Evaluation of the applicability of acid leaching for the 238U-230Th internal isochron method
Evaluation of the applicability of acid leaching for the 238U-230Th internal isochron method
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238U-230Th内部等时线法酸浸适用性评价
DOI：
10.1016/j.chemgeo.2014.12.025
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发表时间：
2015
期刊：
影响因子：
3.9
通讯作者：
E.
中科院分区：
文献类型：
--
作者：
Tanaka;R.;Yokoyama;T.;Kitagawa;H.;Tesfaye;D.B.;and Nakamura;E.
238 U-230 Th内等时线法在确定年轻火山物质喷发年龄方面具有很大的潜力，只要分离的基质相具有足够宽的U/Th比值范围。我们研究了U和Th在老化（即，> 0.5Ma）玄武岩样品，以评价酸浸处理对238 U-230 Th内等时线法的适用性。在238 U-234 U-230 Th长期平衡条件下，在整体岩石尺度上对古老的玄武岩进行酸浸，导致浸出物和残渣中230 Th-238 U和234 U-238 U放射性不平衡。这些放射性不平衡现象可以用α反冲导致234 Th（234 U的母体）和230 Th在酸溶相和耐酸相之间的再分配来解释。通过使用234 U原子的质量守恒方程以及反冲的230 Th和234 Th的相对量（后者与反冲核素的动能成正比），可以计算出被α反冲重新分配的230 Th原子的数量。当~（238）U α反冲再分布后，残留在残渣或浸出液中的~（234）U子体分数足够大（> 95%）时，浸出液和残渣的~（230）Th/~（238）U校正值显示放射性平衡。这一结果表明，如果不对α反冲径迹进行选择性刻蚀，玄武岩样品在酸浸过程中不会发生U和Th之间的优先分馏。本研究表明，酸浸法与238 U-230 Th内等时线法相结合，可以通过评价234 U的α反冲再分配程度来确定年轻火山岩的年龄。
The238U–230Th internal isochron method has great potential for determining eruption ages of young volcanic materials if the separated groundmass phases have a sufficiently wide range of U/Th ratios. We examined the fractionation behavior of U and Th in aged (i.e., > 0.5 Ma) basaltic samples by 6 M hydrochloric acid leaching to evaluate the applicability of acid-leaching treatment for the238U–230Th internal isochron method. Acid leaching of aged basaltic rocks in238U–234U–230Th secular equilibrium at the bulk-rock scale results in230Th–238U and234U–238U radioactive disequilibria for both leachates and residues. These radioactive disequilibria can be explained by redistribution of234Th (parent of234U) and230Th between acid-soluble and acid-resistant phases due to α-recoil. The number of230Th atoms redistributed by α-recoil can be calculated by using a mass conservation equation for234U atoms and by the relative amount of recoiled230Th and234Th, the latter proportional to the kinetic energy of the recoiled nuclide. When the fraction of daughter nuclide234U remaining in either the residue or leachate, after α-recoil redistribution of238U, is large enough (> 95%), the corrected (230Th/238U) values of leachate and residue show radioactive equilibria. This result demonstrates that preferential fractionation between U and Th does not occur during acid leaching for basaltic samples if there is no selective etching of the α-recoil track. This study implies that acid-leaching can be used in conjunction with the238U–230Th internal isochron method for dating young volcanic rocks by evaluating the degree of the α-recoil redistribution of234U.