concentrations have
also been studied in the laboratory and were determined by Baker and Kastner (1981) to inhibit
dolomitization, even at very small concentrations (less than 5%). As sulfate is reduced, alkalinity
increases, NH4+ is produced, and, in exchange, additional Mg2+ is released from solid silica and
dissolves into the dolomitizing fluid (Baker and Kastner, 1981; Morrow and Rickets, 1988).
The application of high-temperature experiments to understanding the factors affecting the
formation of natural near-surface dolomites is supported in literature by the observed similarities
in mineralogy, stoichiometry, and crystal morphology of both natural and synthetic dolomites
(Sibley et al., 1987; Zempolich and Baker, 1993; Kaczmarek and Sibley, 2014, 2017; Kaczmarek
and Thornton, 2017). The disadvantage of using experiments as an analogue to understanding
ancient dolomites is that high reaction temperatures speed up the reaction and may impact the
dolomitization reaction in a way that is not completely understood (Kaczmarek and Sibley, 2014;
Kaczmarek at al., 2017). The petrological similarities between experimental and natural
dolomites in literature imply that experimental dolomites can be used as an effective analogue
for natural sedimentary dolomites. The mimetic replacement of allochems in high-temperature
dolomitization experiments appears to be the same as the textural preservation in some naturally
dolomitized allochems (Sibley et al., 1987). Sibley et al. (1987) observed similarities in the
product dolomite, including rhombic morphology of dolomite formed in high temperature
experiments that matches natural dolomite. Sibley et al. (1897) also recognized the scarcity of
partially dolomitized rocks due to the reaction rate observed in high-temperature experiments
10
and an increase in stoichiometry in completely dolomitized limestone (Sibley et al., 1987).
Zempolich and Baker (1993) and Kaczmarek and Thornton (2017) observed textural similarities
in dolomitized ooids from nature and those produced in high-temperature experiments. The
nanotopography of the surface of natural and experimental product dolomite surfaces is also the
same (Kaczmarek and Sibley, 2007, 2014).
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CHAPTER III
METHODS
Experimental Procedure
To evaluate the hypotheses, 189 high-temperature dolomitization (dissolution-
precipitation) experiments were carried out to assess the effects of Na, K, Ca, and Mg
concentrations on dolomite composition and reaction rate. The laboratory experiments followed
a similar approach used previously in Kaczmarek and Sibley (2007, 2011, 2014) and Kaczmarek
and Thornton (2017). Batch dolomitization experiments were conducted whereby aragonite
ooids were dolomitized in Mg-Ca-Cl fluids exhibiting a range of salinities. A stock electrolyte
solution was produced by dissolving 10 moles (2033 g ± 10 g) of magnesium dichloride
hexahydrate and 10 moles (1110 g ± 10 g) of calcium dichloride dehydrate in 10 liters of ultra-
pure H2O. The mixtures were mechanically stirred for approximately 10 minutes to ensure all
solutes were dissolved. The calculated molarity of the stock solution with respect to Mg2+ and
Ca2+ is 0.86 mol/L. The Mg2+/Ca2+ ratio of the stock solution is 1.0.
The stock solution was divided equally into five Erlenmeyer flasks and different
quantities of salt ([NaCl] or [KCl]) were dissolved in each flask to obtain experimental solutions.
The experimental solutions were created to mimic the [Na], [K], [Mg], and [Ca] concentrations
of common dolomitization fluids (e.g., seawater and hypersaline brines) (Table 1; Logan, 1987;
Jones and Xiao, 2005). Cation concentrations are based on previously published data from
surface waters in Lake MacLeod, an intracratonic evaporite basin in western Australia (Table 1;
Logan, 1987; Jones and Xiao, 2005). Lake MacLeod is a modern lake exhibiting continued
reflux due to continuous evaporation of seawater coupled with the periodic flooding of meteoric
water (Logan, 1987). The fluid chemistry of Lake MacLeod was previously used for reactive
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transport modeling (RTM) (Jones and Xiao, 2005; Gabellone and Whitaker, 2016). Gabellone
and Whitaker (2016) used reactive transport modeling (coupled chemical and hydrological
numerical model) and hypothesized an expected increase in dolomitization rate with an increase
in concentration of total dissolved solids. Later work by Kaczmarek et al. (2016) showed that the
geochemical models used in RTM do not specifically consider [NaCl] and [KCl]. In this study, a
series of high-temperature experiments are used that directly measure the effect of [NaCl],
[KCl], [Mg], and [Ca] on the stoichiometry and rate of dolomitization.
Table 1: Experimental values for each long-term solution. Values based on those of Logan (1987).
Two sets of experiments – long-term and short-term – were used to determine the effects of
solution chemistry on dolomite composition and reaction rate. Long term experiments were
conducted to elucidate the effects of different fluid [NaCl] and [KCl] on long-term
dolomitization rate and how dolomite stoichiometry and cation ordering evolve during the multi-
stage reaction. 160 experiments lasting between 2 and 800 hours were conducted. In these
experiments, aragonite ooids were reacted in high-temperature (215°C) dolomitizing fluids that
were synthesized to mimic zero salt, seawater, and evaporated seawater (hypersaline) conditions
(Table 1). The products were analyzed and percent dolomite, stoichiometry, and cation ordering
[Mg]
[Ca]
[Na]
[K]
Stock Solution Modern Seawater Hypersaline KCl (NaCl)
0.86 mol/L
0.86 mol/L
N/A
0.484 mol/L
0.86 mol/L
0.86 mol/L
2.93 mol/L
N/A
0.86 mol/L
0.86 mol/L
0.485 mol/L
N/A
0.86 mol/L
0.86 mol/L
N/A
N/A
13
were measured to address the long-term effect of changing fluid [NaCl] and [KCl] on
dolomitization (Appendix A).
