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www.elsevier.com/locate/epsl
Earth and Planetary Science Le
Biological control of Cl/Br and low sulfate concentration in a
3.5-Gyr-old seawater from North Pole, Western Australia
Julien Foriela,*, Pascal Philippota, Patrice Reyb, Andrea Somogyic,
David Banksd, Benedicte Meneza
aaLaboratoire de Geosciences Marines (UMR CNRS 7097), Institut de Physique du Globe de Paris, Tour 14-15E5,
case 89, 4 Place Jussieu, 75252 Paris cedex 05, FrancebThe University of Sydney, School of Geosciences, Division of Geology and Geophysics, Sydney NSW 2006, Australia
cEuropean Synchrotron Radiation Facility, beamline ID-22, BP 220, 38043 Grenoble cedex 9, FrancedSchool of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Received 7 June 2004; received in revised form 17 September 2004; accepted 23 September 2004
Available online 6 November 2004
Editor: V. Courtillot
Abstract
The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the
evolution of Earth’s hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean
seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and
hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz–carbonate pods that formed during
oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods
seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry
and synchrotron radiation X-ray microfluorescence (A-SR-XRF) techniques. Bulk chemical analyses were also performed using
crush-leach method.
Microthermometric data combined with crush-leach and A-SR-XRF analyses yielded a model composition of 1100 mM Na,
2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration
(ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin
environment. Individual fluid inclusion analysis using A-SR-XRF revealed the presence of three main fluid populations: a
metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe–S-rich end-member. The Cl/Br ratio of metal-depleted fluid
inclusions (630) is similar to the modern seawater value (649). By contrast, Ba- and Fe-rich brines have Cl/Br ratios (350 and
390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metal-
depleted fluid displays low sulfate concentration (0–8 mM compared to 28 mM in present-day ocean). Sulfur content of the Fe-
rich fluids ranges between 41 and 82 mM.
0012-821X/$ - s
doi:10.1016/j.ep
* Correspon
E-mail addr
tters 228 (2004) 451–463
ee front matter D 2004 Elsevier B.V. All rights reserved.
sl.2004.09.034
ding author. Tel.: +33 1 44278209; fax: +33 1 44279969.
esses: [email protected] (J. Foriel)8 [email protected] (B. Menez)8 [email protected] (P. Philippot)8
d.edu.au (P. Rey)8 [email protected] (A. Somogyi), [email protected] (D. Banks).
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463452
Fluid–rock interaction processes occurring in pillow basalts located on the seafloor are unlikely to be the cause of Cl/Br
fractionation between hydrothermal fluids and the bNorth Pole seawaterQ end-member. We hypothesize that high biological
activities associated with the specific geological settings of North Pole (small isolated basin exposed to relatively intense
terrigenous sedimentation) are responsible for the buffering of North Pole seawater Cl/Br value. Although the exact effect of
water–rock interaction, barite precipitation, and biological activity on the sulfate concentration of analyzed fluids is difficult to
asses, the low sulfate concentration recorded for North Pole seawater argues in favor of a poorly oxygenated Archaean
environment.
D 2004 Elsevier B.V. All rights reserved.
Keywords: fluid inclusions; halogens; sulfate; synchrotron X-ray microfluorescence; Archaean seawater; North Pole Dome
1. Introduction
Geological and biological processes occurring on
the early Earth surface may have been recorded by the
composition of seawater. Sulfur is a key element in
Archaean geochemistry, as seawater sulfate concen-
tration is used to constrain the oxidation state of the
early ocean and atmosphere (e.g., [1,2]), and S may
have been a key element for early metabolic processes
and for the evolution of primitive life [3]. The level of
sulfate in Archaean seawater has been evaluated by
indirect approaches [4,5] but remains a subject of
major controversy [6,7]. Cl/Br ratio in fluids is
considered conservative in many geological settings
[8,9] and is therefore widely used to trace the origin of
fluids (meteoric, oceanic, crustal, mantellic) in the
rock record. Analysis of ancient fluid inclusions from
the Kaapvaal Craton, South Africa, suggest that Cl/Br
in Archaean 3.2 Gyr-old [10,11] (but see [12] for a
controversy on the origin of the samples considered)
and paleoproterozoic 2.2 Gyr-old [13] seawater was
below present-day value and resulted from mantle
buffering. These studies, however, were based on bulk
fluid analyses (i.e., crush-leach [14,15]) which can
result in fluid mixing if several fluid generations occur
in a single sample. Alternatively, detailed chemical
analysis of individual fluid inclusions can be per-
formed by Synchrotron Radiation X-ray micro-Fluo-
rescence (A-SR-XRF) [16–21], thus allowing
independent analysis of different fluids trapped in
the same sample.
