Geochemistry of Precambrian carbonate intercalated in pillows and its host basalt: implications for the REE composition of circa 3.4 Ga seawater
Geochemistry of Precambrian carbonate intercalated in pillows and its host basalt: implications for the REE composition of circa 3.4 Ga seawater
复制标题
DOI：
10.1016/j.precamres.2004.09.006
复制
发表时间：
2004-12
影响因子：
3.8
通讯作者：
Koshi Yamamoto;N. Itoh;Takuya Matsumoto;Tsuyoshi Tanaka;M. Adachi
中科院分区：
文献类型：
--
作者：
Koshi Yamamoto;N. Itoh;Takuya Matsumoto;Tsuyoshi Tanaka;M. Adachi
通过对西澳大利亚皮尔巴拉地块枕状玄武岩中夹层白云石的分析，估算了早太古宙海水的稀土元素组成。我们对地质时期可能影响碳酸盐化学成分的干扰进行了严格的评估。皮尔巴拉地区的白云石样品显示出球粒陨石标准化的轻稀土富集型模式，并具有向下凸起的四分体效应，表明这些稀土元素特征与白云岩沉淀的古老海水有关。根据在白云岩中观察到的小的Eu异常，热液成分对海水特征的贡献似乎可以忽略不计。因此，皮尔巴拉地块的白云石样品似乎保留了古海水固有的稀土元素，这表明在已知稀土元素的碳酸盐-海水分配系数的情况下，可以估计海水的稀土元素组成。根据对日本地质调查局JDO-1白云石标准样品的分析，我们使用了稀土分配系数，结果表明，∼3.4Ga海水中的稀土元素含量可能比现在高出一个或两个数量级。此外，3.4Ga海水明显富含HREE，与现代海水相比，W型四分体效应不明显。这可能反映了太古宙海水特有的古环境条件，如高盐度和[CO32−]。
The REE composition of early Archean seawater has been estimated through analyses of dolomite intercalated in pillow basalts from the Pilbara Block, Western Australia. We have critically evaluated the possible disturbances that have affected the chemical composition of the carbonate through geologic time. Dolomite samples from the Pilbara show a chondrite-normalized LREE-enriched pattern with profound downward convex tetrad effects, suggesting that these REE signatures are related to ancient seawater from which the dolomite precipitated. Contribution of a hydrothermal component to the seawater signature appears negligible based on the small Eu anomalies observed in dolomite. Therefore, dolomite samples from the Pilbara Block appear to preserve REEs intrinsic to ancient seawater, suggesting that the REE composition of seawater could be estimated given that carbonate–seawater partition coefficients of REEs are known. We used REE partition coefficients based on the analyses of a dolomite reference sample (JDo-1 from the Geological Survey of Japan) and show that REE concentrations of ∼3.4Ga seawater could have been one or two orders of magnitude higher than at present. Moreover, it appears that 3.4Ga seawater was significantly enriched in HREE and had less pronounced W-type tetrad effects relative to that of present seawater. These may reflect paleo-environmental conditions particular to seawater in the Archean, such as high-salinity and [CO32−].