A series of short term (5 and 10 hour) dolomitization experiments were also conducted to
evaluate the sensitivity of stoichiometry with smaller intervals of concentrations of Na, K, Mg,
and Ca. The Na experiments were 5 hours in length while the Mg and Ca experiments were 10
hours in length. Five-hour [Na] experiments were intended to evaluate the stoichiometry and
reaction rate of the initial product. For the 5 hour experiments, varying amounts (0.0-3.5 g) of
NaCl were added in 0.5 g increments to 15 mL of stock solution. This included replicates of the
solutions created for the long-term experiments. For the short-term experiments evaluating [Mg]
and [Ca], concentrations of Mg and Ca were added to the stock solution up to a molarity of 1.5
mol/L. The experimental solutions used in these experiments had Mg/Ca ratio of 1. Short-term
experiments evaluating the effects of [Mg] and [Ca] were ten hours in duration to analyze the
initial product.
Naturally occurring modern aragonite ooids were used as the solid reactants in all
experiments (Appendix A). Ooids were harvested from the Ambergris Shoal, Turks and Caicos
Islands (Kaczmarek and Thornton, 2017). The ooids were sieved to obtain the 355-425µm size-
fraction to control for reactant surface area.
Each Teflon-lined stainless steel reaction vessel (bomb) was loaded with 100 mg of
aragonite ooids and 15 mL of the experimental solution (Figure 2). Reaction vessels were sealed,
immediately placed in a high precision Thermo Scientific Heratherm convection oven preheated
to 215°C. Individual vessels were removed periodically between 2-800 hours, and were force
cooled to room temperature with compressed air. Solids were obtained by filtering using a
vacuum flask. Solids were rinsed with ~500 mL of de-ionized water to remove any salt
14
precipitates. Experimental fluids were discarded. Solids were dried for a minimum of 30 minutes
in a vacuum desiccator prior to further analysis (Figure 3). These methods have been previously
described in Cohen and Kaczmarek (2017) and Cohen and Kaczmarek (2018).
Figure 2: Illustration of experiment set up. Teflon-lined stainless steel reaction vessel with a flanged closure loaded with experimental solution (blue) and aragonite ooids (white) Adapted from Parr Instrument Company (2019).
Figure 3: Illustration of experimental procedure. Procedure follows: (1) solution preparation, sieving of ooids, and loading of bombs, (2) placement of bombs in oven, (3) removal of samples to dry and desiccate, (4) powder products, and (5) analyze powdered products with x-ray diffractometer. Adapted from Thermo Fisher Scientific Inc. (2012), DWK Life Sciences LLC. (2017), Cole-Parmer Instrument Company, LLC (2019), and Parr Instrument Company (2019).
15
Analytical Techniques
The mineralogical composition of the solid products was analyzed using standard powder x-
ray diffraction techniques. X-ray diffraction (XRD) is an analytical tool routinely used to
determine the relative proportions of carbonate minerals in a sedimentary assemblage. It is also
routinely used to determine the stoichiometry and cation ordering of dolomites (Royse et al.,
1971; Lumsden, 1979; Reeder and Sheppard, 1984; Zhang et al., 2010; Gregg et al., 2015). To
homogenize the samples, solid products were powdered by hand using an agate mortar and pestle
and then mounted on a Boron-doped silicon P-type zero background diffraction plate. Each
sample was analyzed with a Bruker D2 Phaser diffractometer at Western Michigan University
using CuKα radiation (λ=1.54184 Å) at 300 Watts (30 kV, 10 mA). The diffractometer uses
Bragg-Brentano geometry with a fixed linear LYNXEYE 1D line detector. The samples were
scanned over the range of 20-55°2θ with a 0.008°2θ step size increment (4327 steps), at a speed
of 1.0 sec/step. Samples were rotated at 10 revolutions/minute to ensure that the entire sample
was analyzed. Peak positions were confirmed with an internal standard. The scan parameters
were modified from those used in Kaczmarek and Thornton (2017).
XRD patterns were analyzed with the Bruker EVA toolkit with the Crystallography Open
Database. Patterns were processed to remove k-beta, background, and smoothed using the
Bruker EVA toolkit and are consistent with conventional data processing techniques (Kaczmarek
and Thornton, 2017). These data were also used to determine the mineralogical composition (i.e.
percent Ca-Mg-carbonate products relative to aragonite reactants), dolomite stoichiometry, and
cation ordering based on the positions and intensities of specific reflections (Figure 4; Royse et.
al., 1971; Goldsmith and Graf, 1958; Lumsden, 1979). The percent product was determined
using the ratio of the [104] peak intensity to the sum of the intensities of the [104] and aragonite
16
[111] peaks (Equation 3). Dolomite stoichiometry was quantified based on the position of the
[104] peak using the empirical relationship (Equation 4) established by Lumsden (1979). Mg
ions are smaller than Ca ions and thus cause a reduction in the d-spacing of the layer in the
crystal structure and an increase in the 2-theta value for the dolomite [104] peak, per Bragg’s
Law (Lumsden, 1979). Cation ordering was determined by the intensity ratio of the dolomite
[015] ordering peak to the dolomite [110] peak (Equation 5), as detailed in Goldsmith and Graf
(1958).