The main advantages of the A-SR-XRF technique
reside in its nondestructive character, high spatial
resolution, multielement analytical capability (from S
to Pb) and high sensitivity [16–20,22–24]. Hence, A-SR-XRF can analyze small, diluted individual fluid
inclusions, discriminate distinct inclusion populations
trapped in a single crystal, and provide information
about complex histories of fluid circulations. Pro-
gress toward quantitative analysis of single-fluid
inclusions has been realized by our group in the
recent years [16,20,21]. In this work, we investigated
a suite of well-preserved intrapillow quartz–carbo-
nate pods that formed during oceanic hydrothermal
alteration of the 3.49 Ga Dresser Formation, North
Pole Dome, Western Australia. The pods contain
primary fluid inclusions which were analyzed indi-
vidually using microthermometry and A-SR-XRFtechnique. Complementary bulk fluid analyses were
performed by crush-leach method [15].
2. Geological settings and samples
Studied rocks are from the Dresser Formation
(Warrawoona Group) of the North Pole Dome,
Pilbara Craton, Western Australia. The Dresser
Formation consists of pillow basalts and metabasalts
and five interbeds of cherty sediments deposited in a
shallow water environment [25–27]. Stratigraphi-
cally, lowest horizon consists of a ca. 3.49 Ga [28]
chert–barite–carbonate unit where stromatolites have
been described [26,27]. Deposition of the bedded
chert–barite was linked to a swarm of independent
sets of hydrothermal chert and barite veins, episodic
volcanoclastic, and detrital sediments [26,27]. North
Pole Dome rocks have only experienced very low
grade metamorphism [29] and slight deformation. In
the Dresser Formation, extensive hydrothermal alter-
ation has affected basalts located immediately under
chert beds, whereas above chert units, basalts show
very little alteration [30].
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463 453
Our samples came from the Northernmost part
of the Dresser Formation on the Western flank of
the North Pole Dome. Undeformed pillow basalts,
showing no signs of significant alteration, are found
above the contact with the chert–barite horizon.
Inside some of these pillow basalts, quartz–carbo-
nate aggregate forming isolated pods not connected
to each other can be found. Intrapillow pods are
ovoid in shape, with their long dimension (up to 10
cm in length) oriented parallel to the top margin of
the pillow. Intrapillow pods resemble typical
mineralizations associated with passive hydrother-
mal circulation of seawater through shallow oceanic
crust [31].
Intrapillow quartz crystals contain abundant, 1–25
Am, two-phase (liquid and b5% vapor) aqueous
inclusions (Fig. 1b). Inclusions are randomly dis-
tributed throughout their host crystals, arguing for a
primary origin (i.e., fluid inclusion trapped during
mineral growth). Scanning electron microscopy
luminescence imaging (Fig. 1c) shows a very
complex micrometer-scale brecciated texture,
although optical observation of quartz grains does
not reveal any apparent heterogeneity. Such a texture
probably results from a succession of hydrofracturing
and sealing events of the quartz grains attending
fluid infiltration and crystallization.
The absence of crosscutting veins, metamorphic
overprint, and deformation features affecting the
studied basalt and associated pods indicate that they
did not experience significant fluid remobilization
Fig. 1. (a) Field exposure of a 3.49-billion-years-old North Pole pillow ba
hydrothermal circulation of seawater; scale bar is 10 cm. (b) Optical micro
containing myriads of randomly distributed primary fluid inclusions; scal
image of two quartz crystals from Pi02-39 intrapillow pod. Different lum
successive events of fluid infiltration and quartz precipitation. Dashed lin
and circulation after deposition and crystallization.
Recognition that the intrapillow pods form well-
defined ovoid shape isolated in the core of the
pillow indicates that fluid circulation processes
driving mineral precipitation should have occurred
through a porous unconsolidated media shortly after
basalt deposition. The presence of a texturally
unique fluid inclusion population in intrapillow
pods further suggests an early stage of fluid
trapping attending crystallization and basalts con-
solidation.
3. Methods
A-SR-XRF experiments were carried out at the A-fluorescence, A-imaging and A-diffraction beamline
(ID22) of the European Synchrotron Radiation
Facility (ESRF). A double-crystal fixed-exit mono-
chromator was used to create a monochromatic beam
of 17 keV. The beam was focused with a KB mirror
to a 1.5�3 Am spot. Focused beam intensity was
3�1011 photons/sec. X-ray fluorescence spectra were
recorded with a Gresham Si(Li) detector of 140 eV
resolution, placed at a 908 angle to the incident beam
in the polarization plan to reduce X-ray scattering.
Detection of S, Cl, K, and Ca was improved by
placing the sample and the semiconductor detector
under a Helium atmosphere, which greatly reduces
the absorption of low energy fluorescence emitted by
light elements.
salt containing a quartz–carbonate pod (arrowed) formed by passive
scope photomicrograph of a quartz crystal from an intrapillow pod
e bars are 20 Am. (c) Scanning electron microscopy luminescence
inescence intensities reflect different grain orientation attributed to
e represents the boundary between crystals; scale bar in 20 Am.
