Distribution and interpretation of rare earth elements and yttrium in Cenozoic dolostones and limestones on Cayman Brac, British West Indies
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- 作者：Hongwen Zhao ; hongwen@ualberta.ca ; Brian Jones
- 关键词：Dolomite ; Rare earth element ; Yttrium ; Secular change ; Cayman Brac
- 刊名：Sedimentary Geology
- 出版年：2013
- 出版时间：15 February, 2013
- 年：2013
- 卷：284-285
- 期：Complete
- 页码：26-38
- 全文大小：2385 K
文摘
The Cenozoic carbonate succession on Cayman Brac, a small island (19 km long and 1.5 to 3 km wide) located in the Caribbean Sea, has been dolomitized to varying degrees. The rare earth element (REE) and yttrium (Y) concentrations (¡Æ REE + Y) of 134 carbonate samples (127 from Cayman Brac and 7 supplementary samples from Grand Cayman), vary from 0.2 to 7.5 ppm (average 2.8 ppm, 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 ¡Æ 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).