Maturation of Neogene dolomite from Xuande Atoll of Xisha archipelago, the South China Sea

Under cathodoluminescence, dolomite cements are mostly dull red to nonluminescent or zoned, whereas the replacive matrix dolomite exhibits a homogeneous greyish-red cathodoluminescence. Whole-rock elemental analyses show that dolomite rock is characterized by low concentrations of Mn (<30 ppm), Fe (<250 ppm), and Sr (<200 ppm), implying that dolomitization fluid chemistry derived from seawater. The δ13C values (1.76-3.49‰) indicate that diagenetic fluids were buffered by carbonate rocks. Positive δ18O values of dolomite, associated with the absence of primary evaporate deposits, strongly suggest that dolomites were precipitated from solutions that were dominated by slightly evaporated seawater. The results of geochemical and petrographic studies of Neogene dolomite from the Xuande Atoll have demonstrated that the early-formed metastable dolomite may occur in near-surface environment involving certain nucleation events but the seawater-derived fluids and elevated temperature may be responsible for the maturation of dolomite in deeper burial diagenetic environments.