Dolomitization (AAPG Course Notes 24)

Dolomitization

Presented at the 1982 AAPG Fall Education Conference in Denver, Colorado.

Education Course Note Series #24 Lynton S. Land University of Texas at Austin

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Dolomite It is probably safe to state that in 1982 no single model of dolomitization unequivocally accounts for all aspects of any massively dolomitized ancient limestone. All models have significant flaws, and our understanding of the dolomitization process and its relation to other diagenetic processes (silicification, stylolitization, organic maturation, etc.) is imperfect. Rather than advocate one solution over another, I will try to summarize some of the strengths and weaknesses of several of the models which have been proposed. As a starting point I will review several important aspects of dolomite mineralogy and chemistry that place constraints on all models and that are sometimes overlooked. Mineralogy Dolomite is a rhombohedral carbonate with the ideal formula CaMg[C03)2 in which calcium and magnesium occupy preferred sites. In the ideal mineral, planes of C03 anions alternate with planes of cations with the c-axis of the crystal perpendicular to the alternating stacked anion and cation planes. Ordering occurs by the additional alternation of cation planes containing only calcium with cation planes containing only magnesium (Fig. 1). It is possible to conceive of a mineral having the same composition as ideal dolomite ((Cao.6Mg0 6)C03) in which all cation planes are alike, containing equal numbers of calcium and magnesium ions. Such a mineral is not dolomite. Such a disordered arrangement of ions occupies more volume than that of the ideal dolomite structure and is unstable with respect to an ordered phase. Perhaps surprisingly, the two compounds just described, ideal dolomite and a disordered 1-to-l ratio Ca-Mg carbonate, are both rare in sedimentary rocks. Ideal dolomite rarely comprises ancient dolomitic sediments and never modern sediments, and the completely disordered polymorph does not occur at all. The dolomite which does occur in sedimentary rocks is commonly Ca-rich, having compositions which range from about Ca(Cao.16Mgog4)(C03)2 to ideality, and/or exhibits weak, diffuse, X-ray diffraction, suggesting considerably less structural order than its composition should dictate. With respect to ideal dolomite, all such naturally occurring dolomite is metastable, and the capacity exists for reactions to occur toward a more stable (more stoichiometric or better ordered) phase. The term protodolomite was defined by Graf and Goldsmith (1956) as "single-phase rhombohedral carbonates which deviate from the composition of the dolomite that is stable in a given environment, or are imperfectly ordered, or both, but which would transform to dolomite if equilibrium were established." Gaines (1977) modified the definition to include only ordered phases. I recommended (1980) that the term be dropped altogether, since almost all sedimentary dolomite is really protodolomite by Gaines' definition. What is important is not what we call these natural materials, but what they really are. 1

o

J^ CARBONATE

MAGNESIUM

CALCIUM

Figure 1 — Schematic representation of the crystal structure of dolomite showing the alternation of cation and anion (carbonate) planes, and the alternation of calcium and magnesium planes. 2

Hydrothermal experiments (Graf and Goldsmith, 1956; Goldsmith and Heard, 1961), extrapolated to low temperature, demonstrate that calcite and dolomite are essentially ideal in composition at 25 °C (Fig. 2). In other words, any double carbonate crystal of Ca and Mg at 25 °C which is not essentially pure dolomite is either metastable or unstable with respect to a mixture of pure calcite plus pure dolomite. The same thing is true with respect to ideal dolomite plus magnesite. The composition of phases which we observe at Earth's surface define the range of metastability. Unstable phases are only observed as transient states in the laboratory. In the case of dolomite, few phases containing more than about 8% excess calcium (on a molar basis) have been reported to date, although the data are admittedly sparce. Reeder (1981) has shown that the structure of various kinds of dolomite revealed by transmission electron microscopy and electron diffraction can be classified into at least three types. All structures are ordered, although the degree of order is variable and difficult to quantify. The first, characteristic only of Holocene dolomite, consists of irregular "mosaics" on a scale of tens or hundreds of Angstroms. The crystals are characterized by extremely high densities of crystallographic faults and dislocations, and can be thought of as an aggregate of "micro-crystals" whose compositions may vary, forming a very discontinuous lattice. This leads to many unsatisfied or strained chemical bonds and to X-ray diffraction patterns with broad, generally weak reflections. This kind of dolomite is also characterized by large trace element substitutions, especially strontium (Behrens and Land, 1972), and sodium (Land and Hoops, 1973). Qualitative data suggest that this material is extremely soluble compared to better ordered forms of dolomite. My attempts to beneficiate samples composed of mixtures of this kind of dolomite and aragonite (for example, supratidal crusts from Florida and the Bahamas) by slow leaching in acetic acid resulted in only slight concentration of the dolomite by selective solution of aragonite. C0 2 for isotopic analyses of Holocene dolomite is evolved much faster than from finely ground ancient dolomite. All evidence suggests that Holocene dolomite is a unique, highly soluble material. It is clearly a metastable phase, unknown (in an unmodified form) in ancient rocks. The second and most common kind of sedimentary dolomite exhibits a lamellar or "tweed" structure when examined by transmission electron microscopy and electron diffraction, which Reeder (1981) has interpreted as a structural and/or compositional modulation on a scale of several hundred Angstroms (Fig. 3). At present this kind of dolomite is thought to consist of two intimately intergrown lamellar domains parallel to the rhomb face with slightly different structures and/or compositions. The texture resembles spinoidal decomposition, or solid state unmixing on a scale of a few hundred angstroms from a single homogeneous precursor. The exact structure and composition of the two domains or lamellae is not known, although one must be more stable (and presumably more magnesium rich) than the other. This type of dolomite is clearly metastable, but continued stabilization cannot proceed spontaneously because it is limited by solid state diffusion. Continued stabilization can occur as a result of solution-reprecipitation processes however, and it has been demonstrated that bulk Ca-rich dolomites dissolve more rapidly than ideal dolomite (Busenberg and Plummer, 1982). Continued stabilization toward a more stoichiometric dolomite would presumably be promoted if pore fluids in the rock changed to enable dissolving out of the less stable, Ca-rich domain. Porosity could easily increase under these conditions.

