Strontium Geochemistry of Modern and Ancient Calcium Sulphate Minerals

Abstract

The study of Sr⁺² distribution in calcium sulphate rocks has been extensive, but largely descriptive. This paper demonstrates the complexity of Sr⁺² diagenesis in sedimentary gypsum and anhydrite in several modern evaporite environments. Distribution of Sr⁺² in these sedimentary sulphates accumulated during the progressive evolution of sea water to a brine is controlled by the solubility products and partition coefficients of aragonite, gypsum, celestite, dolomite, and anhydrite. In arid, supratidal, evaporite environments, this control is modified according to whether the supratidal sediments are dominantly either silici-clastic or carbonate in composition. The absence of dolomitization as in silici-clastic evaporite environments, or presence of dolomitization as in certain carbonate evaporite environments exerts a dominant control on the path of Sr⁺² diagenesis. In the absence of dolomitization, the mass ratio Sr⁺²/Ca⁺² in brines saturated with respect to aragonite, gypsum/anhydrite and celestite rapidly stabilizes at ^~2.2 × 10−² and remains at this value into, and probably beyond, halite saturation. Dolomitization in similar brines causes the ratio to fall to a value of less than 2.2 × 10−². The magnitude of the departure is dependent on the degree of dolomitization. In carbonate-evaporite sediments which approach 100% dolomitization, the ratio Sr⁺²/Ca⁺² in brines stabilizes between 0.9 × 10−² and 1.0 × 10−². Because the concentration of Sr⁺² in gypsum/anhydrite is a product of their respective distribution coefficients and the Sr⁺²/Ca⁺² ratio in brines, Sr⁺² in sedimentary sulphates is dependent on the absence or the degree of dolomitization. In other words, Sr⁺² concentration in sulphates is a function of the nature of their environment of formation.

Mass balance calculations between Sr⁺² in brines and Sr⁺² in gypsum/anhydrite from modern evaporite environments permit an evaluation of the average values for the partition coefficient of gypsum (k ^G_Sr ≃ 0.18) and anhydrite k ^A_Sr ≃ 0.37).

Strontium concentrations in anhydrite rocks can be broadly grouped according to the structural type of anhydrite (mosaic vs. laminated). Certain laminated, dolomite free anhydrites have precipitated/equilibrated with brines with estimated Sr⁺²/Ca⁺² ratios of ^~2.3 × 10−²; mosaic anhydrites with dolomite matrix have precipitated/equilibrated with brines with estimated Sr⁺²/Ca⁺² ratios of between 0.9 × 10−² and 1.4 × 10−².

Fluid chemistry, and structural type anhydrite-dolomite association in modern evaporite environments suggest 1) that most laminated anhydrites precipitated/equilibrated with brines saturated with respect to gypsum/anhydrite and celestite, and that accumulation probably occurred at a sediment-free brine interface, and 2) that most mosaic anhydrites precipitated/equilibrated with brines saturated with respect to gypsum/anhydrite, celestite and dolomite and that accumulation probably occurred in an arid, supratidal setting.