Table 1
Crush-leach and A-SR-XRF results and comparison with seawater composition
Sample Inclusion S Cl K Ca Cr Mn Fe Ni Cu Zn Ga Br Rb Sr Ba Pb Cl/Br
Error
(2r)23–36
(%)
20
(%)
27
(%)
27
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
32
(%)
32
(%)
32
(%)
32
(%)
21
(%)
Crush-leach
Pi02-21 2300 60 500 1.0 20 3.7 8.0 3.3 2.1 620
Pi02-39 2300 61 430 8.5 14 3.3 3.3 9.9 6.3 690
l-SR-XRFAverage for Fe-rich inclusions 57 2100 318 510 0.33 0.27 10 0.23 2.0 2.2 1.2 5.9 0.46 3.3 2.6 0.06 390
Pi01-21 91 770 500 0.50 16 0.25 0.46 4.6 1.5 1.8 1.2 1.0 2.1 0.06 430
Pi02-39 185 82 1600 54 560 0.42 0.16 6.3 0.61 0.48 6.0 0.12 2.3 0.02 270
Pi02-39 186 3000 120 600 0.40 0.15 5.8 0.47 0.36 9.2 0.20 3.5 1.4 0.02 320
Pi02-39 190 2300 65 420 0.39 2.6 2.7 2.0 0.01 4.7 0.21 3.1 0.62 0.03 489
Pi01-21 224 2800 1800 970 0.29 12 3.7 2.5 5.5 11 1.7 8.4 5.1 0.04 254
Pi01-21 226 2000 210 430 0.24 0.38 6.2 1.6 1.3 0.67 4.1 0.39 2.7 2.0 0.02 485
Pi01-21 257 41 2100 260 52 0.19 0.06 28 0.21 4.6 6.6 0.37 7.7 0.29 0.80 8.3 0.23 277
Pi02-39 408 2400 58 430 0.28 2.2 0.07 4.6 0.19 2.2 0.75 517
Pi02-39 411 47 2200 44 710 1.0 0.02 5.7 0.18 6.2 380
Pi02-39 413 1900 46 390 0.36 20 0.15 4.1 0.19 2.5 0.47 453
Average for Ba-rich inclusions 1700 640 460 0.38 0.66 0.56 0.92 0.66 0.08 4.8 1.6 3.9 8.4 0.03 350
Pi02-39 103 1000 1700 580 0.70 0.69 0.05 0.09 0.03 8.1 6.3 6.2 10 0.03 550
Pi02-39 174 3200 100 620 0.39 0.45 0.07 0.01 0.21 6.9 0.32 4.1 2.9 0.04 450
Pi02-39 178 420 160 0.01 0.01 1.5 0.06 2.8 12 0.01 280
Pi02-39 183 2700 84 550 0.37 0.56 0.01 6.2 0.21 3.5 3.6 440
Pi02-39 441 420 1300 160 0.62 0.52 2.2 1.6 1.7 2.7 1.3 16 250
Pi02-39 444 2300 70 670 2.3 1.6 4.4 0.16 5.2 6.0 530
Metal-depleted inclusions
Average for Pi01-21 2500 53 500 0.79 0.10 0.27 1.4 1.0 0.07 3.8 0.16 3.8 1.1 0.04 630
Pi01-21 77 4000 81 390 0.28 2.3 1.5 0.01 4.3 0.17 4.4 0.17 940
Pi01-21 219 2200 63 590 2.1 1.5 5.3 0.18 5.0 0.93 0.04 420
Pi01-21 227 910 62 180 0.10 0.54 1.2 0.90 0.18 3.2 0.17 1.6 0.01 280
Pi01-21 236 1900 31 470 0.68 0.11 0.50 0.43 3.3 0.13 3.3 0.01 600
Pi01-21 239 4300 72 1100 1.40 0.32 3.3 2.2 4.7 0.20 6.4 1.73 0.03 920
Pi01-21 251 1700 26 400 0.53 0.16 0.20 0.21 2.7 0.11 2.7 0.01 640
Pi01-21 253 2100 34 450 0.56 0.21 0.44 0.43 0.01 3.4 0.13 3.0 0.65 0.02 630
Average for Pi02-39/1 1800 43 390 0.42 0.24 0.26 0.49 0.35 0.02 2.7 0.11 2.5 0.84 0.02 640
Pi02-39/1 111 2300 42 610 0.49 0.10 0.03 0.01 4.6 0.12 3.4 0.63 0.01 490
Pi02-39/1 115 2100 58 530 0.39 0.19 1.7 1.2 3.7 0.11 3.7 0.98 0.03 560
Pi02-39/1 119 540 11 120 0.05 0.03 0.16 1.1 0.05 0.93 0.01 480
J.Foriel
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andPlaneta
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228(2004)451–463
454
Sample Inclusion S Cl K Ca Cr Mn Fe Ni Cu Zn Ga Br Rb Sr Ba Pb Cl/Br
error
(2j)23–36
(%)
20
(%)
27
(%)
27
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
29
(%)
32
(%)
32
(%)
32
(%)
32
(%)
21
(%)
l-SR-XRFPi02-39/1 125 1200 21 210 0.03 0.04 0.01 2.0 0.12 1.4 0.39 0.01 610
