Distribution and interpretation of rare earth elements and yttrium in Cenozoic dolostones and limestones on Cayman Brac, British West Indies

Abstract

The Cenozoic carbonate succession on Cayman Brac, a small island (19km long and 1.5 to 3km wide) located in the Caribbean Sea, has been dolomitized to varying degrees. The rare earth element (REE) and yttrium (Y) concentrations (∑REE+Y) of 134 carbonate samples (127 from Cayman Brac and 7 supplementary samples from Grand Cayman), vary from 0.2 to 7.5ppm (average 2.8ppm, n=134). The shale-normalized REE+Y patterns of the carbonate samples, akin to those of oxygenated seawater, are characterized by: (1) LREE depletion relative to HREE (average DyN/SmN=1.7, n=125), (2) positive La anomalies, (3) negative Ce anomalies (average Ce/Ce*=0.4, n=133), and (4) superchondritic Y/Ho molar ratios (average Y/Ho=85, n=86). The lack of correlation between the dolomite content of the samples and their ∑REE+Y, DyN/SmN, LaN/NdN, Ce/Ce*, or Y/Ho indicates that dolomitization did not have a major impact on REE+Y signatures and that dolomitization was probably mediated by seawater-like fluids.The shale-normalized patterns (e.g., DyN/SmN, LaN/NdN, Ce/Ce*, and Y/Ho) vary from formation to formation. For samples from the Cayman Formation, the Pedro Castle Formation, and the Ironshore Formation, there are subtle but gradual changes in DyN/SmN, LaN/NdN, Ce/Ce* with depth. In contrast, there are marked changes in LaN/NdN, Ce/Ce*, Y/Ho, and Sm/Nd across the Brac Unconformity, which forms the boundary between the Lower Oligocene Brac Formation and the overlying Miocene Cayman Formation. Variations in the REE+Y patterns reflect diagenetic processes (e.g., DyN/SmN) and possibly secular changes in the REE+Y composition of seawater (e.g., LaN/NdN, Y/Ho, and Sm/Nd).