Geochimica et Cosmochimica Acta, Vol. 65, No. 14, pp. 2293–2300, 2001
Copyright © 2001 Elsevier Science Ltd
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0016-7037/01 $20.00 ⫹ .00
Pergamon
PII S0016-7037(00)00591-9
Evaluating seawater chemistry from fluid inclusions in halite: Examples from modern
marine and nonmarine environments
M. N. TIMOFEEFF,1 T. K. LOWENSTEIN,1,* S. T. BRENNAN,1 R. V. DEMICCO,1 H. ZIMMERMANN,2 J. HORITA,3 and L. E.
VON
BORSTEL4
1
Department of Geological Sciences and Environmental Studies, State University of New York at Binghamton, Binghamton, NY 13902, USA
2
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 03138, USA
3
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6110, Oak Ridge, TN 37831, USA
4
IGDL, Universitaet Goettingen, Goldschmidtstrasse 3, D-37077 Goettingen, Germany
(Received June 30, 2000; accepted in revised form January 29, 2001)
Abstract—Fluid inclusions from marine halites have long been studied to determine the chemical composition of ancient seawater. Chemical analyses of the major ions in fluid inclusions in halites from the solar
saltwork of Great Inagua Island, Bahamas, and from the supratidal sabkha, Baja California, Mexico, show that
modern marine halites faithfully record the chemical signature of seawater. The major ions in Great Inagua
and Baja California fluid inclusions display distinctive linear trends when plotted against one another (ie.,
Na⫹, K⫹, and SO2⫺
vs. Mg2⫹ and Cl⫺), which track the evaporation path of seawater as it evolved during
4
the crystallization of halite. These evaporation paths defined for the major ions by fluid inclusions in halite
overlap findings of computer simulations of the evaporation of modern seawater by the Harvie, Møller, and
Weare (HMW) computer program. The close match between the HMW seawater evaporation paths and the
Great Inagua fluid inclusion data is not surprising considering the carefully controlled inflow, evaporation, and
discharge of seawater at the Great Inagua saltwork. The major ion chemistry of fluid inclusions from the Baja
California halites matches the HMW seawater evaporation paths in most respects, but one Baja fluid inclusion
has lower concentrations of Mg2⫹ than evaporated seawater. Nonmarine inflows and syndepositional recycling of preexisting salts in the Baja California supratidal setting were not large enough to override the
chemical signature of evaporating seawater as the primary control on the Baja fluid inclusion compositions.
Fluid inclusions in halites from the nonmarine Qaidam Basin, Qinghai Province, western China, have a
distinctly different major ion chemical signature than does “global” seawater. The fluid inclusion chemistries
from the Qaidam Basin halites do not lie on the evaporation pathways defined by modern seawater and can
clearly be differentiated from fluid inclusions containing evaporated seawater. If fluid inclusions in halites
from modern natural settings contain unmistakable samples of evaporated seawater, then evaluation of the
chemistry of ancient seawater by chemical analysis of fluid inclusions in ancient marine halites by means of
the same approach should be valid. Copyright © 2001 Elsevier Science Ltd
existing salts, and then mixes with “fresh” seawater or begins
to evaporate by itself. The dissolution preferentially increases
the proportion of those ions of the more soluble saline minerals
(e.g., Na⫹ and Cl⫺ from halite; K⫹, Mg2⫹, and Cl⫺ from
carnallite; etc.) in the evaporating waters. Evaporative concentration of this altered seawater will produce halite-carrying
fluid inclusions with a chemical composition different from that
of brines evolved from the unaltered parent seawater. Clearly,
halite deposits that formed from nonmarine inflow waters or
formed in paleoenvironments in which significant recycling
occurred cannot be used for study of ancient seawater chemistry. The problem of the marine vs. nonmarine origins of evaporite deposits and the recognition of syndepositional recycling
in ancient evaporites has been dealt with elsewhere (Holser,
1979a,b; Hardie, 1984; Hardie et al., 1985; Lowenstein et al.,
1989; Smoot and Lowenstein, 1991; Attia et al., 1995), and the
reader is referred to these works. However, it is important to
note that few studies have shown that modern marine halites
contain fluid inclusions that track the evaporation chemistry of
evolving seawater brines.
The second problem with the use of fluid inclusions in halite
to evaluate paleoseawater composition is that fluid inclusions in
halite contain brines saturated with respect to halite that are not
simply the parent seawaters concentrated into brines by evaporation. This is due to the complex compositional changes in
1. INTRODUCTION
Fluid inclusions from halite deposits are direct samples of
ancient surface brines. Fluid inclusions from ancient halite
deposits interpreted to be marine in origin have long been
studied to determine the chemical composition of paleoseawater (cf. Holser, 1963; Kramer, 1965; Petrichenko, 1973; Lazar
and Holland, 1988). There are a number of analytical techniques currently available that provide chemical analyses of
fluid inclusions in halite with the accuracy necessary to identify
the composition of the parent water. However, no matter which
analytical technique is used, researchers analyzing fluid inclusions for ancient seawater face two major problems.
The first problem of the use of fluid inclusions in halite to
evaluate paleoseawater composition is the possibility that the
brines trapped as inclusions were not derived from the evaporation of uncontaminated seawater. In this regard, the chemistry
of evaporating seawater changes if there is inflow of nonmarine
water or if “syndepositional recycling” has altered the chemistry of the brines from which the halite precipitated. Syndepositional recycling occurs when seawater or meteoric water
inundates a shallow marine evaporite basin, dissolves the pre*Author to whom correspondence
([email protected]).
should
be
addressed
2293
2294
M. N. Timofeeff et al.
the brine that take place during the successive precipitation of
saline minerals, such as calcite, gypsum, and halite, during
brine evolution (Harvie et al., 1980; Hardie, 1984; Ayora et al.,
1994a,b). Thus, individual ion compositions cannot be extrapolated directly back from the concentrated brine analyses to
recover the composition of the low-salinity parent waters. Instead, to assess the chemistry of ancient seawater, it is necessary to use forward-modeling techniques based on thermodynamic brine evolution models to back-calculate the major ions
in seawater from the chemical composition of brines in fluid
inclusions that have undergone evaporative concentration.
