Modern carbonate ooids preserve ambient aqueous REE signatures
Modern carbonate ooids preserve ambient aqueous REE signatures
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现代碳酸盐类物质保留了环境水性稀土元素特征
DOI：
10.1016/j.chemgeo.2019.01.015
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发表时间：
2019
期刊：
影响因子：
3.9
通讯作者：
Xiucheng Tan
中科院分区：
文献类型：
--
作者：
Fei Li;Gregory E. Webb;Thomas J. Algeo;Stephen Kershaw;Chaojin Lu;Am;a M. Oehlert;Qiaolin Gong;Ali Pourm;Xiucheng Tan
为了重建古海水的稀土元素（REE）和钇（Y）组成，人们对海相沉积岩的骨架和非骨架组分进行了分析，但骨架碳酸盐常常被证明是不可靠的海水化学记录物。在这里，我们提出了一个系统的多技术评估的稀土和其他微量元素的现代大巴哈马银行（GBB海洋）和大盐湖（GSL大陆）的基础上，强酸（氢氟酸和硝酸）和弱酸（乙酸）diglitazone，以及激光消融（LA）的卵状皮质和细胞核的卵状砂。结果表明，巴哈马的卵状皮质具有页岩归一化的REE + Y的功能几乎相同的浅水海水，包括有限的硅质稀土元素的污染。甚至0.2%的碎屑来源的材料的混合物可以修改的主要海洋稀土+ Y模式，例如，增加轻稀土元素（LREE）的含量。巴哈马卵状皮质LA数据的平均值表现出类似的REE + Y签名，由乙酸消化产生的，但LA数据一般是嘈杂的，主要是由于低REE浓度和小体积的碳酸盐烧蚀分析。筛选出REE <0.9ppm的样品，结果表明，相对于环境海水，单个卵状皮层的REE + Y模式更均匀，更接近海水，REE分配系数也更高，更均匀（116 ± 21，从La到Lu）。鲕粒纹层没有明显的交替之间的好氧和缺氧孔隙流体的特点，这表明，不断增长的鲕粒主要形成在碳酸盐浅滩设置的好氧条件下，即使在他们的休息阶段埋。不像巴哈马的卵，页岩归一化的稀土+ Y模式的GSL卵消化弱酸显示轻微亏损的轻稀土组合物，可能会偏离环境流体，由于释放稀土元素从所包括的粘土馏分之间的比较消化和LA方法。同一皮质内的多个消融点允许直接比较单个卵圆的更多和更少污染的部分。几乎一致的Ce正异常可能与强碱性水环境有关。个别烧蚀点，部分叠印的硅质碎屑衍生的稀土元素包含可变的锆含量（从0.096至~4 ppm）和扁平页岩归一化轻稀土模式。污染最少的GSL卵皮质产生LREE贫化REE + Y分布。使用可靠的GBB类卵分布（即，重稀土元素> 0.9 ppm）返回一个平坦的模式，表明在大盐湖和巴哈马沃茨水域相似的水碱度条件下，碳酸盐络合作用控制的轻稀土元素亏损程度相似。总之，ooids可以是一个可靠的代理环境表层沃茨的REE + Y特性时，采用适当的分析方法，包括激光烧蚀，允许识别和隔离的污染信号，从silicicicoparticulate碎屑。
Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.