ABSTRACT

Dolomitization is the most striking diagenetic process affecting marine carbonates from the Quaternary-Neogene strata of the Xisha islands in the South China Sea. Petrographic and geochemical evidence from previous studies outline a diagenetic history in which dolomite has experienced a progressive maturation process indicated by textural and compositional changes from shallow to deep depth. With the goal of providing valuable insight into the role of dolomitization on REY geochemistry, rare earth element and yttrium (REY) concentrations were determined in 56 Neogene carbonate samples from the ZK-1 well in this study. The PAAS-normalized REY patterns of these carbonate samples are characterized by the following: 1) pronounced light rare-earth element (LREE) depletion with Pr_N/Yb_N ranges between 0.28 and 0.94 at an average of 0.57; 2) real negative Ce anomalies and positive La anomalies; 3) superchondritic Y/Ho molar ratios with an average of 85; and 4) moderately negative Eu anomalies with an average of 0.56. Except for the Eu anomalies, these REY distributions are similar to those found in modern seawater and are considered to be indicative of a primary marine REY signature. The presence of hydrothermal input is ruled out by negative Eu anomalies in the carbonates that likely reflect that the diagenetic environment was characterized by more reducing conditions as compared to modern seawater. Dolomitization itself did not necessarily modify the REY signatures of the precursor limestones because the REY patterns of different rock types are uniform, and no correlation exists between dolomite content and REY parameters. Gradual variations in Y/Ho values and LREE depletion with depth indicate that the REY signatures of ZK-1 carbonates depend not only on the REY source of the sedimentary fluids, but also on the degree of diagenetic REY uptake at different stages of diagenesis, which is related to the rate of meteoric influence in early diagenesis versus marine-derived alteration in late diagenesis. The latter process involves the progressive recrystallization or remobilization of dolomite during increasing burial.