It follows from the previous discussion that reconstructing
the chemistry of ancient seawater from fluid inclusions in halite
requires three steps. First, an accurate technique for the chemical analysis of individual fluid inclusions is required. Second,
criteria for establishing that the fluid inclusions contain evaporated seawater, uncontaminated by nonmarine inflow and syndepositional recycling, are required. Third, methods for backcalculating seawater composition from the chemical
composition of brines in fluid inclusions that have undergone
evaporative concentration are required. The purposes of this
article are to present details of the methods we use for steps 1
and 3, which are necessary to determine the major ion composition of ancient seawater from the chemical analysis of fluid
inclusions in halite; and to document that fluid inclusions in
modern marine halites accurately reflect the chemical compositions of evolving seawater brines. We use halites from two
modern marine settings, Baja California, Mexico, and Great
Inagua Island, Bahamas, to illustrate that fluid inclusions from
marine halites do indeed record the major ion composition of
evaporated modern seawater. We also include analysis of a
nonmarine setting, the Qaidam Basin, western China, for comparison.
The ultimate goal of this research is to identify the composition of ancient seawater from ancient halite deposits. If fluid
inclusions in halites from modern natural settings contain unmistakable samples of evaporated seawater (which we document here), then evaluation of the chemistry of ancient seawater by chemical analysis of fluid inclusions in ancient marine
halites by means of the same approach should be valid.
2. METHODS
2.1. Analytical Techniques
Three analytical techniques are capable of providing chemical analyses of fluid inclusions in halites with the accuracy necessary to
identify the composition of the parent seawater.
2.1.1. Cryo-SEM/ESEM-EDS
The first of these techniques uses either a scanning electron microscope (SEM) or an environmental SEM (ESEM) with X-ray energy
dispersive analysis (EDS) capabilities (Ayora and Fontarnau, 1990;
Ayora et al., 1994a,b; Garcia-Veigas et al., 1995; Fanlo and Ayora,
1998; Timofeeff et al., 2000). The Cryo-SEM and ESEM X-ray EDS
technique measures absolute ionic concentration for major cations and
anions in frozen inclusions with concentrations greater than ⬃0.1 wt%.
This technique is capable of producing accurate major element chemical analyses of individual frozen fluid inclusions in halite greater than
⬃30 ␮m in size (Ayora et al., 1994a,b; Timofeeff et al., 2000).
2.1.2. Direct Extraction
The second technique for fluid inclusion analysis is extraction of
fluid from selected inclusions followed by ion chromatography (IC),
inductively coupled plasma mass spectrometry (ICP-MS), or microtitration techniques (Lazar and Holland, 1988; Das et al., 1990; Horita et
al., 1991, 1996; Land et al., 1995; Kovalevich et al., 1998; von Borstel
et al., 2000). The extraction technique requires drilling a small hole into
a halite crystal and is limited to large inclusions more than ⬃200 ␮m
in size. The inclusion brine is extracted with a microsyringe. The
volume of extracted brine is measured. The major ions can be measured
with the microtitration technique of Petrichenko (1973), or both the
major and minor ions can be analyzed via IC or ICP-MS.
2.1.3. Laser Ablation–ICP–MS
The final technique for fluid inclusion analysis is laser ablation–ICP–
MS. This technique uses a laser beam to mine into and vaporize fluid
inclusions as small as ⬃20 ␮m, allowing analysis for major and trace
elements via ICP-MS (Shepherd and Chenery, 1995; Shepherd et al.,
1995, 1998; Moissette et al., 1996). No fluid inclusion volume determination is possible with the laser ablation ICP-MS technique, so it is
not possible to measure absolute ionic concentrations.
2.2. Computer Modeling
Computer models have become essential for the quantitative treatment of evaporative concentration of natural waters and the accompanying mineral precipitation and dissolution. We use the ion interaction
model and parameters of Harvie, Møller, and Weare (1984) (HMW) for
several aspects of this study.
First, the HMW computer model is used to model the evaporation of
seawater (Harvie et al., 1980). The HMW computer program calculates
the concentrations, in molalities, of all the major ions at each evaporation step, along with the number of moles of salts precipitated during
each evaporation step. Such simulated evaporation paths have been
shown to closely match the chemical compositions of natural brines
and fluid inclusion waters, as well as the mineralogical sequences in
bedded evaporites (Harvie et al., 1980, 1984; Davis et al., 1990; Casas
et al., 1992). Evaporation of seawater for this work was done at 25°C,
assuming equilibrium conditions for precipitation of all the major salts
(Harvie et al., 1980). Evaporation of seawater at different (higher)
temperatures does not significantly change the 25°C evaporation paths
for the following reasons: (1) for the interval of the seawater evaporation path under consideration (carbonate– gypsum/anhydrite– halite),
no new minerals are introduced at higher temperatures; (2) the relatively low solubilities of gypsum and anhydrite and changes in their
solubilities with temperature are not significant enough to influence the
seawater evaporation paths; and (3) halite solubility does not change
enough with temperature (⬍1% in system NaCl-H2O between 25°C
and 40°C; Linke, 1965) to affect the seawater evaporation paths. To
more realistically simulate conditions at Great Inagua Island, Bahamas,
where brines are continually pumped through crystallizer ponds (see
below), the HMW evaporation program was run without allowing
back-reaction of evolved brines with some precipitated minerals—for
example, reaction of brine with gypsum or anhydrite to produce glauberite (CaSO4 䡠 Na2SO4).
