Geochemistry and origin of formation brines from the Paris Basin, France: 1. Brines associated with Triassic salts

Abstract

Small amounts of brines associated with Triassic (Keuper) layered halite from the eastern Paris Basin have been collected during salt mining operations. Critical consideration of the possible reactions and interactions involving Br − indicates that Cl − Br − ratio values are conservative except in two cases: (1) precipitation or dissolution of solid chlorides; and (2) convective or diffusive mixing of saline solutions. Calculations suggest that halite recrystallization could be a source of aqueous Br −, but this process is probably not significant under natural conditions as Triassic salt has retained a Br − content higher than calculated for equilibrium conditions. Bromide-chloride data for the brines associated with Triassic salts are presented on a log-log diagram of Cl − Br − vs. Cl −, including the evolution of present-day marine solutions and literature values for the chloride salts between halite and tachyhydrite ((CaCl 2, 2MgCl 2)·12H 2O) deposition. This diagram allows discernment of primary or secondary origins of brines. Chemical variations of major and trace elements during the evaporative concentration of seawater are estimated from selected literature and from additional measurements of salt ponds on the Mediterranean shore. Low values of the Cl − Br − ratios in brines from Triassic salt can be reconciled with high Na + contents if the sampled solutions are mixtures of a secondary brine (halite dissolution) and a highly evolved primary brine. Mass-balance considerations indicate that the primary brine component was more concentrated than the stage of bischofite (MgCl 2·6H 2O) deposition. Contents of Br −, Cl −, Na + and K + can be accounted for by a conservative behaviour of these ions, possibly slightly affected by some precipitation of NaCl induced by the mixing of the two types of solutions. The secondary brine component would have contributed all Na + and K + through halite and sylvite (KCl) dissolution. Several hypotheses are discussed for the origin of the high contents in Li +: diagenesis, halite dissolution, global or local enrichment of Triassic seawater, and contribution from the primary brine. The high Li + concentration is attributed to the highly concentrated brine component. Rubidium would also behave conservatively after all K + has been removed by sylvite and carnallite ((KCl,MgCl 2)·6H 2O) precipitation. Boron, Ca 2+, Sr 2+, Mg 2+ and SO 2− 4 are not conservative. Mass-balance calculations indicate that the high content in Ca 2+ and in Sr 2+ imposes a diagenetic origin. The values of the 87 Sr 86 Sr isotope ratio are clearly higher than the Keuper marine values of the literature, which could be explained by a major contribution of reworked Permian evaporites with high 87Sr content. Such hypothesis would be in agreement with the low heavy-isotope content of aqueous SO 2− 4. Dolomitization, either by the primary brine or by the mixing of primary and secondary brines, would have implied Ca-sulphate precipitation, in agreement with the observed additional depletion in heavy isotopes of aqueous SO 2− 4, relative to anhydrite deposits from Triassic layers of the Paris Basin. Stable isotope contents ( 2H and 18O) of the brines can be accounted for by mixings of a secondary brine of continental origin with a primary brine enriched in 18O by evaporation. In conclusion, emphasis is put on the conservation of Li +, the near-conservative behaviour of the elements Cl −, Br −, Na +, K + and Rb +, whereas diagenetic reactions control B 3+, Mg 2+, Ca 2+, Sr 2+ and SO 2− 4.