THE EVOLUTION OF THE LORRAINE EVAPORITE BASIN: IMPLICATIONS FOR THE CHEMICAL AND ISOTOPE COMPOSITION OF THE TRIASSIC OCEAN

Abstract

During the Upper Triassic times about 75 m of halite with minor anhydrite was deposited in the Lorraine basin. Samples of halite and sulfates were systematically collected from a borehole intersecting the sequence. The chemical evolution of the original brine was investigated by X-ray microanalysis of primary inclusions trapped in halite. The original brines were of the Na-Mg-K-Ca-Cl type, being consistent with the mineral association of halite and minor anhydrite. Their solute content can be reproduced by evaporation of present day ocean composition in an open basin. However, some modifications are required. The inflow of a small excess of CaCl2 into the basin (1.2 to 1.9% of the total recharge) is required to explain the low SO4 values analyzed in fluid inclusions. The same inflow helps to explain more satisfactorily the low Na and high Cl contents. The back-reaction of the brine with previous Ca-sulfate that forms polyhalite could explain the low and variable K values analyzed. The excess of Ca with respect to present day seawater, and the formation of SO4-poor evaporite sequences is the rule rather than the exception in the geological record. We have not been able to assess the ultimate origin of the Ca excess in the basin. Nevertheless, the great variation of the SO4 content within short vertical distances (centimeter scale), suggests that these variations are due to processes taking place within the evaporite basin, rather than to changes in the composition of the ocean at a global scale. The isotopic composition of sulfur and oxygen was determined using samples of sulfates from the same borehole. The δ34S values of 21 samples analyzed show a narrow variation: δ34S=+15.5+/-0.4%%. The δ18O values of eight samples analyzed show a wider range of variation: δ18O=+13.0+/-1.1%%. None of the processes which modify the δ values, such as sulfate reduction or additional sulfate sources (continental, recycling of sulfates from the margins), were clearly identified. The values of δ34SSW=+15.7 to +14.5%% and δ18OSW=+13.7 to +12.2%% for the sulfate dissolved in the Triassic ocean explain the δ values of the sulfates interbedded in the halite of the Lorraine basin, and most analyses reported in the literature.