Second, the HMW computer program was used to calculate the Na⫹
and Cl⫺ concentrations in fluid inclusions measured by two of the
techniques described above. Fluid inclusions less than ⬃150 ␮m analyzed by the ESEM X-ray EDS technique produce spectra for Na⫹ and
Cl⫺ that are anomalously high. This is because scattered electrons from
the primary beam excite the host halite crystal. In these cases, Na⫹ and
Cl⫺ concentrations cannot be determined from the ESEM-EDS data but
must be computed for halite-saturated conditions at the specified mo2⫹
lalities of SO2⫺
, Ca2⫹, and K⫹ by the HMW computer program
4 , Mg
(Timofeeff et al., 2000). Na⫹ and Cl⫺ concentrations from fluid inclusions in halite from Great Inagua, analyzed via extraction-IC by von
Borstel et al. (2000), and from the Qaidam Basin, analyzed by extraction-IC, were also adjusted by the HMW computer program.
Evaluating seawater chemistry from halite
2295
Table 1. Extraction ion chromatographic analysis of 16 fluid inclusions from halite samples from Great Inagua Island, Bahamas (von
Borstel et al., 2000). Na and Cl values were adjusted for halite saturation by the HMW computer model (see text). All measurements are
reported as mmol/kg H2O.
Sample
Na
K
Mg
SO4
Cl
F2_FI_1
F2_FI_2
F2_FI_3
F2_FI_4
F2_FI_5
F2_FI_6
F2_FI_7
F2_FI_8
4952
5058
4713
4850
4643
4947
4909
4948
155
148
198
166
209
159
159
159
761
692
905
818
946
757
772
756
319
293
349
319
359
333
305
316
5992
6005
6024
6014
6025
5954
6003
5986
G2_FI_1
G2_FI_2
G2_FI_3
G2_FI_4
G2_FI_5
G2_FI_6
G2_FI_7
G2_FI_8
3687
4224
3899
4089
3387
3910
4069
3800
366
247
308
282
360
306
287
305
1573
1100
1436
1348
1808
1486
1384
1519
535
423
552
500
647
624
573
590
6129
5824
5975
6069
6070
5941
5976
5962
2.3. Description of Sample Sites for Evaluation of
Modern Halite Fluid Inclusions
2.3.1. Great Inagua Island, Bahamas
von Borstel et al. (2000) analyzed fluid inclusions from halite samples from the solar saltwork of Great Inagua Island, Bahamas. The
Morton solar saltwork on Great Inagua Island pumps seawater through
a series of 61 halite crystallizer ponds, up to 45 cm in depth, in which
the seawater evaporates and eventually precipitates halite (McCaffrey
et al., 1987). Halite precipitated in the crystallizer ponds reaches a
thickness of up to 20 cm (von Borstel et al., 2000). The brines that flow
through the series of crystallizer ponds are discharged into the ocean
when they reach a density of 1.248 g/cm3, before they become concentrated enough to form any late-stage potash minerals. The movement of brines through the crystallizer ponds also prevents any syndepostional “recycling” or any back-reaction of evolved seawater brines
with earlier-formed minerals—for example, reaction of evolved seawater with gypsum to form glauberite or polyhalite.
The carefully controlled flow of water and precipitation of halite at
the Morton solar saltwork is clearly an ideal setting for the study of
seawater evaporation and the fluid inclusions of evolved seawater
trapped in halite crystals (McCaffrey et al., 1987; von Borstel et al.,
2000). von Borstel et al. (2000) analyzed 23 fluid inclusions greater
than 200 ␮m in size by use of the extraction-IC and extraction-ICP-MS
techniques from three samples of “hopper” halite collected at the
bottom of halite crystallizer ponds (Table 1). They found that fluid
inclusions from the cores of hopper crystals had the same chemical
composition as the evaporated seawater brines from which the crystals
formed. The hopper crystals tend to not have the well-developed
protective coating of clear halite that exists on chevron crystals. Fluid
inclusions from the outer parts of halite hopper crystals from one
sample had “network” inclusions that apparently evaporated after sampling because the inclusion brine chemistries were highly variable and
more concentrated than the brine from which they crystallized (von
Borstel et al., 2000). The seven chemical analyses of the probable
evaporated fluid inclusions are not reported here.
The precision of the Great Inagua Island fluid inclusion data shown
in Table 1, in relative standard deviation (%), is as follows: Na⫹
(4.7–9.5%), K⫹ (12.4 –12.8%), Mg2⫹ (10.5–13.4%), Cl⫺ (4.1– 4.3%),
and SO2⫺
(6.6 –12.3%) (von Borstel et al., 2000). The accuracy
4
[(mean ⫺ expected)/expected] ⫻ 100 is as follows: Na⫹ (0.7–3.2%),
⫹
K (2.3–3.5%), Mg2⫹ (14.3–17.4%), Cl⫺ (5.2–7.1%), and SO2⫺
4 (1.5–
5.5%) (von Borstel et al., 2000).
Fig. 1. Photograph of a thin section of modern halite from Salina
Omotepec, Baja California, Mexico, showing alternating layers of
cumulates of halite (sunken rafts, plates, hoppers cubes) and coarse,
vertically oriented chevron crystals that have trapped parallel bands of
tiny fluid inclusions (gray) during crystal growth. Black areas are voids.
Horizontal field of view is 5 cm.
2.3.2. Salina Omotepec, Baja California, Mexico
Halite from the marginal marine saline pan of Salina Omotepec, Baja
California, Mexico, was collected in 1980. Salina Omotepec, a saline
pan on a shallow depression of the supratidal flat, is located on the
eastern side of Baja California, in the western Colorado River delta.
