MG ISOTOPIC SYSTEMATICS OF RIBBON ROCKS AND ITS IMPLICATIONS IN CONSTRAINING THE DIAGENETIC PROCESSES

Ribbon rocks, or marlstone-limestone alternations, consisting of alternating depositions of limestone and marlstone layers of mm- to cm thick, are widely discovered in the rock records. It has been proposed that the rhythmic depositions of limestone or marlstone layers reflect the cyclic changes in the depositional environments, and limestone and marlstone layers have undergone different diagenetic processes. Thus, ribbon rocks can be used to constrain high frequent environmental changes and diagenesis of carbonate rocks. The seawater magnesium (Mg) isotopic composition (δ²⁶Mg_SW) is controlled by the marine Mg cycle, i.e., the intensity of continental weathering, marine dolostone formation, authigenic clay mineral formation, and hydrothermal activities. Reconstruction of δ²⁶Mg_SW can be approached with Mg isotopes of marine carbonate (δ²⁶Mg_carb), but carbonate rocks would undergo various diagenetic alterations. In this study, we measured δ²⁶Mg_carb of ribbon rocks from three Formation, the middle Cambrian Xuzhuang Formation, the early Ordovician Yeli Formation, and the middle Triassic Qingyan Formation. Ribbon rock samples from three formations show wide range of variation in δ²⁶Mg_carb, ranging from from -1.73‰ to -3.51‰, -2.14‰ to -3.21‰ and -1.23‰ to -3.39‰. The limestone layers normally have lower δ²⁶Mg_carb values than the marlstone layers. In addition, δ²⁶Mg_carb values show positive correlations with Mg/Ca ratios, reflecting the effect of dolomitization, and with K and Al content, presumably indicating the effect of authigenic clay mineral formation. Our study indicates that the Mg isotopic system of ribbon rocks can be applied to constrain diagenetic processes of carbonate rocks..