Strontium isotopes as indicators of diagenetic recrystallization scales within carbonate rocks

Abstract Sr isotopic compositions and trace-element concentrations of limestones at millimetre and metre scales were investigated to provide information on diagenetic recrystallization of carbonate rocks. Several samples spaced a few millimetres apart around a 3-mm-thick calcite vein in a hand-sized specimen of upper Cambrian micrite limestone were examined to assess the effect of restricted fluid flow on the distribution of trace elements during late-diagenesic recrystallization of the rock. The recrystallization effect was confined to a zone ∼ 8-mm wide, characterized by an increase in 87 Sr/ 86 Sr from 0.7100 to 0.7112, an increase in Rb content from ∼ 0.7 to ∼ 2 μ g −1 , and a decrease in Sr content from ∼ 319 to ∼ 105,μg g −1 in the wall rock toward the vein. The profile of Sr distribution in the wall rock, which experienced recrystallization under conditions of restricted fluid flow, can be explained in terms of distribution coefficient, while that of the Rb probably relates to precipitation of an alkali-element-enriched aluminosilicate phase next to the vein. A lower Permian micritic limestone bed, ∼ 1 m thick and intercalated between shales, was found to have an upward increase of Sr contents from 193 to 261μ g −1 , and correlated decreases in both Rb and Mn contents from 1.7 to 0.2 μ g −1 and from 41 to 31 μ g −1 , respectively. The 87 Sr/ 86 Sr remained nearly constant at ∼0.70825, which is nearly the Sr isotopic value of seawater during the time of deposition of the limestone. The chemical and Sr isotopic variations across the limestone bed are attributed to early diagenesis under conditions of varied restricted fluid movements.