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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

[email protected]

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

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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.

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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.


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

.

.

,


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.


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.


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.


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-


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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