Pi02-39/1 134 2500 26 590 0.53 2.3 1.6 3.0 0.11 3.4 0.04 850
Pi02-39/1 141 2600 89 660 0.91 0.18 0.93 0.66 0.01 3.7 0.18 4.2 1.25 0.04 690
Pi02-39/1 169 1700 21 280 0.34 0.04 0.06 1.8 0.06 1.8 0.01 930
Pi02-39/1 173 1300 18 200 0.21 0.39 0.09 0.04 0.06 2.3 0.07 1.3 0.02 560
Pi02-39/1 175 400 11 120 0.20 0.09 0.07 0.05 1.3 0.04 1.0 0.01 320
Pi02-39/1 176 3400 97 600 0.53 0.56 0.09 0.06 0.02 4.0 0.16 3.5 1.10 0.02 870
Pi02-39/1 180 2300 54 500 0.32 0.10 0.34 0.06 0.04 0.02 3.3 0.18 3.0 0.68 0.03 720
Pi02-39/1 187 1600 60 350 0.30 0.38 0.35 0.01 2.3 0.15 2.1 0.01 700
Pi02-39/1 189 1200 56 330 0.19 0.01 2.3 0.16 2.2 550
Metal-depleted inclusions
Average for Pi02-39/2 2600 78 680 0.14 0.06 0.06 2.0 1.4 0.01 4.2 0.17 4.2 0.31 610
Pi02-39/2 423 b15 3200 55 860 0.70 0.46 5.0 0.20 5.8 630
Pi02-39/2 424 b8.6 1700 65 380 0.48 0.29 2.3 0.09 2.2 740
Pi02-39/2 426 770 200 0.33 0.17 0.01 1.7 0.05 1.4 450
Pi02-39/2 427 4500 110 1300 0.32 0.11 0.17 4.2 2.9 6.5 0.25 7.1 0.55 700
Pi02-39/2 428 b5.4 3600 140 920 0.04 0.08 4.0 2.8 5.8 0.34 5.7 610
Pi02-39/2 431 960 410 0.09 0.03 0.04 2.9 2.0 2.9 0.04 2.9 330
Pi02-39/2 432 2800 28 790 1.0 0.61 4.5 0.14 4.7 0.14 640
Pi02-39/2 433 2300 73 670 0.12 0.07 1.4 0.95 0.01 4.2 0.25 4.3 0.18 540
Pi02-39/2 434 1900 110 490 0.10 0.01 1.4 0.91 3.4 0.24 3.4 0.36 570
Pi02-39/2 435 3000 530 1.3 0.85 3.2 0.05 3.1 940
Pi02-39/2 436 4400 1000 0.03 2.6 1.8 7.0 0.15 6.5 630
Pi02-39/2 437 b6.9 2300 100 610 0.16 0.06 0.06 4.9 3.5 3.6 0.25 3.9 630
Pi02-39/2 439 2100 22 670 0.01 0.86 0.58 4.4 0.14 4.2 480
Average for Pi02-39/3 2100 53 480 0.11 0.11 1.4 0.66 0.01 3.4 0.13 3.0 0.45 650
Pi02-39/3 401 2900 82 600 0.04 7.2 0.23 3.9 0.47 410
Pi02-39/3 402 2400 44 480 0.02 4.7 0.15 2.5 520
Pi02-39/3 403 b15 1700 57 530 0.18 0.19 2.2 0.21 2.5 0.42 760
Pi02-39/3 405 2200 340 0.20 2.2 1.6 0.02 2.2 0.02 3.0 980
Pi02-39/3 406 810 290 0.01 0.50 0.35 2.0 0.09 2.0 400
Pi02-39/3 416 2400 50 590 0.18 0.01 3.3 0.17 3.5 730
Pi02-39/3 418 1800 51 540 0.07 2.8 0.12 3.3 640
Pi02-39/3 445 b7.2 2300 31 510 0.02 3.1 0.06 2.9 750
Modern seawater 28 550 10 10 6�10�6 5�10�7 10�6 8�10�6 2�10�6 8�10�6 3�10�8 0.85 1.4�10�3 0.09 1.5�10�5 10�8 647
All concentrations in mM, C1/Br is the molar ratio.
J.Foriel
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andPlaneta
ryScien
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228(2004)451–463
455
Fig. 2. A-SR-XRF spectra of two individual fluid inclusions (thick
gray line, metal-depleted inclusion; black line, Fe-rich fluid)
overlain with a control spectrum obtained from an adjacent
inclusion-free, zone of the host quartz (thin gray line). Differen
Cl count rates reflect different fluorescent X-ray absorption by the
host quartz due to different thickness of quartz above the inclusion
Insert is a magnification around sulfur peak energy (arrow)
Absence of sulfur peak in the metal-depleted inclusion, which also
shows a Cl count rate four times higher than the Fe-rich inclusion
indicates a very low S concentration level.
