Change of Sm-Nd isotope composition during weathering of till
Change of Sm-Nd isotope composition during weathering of till
复制标题
DOI：
10.1016/s0016-7037(99)00365-8
复制
发表时间：
2000-03
影响因子：
5
通讯作者：
B. Öhlander;J. Ingri;M. Land;H. Schöberg
中科院分区：
文献类型：
--
作者：
B. Öhlander;J. Ingri;M. Land;H. Schöberg
瑞典北部耕地的风化导致了发育良好的斜土的形成。耕耘区主要由 1.9–1.8 Ga 花岗岩材料组成。稀土元素是风化过程中损耗最严重的元素之一，并且随着原子序数的增加，稀土元素从 E 视界的损失减少。为了研究风化是否会导致 Nd 同位素组成的变化，我们分析了瑞典北部两个风化耕层（典型单倍体）剖面的活植物和腐殖质等不同层位的 Nd 同位素组成。 <0.2 毫米部分中的 Sm 和 Nd 多达 65.6% 至 75.3% 已从 E 水平线中损失，而 B 水平线中则有 32.5% 至 54.7% 损失。 Nd 的损失程度略大于 Sm。两个 C 视界样本的 εNd(0) 值为 -22.1 和 -23.2。相应的E-地平线值为-18.1和-20.2。 B 地平线值介于 E 和 C 地平线值之间。结论是风化导致Sm/Nd比的变化，从而导致Sm-Nd同位素组成的变化。植物和腐殖质样品与未风化的耕种的偏差更大。对于一个站点，结果可以解释为植物吸收的 Sm 和 Nd 与 E 地平线风化释放的这些元素的量具有相似的同位素特征。对于另一个站，有机样品的 Nd 同位素组成可能主要是由蒂尔风化释放的 Nd，然而，它与另一种 Nd 源（可能是空气中的成分）混合。对耕层中同位素组成变化的解释是，风化释放的 Nd 中，与土壤中 Sm/Nd 比值较低的矿物相比，Sm/Nd 比值较高的矿物释放的 Nd 比例更大。尽管各种携带稀土元素的矿物具有相同的初始 Nd 同位素组成，但 147 Sm 到 143 Nd 的 1.8-1.9 Ga 衰变导致具有较高 Sm/Nd 比的矿物中存在较高的 143 Nd/144 Nd 比。
Weathering of till in northern Sweden results in the formation of well-developed spodosols. The till is dominated by 1.9–1.8 Ga granitic material. The REE are among the elements most strongly depleted during weathering, and the loss of REE from the E-horizon decreases as the atomic number increases. To study if weathering leads to a change of the Nd isotope composition, we have analysed the Nd isotopic composition of the various horizons including living plants and humus of two profiles of weathered till (typic haplocryods) in northern Sweden. As much as between 65.6 and 75.3% of the Sm and Nd in the <0.2 mm fraction has been lost from the E-horizon, and between 32.5 and 54.7% from the B-horizon. Nd has been lost to a slightly greater extent than Sm. The two C-horizon samples have εNd(0)values of −22.1 and −23.2. Corresponding E-horizon values are −18.1 and −20.2. The B-horizon values are intermediate between the values of the E and C horizons. It is concluded that the weathering leads to a change in the Sm/Nd ratio resulting in a change of the Sm-Nd isotope composition. The plant and humus samples deviate even more from the unweathered till. For one station the results could be interpreted as if the Sm and Nd taken up by the plants had similar isotope characteristics as the amounts of these elements released by weathering in the E-horizon. For the other station it is probable that the Nd isotope composition of the organic samples is dominated by Nd released by till weathering which, however, is mixed with another Nd-source, possibly an airborne component. The explanation to the change of isotope compostion in the till is that a larger proportion of the Nd released by weathering is released from minerals with a lower Sm/Nd ratio than the bulk soil, compared with the amount released from minerals with a higher Sm/Nd ratio. Although the various REE-carrying minerals had the same initial Nd isotopic composition, 1.8–1.9 Ga of decay of147Sm to143Nd has resulted in a higher present143Nd/144Nd ratio in the minerals with a higher Sm/Nd ratio.