Unlike the controlled evaporation ponds of Great Inagua, this salina
relies on storm flooding to deliver seawater and has the potential for
both nonmarine inflow and syndepositional recycling (Martini and
Walter, 1993). Halite, up to ⬃25 cm in thickness, formed deposits over
an area of several square kilometers in 1970 and 1980 (Shearman,
1970; Lowenstein and Hardie, 1985). The supratidal flats at Salina
Omotepec are flooded by seawater during spring tides and onshore
storms as well as by meteoric waters, derived from inland storms
(Shearman, 1970; Lowenstein and Hardie, 1985). Flooding by seawater
and freshwater both produce ephemeral water bodies that partially to
completely dissolve the more soluble surficial evaporites and recycles
the dissolved salts back into surface waters. The halite samples collected from Salina Omotepec in 1980 formed after a seawater flooding
event just before sampling; they consist of 8 cm of layered halite
cumulates formed at the brine surface and halite chevrons, crystallized
at the brine bottom (Fig. 1). The surface halite layer used in this study
was pristine and showed no evidence of dissolution such as the voids
and clear diagenetic cements common in shallow subsurface halite
crusts from Salina Omotepec (Lowenstein and Hardie, 1985). Fluid
inclusions analyzed came from chevron halite crystals with alternating
bands rich (cloudy) and poor (clear) in fluid inclusions. Clear growth
zones form a protective armor around the numerous fluid inclusions of
the cloudy zones. This protective armor reduces the possibility that the
fluid inclusions will leak or evaporate. Ten primary fluid inclusions
from chevron halite were analyzed with the cryo-ESEM-EDS technique (Table 2). Accuracies for major elements on fluid inclusions in
laboratory-grown halite [(mean ⫺ expected)/expected] ⫻ 100 were
between 6 and 10%, and precisions (relative standard deviation percentage) were 3% for K⫹, 6% for Ca2⫹, and 16% for Mg2⫹ (Timofeeff
et al., 2000).
2.3.3. Qaidam Basin, Qinghai Province, Western China
The Qaidam Basin, Qinghai Province, western China, is a closed,
nonmarine basin, 120,000 km2 in area and 2800 m above sea level
(Lowenstein et al., 1989, 1994; Casas et al., 1992). The center of the
Qaidam Basin contains dry saline pans and a number of shallow saline
2296
M. N. Timofeeff et al.
Table 2. Cryo-ESEM-EDS analysis of 10 fluid inclusions from
Salina Omotepec, Baja California, Mexico. Calcium was below the
detection limits of 0.1 wt% in all cases. Na and Cl values were adjusted
for halite saturation by the HMW computer model (see text). All
measurements are reported as mmol/kg H2).
Inclusion
Na
K
Mg
SO4
Cl
Jun09_Aba1
Jun09_Aba2
Jun09_Aba3
Jun09_Aba4
Jun09_Aba5
Jun09_Bba1
Jun09_Bba2
Jun09_Bba3
Jun09_Bba4
Jun09_Bba5
5580
5150
5230
5260
5170
4880
4680
4780
4260
3740
150
150
140
130
130
170
160
160
190
220
370
630
570
550
600
770
890
840
1140
1460
250
320
280
260
270
310
320
330
380
450
6010
5980
6000
6010
6010
6020
6020
6000
6020
6040
lakes. The largest area underlain by salt is the 6000 km2 Qarhan Salt
Lake plain (Lowenstein et al., 1989). Dabusun Lake is a ⬃250 km2
perennial saline lake on the Qarhan Salt Plain. Dabusun Lake was less
than 1 m deep in the late 1980s and was composed of Na⫹-Mg2⫹-Cl⫺–
2⫹
rich brines near halite saturation with minor K⫹, SO2⫺
4 , and Ca
(Lowenstein et al., 1989; Casas et al., 1992). Two borehole cores were
drilled in the Qarhan area in 1988 and 1989, near the north shore of
Dabusun Lake. Core 88-01 (total depth of 46.9 m) and core 89-04
(depth of 45.0 m) contain Late Pleistocene bedded halite interlayered
with mud (Lowenstein et al., 1994). One sample of clear halite cement
from core 88-01, taken from a depth of 28.3 m, formed in the shallow
subsurface from groundwater brines, was analyzed by J.H. with the
extraction-IC technique. This halite sample occurs 0.5 m below a
stratigraphic interval with a uranium-series age date of 31,200 ⫾ 1070
yr (Lowenstein et al., 1994). Five large fluid inclusions, 500 ␮m in size,
were analyzed for major ions (Table 3). Estimated total analytical
errors are ⫾4% for major ions (Na⫹, Mg2⫹, Cl⫺), ⫾6% for K⫹, and
⫾10% for the minor elements SO2⫺
and Ca2⫹ (Lazar and Holland,
4
1988; Horita et al., 1991).
3. RESULTS
⫹
The concentrations of Na⫹, Cl⫺, SO2⫺
from the
4 , and K
fluid inclusions from Great Inagua, Bahamas, Baja California,
Mexico, and the Qaidam Basin, western China, are shown in
plots vs. Mg2⫹ (Figs. 2A–D), vs. Cl⫺ (Figs. 2E–G), and vs.
SO2⫺
(Fig. 2H). Also shown in these figures are measured
4
brine compositions produced during the evaporation of seawater at Great Inagua Island (McCaffrey et al., 1987), as well as
curves tracking the compositional paths predicted for evaporative concentration of modern seawater calculated by the HMW
brine evolution computer model. The minerals predicted to
precipitate during the equilibrium evaporation of seawater by
the HMW computer program are also shown on the figures.
Table 3. Five large fluid inclusions from Qaidam Basin, western
China, core 88-01, depth of 28.3 m, analyzed using the extraction-ion
chromatography technique. Na and Cl values were adjusted for halite
saturation by the HMW computer model (see text). All measurements
are reported as mmol/kg H2O.