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463456
The procedure used to achieve reliable quantitative
fluid inclusion analysis using A-SR-XRF is summar-
ized below. Detailed information on the absorption
correction, calibration procedures, and error calcula-
tions can be found in [16,20,21] and will not be
renewed here. A �70 microscope placed on the
beamline is used to recognize previously selected
inclusions. The analyzed inclusions were 8–15 Am in
diameter and were brought to within 1–10 Am of the
quartz surface by hand polishing to reduce absorption
by the mineral matrix. The exact position of the
beamspot with respect to the microscope is determined
using the so-called knife edge technique using a thin
gold cross. Before each analysis, a series of short X-ray
fluorescence profiles using element specific of the fluid
phase (Cl, Br) and which are known to be absent from
the host mineral, are acquired across the selected
inclusion. The final acquisition is then performed at the
point of highest count rate for these elements.
Calibration of the X-ray spectrum was achieved by
using the Cl concentration measured by microther-
mometry in each inclusion. When this data was not
available, another inclusion was used as an external
standard (see [20] for details). Error associated with the
calibration procedure was ca. F20%. The thickness of
each inclusion was calculated using absorption line
scans [21] with an error of F1 Am which represents a
F10% error for a 10 Am thick inclusion. Finally, as the
fluorescence of each element is absorbed differently by
the mineral matrix depending on its mass, elemental
ratios are strongly dependent of the pathlength of the X-
ray beam through quartz (i.e., inclusion depth).
Inclusion depth was estimated directly from the X-ray
spectrum using the Ka/Kh method [16] and by optical
means using a 3-D spindle stage. Both methods
provided consistent error ofF1 Am formost inclusions,
which translates into a 9–22% error, depending on the
element considered. Total errors were calculated using
the propagation of error formula. Errors on calibration
and inclusion thickness are introduced only when
concentrations are calculated. This implies that errors
on elemental ratios are smaller than on concentrations.
Precise error calculations performed on a limited
number of inclusions show that, with the exception of
S, uncertainties on concentration estimates were
similar in all inclusions. Calculated errors (2r) wereextended to the whole data set (Table 1). These are
F17–19% for Cl/K and Cl/Ca andF21–25% for other
Cl/Z ratios (Z representing any elements except S).
Errors for absolute concentrations are F20% for Cl
(from microthermometry), F27% for K and Ca, and
F29–32% for other elements.
The fluorescence of a very light element like
sulfur is strongly absorbed by the quartz matrix and
a specific acquisition procedure was required. S
analysis has been restricted to a limited number of
inclusions (9), located within 1 to 3 Am from the
sample surface. Acquisition time was increased to
obtain a reliable statistic on S X-ray peak. Sulfur
could be detected in Fe-rich inclusions, but not in
metal-depleted inclusions (Fig. 2). In the latter case,
3MBS, where BS is the background fluorescence
intensity at S energy, was used instead of S
fluorescence intensity to calculate detection limits.
This ensures that S content in the fluid inclusion is
smaller than calculated detection limits with a
N99% confidence level. Errors (2r) on measured
S concentrations (Fe-bearing fluid) and calculated
detection limits (metal-depleted inclusions) are
F25–36% and F23–26%, respectively.
,
t
.
.
,
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463 457
Optically invisible, microscopic solid phases can occur
in fluid inclusions and alter the interpretation of fluid
composition results [17,32]. In order to asses the
presence of such solids, elemental distributions were
mapped in several inclusions; results for one inclusion
are shown in Fig. 3.
Crush-leach analyses were performed according to
[15] on selected quartz grains from two intrapillow
pod samples (Pi01-21 and Pi02-39).
4. Results
Temperatures of first melting of fluid inclusions
range between �50 and �40 8C, which indicates
a composition dominated by H2O+NaCl+CaCl2.
Fig. 3. Optical microscope photography and A-SR-XRF elemental maps of
corrected for absorption in quartz and inclusion thickness. Different co
concentrations but also variations of the depth of each analyzed spot
homogeneous distribution patterns mimicking the inclusion geometry indic
and do not occur as solid phases. Scale bar on microphotography is 10 A
Distribution of freezing point depressions (from
�11 to �4 8C, mean =�8.0 8C, Fig. 4) may be attri-
buted to the presence of different fluids. However,
no distinct populations could be recognized on the
base of microthermometric data and optical observa-
tions. Homogenization temperatures range between
90 and 175 8C with no observable correlation to first
melting temperature data. Because fluid inclusions
formed at shallow depths, homogenization temper-
atures must be close to trapping temperatures.
Results from microthermometry, crush-leach, and
A-SR-XRF analyses (Table 1, Na/Cl molar ratio,
not shown, is 0.55 for both crush-leach solutions)
yielded a model composition of 1150 mM Na,
2100 mM Cl, and 480 mM Ca, which corresponds
to a bulk fluid salinity of 12 wt.% salt equivalent.
a fluid inclusion. Maps show normalized fluorescence counts, non-
unts rates can therefore reflect not only variations of elemental
below the quartz surface and the volume of fluid analyzed. The
ates that the elements occur as diluted species within the fluid phase
m.
