Rare earth element geochemistry of Late Devonian reefal carbonates, canning basin, Western Australia: Confirmation of a seawater REE proxy in ancient limestones
Rare earth element geochemistry of Late Devonian reefal carbonates, canning basin, Western Australia: Confirmation of a seawater REE proxy in ancient limestones
复制标题
DOI：
10.1016/s0016-7037(03)00422-8
复制
发表时间：
2004-01-01
影响因子：
5
通讯作者：
Kamber, BS
中科院分区：
文献类型：
--
作者：
Nothdurft, LD;Webb, GE;Kamber, BS
测定了西澳大利亚Canning盆地Lennard陆架49个晚泥盆世礁碳酸盐岩样品中稀土元素和钇（REE+Y）的含量。泥盆世晚期样品的页岩标准化（SN）REE+Y模式显示出与富氧浅海地球化学特征一致的特征。各种不同的古代石灰岩成分，包括微生物岩，一些骨骼碳酸盐（层孔类），和水泥，记录海水一样的REE+Y的签名。与磷酸盐，铁氧化物和页岩的污染进行了定量测试，并可以折扣的REE+Y模式的来源。可能来自同一海水的共生碳酸盐组分具有不同的相对REE浓度，但REE+Y模式一致。干净的泥盆纪早期海相胶结物（n = 3）显示出与现代开阔洋海水最相似的REE+Y特征和最高的Y/Ho比值（例如，59)轻稀土（LREE）亏损最大（平均Nd-SN/Yb-SN = 0.413，SD = 0.076）。然而，同沉积胶结物具有最低的REE浓度（例如，405 ppb）。未受污染的泥盆纪微生物岩样品中含有的calcimicrobe Renalcis和泥晶血栓石聚集体在早期海洋水泥（n = 11）的混合物具有最高的相对稀土浓度的测试碳酸盐（平均总稀土= 11.3 ppm）。与现代珊瑚、藻类和软体动物不同，层孔虫骨骼也含有发育良好的海水状REE模式。河口岸礁样品具有不同的REE+Y模式，LREE富集（Nd-SN/Yb-SN > 1），可能反映了河口胶体物质的包体，其中含有优先清除的来自附近河流输入源的LREE。因此，泥盆纪石灰岩提供了一个代理海洋稀土元素地球化学，并允许在古代Lennard架上的共存水团的分化。虽然适当的分配系数量化泥盆纪海水稀土元素浓度的数据是未知的，假设泥盆纪坎宁盆地海水稀土元素模式与现代自然代理和实验值的系数。泥盆纪海水模式略有丰富的轻稀土相比，大多数现代海水，并建议更高的整体稀土元素浓度，但非常相似的高陆源输入地区的海水。我们的研究结果表明，大多数石灰岩应记录重要方面的稀土元素地球化学的沃茨，他们沉淀，提供他们是相对自由的陆源污染和主要成岩蚀变流体高，非海水一样的稀土元素含量。因此，我们期望许多其他古灰岩可以作为海水稀土元素的替代物，从而提供古海洋学、古地理学和海洋地球化学演化的信息。版权所有（C）2004 Elsevier Ltd.
Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements (n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd-SN/Yb-SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement (n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd-SN/Yb-SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modem seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans. Copyright (C) 2004 Elsevier Ltd.