Low-sulphate concentration and biological control of Cl/Br ratio in the early Archaean ocean

J. Foriel, P. Philippot, P. Rey, A. Somogyi*, D. Banks**** and B. Menez*

* IPGP, Laboratoire de Géosciences Marines,
UMR 7097, Tour 26, 4 Place Jussieu, 75252 Paris Cedex 05, France.
School of Geosciences, Edgeworth Building F05,
The University of Sydney, Sydney NSW 2006, Australia
*European Synchrotron Radiation Facility, beamline ID-22, BP 220, 38043 Grenoble cedex 9, France
****School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

in: Earth Planetary Science Letters (2004), Vol.228, 2, pp. 451-463.

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.

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 acknowl edge Pr. C. M. R. Fowl er and t wo anonymous reviewers for their constructive comment s . Thi s wor k was s uppor t ed by a GDR Exobiologie and a GEOMEX grant to P. Philippot and an ARC grant to P. Rey. This is IPGP contribution No. 2920.3.

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