Strontium distribution and origins in a natural clayey formation (Callovian-Oxfordian, Paris Basin, France): A new sequential extraction procedure

摘要

Strontium is a good monitor of geochemical processes in natural clayey formations. In the Callovian-Oxfordian formation of Bure in France, strontium is sorbed on clay minerals and carried by carbonates, detrital minerals and accessory celestite. In order to determine the strontium distribution among these different phases, four-step sequential extractions (I. cobalt hexamine trichloride, 2. acetic acid, 3. EDTA and 4. tri-acid) were performed on samples from different levels of the clayey formation. The leachates were also analyzed for strontium isotopes, in order to determine the strontium origins. This sequential procedure is well suited to determining strontium distribution in claystones, although it is less efficient in clay-rich limestones and in celestite-rich samples. The carbonates (38-47% of the total strontium) show Sr-87/Sr-86 ratios (0.7070-0.7071) that have recorded the isotopic composition of the Callovian-Oxfordian seawater. Diagenetic carbonates (dolomite, ankerite and siderite) have almost not incorporated any strontium, which has been trapped by celestite during the late diagenesis. The major part of the celestite shows Sr-87/Sr-86 ratios (0.7069-0.7070) quite close to the primary carbonates. However, a second generation of celestite (0.7074) shows a slight Sr-87-enrichment and is isotopically in equilibrium with the exchangeable strontium (27-48% of the total strontium with a mean Sr-87/Sr-86 value of 0.70745) and the present-day porewater (0.7074). This very low Sr-87-enrichment could be explained by the partial destabilisation of detrital minerals (feldspars, micas, clays) which exhibit 87Sr/86Sr ratios consistent with their pristine Hercynian origin (0.7229-0.7350). Diffusion of strontium from the subjacent Dogger aquifers (0.7076-0.7082) could also be invoked to explain the slight 87Sr-enrichment.