Two-phase diagenetic alteration of carbonate matrix: Middle Ordovician limestones, Taiyuan City area, China

Abstract

Both pseudospar and micrite are present in carbonate matrix of Middle Ordovician wackestones in the Taiyuan City area, China. Under the scanning electron microscope, pseudospar crystals display unpitted surfaces and an absence of aragonite relics (in polished and slightly etched sections). Micritic crystals show a similar feature, with slightly pitted surfaces and absence of aragonite relics. This possibly indicates that both pseudospar and micrite have a similar original mineralogy (i.e. as calcites). This is further supported by their low Sr contents (<417 ppm). The geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix. Micrite was possibly formed in near-surface meteoric environments, but pseudospar, which is characterized by a population of more depleted δ 18O values (−9.3 to −10.2%. PDB) than that of micrite (−7.4 to −8.7%. PDB), is explained to have formed during shallow-burial diagenesis because its δ 18O composition is too light to be accounted for by near-surface meteoric origin. The lower Sr contents (<417 ppm) of both pseudospar and micrite, in conjunction with their more depleted δ 18O values (−7.4 to −10.2%. PDB), are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites. The more negative and wider range of δ 13C values of pseudospar and micrite (−0.3 to −4.7%.), in comparison with those of other coeval meteoric calcites, was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis. Although the contents of Fe and Mn between pseudospar and micrine overlap widely, an inverse relationship exists between the concentrations of Fe and Mn and the δ 13C values in both pseudospar and micrite. This can be accounted for by variable redox by variable redox conditions during diagenesis. The δ 18O values of void-fill sparry calcite cements in micrite (−8.2 to −8.6%. PDB) fall into the category of near-surface meteoric diagenesis, possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations, so that only minor amount of pseudospars were formed during shallow-burial diagenesis by recrystallization.