研究生知识库

重金属元素即（原子序数大于23的金属元素）的短半衰期体系包括182Hf-182W、205Pb-205Tl、247Cm-235U等。这些体系的衰变周期都小于100百万年，在示踪和制约太阳系早期地质体的分异和演化、高温改造事件、表面挥发和冷凝等过程中发挥了重要作用。子体同位素在不同地质体中分布的不均匀性是放射性衰变、核合成过程和同位素的分馏共同作用的结果。由于受到核体积效应的作用，重金属元素体系存在可观的同位素分馏。但是到目前为止，将重金属同位素的核体积效应运用在太阳系和地球早期高温条件下的地质过程（如核幔分异）中的研究还非常有限。本文从核体积效应对重金属稳定同位素体系分馏的影响的角度入手，探讨核体积效应对于这些短半衰期体系分异、演化过程的影响。结果表明：核体积效应对于原子序数大于40的同位素体系分馏存在显著影响。且原子序数越大，核体积效应越明显。重元素的同位素平衡分馏是由核体积效应和质量依赖分馏共同组成的。因为核体积效应随温度的关系是与1/T成正比，质量依赖分馏是与1/T2成正比（高温下），所以在高温条件下，核体积效应是同位素平衡分馏的主导因素。因此，在研究高温地质过程中的重金属同位素分馏时，精确计算核体积效应是至关重要的。本文的研究结果表明，对于重元素体系而言，其电子结构中s层电子的减少，或p、d、f层电子的增加，都会减小目标元素的在核电子密度。更小的在核电子密度造成的核体积效应会使体系富集重同位素。因此对于变价元素体系来说，价态的变化会引起较大的核体积效应的分馏。例如，W在自然界中的常见氧化态为四价和六价态。W的六价态W[VI]比四价态W[IV]少了2的d层电子，所以W[VI]体系比W[IV]体系富集W的轻同位素，182W/184W的比值增大。这种情况在W的不同离子相之间，25℃的温度下，182W分馏值可以达到170 ppm。在W 的不同固相之间，3000℃的温度下，依然有25 ppm的分馏。对于U，四价态U[IV]和六价态U[VI]之间相差2个f层电子，因此U[VI]体系比U[IV]体系富集U的轻同位素，造成两者之间235U/238U的分馏值在300℃下可以达到3.11‰。核体积效应在这两个体系中同位素的富集规律正好与质量依赖分馏规律相反，即在较为氧化的价态体系中二者倾向于富集轻同位素。另外，由于对于价态变化反应明显，核体积效应也可以作为衡量氧化还原状态变化的指标。核幔分异或地幔分异过程中，核体积效应会引起在不同地质体之间重金属同位素的分馏。它对于使用地幔的同位素异常值限制核幔分异的时间和后期加积量有一定的影响。例如，前人利用Hf-W定年体系限制了类地行星的核形成年龄。本文的研究结果表明，在金属相与硅酸盐相分异过程中，核体积效应对于地幔182W的贡献值会对Hf-W定年产生影响：如果扣除掉核体积效应作用在地幔中的182W的富集值(约20 ppm)，会使校正后的单次核形成年龄(29百万年)推迟1.43百万年。核体积效应作用在地幔中的182W正异常值需要更多的后期加积的物质量，才能把原始地幔182W值拉低到现今值(0 ppm)。如果有20 ppm的核体积效应分馏的加入，1.91wt.%的后期加积物质量会使原始地幔30 ppm的正异常降低为0 ppm。根据本文的研究结果可知，核体积效应会广泛参与到重金属同位素的所有地质过程中。在有氧化还原状态改变时，核体积效应尤为明显。另外，金属同位素在实验提纯测试的过程中也会伴随着核体积效应分馏，但实验测试值往往只去除了质量依赖效应的影响，却很少考虑核体积效应。因此，对于各地质体中重金属同位素组成的异常值，考虑核体积效应的影响才能全面解析该地质体经历的地质过程。核体积效应的重要意义在于，它是引起重金属同位素在地质体中分布不均匀的重要因素之一，并且可以作为氧化还原状态的指标，以及影响定年、后期加积作用的物质量和实验精度。

The short half-lived systems for heavy metal systems included 182Hf-182W, 146Sm-142Nd, 205Pb-205Tl and 247Cm-235U. The decay cycles of these systems are less than 100 Ma and play an important role in tracing the differentiation and evolution of the early solar system, and constraining the ages of geological bodies. These systems are used to trace the events of early solar system, such as the crystallization and differentiation of the Magma Ocean, high-temperature modification, volatilization and condensation, etc. The heterogeneous distributions of 182W, 142Nd, 205Tl and 235U in various geological bodies is a combined result of radioactive decay, nuclear synthesis and isotope fractionation. Heavy metal systems have been shown to have considerable isotope fractionation driven by nuclear volume effect (NVE). However, the application of the nuclear volume effect of heavy metal isotopes in the geological processes of the early Earth has been minimal at high temperature. In this thesis, the effects of NVE on the differentiation and evolution of short half-lived systems are discussed in several aspects. First, the NVE is significant for fractionation of isotopic systems with atomic number greater than 40. The NVE becomes prominent, when the atomic number is increasing. Isotope equilibrium fractionation of heavy elements is composed of NVE and mass-dependent fractionation. Since the NVE scales with 1/T and mass-dependent fractionation scales with 1/T2, the NVE becomes the major factor that dominates total equilibrium fractionation in the