PRECAMBRIAN CHERTS AND THE EVOLUTION OF SEAWATER HALOGEN RATIOS

Organisms play an important role in modifying the chemistry of ocean waters, notably halogen abundances. Chlorine and, to a lesser extent, bromine behave conservatively, whereas iodine is strongly depleted by biogenic uptake, principally by marine algae (1). The consequence is an I/Cl ratio in modern seawater two orders of magnitude lower than the chondritic value (Br/Cl ratio is two thirds chondritic). Thus changes in seawater I/Cl potentially track the evolution of metabolic processes.

Recent fluid inclusion studies of ironstones and pillow lavas have attempted to determine halogen ratios in Precambrian seawater. However, the low concentration of iodine, the effects of basalt-seawater chemical exchange reactions, and the possibility of later fluid entrapment have hampered the interpretation of these fluids in terms of early oceans (2, 3). Our approach has been to study Precambrian chert samples. We use noble gas mass spectrometry of neutron-irradiated samples, allowing simultaneous determination of Ar and neutron-produced noble gas isotopes derived from Cl, Br and I .

It is well known that cherts trap ions during formation and subsequent diagenesis, but the extent to which present-day ratios reflect the ambient concentrations at the time of their formation is uncertain. A detailed study of chert from the Dales Gorge Member of the Hamersley Group (Western Australia) has revealed Br/Cl and 36Ar/Cl ratios similar to modern seawater and an I/Cl ratio ~40 times the seawater value. There is no correlation between iodine and iron, suggesting that adsorption on iron oxide bands within the chert is not responsible for high I/Cl.

Preliminary data from 17 Precambrian cherts indicate a significant decrease in I/Cl from 3.5 to 2.8 Ga. Elevated ratios may reflect a higher seawater I/Cl ratio or may be due to uptake of iodine into organic matter incorporated into the chert.

1. G. T. F. Wong, Reviews in aquatic science 4, 45 (1991).

2. D. M. D. Channer et al., Earth and Planetary Science Letters 150, 325 (1997).

3. J. Gutzmer et al., Chemical geology 201, 37 (2003).