U–Pb geochronology and clumped isotope thermometry study of Neoproterozoic dolomites from China
Lei Jiang, Anjiang Shen, Zichen Wang, Anping Hu, Yongsheng Wang, Xianying Luo, Feng Liang, Karem Azmy, Liyin Pan
Abstract
Abstract The higher abundance of dolomite in the Neoproterozoic compared with that of the Phanerozoic has been linked to elevated marine Mg/Ca ratios, and a hypothesized ‘dolomite sea’ that promoted widespread syn‐sedimentary fibrous dolomite cement precipitation. This article provides new uranium–lead (U–Pb) chronology, clumped isotope thermometry ( T Δ47 ), combined with δ 13 C and δ 18 O, and trace element geochemistry measurements for different types of dolomite, including bulk dolomite matrix (BD), fibrous dolomite cements (DC1) and late sparry dolomite cements (DC2 and DC3), in three Neoproterozoic dolostone profiles from China. The Ediacaran U–Pb ages coupled with nearly identical, low T Δ47 temperatures (average at ca 55‐60°C) for BD and DC1 dolomites, imply that these dolomites were either syn‐depositional or early diagenetic. Some DC1 dolomites yield slightly younger ( ca 20 to 50 Ma) U–Pb ages than the BD dolomites, suggesting that they were diagenetic products formed after matrix dolomitization. The estimates of δ 18 O water values for dolomitizing fluids suggest that both DC1 and BD dolomites were precipitated from highly evaporated seawater. In contrast, DC2 and DC3 dolomites filling in vugs and fractures have much younger U–Pb ages (mostly <525 Ma), higher T Δ47 temperatures (>95°C), and lower Sr and U contents, which is consistent with the paragenetic sequence that they were deeper burial diagenetic products. A synthesized diagenetic model with evaporative dolomitizing water evolved from anoxic, ferruginous to sub‐oxic and/or oxic, non‐ferruginous conditions is responsible for the decrease of Fe and Mn concentrations in global DC1 and BD dolomites throughout Neoproterozoic time. This study suggests that the widely distributed Neoproterozoic fibrous dolomite cements may be a later diagenetic feature, and thus calls into question their use in palaeoclimate reconstruction and as a principal line of evidence for the Neoproterozoic ‘dolomite sea’ hypothesis.