Funifuti, Kita-daito-jima, and Enewetak and made several
deductions concerning dolomite formation. Clearly, dolomite formation was not a function of depth. No relationship between its occurrence and duration of immersion
in seawater could be established. There was no obvious
relationship between the presence of porosity and the
occurrence of dolomite. He stated that “each atoll displays
a unique pattern of distribution of dolomite.” In the borings at Mururoa Atoll, Aissaoui et al. (1986) have demonstrated an extensive thickness of dolomite where the
dolomite attains maximum thickness at the periphery of
the atoll and thins toward the former volcanic edifice
(Bardintzell et al., 1985). Trichet et al. (1984) have
described in great detail the dolomite occurrences and the
hydrological model responsible for the dolomitization
which essentially involves dissolution – precipitation processes (Aissaoui et al., 1986; Aissaoui, 1988). The dolomite
occurrences at Niue have been comprehensively
documented by Aharon et al. (1987) and Wheeler and
Aharon (1997) who report that the dolomite is “generally
limited to two vertically distinct units which are separated
by a 120 m thick interval of undolomitized limestone.”
They proposed that in the upper dolomite layer tidal
pumping through the more permeable layers could have
produced the tabular bodies of dolomite, whereas in the
lower dolomite unit it is more likely to have been produced
from ocean-derived saline groundwater flow and reheating.
In Resolution, Wodejebato and MIT guyots of the
northern and northwestern Pacific Ocean where drilling
was conducted in 1992 (Ocean Drilling Program, Legs
143 and 144) dolomite was recorded at considerable depth
in excess of 2,000 m (Flood, 1998; Flood and Chivas,
1995; Flood et al., 1996). In hole ODP 866A on Resolution Guyot, the dolomite rock preserves the original depositional texture and also exhibits ghosts of the preexisting
carbonate grains. The isotopic composition of the dolomite display oxygen, carbon, and strontium values indicative of precipitation from slightly modified seawaters.
Here two distinct dolostone units occur; one is brown in
color with a sugar-like texture, and the other is white with
a massive crystalline appearance. The former appears to
have formed slightly after deposition of the carbonate
sediments whereas the second pulse of dolomitization
responsible for the massive white dolomite occurred some
100 million years after the depositional age of the
enclosing carbonates.
There are many other reported occurrences of dolomite
in reefs, but they are too many to record herein. However,
some will be mentioned to direct the reader to the extensive literature. They are Mitchell et al. (1987), who have
described modern marine dolomite cement in a north
Jamaican fringing reef; Strasser and Strohmenger, (1997)
who have described dolomite occurrences in the Pleistocene reefs of the Southern Sinai, Egypt; Shinn et al.
(1965) and Vahrenkamp and Swart (1994), who have
reported upon Recent supratidal dolomite from Andros
Island, Bahamas; and Bourrouilh (1973), Chevalier
(1973), and Coudray (1971), who have described dolomite in modern reef environments in New Caledonia.
Summary
Dolomite, CaCO 3 ÁMgCO 3 , is a mineral commonly
found with limestone and carbonate sediments of coral
reefs. Dolomite does not precipitate from normal seawater or solutions at low temperature and atmospheric pressure, yet it is a very common carbonate mineral in
sedimentary rocks throughout the geological record.
This paradox is referred to as the Dolomite problem.
Dolomite is not a product of a CaCO 3 -MgCO 3 solid
solution series but it is a mixed lattice crystal whose
nucleation cannot be explained in terms of classical
(equilibrium) thermodynamics where Ostwald’s step rule
governs the course of irreversible geochemical reactions.
It was as recently as 1999 that Deelman suggested that
Ostwald’s Rule can be broken when fluctuations of sufficient amplitude, frequency, and duration occur and
they involve changes in pH conditions related to variable pCO 2 . Under such conditions a metastable precursor to dolomite (such as magnesium calcite) forms and
through repeated alternations of dissolution and precipitation, change to more stable phases (such as magnesite
or dolomite). He proposes that such fluctuations allow
for both the metastable and stable phases to nucleate
simultaneously and that during such fluctuations, conditions opposing the subsequent growth of the metastable
phase will favor continued growth of the stable phase.
It has recently been suggested by other researchers that
the presence of microbial mediation could overcome
the thermodynamic and kinetic barriers to dolomite
nucleation in the natural environment.
The majority of hydrological settings in which dolomite is known to occur involve special conditions which
are characteristically highly variable and fluctuating and
are dominated by enhancement of fluid flow, usually of
modified seawater through preexisting carbonate sediments and sedimentary rocks, especially coral reefs.