Figure 4: X-ray diffraction overlapping diffractograms for a pure aragonite sample and a pure dolomite sample. Aragonite sample is in light blue and dolomite sample is in dark blue. Peaks used for determining percent dolomite, stoichiometry, and cation ordering labeled.
% dolomite = !"#$%(()*+),-*.)!"#$% ()*+),-*. 01"###(()*+),-*.)
∗ 100 (Equation 3)
mole% MgCO3 = 1 – ((333.33*(D {104} d-space))-911.99) (Equation 4)
cation ordering = {015} / {110} (Equation 5)
17
CHAPTER IV
RESULTS
To determine experimental error, 7 replicates were run for 18 hours in the stock solution.
Based on the data from these runs, the analytical uncertainty for dolomite stoichiometry and
percent product is ±0.538 mol% MgCO3 and ±2.38% product, respectively (Appendix A).
Replacement Reaction
Replacement reaction rate data from the long-term high-temperature dolomitization
experiments indicate an increased abundance of Ca-Mg-carbonate product with reaction time.
Data plotted in Figure 5 show a characteristic S-shaped reaction curve proceeding in a non-linear
fashion. Reaction data from all four experimental solutions (stock solution, NaCl seawater, NaCl
hypersaline, and KCl seawater) follow the same general pattern previously defined by
Kaczmarek and Sibley (2014), whereby the reaction proceeds through four distinct stages that
represent a progressive replacement of aragonite by a protodolomite, poorly ordered dolomite,
and well-ordered dolomite (Figure 5). The four distinct reaction stages previously defined by
Kaczmarek and Sibley (2014) will be referred to here as the induction stage, rapid replacement
stage, primary recrystallization stage, and secondary recrystallization stage (Figure 5). During
the induction stage, no products are detected by XRD. The induction stage in all three
experiments lasted between three and four hours (Appendix A). This means that in the
experiments which ended within 3 hours of reaction time, no products were detected. After 4
hours, however, Ca-Mg-carbonate products were detected by XRD in all experiments. The
stoichiometry of these initial Ca-Mg-carbonate phases ranges from 41.0 – 46.2 mole% MgCO3.
No cation ordering in these products was detected with XRD. Based on the dolomite like
18
composition, but lack of cation ordering, these phases can be referred to as either very high-
magnesium calcite, sensu Gregg et al. (2015), or protodolomite, sensu Graf and Goldsmith
(1956). Herein, the initial Ca-Mg-carbonate phases observed in these experiments will be
referred to as protodolomite because it can be demonstrated that they become ordered dolomite
given enough time. The rapid replacement stage is characterized by fast protodolomite formation
as evidenced by a steep slope in Figure 6. During the rapid replacement stage the products
maintain a relatively constant stoichiometry until ~95% product as shown by the nearly flat
slopes (m<0.02) and low correlation coefficients shown in the plot of dolomite stoichiometry vs.
time (Figure 6, Appendix A). The subsequent primary recrystallization stage, in contrast, is
characterized by a marked decrease in reaction rate as evidenced by the reduction in slope in
Figure 5. A rapid increase in stoichiometry to 50 mol% MgCO3 occurs during the primary
recrystallization stage, as indicated by the steep slopes in Figure 6. The secondary
recrystallization stage is characterized by dolomite products that are have become fully
stoichiometric but cation ordering that continues to increase with time to some maximum value
and then levels off (Appendix A). The end products from the longest experiments, which lasted
800 hours, were well-ordered and stoichiometric dolomite (Appendix A).
19
Figure 5: Percent product as a function of reaction time. Percent product refers to dolomite relative to aragonite. Data is consistent with previously published model (Kaczmarek and Sibley, 2014).
Figure 6: Stoichiometry as a function of percent Ca-Mg-carbonate product. Prior to recrystallization, stoichiometry is relatively constant. Data is consistent with previously published model (Kaczmarek and Thornton, 2017).
Aragonite Poorly Ordered DolomiteProtodolomite Well Ordered
Dolomite100
80
60
40
20
00 10 100 1000
Reaction time (hours)
% P
rodu
ct
Induction
Rapi
d Re
plac
emen
t
Primary and Secondary Recrystallization
Mg2+ + 2CaCO3 --> CaMg(CO3)2 + Ca2+
Hypersaline: m=51.67, R2 = 0.9217Modern Seawater: m=48.12, R2 = 0.9654Stock Solution: m=49.02, R2 = 0.9721
1. Induction: nucleation and slow growth2. Rapid Replacement: protodolomite growth3. Primary Recrystallization: dolomite stoichiometry and cation ordering increase4. Secondary Recrystallization: fully stoichiometric dolomite, cation order increases to maximum
52
50
48
46
44
42
40
% Product
MgC
O3 m
ol %
0 20 40 60 80 100
Stoichiometry vs. % Product Rapid Replacement Primary RecrystallizationHypersaline: y=0.01x+44.6 R2 = 0.119 y=0.85x-34.1 R2 = 0.657Modern Seawater: y=-0.0007x+43.8 R2 = 0.009 y=0.68x-17.2 R2 = 0.607Stock Solution: y=0.02x+42.3 R2 = 0.297 y=1.19x-67.2 R2 = 0.778
Rapid Replacement
Prim
ary
Rec
ryst
alliz
atio
n
20
Protodolomite Reaction Rate
Reaction rates during the rapid replacement stage were determined based on the percent
product per reaction time (Figure 5; Figure 7; Appendix A). The rapid replacement stage in these
experiments is defined as the end of the induction period until 60% protodolomite (Figures 5–7).