3

Ordered Dolomite

1000 Dolomite + Magnesite

800

600

-

TEMP. (°C)

Lower limit of experimental data

400 -

Ranges of metastable phases observed in nature (

10

20

30

40

50

MOLE % MgC03

Figure 2 — Stability relations in the system CaC03 • MgC03

60

Figure 3 — Dark field transmission electron micrograph of a calcian dolomite (Caj i2Mg0 88(C02)2) of Eocene age. The prominent modulated structure is typical of sedimentary dolomite, and such crystals are metastable with respect to ideal stoichiometric dolomite. Photograph by Richard Reeder. A third kind of dolomite is nearly ideal in composition, and when examined by transmission electron microscopy and electron diffraction is observed to be homogeneous, consisting of large single domains. This kind of dolomite is presently known mostly from ancient, deeply buried sequences and from metamorphic rocks. The philosophy that, like limestone, the diagenesis of dolomite is dominated by the stabilization of metastable dolomitic phases, is relatively new. There is no question that calcium-rich dolomite has the capacity to react to form crystals with a more stoichiometric composition, but many important questions remain. What kinds of diagenetic environments promote the reaction? Does stabilization to ideal dolomite take place all at once or in several stages? How far from ideality must a phase be before it is prevented from further reaction for kinetic reasons? Many of these questions must be answered both by laboratory work and by careful mineralogical analysis of particular dolomites under investigation before thinking can advance much further. 5

Aqueous solution equilibria Several lines of evidence have been used to determine the solubility of dolomite at sedimentary and early burial conditions. The data are complicated by the mineralogical variations in dolomite already discussed. All metastable phases must be more soluble than ideal dolomite, and variations in the degree of metastability can obviously occur. Data have been derived from two sources, (1) high temperature experiments and (2) natural dolomite aquifers. Of interest is the equilibrium constant, K, for reactions between the ideal solids, 2CaC0 3 + Mg + + ^

CaMg(Co3)2 + Ca ++

K = (Ca ++ )/(Mg ++ )

or the calcium-to-magnesium activity ratio of a solution at equilibrium with calcite + dolomite (as a function of temperature). Solutions more magnesium-rich than the equilibrium solution should cause dolomitization of calcite, while solutions more calcium-rich should cause dedolomitization. Dolomite is easily synthesized hydrothermally at about 300 °C, with reaction times of only a few days. Progressively slower reaction is observed at lower temperatures and below about 100°C very long experiments are required. Nobody has yet synthesized dolomite at Earth-surface conditions (although a Dalmatian has!, Mansfield, 1980). Experimental data are in reasonable agreement around 300°C, and the molar Ca/Mg ratio of a solution in equilibrium with calcite and dolomite is about 15. In other words, as temperature increases, dolomite becomes increasingly less soluble than calcite. Any solution with a molar Ca/Mg ratio of less than 15 is capable of dolomitizing at 300 °C (Fig. 4)! At lower temperatures, experimental data become more conflicting, the reason being, I suspect, that metastable Ca-rich phases are much more easily formed. Kinetic experiments (Land, 1967) have shown that the formation of a Ca-rich (metastable) dolomite rather than the ideal phase is favored (within the stability field of dolomite) by (1) higher Ca/Mg ratio of the solution, (2) lower solution concentration, and (3) lower temperature. Metastable Ca-rich phases are more soluble and therefore will coexist with more magnesium-rich fluids (Helgeson et al, 1978). Hsu (1963), Holland et al, (1964), Barnes and Back (1964) and Langmuir (1971) all studied the Ca/Mg ratio of natural dolomite aquifers, reasoning that equilibrium with dolomite would eventually be reached as water recharged a dolomite aquifer and moved downdip at rates typical for groundwater flow. Langmuir's compilation is plotted on Figure 4. The extrapolation of Rosenburg and Holland's (1964) data to intercept Langmuir's low temperature data is not too unreasonable if one accepts that the lower temperature hydrothermal data points of Rosenburg and Holland may be displaced toward magnesium-rich compositions because of formation of a non-ideal (more soluble) phase. The reasonable agreement between low temperature and high temperature data ignores non-ideal solution behavior, which is significant in the saline solutions Rosenburg and Holland used. But at 300°C, experiments at 2M, 1M and 0.5M solutions all yield similar results, suggesting the effects are not large. Further support for extrapolation between the two types of data was 6

