Diffusive fractionation of K isotopes in molten basalts
Diffusive fractionation of K isotopes in molten basalts
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DOI：
10.1016/j.epsl.2022.117405
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发表时间：
2022-03
影响因子：
5.3
通讯作者：
Youxue Zhang
中科院分区：
文献类型：
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作者：
Youxue Zhang
用二次离子质谱（西姆斯）测量了扩散偶实验中的41 K/39 K同位素比值剖面。本研究的目的之一是推动西姆斯在非传统稳定同位素比值测量中的应用。第二个更重要的目标是首次量化K同位素的扩散分馏及其对温度和不同反扩散元素的依赖性。数据表明，西姆斯测量的单昼夜精度可以达到0.2‰（以下简称1σ），不使用同位素比值标准的长期精度约为2.5‰。在扩散偶中，当初始浓度比（高浓度与低浓度之比）约为70时，41 K/39 K分馏（最大值与最小值之差）约为10‰。41 K/39 K比值分布最初是通过假设K2 O的有效二元扩散系数（D）恒定来拟合的，这导致了微小的失配，更重要的是，基于化学扩散和同位素扩散分布的扩散系数之间存在很大的不一致。发现K_2O的D随浓度的变化而变化。然后通过假设K2 O的D随K2 O浓度呈指数级增加来拟合曲线，这解决了不匹配和不一致的问题。也就是说，结合浓度和同位素比曲线，能够区分微妙的浓度依赖性扩散。对于SiO_2-K_2O互扩散偶，经验扩散同位素分馏参数β随温度的升高略有增加，从1260 °C时的0.104 ± 0.003增加到1500 °C时的0.116 ± 0.003。对于MgO-K2 O互扩散偶，排除1260 °C下的异常点，β从1350 °C下的0.090 ± 0.005增加到1500 °C下的0.100 ± 0.003。这些β值与SiO2-K2 O互扩散偶中NaKO与SiO2交换或MgO-K2 O互扩散偶中NaKO与MgO交换的扩散机制大致一致。应用β值模拟自然界中的扩散同位素分馏，岩浆混合过程中的扩散K同位素分馏有望达到西姆斯所能分辨的程度。在采集样品进行K-Ar或K-Ca定年时，通过测量样品中的K同位素比值来校正可能的K同位素分馏效应是很重要的。当采用扩散和对流扩散模型来评估通过扩散和蒸发的挥发性损失期间的同位素分馏时，发现80%的钾损失将导致δ 41 K增加6.3‰至8.9‰，比月球相对于地球的δ 41 K富集度高10倍以上。因此，月球相对于地球的K贫化和δ 41 K的富集不太可能是扩散控制的。
The41K/39K isotope ratio profiles in diffusion couple experiments have been measured by Secondary Ion Mass Spectrometry (SIMS). One goal of this research is to push the use of SIMS in measuring non-traditional stable isotope ratios. The second, more important goal, is to quantify for the first time diffusive fractionation of K isotopes and its dependence on temperature and different counter-diffusion elements. The data show that the precision in a single day-night session of SIMS measurements can reach 0.2‰ (1σhereafter) with effort, and the long-term accuracy without using any isotope ratio standard is about 2.5‰. At an initial concentration contrast (ratio of high concentration to low concentration) of about 70 in a diffusion couple, the total41K/39K fractionation (maximum minus minimum) is about 10‰. The41K/39K ratio profiles were initially fit by assuming constant effective binary diffusivity (D) of K2O, which led to minor misfits and more importantly, to large disagreement between diffusivities based on chemical diffusion and isotope diffusion profiles. It was found thatDfor K2O varies with its concentration. The profiles were then fit by assumingDfor K2O increases exponentially with K2O concentration, which resolved the misfits and disagreements. That is, combining concentration and isotope ratio profiles enables distinguishing subtle concentration-dependent diffusivity. For the SiO2-K2O interdiffusion couples, the empirical diffusive isotope fractionation parameterβincreases slightly with temperature from 0.104 ± 0.003 at 1260 °C to 0.116 ± 0.003 at 1500 °C. For the MgO-K2O interdiffusion couples, excluding an outlier point at 1260 °C,βincreases from 0.090 ± 0.005 at 1350 °C to 0.100 ± 0.003 at 1500 °C. Theseβvalues are roughly consistent with a diffusion mechanism of NaKO exchanging with SiO2in the SiO2-K2O interdiffusion couples, or NaKO exchanging with MgO in the MgO-K2O interdiffusion couples. Applying the obtainedβvalue to model diffusive isotope fractionation in nature, diffusive K isotope fractionation during magma mixing is expected to be large enough to be resolvable by SIMS. When collecting samples for K-Ar or K-Ca dating, it is important to correct for the effect of possible K isotope fractionation by measuring K isotope ratio in the sample. When diffusive and convective-diffusive modeling was applied to evaluate isotope fractionation during volatile loss through diffusion and evaporation, it is found that a loss of 80% potassium would lead to an increase ofδ41K by 6.3‰ to 8.9‰, more than 10 times greater than the enrichment ofδ41K in the Moon relative to the Earth. Hence, the depletion of K and the associated enrichment ofδ41K in the Moon relative to the Earth are unlikely diffusion controlled.