Sample
Q1
Q2
Q3
Q4
Q5
Na
K
Mg
Ca
SO4
Cl
2804
4731
2938
4857
3186
188
82
167
69
124
1780
663
1770
604
1540
78
76
40
68
87
13
25
76
25
11
6682
6241
6573
6220
6542
3.1. Great Inagua Island, Bahamas
Concentrations of Na⫹ and Cl⫺ for the Great Inagua fluid
inclusions plotted against Mg2⫹ trace out well-defined paths
that closely parallel the evaporation paths of modern seawater
⫹
and Inagua brines (Figs. 2A,B). The molalities of SO2⫺
4 and K
2⫹
vs. Mg in Great Inagua fluid inclusions show more scatter,
but they still follow orderly trends close to the seawater evaporation paths defined by the Great Inagua brines and the HMW
computer program (Figs. 2C,D). The concentration of K⫹ in
Great Inagua fluid inclusions defines an evaporation path
slightly higher than the evaporation path of the Inagua brines
and the HMW computer program (Fig. 2D).
⫹
The plots of Na⫹, SO2⫺
vs. Cl⫺ and K⫹ vs. SO2⫺
4 , and K
4
show that the fluid inclusions from Great Inagua completely
overlap the seawater evaporation trend of the HMW computer
program (Figs. 2E–H). The sharp decrease in Na⫹ molalities
and sharp increases in SO2⫺
and K⫹ reflect precipitation of
4
halite when saturation is reached at ⬃5154 mmol Na⫹ and
6035 mmol Cl⫺. The Great Inagua fluid inclusions from halite
samples fall along the halite precipitation segment of the seawater evaporation curve, as they should. The measured Inagua
brines show uniformly low Cl⫺ concentrations on these plots,
slightly below the halite saturation calculated by the HMW
computer program. Seawater evaporated to halite saturation,
analyzed by means of the HMW computer program, has Cl⫺ of
more than 6000 mmol, whereas 14 out of 18 natural brines
reported by McCaffrey et al. (1987) from the Great Inagua
halite crystallizer ponds had Cl⫺ less than 6000 mmol. The
systematically low Cl⫺ concentrations of McCaffrey et al.
(1987) probably reflect analytical errors.
3.2. Baja California, Mexico
Fluid inclusions in halites from Baja California, collected
from the supratidal flats, generally track the HMW seawater
evaporation paths despite the fact that waters flooding Salina
Omotepec must result in the syndepositional recycling of previously deposited salts. This is because a subaerially exposed
surface crust of halite and gypsum underlies the area where
floodwaters normally become ponded (Shearman, 1970). Unfortunately, no surface brines remained when the salt samples
were collected in 1980, so we cannot compare the chemistry of
the fluid inclusions with the composition of the parent waters
from which the halite crystals precipitated.
Concentrations of Na⫹ and Cl⫺ in the Baja California fluid
inclusions plotted against Mg2⫹ trace out well-defined paths
that parallel the evaporation paths of modern seawater (Figs.
2A,B). One fluid inclusion, with relatively low concentration of
Mg2⫹, plots well away from the nine other Baja California fluid
inclusions and the HMW seawater evaporation paths. The plots
of SO2⫺
and K⫹ vs. Mg2⫹ in Baja fluid inclusions show more
4
scatter but define a rough evaporation path that lies close to the
HMW seawater evaporation paths (Figs. 2C,D). The one fluid
inclusion with relatively low values of Mg2⫹ again plots further from the seawater evaporation paths. The plots of SO2⫺
4
and K⫹ vs. Cl⫺ and K⫹ vs. SO2⫺
for the fluid inclusions from
4
Baja California completely overlap the seawater evaporation
trend of the HMW computer program and lie on the halite
precipitation segment of the seawater evaporation curve (Figs.
Evaluating seawater chemistry from halite
Fig. 2. Elemental plots of the brine chemistries from fluid inclusions in marine halite samples from Salina Omotepec, Baja
California, Mexico, and from the solar saltwork of Great Inagua Island, Bahamas. Brine samples from the Inagua crystallizer
ponds were analyzed by McCaffrey et al. (1987) and plotted for comparison with the fluid inclusion data. Fluid inclusion
brine chemistries from the modern nonmarine closed basin halite deposits of the Qaidam Basin, China, are also plotted.
Solid curves track the evaporation of modern seawater simulated by the HMW computer brine model. The HMW program
was run without back reaction of evolved brines with gypsum/anhydrite (to form glauberite) to more realistically simulate
conditions at Great Inagua Island. The salts predicted to precipitate during equilibrium HMW evaporation of seawater are
shown as horizontal bars (calcium carbonate, not shown, precipitates in small amounts throughout).
2297
2298
M. N. Timofeeff et al.
2F–H). The plot of Na⫹ vs. Cl⫺ also shows that the Baja
California fluid inclusions lie along the seawater evaporation
trend (Fig. 2E). The one anomalous fluid inclusion with low
Mg2⫹ falls above the HMW seawater evaporation curve. Aside
from this one fluid inclusion with anomalously low Mg2⫹
concentrations, there are no significant differences between the
evaporation paths defined by the Baja California fluid inclusions and by the HMW computer program.
3.3. Nonmarine Setting: Qaidam Basin, Western China
The major ions Na⫹, Mg2⫹, K⫹, Cl⫺, and SO2⫺
in fluid
4
inclusions from the nonmarine Qaidam Basin, China, show a
distinctly different trend from the evaporation path of modern
seawater as it evolves during the crystallization of halite. The
five fluid inclusions in halite from core 88-01, Qaidam Basin,
contain nonmarine brines evolved by evaporation of parent
waters relatively depleted in SO2⫺
(Figs. 2C,F,H), K⫹ (Figs.