In long-term experiments, a general trend is observed of slightly faster protodolomite formation
in the stock solution than in the solutions with added salt (i.e. NaCl seawater, NaCl hypersaline,
and KCl). Experiments conducted in the stock solution display a reaction rate slope of 7.19
whereas the slopes of the reaction rate for the NaCl seawater, NaCl hypersaline, and KCl
experiments are 7.12, 6.96, and 6.80, respectively. Data from the individual experiments have
high correlation coefficients (R2 = 0.97 – 0.99), and the addition of NaCl and KCl decreases the
rate of dolomitization in comparison to the stock solution with no added salt (Figure 7).
Figure 7. Percent product as a function of reaction time. Data plotted from the four long-term experiments up to 60% protodolomite. The zero salt shows the fastest rate, but the slopes are very similar indicating only minor differences.
60
50
40
30
20
10
00 2 4 6 8 10 12 14 Time (Hrs)
% p
roto
dolo
mite
NaCl Hypersaline y=6.9x - 28.1 R2 = 0.99NaCl Seawater y=7.1x - 23.4 R2 = 0.99KCl y=6.8x - 23.5 R2 = 0.98Stock (No salt) y=7.2x - 20.9 R2 = 0.97
21
Results from the 5-hour Na experiments also show that reaction rate is inversely
proportional to the NaCl concentration in the solution (Figure 8). After 5 hours, 6.86%
protodolomite formed in the solution with 2.57 g NaCl and 15.66% protodolomite formed in the
solution with 0.25 g NaCl (Figure 8; Appendix A).
Figure 8: Percent product as a function of sodium concentration in the dolomitizing fluid. Percent product determined by x-ray diffraction analysis. Data plotted from short-term experiments evaluating [NaCl]. Regression line shows rate of dolomitization in the early stage of the reaction decreases with increasing concentrations of Na. [KCl] data has a similar trend. Blue points show results of 300-555 µm Iceland spar calcite experiments (Martin et al., 2013; Kaczmarek et al., 2016) Black dots show results of 355-425 µm aragonite ooid experiments (this study).
Stoichiometry (5 h)
MgC
O3 m
ol %
NaCl mass in experimental solution (g/15mL)
49
48
47
46
45
44
43
42 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
300-355 μm: R2 = 0.6900355-425 μm: R2 = 0.5159
30%
25%
20%
15%
10%
5%
0%
NaCl mass in experimental solution (g/15mL)
% P
rodu
ct
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Rate of Dolomitization (5 h)300-355 μm : R2 = 0.7800355-425 μm: R2 = 0.8055
22
Results from the 10-hour Mg and Ca experiments show that as concentrations of Mg and
Ca increase, the rate of dolomitization increases (Figure 9) below concentrations of 1.0 mol/L of
Mg and Ca. After 10 hours, 9.52% initial product formed in the 0.05 M solution and 56.41%
initial product formed in the 0.87 M solution (R2=0.91) (Figure 9; Appendix A).
Figure 9: Percent product as a function of magnesium and calcium concentrations in the dolomitizing fluid. Percent product determined by x-ray diffraction analysis. Data plotted from short-term experiments evaluating [Mg] and [Ca] demonstrating an increased rate of dolomitization with increasing concentrations. The shaded portion represents values that exceed those found in natural environments.
Protodolomite Stoichiometry
In the long-term experiments the stoichiometry of the initial Ca-Mg-carbonate products,
i.e. protodolomite, (Figure 6) is higher with increased concentrations of salt in the fluid. The
average stoichiometry of the protodolomite produced in the hypersaline solution is 44.93 mol%
MgCO3. In contrast, the average stoichiometry of the protodolomite produced in the seawater
and stock solutions is lower at 43.69 mol% MgCO3 and 43.19 mol% MgCO3, respectively
(Appendix A). The average stoichiometry of the protodolomite produced in the KCl solution is
23
43.55 mol% MgCO3. The addition of NaCl and KCl both increase the average stoichiometry of
the initial product in comparison to the stock solution with no added salt.
Data from the short-term experiments show a similar relationship between salt
concentration and stoichiometry (Figure 10). The 5-hour Na experiments show that the
stoichiometry of the initial products positively correlates with the amount of NaCl added to the
solution (0-2.57 g per 15 ml). The protodolomite products of the 5-hour experiments have
stoichiometry values that range from 43.80 – 45.89 mol% MgCO3 over the range of NaCl
examined and have a corresponding R2 value of 0.52 (Figure 10).
Figure 10: Protodolomite stoichiometry as a function of sodium concentration in the dolomitizing fluid. Fluid chemistry plotted from short-term experiments showing an increase in stoichiometry with increasing [NaCl] and [KCl]. Only [NaCl] data is plotted. [KCl] data is consistent, but not show on the graph. NaCl concentration (g NaCl/15 mL stock solution) is plotted on the x-axis and stoichiometry is plotted on the y-axis. Solid black dots are from the experiment using 355-425 µm aragonite ooids. Solid blue dots are from experiments with an identical procedure but smaller ooids.
