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

Dolomitization is a remarkable diagenetic process occurring in the subsurface carbonate beds at Xuande Atoll of Xisha islands in the South China Sea. Dolomite crystals initially appear at the depth of similar to 180m in HolocenePleistocene carbonate with relatively small concentrations, whereas extensive dolomitization has developed at the minimum burial depth of 290 m in Pliincene strata of the ZK-1 well. Petrographically, ZK-1 dolomites have undergone, from shallow to deep depth, a progressive maturation process indicated by fabric evolvement of dolomitized rocks, rock lithification induced by textural coarsening and continued growth of cements, increase in stoichiometry and cation ordering of dolomite downcore, and alteration of metastable dolomite to stable one, involving preferential dissolution and subsequent reprecipitation process. 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 delta C-13 values (1.76-3.49 parts per thousand) indi parts per thousand te that diagenetic fluids were buffered by carbonate rocks. Positive delta O-18 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.