REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite and aqueous solution
REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite and aqueous solution
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DOI：
10.2343/geochemj.40.425
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发表时间：
2006-10
影响因子：
0.8
通讯作者：
Kazuya Tanaka;I. Kawabe
中科院分区：
文献类型：
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作者：
Kazuya Tanaka;I. Kawabe
在我们以前的工作中，我们测定了方解石和水溶液(CaCl2+NaCl)之间的相对REE分配系数，但由于方解石的小范围生长，无法获得绝对值。在本研究中，我们进行了类似的实验，以确定稀土在方解石和水溶液之间的绝对分配系数，并估算了方解石和海水之间的分配系数。在CO_2+N_2气体中，方解石-稀土掺杂过饱和溶液不断地被泵入反应器中，在种子上形成方解石过度生长。得到了稀土分配系数的绝对值Kd(REE)=(XREE/XCa)方解石/([REE，Tot.]/[Ca])溶液。Kd(REE)的模式表现为凸四元效应和Y/Ho分馏，这与我们以前的工作是一致的。利用海水成分校正的分配系数和二叠纪海山型石灰岩的稀土元素分析，计算了二叠系海水中的稀土元素丰度。二叠纪海水中稀土元素丰度的估算均表现为凹面四分体效应、大的Y/Ho分馏和负的Ce、Eu球粒陨石归一化模式，这与现代海水的特征十分相似。然而，据估计，二叠纪海水中的稀土元素丰度比现在海水中的稀土元素丰度低了大约一个数量级。这表明，在原始低稀土生物成因碳酸盐沉积物的渗透率和孔隙度降低到一定的临界水平后，大部分海相石灰岩不再含有海水稀土。
In our earlier work, we determined relative REE partition coefficients between calcite and aqueous (CaCl2 + NaCl) solution, but absolute values could not be obtained because of small overgrowths of calcite. In this study, we have made similar experiments in order to determine the absolute REE partition coefficients between calcite and aqueous solution, and have evaluated partition coefficients between calcite and seawater. Calcite-supersaturated solution doped with REEs was constantly pumped into a reactor to form calcite overgrowths on the seeds under CO2 + N2 gas. Absolute values of REE partition coefficients, Kd(REE)=(XREE/XCa)calcite/([REE,tot.]/[Ca])solution, have been obtained. The patterns of Kd(REE) show convex tetrad effects and Y/Ho fractionation, which is consistent with our earlier work. We have calculated REE abundances in Permian seawater using the partition coefficients corrected to seawater compositions and REE analyses of Permian seamount-type limestones. All the estimated REE abundances in Permian seawater indicate concave tetrad effects, large Y/Ho fractionation and negative Ce and Eu anomalies in their chondrite-normalized patterns, which are quite similar to those for present-day seawater. The estimated REE abundances in Permian seawater, however, are lower than those in present-day seawater by approximately one order of magnitude. This suggests that significant parts of marine limestones could not incorporate seawater REEs after the permeability and porosity of the original biogenic carbonate sediments with low-REE contents had been reduced to certain critical levels.