Stoichiometry (5 h)
MgC
O3 m
ol %
NaCl mass in experimental solution (g/15mL)
49
48
47
46
45
44
43
42 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
300-355 μm: R2 = 0.6900355-425 μm: R2 = 0.5159
30%
25%
20%
15%
10%
5%
0%
NaCl mass in experimental solution (g/15mL)
% P
rodu
ct
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Rate of Dolomitization (5 h)300-355 μm : R2 = 0.7800355-425 μm: R2 = 0.8055
24
Results from the 10-hour Mg and Ca experiments show that as concentrations of Mg and
Ca increase, dolomite stoichiometry increases below [Ca] and [Mg] of 1.0 mol/L, but increases
above 1.0 mol/L (Figure 11). Protodolomite stoichiometry in the 0.05 M [Ca] and [Mg] solution
is 41.19 mol% MgCO3 compared to 44.27 mol% MgCO3 in the 0.87 M [Mg] and [Ca] solution.
An R2 value of 0.87 is shown for the data below 1.0 mol/L (Figure 11; Appendix A).
Figure 11: Protodolomite stoichiometry as a function of magnesium and calcium concentrations in the dolomitizing fluid. Fluid chemistry from short-term experiments evaluating [Mg] and [Ca] demonstrating an increase in stoichiometry with increasing concentrations. The shaded portion represents values that exceed those found in natural environments.
Dolomite Cation Ordering
The 5-hour Na experiments and the 10-hour Mg and Ca experiments only produced
initial products and no cation ordering was observed. All solutions required at least 22 hours
before XRD evidence of cation ordering was observed (Appendix A). The long-term
experiments show that the cation ordering of the dolomite formed is directly influenced by the
25
concentration of salt in the fluid. The dolomite formed from the stock solution with zero salt
added reaches a maximum cation ordering of 0.34. The dolomite formed from the seawater
solution reaches a maximum cation ordering of 0.34. The dolomite formed from the hypersaline
solution reaches a maximum cation ordering of 0.31. The dolomite formed from the solution
with added KCl has reaches a maximum cation ordering of 0.32. The addition of NaCl and KCl
both decrease the cation ordering of the dolomite after 800 hours in comparison to the stock
solution with no added salt.
26
CHAPTER V
DISCUSSION AND CONCLUSIONS
Three key observations help drive this research: (1) dolomite formed in peritidal settings
is more abundant and more stoichiometric than dolomite formed in subtidal settings (Lumsden
and Chimahusky, 1980), (2) dolomites formed in association with evaporite minerals are more
stoichiometric (Sass and Bein, 1988; Figure 6, 10, 11), and (3) numerous physicochemical
factors have been shown to impact dolomite stoichiometry (e.g., temperature, burial depth, water
depth, degree of evaporation, presence of [SO42-], fluid chemistry ([NaCl], [KCl], [Ca], and
[Mg]), and the Mg/Ca ratio in the fluid (Graf and Goldsmith, 1956; Glover and Sippel, 1967;
Land, 1985; Kaczmarek and Sibley, 2011; Kaczmarek and Thornton, 2017).
Comparison to Previous Work
Reaction Curve
The data presented in Figure 5 is consistent with previous data of Kaczmarek and Sibley
(2014). The general S-shaped pattern of the reaction curve and identification of four distinct
stages (induction, rapid replacement, primary recrystallization, and secondary recrystallization)
are observed in data from each long-term experiment conducted in this study. Induction time,
slope (rate), change in slope (increasing and decreasing rates), stoichiometry increases, and
cation ordering increases, are each observed over time and are consistent with previous work
(Kaczmarek and Sibley, 2014; Kaczmarek and Thornton, 2017). This characteristic reaction
curve provides additional information regarding trends that are consistent with previous work.
27
Rate of Dolomitization (Protodolomite)
The data presented here are remarkably consistent with previous ooid temperature data.
Figure 12 (modified from Kaczmarek and Thornton, 2017) is a linear cross plot of percent
protodolomite (no ordering reflections) as a function of reaction time. Kaczmarek and Thornton
(2017) followed a similar procedure in which they dolomitized aragonite ooids at various high
temperatures (160°C, 180°C, 200°C, 218°C, 235°C, and 250°C). The experiments in this study
were conducted at 215°C and the data points from the zero salt experiments at this temperature
plot directly between the 200°C and 218°C data from Kaczmarek and Thornton (2017) (black
dots). The consistency provides added confidence that the experiments were well controlled and
performed in a manner consistent with accepted methodologies.
Figure 12: Percent protodolomite (no ordering reflections) as a function of reaction time. Linear cross plot modified from Kaczmarek and Thornton (2017). The black data points represent experiments from this study. Figure on previous page (35).
y=0.87ln(x)- 0.65R²=0.97
y=0.65ln(x)- 0.53R²=0.97
y=0.52ln(x)- 0.49R²=0.97
y=0.50ln(x)- 0.80R²=0.98
y=0.38ln(x)- 0.94R²=0.96
y=0.14ln(x)- 0.49R²=0.91
y=0.50ln(x)- 0.64R²=0.97
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1 10 100 1000
Percen
tProtodo
lomite
(noorde
ringrefle
ctions)
Time(Hours)
250
235
218
200
180
160
215
28
Ooid Size
Data from Martin et al. (2013) and Kaczmarek et al. (2015; 2016) that used ooids sieved
to a smaller size fraction (300-355 µm) were utilized in this comprehensive analysis. In
comparison to the ooids analyzed in this study (355-425 µm), the smaller ooid reactants exhibit a
slightly faster reaction rate than the larger ooids. This follows Zempolich and Baker (1993) as
dolomitization is described as a mimetic (fabric preserving) process in which, over time, the
dolomitization front will progressively move inwards as solution seeps into the inner layers of
the ooid. Inherently, smaller ooids will dolomitize at a faster reaction rate. Even though the ooids
used in this study were a larger fraction size, the reaction rate data is generally consistent for
both ooid sizes, which suggests that ooid size is not as strong of a control on reaction rate as is
temperature.
