GEOCHEMISTRY AND ORIGIN OF FORMATION BRINES FROM THE PARIS BASIN, FRANCE .1. BRINES ASSOCIATED WITH TRIASSIC SALTS

Small amounts of brines associated with Triassic (Keuper) layered hali te from the eastern Paris Basin have been collected during salt mining operations. Critical consideration of the possible reactions and inte ractions involving Br- indicates that Cl-/Br- ratio values are conserv ative except in two cases: (1) precipitation or dissolution of solid c hlorides; and (2) convective or diffusive mixing of saline solutions. Calculations suggest that halite recrystallization could be a source o f aqueous Br-, but this process is probably not significant under natu ral 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 diagr am of Cl-/Br- vs. Cl-, including the evolution of present-day marine s olutions and literature values for the chloride salts between halite a nd tachyhydrite ((CaCl2,2MgCl2).12H2O) deposition. This diagram allows discernment of primary or secondary origins of brines. Chemical varia tions of major and trace elements during the evaporative concentration of seawater are estimated from selected literature and from additiona l measurements of salt ponds on the Mediterranean shore. Low values of the Cl-/Br- ratios in brines from Triassic salt can be reconciled wit h high Na+ contents if the sampled solutions are mixtures of a seconda ry 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 (MgCl2.6H2O) deposition . Contents of Br-, Cl-, Na+ and K+ can be accounted for by a conservat ive behaviour of these ions, possibly slightly affected by some precip itation of NaCl induced by the mixing of the two types of solutions. T he secondary brine component would have contributed all Na+ and K+ thr ough halite and sylvite (KCl) dissolution. Several hypotheses are disc ussed for the origin of the high contents in Li+: diagenesis, halite d issolution, global or local enrichment of Triassic seawater, and contr ibution from the primary brine. The high Li+ concentration is attribut ed to the highly concentrated brine component. Rubidium would also beh ave conservatively after all K+ has been removed by sylvite and carnal lite ((KCl,MgCl2).6H2O) precipitation. Boron, Ca2+, Sr2+, Mg2+ and SO4 2- are not conservative. Mass-balance calculations indicate that the h igh content in Ca2+ and in Sr2+ imposes a diagenetic origin. The value s of the Sr-87/Sr-86 isotope ratio are clearly higher than the Keuper marine values of the literature, which could be explained by a major c ontribution of reworked Permian evaporites with high Sr-87 content. Su ch hypothesis would be in agreement with the low heavy-isotope content of aqueous SO42-. Dolomitization, either by the primary brine or by t he mixing of primary and secondary brines, would have implied Ca-sulph ate precipitation, in agreement with the observed additional depletion in heavy isotopes of aqueous SO42-, relative to anhydrite deposits fr om Triassic layers of the Paris Basin. Stable isotope contents (H-2 an d O-18) of the brines can be accounted for by mixings of a secondary b rine of continental origin with a primary brine enriched in O-18 by ev aporation. 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 B3+, Mg2+, Ca2+, Sr2+ and SO 42-.