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

During the Upper Triassic times about 75 m of halite with minor anhydr ite was deposited in the Lorraine basin. Samples of halite and sulfate s were systematically collected from a borehole intersecting the seque nce. The chemical evolution of the original brine was investigated by X-ray microanalysis of primary inclusions trapped in halite. The origi nal brines were of the Na-Mg-K-Ca-Cl type, being consistent with the m ineral 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 rech arge) is required to explain the low SO4 values analyzed in fluid incl usions. The same inflow helps to explain more satisfactorily the low N a and high Cl contents. The back-reaction of the brine with previous C a-sulfate that forms polyhalite could explain the low and variable K v alues 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 distanc es (centimeter scale), suggests that these variations are due to proce sses taking place within the evaporite basin, rather than to changes i n the composition of the ocean at a global scale. The isotopic composi tion of sulfur and oxygen was determined using samples of sulfates fro m the same borehole. The delta(34)S values of 21 samples analyzed show a narrow variation: delta(34)S = + 15.5 +/- 0.4 parts per thousand. T he delta(18)O values of eight samples analyzed show a wider range of v ariation: delta(18)O = + 13.0 +/- 1.1 parts per thousand. None of the processes which modify the S values, such as sulfate reduction or addi tional sulfate sources (continental, recycling of sulfates from the ma rgins), were clearly identified. The values of delta(34)S(SW) = + 15.7 to + 14.5 parts per thousand and delta(18)O(SW) = + 13.7 to + 12.2 pa rts per thousand for the sulfate dissolved in the Triassic ocean expla in the delta values of the sulfates interbedded in the halite of the L orraine basin, and most analyses reported in the literature. (C) 1998 Elsevier Science B.V. All rights reserved.