Temp. (°C) 3.5

— 10

Langmuir, 1971

-

— 25 CALCITE

— 50 3.0

-

DOLOMITE — 100 2CaC03 + Mg++ 2.5

CaMg(C03)2 + Ca++

^

-

— 150

1000 T(°K) t 2.0

\

— 200

Rosenberg and Holland, 1964 — 250

-

Gaines (pers. comm.) — 300 Land. Rosenberg, Burt and Holland

1.5 -0.2

0.0

0.2

0.4

0.6

0.8 I 5

1.0 I 10

1.2

1.4 logCa/Mg I 25 Ca/Mg

Figure 4 — Aqueous solution compositions presumed to be in equilibrium with calcite plus dolomite.

obtained by Pakhomov and Kisson (1973) (reproduced in Carpenter, 1980) who plotted the Ca/Mg ratio of saline formation water from the Russian platform versus temperature. Despite the fact that they totally ignored rock composition (calcite plus dolomite may not both have been present to control the solution composition), and obtained considerable scatter, their regression line essentially connects Rosenburg and Holland's and Langmuir's data! Until further experimental work is conducted (which must include characterization of the dolomite phase) the data presented in Figure 4 are all that are available. They are consistent both with a gross oversaturation of seawater with dolomite, and the Mg-depleted nature of most saline formation water. The reason for the gross oversaturation of seawater with respect to dolomite ultimately lies in the kinetic problem of nucleating and growing the ordered crystal (Goldsmith, 1953). The molar Ca/Mg ratio of seawater (0.19) is apparently incapable of causing dolomitization at observable rates. By either decreasing the molar Ca/Mg ratio of seawater (say by gypsum precipitation) or decreasing the activity Ca/Mg ratio the kinetic constraints can be overcome, at least to the point of being able to nucleate and grow a poorly crystalline Ca-rich phase. The activity Ca/Mg ratio of seawater is 0.18 (Berner, 1971), and can be decreased by dehydrating the Mg+ + ion (Usdowski, 1968) or by removing components which form strong ion pairs with Mg+ + (for example, S0 4 = , Baker and Kastner, 1981). These factors do not alter the equilibrium relations (Fig. 4) and only provide the kinetic "push" to form the initial phase. The early-formed phase can then stabilize by further reaction. Another variable in the dolomitization process which needs additional confirmation is the role of organic material, particularly dissolved organic acids. Dissolved organic acids are known to control the kind of calcium carbonate which precipitates from solution. Increased organic acid content favors Mg-calcite over aragonite precipitation (Kitano and Kanamori, 1966). Although algal processes have been invoked as being able to cause dolomitization (Gebelein, 1973), the "organic gremlin" is neither proven nor disproven. Stable Isotopic Geochemistry Most current evidence supports the contention that sedimentary dolomite is enriched in 180 about 3 to 4 ppt with respect to a co-existing calcite in the range of sedimentary and burial diagenetic temperatures of normal interest (Land, 1980). Little evidence exists for dolomite replacement of calcite without change of isotopic composition (Katz and Matthews, 1977). The fact that many ancient dolomites are significantly depleted in 180 is best explained by stabilization of an earlier-formed phase during burial (Fig. 5). The isotopic composition of the dolomite comprising sedimentary rocks is controlled both by the chemistry of the latest recrystallization (stabilization) event and by the chemistry of the precursor (aragonite, Mg-calcite, calcite and/or dolomite). Dolomite rarely recrystallizes homogeneously in an open aqueous chemical system, accurately recording the conditions of recrystallization, just as it almost never accurately retains the chemistry of the precursor. Recrystallization may be incomplete, leaving an inhomogeneous rock, and the composition of the replaced phase may "contaminate" the replacing phase (Land, 1980). The practical problem of analyzing intimate mixtures of dolomite of slightly different compositions is not yet solved. 8

200

160-

Temp. (°C)

12 o

80

40-