The occurrences of dolomite in Modern coral reefs and
atolls has been known for over 100 years, yet the process
of dolomite nucleation has evaded adequate chemical
explanation until 1999 when Deelman proposed a way
that it could happen. The deep borings of the Pacific
Ocean Atolls and Guyots have provided a window to view
the occurrence of dolomite and the processes of its formation. Of special significance have been the results of scientific investigation at Funafuti Atoll, Enewetak Atoll,
Midway Atoll, Niue, Kita-daito-jima Atoll, Mururoa
Atoll, Fangataufa Atoll, Christmas Island and Resolution
Guyot, Wodejebato Guyot, and MIT Guyot. Equally
important have been those deep borings where dolomite
was not reported. They include the Great Barrier Reef,
Bikini Atoll, Makatea Island, Guam Island, to name just
a few. No discernable pattern of dolomite occurrence
could be established. The overarching dolomitization
DOLOMITIZATION
323
deductions concerning dolomite formation. Clearly, dolomite formation was not a function of depth. No relationship between its occurrence and duration of immersion
in seawater could be established. There was no obvious
relationship between the presence of porosity and the
occurrence of dolomite. He stated that “each atoll displays
a unique pattern of distribution of dolomite.” In the borings at Mururoa Atoll, Aissaoui et al. (1986) have demonstrated an extensive thickness of dolomite where the
dolomite attains maximum thickness at the periphery of
the atoll and thins toward the former volcanic edifice
(Bardintzell et al., 1985). Trichet et al. (1984) have
described in great detail the dolomite occurrences and the
hydrological model responsible for the dolomitization
which essentially involves dissolution – precipitation processes (Aissaoui et al., 1986; Aissaoui, 1988). The dolomite
occurrences at Niue have been comprehensively
documented by Aharon et al. (1987) and Wheeler and
Aharon (1997) who report that the dolomite is “generally
limited to two vertically distinct units which are separated
by a 120 m thick interval of undolomitized limestone.”
They proposed that in the upper dolomite layer tidal
pumping through the more permeable layers could have
produced the tabular bodies of dolomite, whereas in the
lower dolomite unit it is more likely to have been produced
from ocean-derived saline groundwater flow and reheating.
In Resolution, Wodejebato and MIT guyots of the
northern and northwestern Pacific Ocean where drilling
was conducted in 1992 (Ocean Drilling Program, Legs
143 and 144) dolomite was recorded at considerable depth
in excess of 2,000 m (Flood, 1998; Flood and Chivas,
1995; Flood et al., 1996). In hole ODP 866A on Resolution Guyot, the dolomite rock preserves the original depositional texture and also exhibits ghosts of the preexisting
carbonate grains. The isotopic composition of the dolomite display oxygen, carbon, and strontium values indicative of precipitation from slightly modified seawaters.
Here two distinct dolostone units occur; one is brown in
color with a sugar-like texture, and the other is white with
a massive crystalline appearance. The former appears to
have formed slightly after deposition of the carbonate
sediments whereas the second pulse of dolomitization
responsible for the massive white dolomite occurred some
100 million years after the depositional age of the
enclosing carbonates.
There are many other reported occurrences of dolomite
in reefs, but they are too many to record herein. However,
some will be mentioned to direct the reader to the extensive literature. They are Mitchell et al. (1987), who have
described modern marine dolomite cement in a north
Jamaican fringing reef; Strasser and Strohmenger, (1997)
who have described dolomite occurrences in the Pleistocene reefs of the Southern Sinai, Egypt; Shinn et al.
(1965) and Vahrenkamp and Swart (1994), who have
reported upon Recent supratidal dolomite from Andros
Island, Bahamas; and Bourrouilh (1973), Chevalier
(1973), and Coudray (1971), who have described dolomite in modern reef environments in New Caledonia.
Summary
Dolomite, CaCO 3 ÁMgCO 3 , is a mineral commonly
found with limestone and carbonate sediments of coral
reefs. Dolomite does not precipitate from normal seawater or solutions at low temperature and atmospheric pressure, yet it is a very common carbonate mineral in
sedimentary rocks throughout the geological record.
This paradox is referred to as the Dolomite problem.
Dolomite is not a product of a CaCO 3 -MgCO 3 solid
solution series but it is a mixed lattice crystal whose
nucleation cannot be explained in terms of classical
(equilibrium) thermodynamics where Ostwald’s step rule
governs the course of irreversible geochemical reactions.
It was as recently as 1999 that Deelman suggested that
Ostwald’s Rule can be broken when fluctuations of sufficient amplitude, frequency, and duration occur and
they involve changes in pH conditions related to variable pCO 2 . Under such conditions a metastable precursor to dolomite (such as magnesium calcite) forms and
through repeated alternations of dissolution and precipitation, change to more stable phases (such as magnesite
or dolomite). He proposes that such fluctuations allow
for both the metastable and stable phases to nucleate
simultaneously and that during such fluctuations, conditions opposing the subsequent growth of the metastable
phase will favor continued growth of the stable phase.
It has recently been suggested by other researchers that
the presence of microbial mediation could overcome
the thermodynamic and kinetic barriers to dolomite
nucleation in the natural environment.
The majority of hydrological settings in which dolomite is known to occur involve special conditions which
are characteristically highly variable and fluctuating and
are dominated by enhancement of fluid flow, usually of
modified seawater through preexisting carbonate sediments and sedimentary rocks, especially coral reefs.
The occurrences of dolomite in Modern coral reefs and
atolls has been known for over 100 years, yet the process
of dolomite nucleation has evaded adequate chemical
explanation until 1999 when Deelman proposed a way
that it could happen. The deep borings of the Pacific
Ocean Atolls and Guyots have provided a window to view
the occurrence of dolomite and the processes of its formation. Of special significance have been the results of scientific investigation at Funafuti Atoll, Enewetak Atoll,
Midway Atoll, Niue, Kita-daito-jima Atoll, Mururoa
Atoll, Fangataufa Atoll, Christmas Island and Resolution
Guyot, Wodejebato Guyot, and MIT Guyot. Equally
important have been those deep borings where dolomite
was not reported. They include the Great Barrier Reef,
Bikini Atoll, Makatea Island, Guam Island, to name just
a few. No discernable pattern of dolomite occurrence
could be established. The overarching dolomitization
DOLOMITIZATION
323