Calcite Dolomitization
The new short-term experimental data are also consistent with previously published data
from previous calcite reactant experiments (Martin et al., 2013; Kaczmarek et al., 2015, 2016),
which show that stoichiometry of the initial protodolomite products increases with salt
concentration in the fluid. The short-term experimental data presented here demonstrate an
increase in stoichiometry with each increasing NaCl and increasing KCl after only 5 hours.
In previous calcite experiments, cation ordering increased with increasing salt
concentration (Martin et al., 2013; Kaczmarek et al., 2015, 2016). In the current study, however,
there is little evidence that salinity has a large impact on cation ordering. The main differences
between the two studies are (i) the mineralogy of the reactant (calcite v. aragonite), and (ii) the
29
calcite study used a higher concentration of NaCl in the experimental fluids. There is no direct
evidence that the mineralogy of the reactant causes this difference.
In Martin et al. (2013) three sets of high-temperature experiments were conducted in
which calcite reacted with dolomitizing fluids with low, medium, and high salinity
concentrations. In these experiments, the low salinity fluid had 0 g of salt added, the medium
salinity fluid had 2.97g of NaCl added per 15 mL, and the high salinity fluid had 5.87 g of NaCl
added per 15 mL stock solution. In comparison, the long-term experiments conducted in this
study used zero salt, seawater, and hypersaline fluids with 0 g, 0.4251 g, and 2.57 g of NaCl
added per 15 mL stock solution, respectively. In comparing these sets of experiments, the low
salinity and medium salinity from Martin et al. (2013) correlate respectively with the zero salt
and hypersaline solutions from the experiments in this study. In Kaczmarek et al. (2013), very
little difference in cation ordering was observed between these two solutions. Similarly, very
little difference in cation ordering was observed in the NaCl solution within a range of 0-2.97 g
per 15 mL. The calcite data from Martin et al., (2013) suggests that at extremely high salt
concentrations (5.87 g/15 mL), cation ordering suddenly increases significantly to values higher
than is typically observed in laboratory dolomites (Kaczmarek and Sibley, 2014) and even higher
than natural dolomites (Manche and Kaczmarek, 2019a) and therefore may be an artifact of the
extremely high salt concentrations.
Mechanisms for Salt Influence
Fluid chemistry ([NaCl], [KCl], [Ca], and [Mg]) affects the kinetics of the dolomite
replacement reaction while the thermodynamic stability of the dolomite replacement reaction is a
function of the fluid Mg/Ca and temperature. Four proposed explanations for the change in
30
dolomitization rate and stoichiometry presented in this dataset include: (i) the competing ion
effect, (ii) solubility of the solid reactant, (iii) surface poisoning, and (iv) Mg-de-hydration.
Per the competing ion effect, as concentrations of competing ions in the fluid are
increased, the solution will lose energy and it becomes increasingly difficult to form highly
ordered minerals such as dolomite (Folk and Land, 1975). Instead, it becomes more likely that
minerals requiring less ordering will form due to the smaller amount of energy required. This can
explain the higher abundances of calcite and aragonite in marine-derived fluids, as these are
minerals with more simple structures than dolomite. In less concentrated solutions, there is less
interference of ions and slower rates of crystallization allows for highly ordered minerals such as
dolomite to form (Folk and Land, 1975). Folk and Land (1975) found that while hypersaline
solutions provide the necessary ions and a high Mg/Ca ratio, which both promote dolomite, it
must be significantly diluted to favor dolomite production and ordering.
Another way to explain the lower reaction rates and higher stoichiometries observed in
the salt experiments is how fluid chemistry can impact the solubility of the solid reactant. In the
scope of this project, as concentrations of NaCl, KCl, Mg, and Ca increase in the fluid, the
solubility of the aragonite ooid reactant will decrease (James and Jones, 2016). Specifically,
James and Jones (2016) discuss how an increase in magnesium ions in the fluid leads to a more
distorted crystal lattice. This also results in less carbonate ions being available in the solution and
decreases the rate of dolomitization due to a decreased availability of carbonate ions to form
dolomite. Dolomite is less soluble in solution because of its higher stability that comes from its
ordered structure and smaller magnesium ions in its crystal lattice in comparison to the calcium
ions in calcite. The findings presented here support the idea that the presence of certain cations
with strong hydration enthalpies appear to promote Mg dehydration in aqueous solutions, thus
31
aiding in the addition of Mg into protodolomite (e.g., Gaines, 1974; Vandeginste et al., 2019).
This explanation is supported by Debeye-Huckel theory, which shows that Mg-complexes are
less stable in solutions with higher ionic strength.
