Conservative behavior of Mg isotopes in massive dolostones: From diagenesis to hydrothermal reworking

Abstract

Mg isotopes in syndepositional dolomite have been suggested to be a potential proxy for understanding seawater chemistry. However, it is argued that the δ26Mg values of dolomite could be complicated by the effects of early diagenesis and later hydrothermal activities. Further investigations into the behaviors of Mg isotopes in dolomitization systems are needed to resolve this controversy. In the present study, we investigated early Ordovician dolostones from the Tarim Basin, including diagenetically altered dolomites, hydrothermally altered dolomites and well-preserved dolostones that precipitated in the slope, margin and interior of a carbonate platform. Different types of dolomites, bulk dolostone and limestone were sampled by microdrilling for analyses of C-O-Mg isotope compositions and REE concentrations. The dolostone δ13C values match those of coeval seawater, and the REE distribution patterns in the dolostones are comparable with those in the limestones, indicating that the dolostones originated from syndiagenetic dolomitization. The δ26Mg values of the various syndiagenetic dolomites that formed in the slope, margin and interior of the carbonate platform are similar, averaging approximately −2.06‰ ± 0.20‰. No stratigraphic variability in the dolomite Mg isotopes can be discerned, which implies that the Mg isotope compositions of the porewater were homogeneous during massive dolomitization and remained in equilibrium with seawater. Additionally, the δ26Mg values of the altered dolostone do not show correlations with diagenetic and hydrothermal signals, demonstrating that dolomite Mg isotopes are insensitive to postdepositional alteration. Given these facts, we propose that Mg isotopes in dolostones have conservative behaviors during diagenesis and late stage hydrothermal reworking.