4
⫹
2D,G), and Na (Figs. 2A,E) compared with seawater, and
relatively enriched in Cl⫺ (Fig. 2B,E). The chemical composition of fluid inclusions in halite from the Qaidam Basin can be
readily distinguished from fluid inclusions in marine halites and
from the HMW seawater evaporation paths.
4. DISCUSSION AND CONCLUSIONS
The chemical analyses of the major ions in fluid inclusions in
modern marine halites from Great Inagua Island, Bahamas, and
Baja California, Mexico, show distinctive linear trends when
2⫹
plotted against one another (i.e., Na⫹, K⫹, and SO2⫺
4 vs. Mg
⫺
and Cl ), which track the evaporation path of seawater as it
evolved during the crystallization of halite (Fig. 2). These
evaporation paths defined for the major ions by fluid inclusions
in halite overlap the computer simulations of the evaporation of
modern seawater by the HMW computer program. It can thus
be concluded that fluid inclusions in modern marine halites
faithfully record the chemical signature of modern seawater.
The fluid inclusion data from the solar saltwork of Great
Inagua Island closely match the predicted seawater evaporation
path calculated by the HMW computer model. This is not
surprising considering the carefully controlled inflow, evaporation, and discharge of seawater at the Inagua saltwork. The
flow of brines through the crystallizer ponds at Inagua, however, precludes back-reaction of evolved seawater brines with
earlier-formed minerals—for example, reaction of evolved seawater with gypsum to form glauberite or polyhalite. In this
regard, the Inagua system differs from most natural evaporite
settings, modern and ancient.
The major ion chemistry of fluid inclusions from the Baja
California halites matches the HMW seawater evaporation
paths in most respects. One of the 10 Baja California fluid
inclusions has relatively low concentrations of Mg compared
with evaporated seawater. One possible explanation of this
anomalous fluid inclusion is syndepositional recycling. The
effects of syndepositional recycling are most commonly found
in ephemeral salt pan evaporite settings, where, during each
flooding event, previously deposited salts such as halite are
dissolved, thereby changing the major ion chemistry of the
evaporating brine. Flooding of the Baja California ephemeral
salt pan by seawater, for example, will dissolve halite, adding
Na⫹ and Cl⫺ in equal molar proportions, which decreases the
relative concentrations of all the other ions in this modified
seawater. This mechanism, however, cannot explain the anomalous Baja fluid inclusion because only Mg occurs in concentrations below that expected of evaporating seawater. Other
ions in this inclusion, for example K⫹ and SO2⫺
4 , occur in
concentrations close to that expected for the evaporation of
seawater (Figs. 2F,G,H). Furthermore, all the fluid inclusions
analyzed came from the same pristine halite crust, so they all
formed during crystallization from the same parent water. The
common origin of all the Baja California halites and their fluid
inclusions also rules out nonmarine parent water contributions
because 9 of the 10 inclusions contain evaporated seawater.
Finally, loss of Mg from brines via dolomitization or clay
mineral formation has been documented in supratidal sabkha
environments (for example, Hover et al., 1999). However, such
Mg removal should apply to all the fluid inclusions in the halite
sample, not just one, because the halite all formed from the
evaporation of a common seawater that flooded the surface
saline pan of Salina Omotepec in 1980. At this point, the low
Mg content of the one Baja fluid inclusion remains unexplained.
That the fluid inclusion chemistries in Baja halites nearly
match the seawater evaporation trend is surprising because
syndepositional recycling of surface evaporite crusts and nonmarine inflow both occur in this sabkha setting (Shearman,
1970; Lowenstein and Hardie, 1985). We conclude that nonmarine inflows and syndepositional recycling of preexisting
salts in the Baja California supratidal setting were not large
enough to override the chemical signature of normal evaporating seawater as the primary control on the Baja fluid inclusion
compositions.
Fluid inclusion data from halites from the nonmarine
Qaidam Basin, Qinghai Province, western China, establish that
fluid inclusions from this continental setting have a distinctly
different major ion chemical signature than “global” seawater.
The fluid inclusion chemistries from the Qaidam Basin halites
do not lie on the evaporation paths defined by modern seawater
and can clearly be differentiated from fluid inclusions containing evaporated seawater. The Qaidam Basin fluid inclusions
occur in cement crystals that precipitated from groundwater
brines. The Na-Mg-Cl–rich fluid inclusions from the Qaidam
Basin cements are very similar in chemical composition to the
surface waters of nearby Dabusun Lake and to the groundwaters of the area (Casas et al., 1992). Therefore, the distinctly
nonmarine chemical composition of the Qaidam Basin fluid
inclusions is representative of the modern lacustrine system,
which is clearly different in chemical composition from evaporated seawater.
In general, fluid inclusions from continental halites should
reflect the fact that nonmarine brines can have a wide variety of
chemical compositions controlled by inflow water sources and
that are commonly chemically distinct from seawater (Hardie,
1984).
5. APPLICATION TO THE ROCK RECORD:
EVALUATION OF PALEOSEAWATER CHEMISTRY
When studying ancient halites, we do not of know whether
the deposit was marine or nonmarine, nor do we know, a priori,
Evaluating seawater chemistry from halite
the chemical composition of the parent waters. The identification and quantification of fluid inclusions containing uncontaminated samples of seawater has become especially important in
light of evidence from Phanerozoic marine carbonate and evaporite mineralogies that the chemistry of ancient seawater has
undergone fluctuations (Hardie, 1996). How do we know fluid
inclusions in ancient halites contain evaporated seawater?
Some constraints on the parent waters may be made with
knowledge of underlying and overlying sedimentary rocks and
lateral facies changes, especially if marine fossils exist in
related strata. Important information for differentiating marine
from nonmarine evaporite parent waters can come from a
variety of geochemical measurements including 87Sr/86Sr in
sulfate salts, halite, and fluid inclusions in halite, the Br and Rb
content of chloride minerals (halite, sylvite, and carnallite) and
fluid inclusions in halite, and the 34S/32S ratios of sulfate
minerals (for example, Holser, 1979a,b; Garcia-Veigas et al.,
1995; Land et al., 1995; Fanlo and Ayora, 1998).