A poisoning effect may also explain how fluid chemistry in the experiments impacts
dolomitization rates and product composition. In this scenario, the surface of the aragonite ooid
reactants are poisoned by the adsorption of impurities and ionic compounds present in the
solution. Deelman (1999), for example, proposed that chlorine ions can easily bind with common
cations in marine fluids to produce compounds such as MgCl2, NaCl, and KCl that can sorb onto
mineral surfaces and thus decrease the reactive surface area of the calcium carbonate reactants.
This would decrease the number of surface sites available for the Mg to fit onto, which decreases
the rate of dolomitization (Deelman, 1999). This is a potential explanation of the observations
presented here as the rate of dolomitization does decrease with an increase in NaCl, KCl, MgCl2,
and CaCl2.
Lastly, Mg-de-hydration effects the dolomitization reaction by limiting the ability of Mg
to participate in the reaction (Lippmann, 1973). Because Ca ions have larger ionic radii than Mg
ions, they will more readily hydrate from the MgCl2 * 6H2O in the solution (Lippmann, 1973).
This results in the dehydrating of the Mg ions and causes either disordered calcium magnesium
carbonates or hydrous magnesium carbonates to form preferentially to dolomite (Lippmann,
1973). In the solutions with added Na and K, the Na and K ions also have larger ionic radii and
will even more readily hydrate from the MgCl2 * 6H2O in the solution. Additionally, as
temperatures increase, the dehydration of Mg will increase and a higher amount of Mg ions will
readily participate in the reaction (Lippman, 1973). Additionally, aqueous Mg ions are more
strongly hydrated than other divalent cations and will have a larger dehydration effect at higher
32
temperatures (Noyse, 1962; Lippman, 1973; Arvidson and Mackenzie, 1999; Stephenson et al.,
2008) As the Mg dehydrates, it will become more available to form into a carbonate lattice and
thus the rate of dolomitization will increase (Lippmann, 1973; Arvidson and Mackenzie, 1999).
Implications
The data presented here provide the basis for an alternative interpretation for the
relationship between evaporative settings and stoichiometry. Specifically, this dataset
demonstrates that precipitation of evaporites is not required for stoichiometric dolomite to form.
By altering the concentrations of major cations in dolomitizing fluids in this series of
experiments, more stoichiometric dolomite formed, without the need for invoking a high fluid
Mg/Ca associated with gypsum precipitation. These results show that fluid chemistry is an
important parameter effecting the rate and composition of the initial dolomite phase.
Dolomite as a Proxy for Understanding Fluid Chemistry
In previous work, evaporative conditions and recrystallization have been noted as the
primary cause of highly stoichiometric dolomites (Fuchtbauer and Goldschmidt, 1965; Schmidt,
1965; Sass and Katz, 1982; Langbein et al., 1984; Sperber et al., 1984; Kaczmarek and Sibley,
2011). These studies each observed that due to both thermodynamic and kinetic controls, higher
Mg/Ca ratios in the dolomitizing fluid resulted in higher amounts of magnesium in the product
dolomite. Glover and Sippel (1967), Kaczmarek at al. (2016), and Kaczmarek and Thornton
(2017) demonstrated with high-temperature experiments that in addition to a high Mg/Ca ratio in
the dolomitizing fluid, temperature can also directly impact the stoichiometry of evaporative
dolomites. Logan (1987) noted natural fluids with higher Mg/Ca ratios also tend to have
33
increased temperatures and salinity, which can exert additional pressure on the stoichiometry of
the product dolomite (Kaczmarek and Thornton, 2017; this study).
This study is part of an ongoing investigation to assess the controls on dolomite
stoichiometry (Kaczmarek and Sibley, 2011; Kaczmarek and Thornton, 2017; Manche and
Kaczmarek, 2019a). Manche and Kaczmarek (2019a) combined laboratory analyses with field
work by utilizing high resolution XRD data to assess stratigraphic variations in dolomite
stoichiometry in natural dolomites. In conjunction with Kaczmarek and Sibley (2011) and
Kaczmarek and Thornton (2017), this study provides a new way to explain the abundance of
stoichiometric dolomites in evaporitic depositional settings without the need for a high Mg/Ca
ratio or gypsum precipitation. This dataset provides evidence that salinity is an additional
parameter that needs to be considered as it directly affects the rate and composition of dolomite.
More stoichiometric dolomites, therefore, should not be solely attributed to increased Mg/Ca and
temperature alone. Instead, the concentrations of Na, K, Mg, and Ca in the dolomitizing fluid
must be considered as additional controlling factors.