high-T system. It has been known that for heavy elements, reducing s-layer electrons or increasing p, d, f-layer electrons can increase the electronic density at the nucleus and enrich heavy isotopes. Therefore, for the elements with variable valences, the change of valence state can lead to major fractionation driven by NVE. The oxidation states of W in nature are dominated by tetravalent and hexavalent. W[VI] has two d-shell electrons less than W[IV], so the W6+-bearing systems enrich light isotope than W4+-bearing systems characterized by higher 182W/184W in the W6+-bearing systems. The fractionation value of 182W between ion phases can reach 170 ppm under 25℃. The fractionation value of 182W between solid phases can reach 25 ppm even under high temperature (3000℃). The difference between the tetravalent U[IV] and the hexavalent U[VI] of U is two f electrons, the light isotope of U is also enriched in the U6+ system than in the U4+ system. The fractionation value of 235U/238U between the two valence states can reach 3.11‰ at 300K. The law of isotopic enrichment of NVE in these two systems is opposite to that of mass-dependent fractionation. The NVE can also be used as an index to the change of redox state in geological history.The nuclear volume effect can result in the fractionation of heavy metal isotopes between different geological bodies in the process of the differentiation between core-mantle segregation or mantle differentiation. It has a certain effect on limiting the age of mantle differentiation and the mass of late accretion by using the anomalous isotope values of mantle samples. Previous researchers used Hf-W dating system to limit the core formation age. The contribution of nuclear volume effect in the process of metal phase and silicate phase differentiation to the 182W value of mantle will affect Hf-W dating. After deducting the enrichment value of 182W in the mantle caused by the nuclear volume effect, which is about 20 ppm, the corrected age of single core formation age (29 million years) will be delayed by 1.43 million years. The 182W positive anomaly in the mantle due to the nuclear volume effect requires more late accretion mass to pull the pre-late accretion mantle value down to 0 ppm.The nuclear volume effect is involved in all geological processes of heavy metal isotopes. It is especially obvious when the redox state changes. The nuclear volume effect fractionation also exists in the process of experimental purification and ionization. However, the experimental method only considers mass bias correction, but neglects the effect of NVE. Therefore, for the abnormal value of heavy metal isotope composition in the different geological bodies, the fractionation value caused by nuclear volume effect needs to be taken into account in order to fully understand the geological process of the heavy metal systems. The significance of nuclear volume effect is that it is one of the critical factors that cause the heterogeneous distribution of heavy metal isotopes in geological bodies, and it can be used as an indicator of the redox state, as well as influencing the dating and experimental accuracy.