Fig. 4. Microthermometry results. Unimodal distribution of temper-
atures of final ice melting argues for the presence of a single fluid
inclusion population although the relatively large spread of data
may indicate the presence of different, yet indistinguishable, fluid
populations. Mean value=�8.0 8C.
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463458
A-SR-XRF as well as crush-leach results (Table 1)
show that all elements are more concentrated in intra-
pillow fluids than in present-day seawater. Cl/Br in
crush-leach solutions, however, is similar to the
modern seawater value. A-SR-XRF results (Fig. 5)
reveal the presence of three fluid populations within
quartz: a metal-depleted fluid, a Ba-rich fluid, and a
Fe-rich end-member (Fig. 5a). Fe-rich inclusions occur
in both Pi01-21 and Pi02-39 rock samples. Inclusions
characteristic of the Ba-rich phase (high Ba and low Fe
contents) were found only in Pi02-39. Cl, Ca, and Sr
concentrations are similar in all three fluids. Compared
to the metal-depleted inclusions, Fe-rich inclusions are
enriched in K, Fe, Ga, Ba, and, to a lesser extent, Ni,
Cu, Zn, Br, and Rb. Ba-rich inclusions contain
significantly more K, Rb, and Ba than metal-depleted
inclusions, but are only slightly enriched in Fe, Mn,
and Br. Metal-depleted fluid in Pi01-21 and Pi02-39
are nearly identical. Sulfur content ranges between 41
and 82 mM in the Fe-rich hydrothermal inclusions.
Detection limit calculations show upper limit concen-
trations of S in six metal-depleted inclusions ranging
from 5.4 to 15 mM (Fig. 5d). No sulfur compound
could be detected using Raman spectroscopy, likely
because detection limits are relatively high (N200 mM
for H2S; Dubessy, personal communication) for vapor
phases as small as those encountered in intrapillow
inclusions (bubble diameter b5 Am).
Concentrations of K, Ca and Br are identical in
crush-leach solutions and metal-depleted inclusions.
Leaching solution are however highly enriched in Mn,
Fe, Ba, and Pb. Pi01-21 crush-leach solution compo-
sition resembles that of the Fe-rich inclusions in terms
of Fe and Ba. Ba concentration in crushed grains from
sample Pi02-39, where all Ba-rich inclusions were
found, is similar to that of the Ba-rich fluid.
High concentrations of Mn and Pb in crush-leach
solutions could result from the contribution of phases
not analyzed by A-SR-XRF, possibly microcrystals.
Elemental mappings show homogeneous distribution
of Cl, Ca, Br, and Sr (Fig. 3); these elements therefore
occur diluted in the liquid. Other elements were not
concentrated enough to obtain reliable distribution
maps, which strongly argues against the presence of
solid phases [32]. It is therefore unlikely that A-SR-XRF results were altered by microscopic solids
present within the analyzed inclusions. High Mn and
Pb concentrations in the crush-leach solutions could
result from contamination by Mn- and Pb-rich phases
present in the mineral matrix but not associated with
the fluid phase.
5. Discussion
Chlorine concentration in intrapillow fluids is too
low for halite (NaCl) precipitation to have taken place
at North Pole (Fig. 5c). NaCl dissolution is unlikely,
since it would result in higher Cl/K ratios at higher Cl
concentrations which is not observed in A-SR-XRFdata. The shallow setting of North Pole seafloor [25–
27] and the relatively low temperatures of fluid
homogenization imply that inclusion trapping took
place in subcritical conditions. Therefore, phase
separation did not alter the Cl/Br ratio significantly
[33] and that Cl and Br can be considered conservative
in solution and used to trace the fluid sources [8,9].
Fluid inclusion populations identified in intra-
pillow quartz show several orders of magnitude of
metal excess when compared to a typical seawater
composition (Table 1), which implies that fluid
compositions were affected by a certain extent of
water-rock interaction. The range of homogenization
temperatures (90–175 8C) may be interpreted as
resulting from a mixing between a cold seawater
end-member and hot hydrothermal fluids (ca. 150 8C;[34]). It is our interpretation that Fe- and Ba-rich
inclusions represent two distinct hydrothermal fluid
end-members, as is also suggested by their Cl/Br
Fig. 5. Fluid inclusion composition results. Crosses, open dots, and black squares correspond to A-SR-XRF results whereas open diamonds
represent crush-leach data. (a) Fe vs. Ba concentration diagram showing the occurrence of three fluid inclusion populations in intrapillow quartz,
a Ba-rich fluid (�), a Fe-rich fluid (n), and a metal-depleted end-member (o) corresponding to bNorth Pole seawaterQ end-member. (b) Cl vs.