Previous studies have proposed the use of stoichiometry as a useful proxy for conditions
of dolomitization (Lumsden and Chimahusky, 1980; Kaczmarek and Sibley, 2011; Ren and
Jones, 2017; Manche and Kaczmarek, 2019a). Lumsden and Chimahusky (1980) pointed out that
the compositional zonation commonly observed in ancient dolomite crystals suggests that
stoichiometry is controlled in part by the chemistry of the dolomitizing fluids. This coupled with
broad trends observed between dolomite stoichiometry and depth, Lumsden and Chimahusky
(1980) also posited that environmental factors and fluid flow may exert additional control on
stoichiometry of stratigraphically related dolomites. Kaczmarek and Sibley (2011) reasoned that
the as the Mg/Ca of the dolomitizing fluids decreased along a flow path (i.e. as dolomite forms,
34
Ca is released back into the fluids – Equation 1), that a decrease in dolomite abundance and
stoichiometry should result. For example, for hypersaline reflux, they suggested that one might
expect a decrease in stoichiometry down section as the fluids become progressively depleted in
Mg. Utilizing this concept, Ren and Jones (2017) cited observed trends in which dolomite
stoichiometry decreased laterally towards the center of Grand Cayman Island to interpret the
inward migration of seawater as the main control on both the rate of dolomitization and the
composition of the dolomite. They argued that as the seawater dolomitized the rock and
progressively lost Mg ions (i.e. decreased the Mg/Ca ratio in the fluids) as it moved toward the
island center, the dolomite produce was both less abundant and less stoichiometric. Most
recently, Manche and Kaczmarek (2019a) took this idea further and presented a high-resolution
vertical logs documenting patterns in dolomite abundance, dolomite stoichiometry, dolomite
crystal size, and stable oxygen isotopes for the Cretaceous dolomites of the Upper Glen Rose
Formation in central Texas. Although no variability was observed laterally on the scale of 100’s
of meters, transgressive and regressive vertical facies successions were observed to oscillate in
conjunction with dolomite abundance, stoichiometry, dolomite crystal size, and δ18O. These
parameters (except for dolomite crystal size) exhibited increases in transgressive facies
successions (Manche and Kaczmarek, 2019a). The opposite trends were observed in the
regressive facies successions. Manche and Kaczmarek (2019a) used these observations to
propose a model of syndepositional dolomitization in which patterns of dolomitization of
individual facies occur prior to the deposition of overlying sediments. Taken collectively, the
studies by Ren and Jones (2017) and Manche and Kaczmarek (2019a) demonstrate the
usefulness of stoichiometric variations in both the lateral and vertical directions to constrain the
conditions of dolomitization. Salt concentrations vary naturally in marine settings and are an
35
additional parameter that need to be considered. The experiments in this study address the
varying salt concentrations when interpreting patterns in stoichiometry. The new dataset
presented here is consistent with what has been observed in nature demonstrating the use of
stoichiometry as a proxy for conditions of dolomitization (Ren and Jones, 2017; Manche and
Kaczmarek, 2019a). These results also add to a growing body of research (Manche and
Kaczmarek, 2019a; Ryan et al., in review; Laya et al., in review) that suggests evaporite-
associated dolomites, though a common association, may not reflect a causal relationship
between highly evaporative (i.e. gypsum saturation) fluids and dolomitization.
Global Correlations
The findings presented here may have additional global implications. Certain
sedimentary basins, such as the Michigan Basin, which is a restrictive and periodically isolated
basin, were periodically evaporative and hence had higher salinity (Rine et al. 2017a; Rine et al.,
2017b). In a parallel study (Manche and Kaczmarek, 2019b), the global scale relationships
between dolomite stoichiometry and secular variations in seawater chemistry and depositional
mineralogy were examined in more detail. Manche and Kaczmarek (2019b) reported patterns of
dolomite abundance, stoichiometry, and δ18O that oscillated vertically with transgressive and
regressive facies successions. As an example, the data presented here may be useful as a
reference to interpret other times in Earth’s history. One example of this includes understanding
icehouse and greenhouse periods in Earth’s history. The global ocean exhibits trends of calcitic
seas during icehouse periods and aragonitic seas during greenhouse periods (Lowenstein et al.,
2001). Lowenstein et al. (2001) used fluid inclusions to interpret these oscillations in seawater
chemistry because of varied evaporite mineralogy. The aragonitic seas of the greenhouse periods
36
exhibit significantly lower salt concentrations in the seawater (Lowenstein et al., 2001). The
study of these varying seawater conditions can be combined with the additional consideration of
salt concentrations presented here to better understand the conditions of dolomitization in Earth’s
history.
Conclusions
The experimental data presented here are broadly consistent with the findings from
previous high-temperature dolomitization studies. For instance, these new data show that
dolomite replaces aragonite reactants in a stepwise process whereby aragonite is replaced by
protodolomite, which is replaced by poorly-ordered dolomite, which is replaced by well-ordered,
stoichiometric dolomite. The data presented here also show that higher concentrations of the
cations Na, K, Cl, Mg, and Ca, which are common constituents in marine fluids, significantly
increase dolomite stoichiometry. The concentrations of Mg and Ca also significantly increase
the rate of dolomitization, but the data for the Na and K experiments show only minor effects on
reaction rate. No effect on cation ordering was observed in the experiments. The finding that
cation concentrations exert positive pressure on dolomite stoichiometry and reaction rate
provides a new way to explain the common observations of higher dolomite abundances and
more stoichiometric dolomites interpreted to have formed in evaporitic settings. Previous studies
have invoked gypsum precipitation during evaporative conditions to cause higher fluid Mg/Ca
ratios, which in turn increase reaction rates and exert positive pressure on dolomite
stoichiometry. These new data show that the conventional explanation may be more nuanced that
previously accepted. Specifically, that higher Mg/Ca ratios driven by evaporite precipitation may
not be needed for higher stoichiometry; only elevated concentrations of common cations. In
37
addition to temperature, Mg/Ca, reactant mineralogy, organic matter, and sulfate, the
concentrations of Na, K, Cl, Mg, and Ca should be considered as additional variables when
interpreting temporal and spatial patterns in dolomite stoichiometry. The observations presented
here add to our understanding of the fundamental controls on dolomite stoichiometry and
reaction rate, and can help further constrain geological interpretations based on dolomite
stoichiometry, which has recently been shown to be a useful proxy in our effort to elucidate the
origin of ancient dolomites.
38
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