Terminal Ediacaran marine dolomite cements: Primary precipitation or mimetic dolomitisation

ABSTRACT

The reliability of marine dolomite cements as archives of seawater chemistry is debated due to the difficulty in distinguishing primary precipitates from diagenetic overprints. According to the optical characteristics of the mineral (e.g. crystal habit, luminescence), length‐slow cements are considered as primary dolomite, while length‐fast dolomites are supposed to be formed by mimetic dolomitisation. However, a diagnostic geochemical tool to differentiate primary precipitation from mimetic replacement is currently lacking. To address this, this study aimed to elucidate the origin of terminal Ediacaran dolomite cements, provide geochemical constraints on the mechanisms of mimetic dolomitisation and primary precipitation, and evaluate their potential for reconstructing oceanic conditions. Based on petrographic and mineralogical analysis, Mg, C and O isotopes were analysed in fibrous marine and later diagenetic sparry dolomite cements from the terminal Ediacaran Dengying Formation in the Sichuan Basin, South China. The δ ²⁶ Mg values of fascicular fast dolomite cements show a narrow range and are heavier than the micritic‐micro dolomite matrix, indicating mimetic dolomitisation of carbonate precursor. The systematic broader range in δ ²⁶ Mg values of length‐slow dolomite might arise from kinetic Mg isotope fractionation resulting from variations in precipitation rates. The δ ²⁶ Mg values of later diagenetic sparry dolomites vary significantly, accompanied by elevated Fe, Mn and Sr contents, indicating alterations of diagenetic fluids during the burial stage. Thus, the primary length‐slow dolomite cement, directly precipitated from the Ediacaran ‘Dolomite Sea’, may be a robust archive for reconstructing Precambrian Ocean conditions. For the reconstruction of isotopic composition that has a strong dependence on kinetic isotope fractionation, it is important to understand the process‐associated fractionation. Therefore, to develop robust paleo‐proxy tools, detailed petrological and diagenetic evaluations, along with the assessment of process‐associated kinetic isotope effects, are highly recommended.