Br concentrations for metal-depleted inclusions compared with modern seawater value (1) and seawater Cl/Br evaporative trend (—). (c) Cl/Br
vs. Cl trend of evaporating seawater compared with crush-leach and A-SR-XRF results. For A-SR-XRF data, each symbol represents the mean
value of each fluid inclusion population analyzed in different samples (data from Table 1). Also shown is the field of Kaapvaal Proterozoic
seawater [13]. (d) S vs. Cl/Br plot for several individual fluid inclusions of bNorth Pole seawaterQ end-member and Fe-bearing fluids. Black
squares indicate detected sulfur concentrations in Fe-rich fluids with calculated 2r uncertainty. Open circles and associated bars represent
calculated sulfur detection limit and possible range of S concentrations in North Pole seawater end-member, respectively.
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463 459
ratios, between 350 and 390, close to bulk Earth value
(420) and possibly indicating mantle buffering.
Although evaporitic gypsum producing barite may
have formed at North Pole [25], low Cl/Br ratios in
Ba-rich inclusions suggest that the Ba-rich fluid
results from hydrothermal venting. Fe-rich fluid may
be linked to the pyrite-bearing chert veins. Because of
their lower metal content, the metal-depleted inclu-
sions could reflect a lower extend of fluid–rock
interaction or a lower degree of mixing between a
seawater end-member and hydrothermal fluids. Con-
servative halogen elements in the metal-depleted
inclusions would therefore have preserved the original
North Pole seawater Cl/Br ratio, which value of 630 is
similar to that of modern-day seawater (647, Fig. 5b).
The high Cl concentration (ca. four times the present-
day value) reflects a typical modern-day seawater
evaporation trend (Fig. 5c) in a shallow marine, closed
basin environment reminiscent of the geological
setting at North Pole.
Hydrothermally derived Cl/Br values are close to
bulk fluid Cl/Br estimates of 2.2 Gyr (=274 [13])
Fig. 6. General model for North Pole 3.5 Ga and Kaapvaal 2.2 Ga
seawaters. At the North Pole, Br is removed by organic matter buria
and the resulting seawater Cl/Br ratio is identical to modern
seawater value. In the Proterozoic Kaapvaal Ocean, halogen
geochemistry is mantle-buffered or reflects a low biologica
productivity. Low sulfate concentration in fluid inclusions sugges
a low sulfate concentration in the 3.5 Ga North Pole seawater
although removal of sulfate from the analyzed fluid by barite
precipitation and fluid–rock interaction cannot be excluded.
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463460
Kaapvaal seawater (Fig. 5c), thus providing support to
the interpretation that Kaapvaal seawater was buffered
by vent fluids and/or lacked sufficient organic matter
to fractionate Cl from Br [13] (see below). The
Kaapvaal results were obtained by bulk analysis and
could represent a mixture of different fluids. In this
work, crush-leach Cl/Br ratio and K and Ca concen-
trations are similar to the A-SR-XRF results for metal-
depleted fluid, indicating that metal-depleted inclu-
sions dominate in the intrapillow quartz pods in terms
of major elements.
In the hydrothermally active setting of North Pole,
changing composition of fluids circulating in basalts
(e.g., after a nearby eruption on the seafloor) could
have triggered local dissolution/recrystallization of
quartz grains, as supported by the complexity of
quartz grains microtexture (Fig 1c). Hence, different
fluid mixings could have been trapped until intra-
pillows sealed, resulting in the observed distribution of
inclusions with different compositions within single-
quartz grains a few tens of micrometers apart.
The intense influx of low-Cl/Br hydrothermal
fluids to the North Pole basin requires that a
balancing process existed to maintain a high Cl/Br
ratio in the seawater. NaCl cannot be invoked (see
above), and a mantle-derived source could not
increase Cl/Br above the bulk Earth value (420).
Modern organic matter is strongly enriched in heavy
halogens (Cl/Brc150 [35,36]) and if it is buried
rapidly, Br can be trapped in sediments resulting in a
Br-poor seawater [36,37]. Several lines of evidence
suggest that the conditions were favorable for this
process to occur at North Pole (Fig. 6). Isotopic
fractionation values recorded by kerogen-bearing
cherts [38–40] and microscopic pyrite [41] show
that vigorous microbial activity was taking place at
North Pole. In addition, the depositional environment
has been interpreted either as subaerial back-barrier
lagoons [25] or as tectonically active calderas linked
with hydrothermal emanations and volcanoclastic
deposits [26,27]. This implies that the water body
was isolated from the main ocean and may have
been small enough to record Br depletion through
organic matter burial in sediments. Mantle-buffered
Cl/Br composition of Proterozoic Kaapvaal seawater
[13] suggests that the composition of North Pole
seawater resulted from a specific process and may
have been different, at least in terms of Cl/Br, than
l
l
t
that of the rest of the Archaean and Proterozoic
ocean.
