期刊
CHEMICAL GEOLOGY
卷 445, 期 -, 页码 208-220出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/j.chemgeo.2016.06.003
关键词
Ribbon rock; Mg isotopes; Dolomite; Doushantuo; Clay mineral
资金
- Division Of Earth Sciences
- Directorate For Geosciences [1340160] Funding Source: National Science Foundation
Modern dolomite formation is conditional and restricted in specific environmental and geochemical conditions, accordingly cannot be used as the analog of ancient dolostones, which normally have platform-wide distribution. The key to understand ancient dolostone formation is to find a dolomitization process that is independent of physical/geochemical environments. It is well-known that ribbon rock (one type of calcareous rhythmites, sensu stricto referring to the alternating limestone and marlstone layers or the limestone-marl alternations) that are deposited in nearly all marine environments throughout the Earth's history, are exclusively, though normally partially, dolomitized. Thus, understanding ribbon rock dolomitization may provide valuable insight into the 'dolomite problem'. In this study, we measured Mg isotopic compositions (delta Mg-26) of ribbon rock samples collected from the Ediacaran Doushantuo Formation in South China. The marlstone layers contain higher dolomite contents and are characterized by higher delta Mg-26 (ranging from - 2.68%. to - 1.84 parts per thousand) than the limestone layers (varying between - 3.21 parts per thousand. and 2.53 parts per thousand). A numerical model calculation indicates that clay minerals cannot provide enough Mg for dolomitization, arguing against the traditional interpretation that dolomitization is resulted from diagenetic Mg release from clays. Instead, contemporaneous seawater might provide sufficient Mg for dolomitization, because both Mg isotopes and dolomite content can be simulated by the Diffusion-Advection-Reaction model. Contemporaneous seawater dolomitization requires kinetic barrier be overcome at shallow depth of sediments. We propose the ribbon rock dolomitization is attributed to differential diagenesis of the clay-rich and clay-poor sediment layers, in which Ca2+-Mg2+ exchange elevates porewater Mg/Ca in clay-rich layers. (C) 2016 Elsevier B.V. All rights reserved.
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