Determination par activation neutronique des rapports Cl/Br des inclusions fluides de divers gypses. Correlation avec les donnees de la microcryoscopie et interpretations genetiques

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RÉSUMÉ

Le gypse, comme la plupart des minéraux, contient en inclusions des liquides contemporains de la cristallisation dont l'analyse permet de connaítre la composition du milieu de genèse—le rôle des circulations secondaires devant être discuté.

Nous avons choisi de déterminer les rapports Cl/Br des inclusions car ce rapport est constant dans l'eau marine actuelle et bien connu dans de nombreux types d'eaux naturelles qu'il caractérise.

L'activation neutronique permet de doser les très faibles quantités de chlore et de brome qui se trouvent dans les cavités mesurant en moyenne 10−6mm3.

Cette technique n'a encore été que très rarement utilisée dans l'etude des inclusions et la méthode mise au point ici écarte tout risque de pollution et donne directement le rapport Cl/Br (á 10% près) après une seule irradiation par échantillon.

Nous rapportons les résultats de 58 manipulations faites sur une douzaine de gisements de types différents; leur étude microcryoscopique avait été effectuée au préalable.

La connaissance des salinités (en équivalents NaCl) et des rapports Cl/Br des inclusions liquides nous permet de proposer des hypothèses génétiques pour les gisements anciens d'origine discutée, aprés comparaison avec les gisements actuels ou récents.

Ont étéétudiés des cristaux de gypse primaires de marais salants de France, de mangrove de Nouvelle-Calédonie, de la Sebkha el Melah en Tunisie, ainsi que des cristaux éocènes du bassin de Paris, oligocènes du Sud de la France et miocènes de Sicile.

ABSTRACT

Gypsum contains liquid inclusions which fill mainly primary cavities. Analysis of inclusions may give some information about the mineral formation process. In places, diagenetic solutions have filled secondary cavities along cleavage planes or replaced initial liquid inclusions in primary cavities; the presence of such secondary inclusions must, therefore, be taken into consideration. This study presents our results on the Cl/Br ratio of liquid inclusions in gypsum.

The Cl/Br ratio is practically constant in present sea water, and has been measured in many different types of waters. The Br content of halite has also been extensively studied to trace the compositional changes of a salt-depositing brine. In gypsum Br is present only in microscopic fluid inclusions. For this reason we used the neutron-activation method. We adopted the technique in order to prevent any contamination and obtain directly Cl/Br ratio within 10% accuracy. In order to ascertain that liquid inclusions reside mainly in primary cavities of gypsum, we selected our samples only after microscopic observation. Fifty-eight samples from different types of gypsum were analysed. The salinities of the inclusions, obtained through the data on freezing, and the Cl/Br ratio, obtained through activation-analysis, give us clues as to the nature of the fluids depositing the gypsum. Recent gypsum deposits from French salt-pans of Mediterranean and Atlantic coasts were sampled. The latter has apparently been diagenetically altered by meteoric water; the salinity of many inclusions having been changed while the Cl/Br ratio (315 and 350) in both remains close to the value of present sea water (#300). Liquid inclusions of Recent crystals from New Caledonia, deposited in mangrove swamps are enriched in Br (Cl/Br = 150 and 200). The enrichment might be related to the presence of abundant organic materials. Inclusions in crystals from Sebkha el Melah (Tunisia) also show high Br content (Cl/Br = 194). In this case, the brine was highly concentrated and was saturated with NaCl.

The Upper Miocene gypsum from Sicily was studied to test the various models for the genesis of this important evaporite formation of the Mediterranean. Freezing data show some decrease of salinities, probably by groundwater diagenesis, but the Br content is high. The Cl/Br ratio is 175, and this value is similar to that of Sebkha el Melah. Samples from three thick Eocene and Oligocene sequences near Paris and in the South of France were studied. Freezing data and Cl/Br ratio of their inclusions indicate that those gypsum deposits have crystallized in marine environments receiving considerable influx of river-waters.

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