One test to identify evaporites formed from seawater is to
compare plots of fluid inclusion major ion chemistries (Na⫹,
K⫹, and SO2⫺
against Mg2⫹ and Cl⫺, as in Fig. 2) from
4
several evaporite basins of the same age. Fluid inclusion data
from halites early in the precipitation sequence, before the
precipitation of potash salts, eliminates the complications of
back reaction of early-formed minerals with evolved brines and
its affect on brine chemistry. In addition, halite samples from
stratigraphic intervals with petrographic textures diagnostic of
perennial subaqueous depositional conditions (Smoot and Lowenstein, 1991) minimize the possibilities for syndepositional
recycling. If each geographically separate evaporite deposit
contains fluid inclusions that fall along a distinctive evaporation path, and if the paths for the various evaporite basins of the
same age overlap one another, the implication is that the parent
water had a uniform “global” seawater chemical composition.
Overlapping evaporation paths also indicate minimal influence
of nonmarine inflow and syndepositional recycling.
The final problem in determining the composition of paleoseawater is the back-calculation of the chemistry of seawater from brine inclusions in halite that have undergone
evaporative concentration. Once an evaporation path is established from fluid inclusions in geographically separate
deposits of about the same age, the next step is to calculate
the chemical composition of the original seawater. This can
be done with the HMW computer program, which can simulate evaporative concentration of parent seawaters that best
fit the evaporation paths of the major ions defined by fluid
inclusion chemistries. We select an initial seawater chemistry, which we evaporate by means of HMW computer program simulation. The calculated evaporation pathways are
plotted and compared with the measured inclusion brines.
The parent seawater chemistry is then adjusted in an iterative process to best fit the fluid inclusion data. The composition of the seawater parent is given by the closest fit
between the selected parent water chemistry and the measured evaporation path obtained from the fluid inclusions in
halite.
Acknowledgments—This research was supported by NSF grant
EAR9725740. Support for J.H. was provided by the U.S. Department
of Energy Geosciences program under contract DE-AC05-960R22464
2299
with Oak Ridge National Laboratory. Many thanks to reviewers Robert
Goldstein, Sarah Gleeson, and Kathleen Benison.
Associate editor: M. A. McKibben
REFERENCES
Attia O. E., Lowenstein T. K., and Wali A. M. A. (1995) Middle
Miocene gypsum, Gulf of Suez: Marine or nonmarine? J. Sediment.
Res.A 65, 614 – 626.
Ayora C. and Fontarnau R. (1990) X-ray microanalysis of frozen fluid
inclusions. Chem. Geol. 89, 135–148.
Ayora C., Garcia-Veigas J., and Pueyo J.-J. (1994a) X-ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins. Geochim. Cosmochim. Acta 58, 43–55.
Ayora C., Garcia-Veigas J., and Pueyo J.-J. (1994b) The chemical and
hydrological evolution of an ancient potash-forming evaporite basin
in Spain as constrained by mineral sequence, fluid inclusion composition, and numerical simulation. Geochim. Cosmochim. Acta 58,
3379 –3394.
Casas E., Lowenstein T. K., Spencer R. J., and Zhang P. (1992)
Carnallite mineralization in the nonmarine Qaidam basin, China:
Evidence for the early diagenetic origin of potash evaporites. J.
Sediment. Petrol. 62, 881– 898.
Das N., Horita J., and Holland H. D. (1990) Chemistry of fluid
inclusions in halite from the Salina Group of the Michigan Basin:
Implications for Late Silurian seawater and the origin of sedimentary
brines. Geochim. Cosmochim. Acta 54, 319 –327.
Davis D. W., Lowenstein T. K., and Spencer R. J. (1990) Melting
behavior of fluid inclusions in laboratory-grown halite crystals in the
systems NaCl-H2O, NaCl-KCl-H2O, NaCl-MgCl2-H2O, and NaClCaCl2-H2O. Geochim. Cosmochim. Acta 54, 591– 601.
Fanlo I. and Ayora C. (1998) The evolution of the Lorraine evaporite
basin: Implications for the chemical and isotope composition of the
Triassic ocean. Chem. Geol. 146, 135–154.
Garcia-Veigas J., Orti F., Rosell L., Ayora C., Rouchy J.-M., and Lugli
S. (1995) The Messinian salt of the Mediterranean: Geochemical
study of the salt from the Central Sicily Basin and comparison with
the Lorca Basin (Spain). Bull. Soc. Geol. France 166, 699 –710.
Hardie L. A. (1984) Evaporites, marine or non-marine? Am. J. Sci. 284,
193–240.
Hardie L. A. (1996) Secular variation in seawater chemistry: An
explanation for the coupled secular variation in the mineralogies of
marine limestones and potash evaporites over the past 600 my.
Geology 24, 279 –283.
Hardie L. A., Lowenstein T. K., and Spencer R. J. (1985) The problem
of distinguishing between primary and secondary features in evaporites. In Sixth International Symposium on Salt (eds. B. C. Schreiber
and H. L. Harner), pp. 11–38. Salt Institute.
Harvie C. E., Weare J. H., Hardie L. A. and Eugster H. P. (1980)
Evaporation of seawater: Calculated mineral sequences. Science 208,
498 –500.
Harvie C. E., Møller N., and Weare J. H. (1984) The prediction of
mineral solubilities in natural waters: The Na-K-Mg-Ca-H-Cl–SO4OH-HCO3-CO3-CO2-H2O system to high ionic strengths at 25°C.
Geochim. Cosmochim. Acta 48, 723–751.
