PACROFI VI - Electronic Program


Changes in brine and fluid inclusion composition during the precipitation of halite out of evaporating seawater

Heide Zimmermann

IGDL, Universität Göttingen, Goldschmidtstr.3, D-37077 Göttingen


Introduction

During the last decade an increasing effort was made to understand paleo seawater composition by studying fluid inclusions in halite (e.g. Horita et al., 1991; Ayora et al., 1994). The evolution of major brine components during the evaporation of modern seawater was thoroughly studied by Herrmann et al. (1973), Brantley et al. (1984), and McCaffrey et al. (1987). But there exists only little information on the composition of fluid inclusions trapped in modern halite compared to the parent brines.

Analytical Procedures

Fluid inclusions > 200 microns in size were extracted using a microdrill and a capillary with a uniform inner diameter of 25 microns. 100 microliters of distilled water were added to the extracted brine. Brines and extracted fluid inclusions samples were analyzed for Cl-, SO42-, Br-, F-, Na+, K+, Mg2+, Ca2+, and Li+ by ion chromatography (IC) using a DIONEX 500. B, Ba, Cs, I, Li, Mo, Rb, and Sr were measured with a FISONS INSTRUMENTS Plasmaquad mass spectrometer PQ2+ (ICP-MS).

Results

The collected brines represent degrees of evaporation (D.E.) between 1 and 30. The precipitation of aragonite starts at D.E.=1.8, gypsum crystallizes at D.E.=4, and finally halite at D.E.=10. This agrees well with the studies of Brantley et al. (1984) and McCaffrey et al. (1987). Herrmann et al. (1973) observe aragonite precipitation at D.E.=4 and gypsum precipitation at D.E.=6. But the re-interpretation of their data seems to show that both minerals start to precipitate more or less simultaneously at D.E.=4. Up to D.E.=30 potassium and magnesium are the only main components that are concentrated quantitatively in the evaporating brines.

Sr, Ba, and I are removed from the brine at a D.E.of about 6. The decrease of Sr and especially Ba contents do not seem to be related to the precipitation of gypsum. It is more likely that there occurs an independant Sr phase, such as celestite. The observed decrease of iodine contents during evporation is not yet resolved. Li, Rb, B, and Br are almost quantitatively accumulated in the evaporating brines, whereas the continuously increasing concentration of Mo reaches only 80 % of the expected value at D.E.=10. During evaporation the molar ratios of Rb/Br (0.0025) and B/Br (0.5) remain fairly constant, so that Carribean seawater and evaporating brines plot on a distinct area in a Rb/Br - B/Br diagram.

To our surprise first analyses of fluid inclusions in modern halite have shown significantly higher K, SO4 ,and Br contents compared to the evaporating parent brine (Zimmermann & Wehebrink, 1995). This might indicate that brine and trapped fluid represent different degrees of evaporation, e.g. due to dilution by rainfall. In this case the observed Mg contents of the fluid inclusions are expected to be higher than observed , since K and Mg show the same trend during evaporation of seawater. If we assume a selective chemical fractionation during the trapping process, this would have an tremendous impact on the interpretation of fluid inclusion data for modern and ancient saltforming processes. To sort this out we present additional and more reliable data.

References