Sulfate concentration provides a direct mean of
evaluating the oxidation state of seawater. It is also a
proxy for atmospheric oxidation state as sulfate
accumulation in the ocean is a result of oxidative
weathering of land sulfides. Detection limit calcula-
tions suggest that the sulfur concentration in North Pole
seawater was between 0 and 8 mM (b8.6 mM in 4
metal-depleted inclusions, Table 1, Fig. 5d), much
lower than in the Fe-rich hydrothermal fluid (41–82
mM). Because A-SR-XRF analysis does not provide
information on the oxidation state of dissolved species
in the inclusions, the origin of the detected sulfur could
be elementary sulfur, H2S, sulfate, or another form. An
indication of the oxidation state of sulfur in the different
inclusion populations can be inferred indirectly using
geochemical and geological arguments. The formation
of massive hydrothermal barite veins and possibly of
evaporitic gypsum [25] requires that SO42� was
available in North Pole waters. Because of the very
low solubility of barium ions in the presence of SO42�,
the two species cannot have been transported in the
same fluid, indicating that the sulfate was primary and
derived directly from ambient seawater. The occur-
J. Foriel et al. / Earth and Planetary Science Letters 228 (2004) 451–463 461
rence of two distinct Ba- and Fe–S-bearing hydro-
thermal fluids suggests that the barite and chert veins
can represent two types of conduits for hydrothermal
fluids of different compositions, where upwelling Ba2+
solutions mixed with seawater sulfate and precipitated
conjointly with Fe–S-bearing fluids to form interlay-
ered barite beds and laminae of macroscopic pyrite. We
also confirm the presence of H2S-bearing fluid
inclusions lining overgrowth zones in interbedded
barite [42]. These inclusions are associated with
microscopic pyrite texturally similar to the highly
depleted 34S pyrite attributed to microbial sulfate
reduction [41]. Furthermore, if 41–82 mM of sulfur
was present as sulfate in the Fe-rich fluid inclusions, the
high concentrations of Ca (mean=500 mM for all Fe-
rich inclusions) or Ba (mean=2.3 mM) would induce
calcium and barium sulfate precipitation, which is not
observed. Therefore, we suggest that the sulfur species
in the Fe-rich vent fluids were most likely reduced
whereas sulfate was the dominant S form in North Pole
seawater.
The sulfate concentration measured in North Pole
seawater (0 to 8 mM, Fig. 5d), is much lower than in
present-day oceans (28 mM) and in the Black Sea
anoxic bottom waters (18 mM [43]), and hence does
not favor an oxic atmosphere model. The volume of
sulfate precipitation and evidences for microbial
sulfate reduction [41] have drawn some authors to
consider North Pole a sulfate-rich pond [25]. How-
ever, given the high Ca and Ba concentrations, even a
sulfate concentration much lower than 8 mM would
be sufficient to precipitate BaSO4 or CaSO4. Under an
oxygen-rich Archaean atmosphere [6,44], the relative
isolation of the North Pole basin and its exposure to
large detrital inputs would make it a very favorable
setting for the accumulation of sulfate produced by
subaerial sulfide oxidation. Therefore, the sulfate
content of North Pole seawater may be regarded as
indicative of local conditions and representing an
upper limit for the rest of the Archaean ocean. This
implies that the Early Archaean ocean as a whole
would have been dramatically depleted in sulfate, with
values possibly lower than 1–0.2 mM as imposed by
the isotopic record of Archaean sedimentary sulfides
[5,41,45]. An alternative source for the sulfate
involved in microbial sulfate reduction and barite
formation could be bacterial oxidation [3,46] of the
sulfide emitted by vent fluids.
6. Conclusion
A-SR-XRF analyses of fluid inclusions can bring
unique and direct information about ancient seawater
and hydrothermal fluids composition and their coevo-
lution with local ecosystem and environment. Our
results suggest that biological activity at North Pole
ca. 3.5 Gyr ago was likely efficient enough to control
the seawater halogen composition. However, the
sedimentary Br-rich phase required by this model
has not been identified and the nature of involved
organisms remains unknown.
Precise analysis of fluid inclusions sulfur content
supports models of a reducing Archaean ocean and
atmosphere. It must be stressed that, although the
composition of the seawater end-member may be
related to a mixing with hydrothermal fluids, a
limited extension of water–rock interaction, which
could have removed SO42� from seawater, cannot be
excluded. Low sulfate concentration could also be
due to an intense activity of sulfate-reducing bacteria
or to the scavenging of water sulfate by barite
precipitation.
This work represents the basis for a much more
thorough analysis of North Pole fluids which will be
required to better constrain the influence of life on
seawater composition and the behavior of sulfate in
the Early Archaean ocean.
Acknowledgments
We thank J. Cauzid for providing A-SR-XRF data
processing software and A. Simionovici and S.
Bohic for assistance during A-SR-XRF experiments.
We acknowledge Pr. C.M.R. Fowler and two
anonymous reviewers for their constructive com-
ments. This work was supported by a GDR
Exobiologie and a GEOMEX grant to P. Philippot
and an ARC grant to P. Rey. This is IPGP contribution
No. 2920.
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