Holser W. T. (1963) Chemistry of brine inclusions in Permian salt from
Hutchinson, Kansas. In Symposium on Salt (ed. A. C. Bersticker),
Vol. 1, pp. 86 –95. Northern Ohio Geological Society.
Holser W. T. (1979a) Trace elements and isotopes in evaporites. In
Marine Minerals (ed. R. G. Burns), Vol. 6, pp. 295–346. Reviews in
Mineralogy. Mineralogical Society of America.
Holser W. T. (1979b) Mineralogy of evaporites. In Marine Minerals
(ed. R. G. Burns), Vol. 6, pp. 211–294. Reviews in Mineralogy.
Mineralogical Society of America.
Horita J., Friedman T. J., Lazar B., and Holland H. D. (1991) The
composition of Permian seawater. Geochim. Cosmochim. Acta 55,
417– 432.
Horita J., Weinberg A., Das N. and Holland H. D. (1996) Brine
inclusions in halite and the origin of the Middle Devonian Prairie
Evaporites of Western Canada. J. Sediment. Res. 66, 956 –964.
Hover V. C., Walter L. M., Peacor D. M., and Martini A. M. (1999)
2300
M. N. Timofeeff et al.
Mg-smectite authigenesis in a marine evaporative environment,
Salina Ometepec, Baja California. Clays Clay Minerals 47, 252–268.
Kovalevich V. M., Peryt T. M., and Petrichenko O. I. (1998) Secular
variation in seawater chemistry during the Phanerozoic as indicated
by brine inclusions in halite. J. Geol. 106, 695–712.
Kramer J. R. (1965) History of seawater: Constant temperature pressure equilibrium models compared to fluid inclusion analyses.
Geochim. Cosmochim. Acta 29, 921–945.
Land L. S., Eustice R. A., Mack L. E., and Horita J. (1995) Reactivity
of evaporites during burial: An example from the Jurassic of Alabama. Geochim. Cosmochim. Acta 59, 3765–3778.
Lazar B. and Holland H. D. (1988) The analysis of fluid inclusions in
halite. Geochim. Cosmochim. Acta 52, 485– 490.
Linke W. F. (1965) Solubilities, Inorganic and Metal-Organic Compounds, 4th ed., Vols. 1 and 2. American Chemical Society and Van
Nostrand.
Lowenstein T. K. and Hardie L. A. (1985) Criteria for the recognition
of salt-pan evaporites. Sedimentology 32, 627– 644.
Lowenstein T. K., Spencer R. J., and Zhang P. (1989) Origin of ancient
potash evaporites: Clues from the modern nonmarine Qaidam Basin
of western China. Science 245, 1090 –1092.
Lowenstein T. K., Spencer R. J., Yang W., Casas E., Zhang P., Zhang
B., Fan H., and Krouse H. R. (1994) Major-element and stableisotope geochemistry of fluid inclusions in halite, Qaidam Basin,
western China: Implications for late Pleistocene/Holocene brine evolution and paleoclimates. In Paleoclimate and Basin Evolution of
Playa Systems (ed. M. R. Rosen), pp. 19 –32. Geological Society of
America Special Paper 289.
Martini A. M. and Walter L. M. (1993) Recent brines for the Salina
Ometepec, Baja California: Chemical evolution of marine brines
from a siliciclastic/evaporite environment. In Programs and Abstracts of the Geological Society of America, Vol. 25, p. 254.
Geological Society of America.
McCaffrey M. A., Lazar B., and Holland H. D. (1987) The evaporation
path of seawater and the coprecipitation of Br- and K⫹ with halite.
J. Sediment. Petrol. 57, 928 –937.
Moissette A., Shepherd T. J., and Chenery S. R. (1996) Calibration
strategies for the elemental analysis of individual aqueous fluid
inclusions by laser ablation-ICP-MS. J. Anal. Atom. Spec. 11, 177–
186.
Petrichenko O. I. (1973) Naukova dumka, Kiev, 98 [in Ukrainian].
Methods of Study of Inclusions on Minerals of Saline Deposits, In
Fluid Inclusion Research Proc. COFFI (ed. E. Roedder), Vol. 12, pp.
214 –274. University of Michigan Press, Ann Arbor.
Shearman D. J. (1970) Recent halite rock, Baja California, Mexico.
Inst. Mining Metal. Trans. B 79, 155–162.
Shepherd T. J. and Chenery S. R. (1995) Laser ablation ICP-MS
elemental analysis of individual fluid inclusions: An evaluation
study. Geochim. Cosmochim. Acta 59, 3997– 4007.
Shepherd T. J., Chenery S. R., and Moissette A. (1995) Optimization of
laser ablation–ICP–MS for the chemical analysis of fluid inclusions
in evaporite minerals [abstract EUG 8]. Terra Nova, Vol. 7, p. 344.
Strasbourg.
Shepherd T. J., Ayora C., Cendon D. I., Chenery S. R., and Moissette
A.1. (1998) Quantitative solute analysis of single fluid inclusions in
halite by LA-ICP-MS and cryo-SEM-EDS: Complementary microbeam techniques. Eur. J. Mineral. 10, 1097–1108.
Smoot J. P. and Lowenstein T. K. (1991) Depositional environments of
non-marine evaporates. In Evaporites, Petroleum and Mineral Resources (ed. J. L. Melvin), Vol. 50, pp. 189 –347. Developments in
Sedimentology. Elsevier.
Timofeeff M. N., Lowenstein T. K., and Blackburn W. H. (2000)
ESEM-EDS. An improved technique for major element chemical
analysis of fluid inclusions. Chem. Geol. 164, 171–182.
von Borstel L. E., Zimmermann H., and Ruppert H. (2000) Fluid
inclusion studies in modern halite from the Inagua solar saltwork. 8th
World Salt Symposium (ed. R. M. Geertman), vol. 2, pp. 673– 678.
Elsevier Science BV, Amsterdam, Netherlands.