地球早期演化的Hf-W同位素制约

Hf-W isotopic constraints on early evolution of the Earth

^(182)Hf-^(182)W作为短周期放射性衰变体系,可有效约束地球早期演化吸积增生和内部分异过程。本文通过系统总结、归纳太阳系形成初期地球核幔分异过程中Hf-W同位素变化规律、月球与地球硅酸盐的W同位素组成,提出利用^(182)Hf-^(182)W体系测定地球核幔分异时间不确定性的主要原因是地球核幔分异的持续性及开放性,大碰撞时间的^(182)Hf-^(182)W同位素限定主要受控于硅酸盐地球和硅酸盐月球的Hf/W比值,讨论了地幔W同位素不均一性的形成机制,与现代地幔不同的^(182)W/^(184)W组成可能代表了后增生作用之前整体硅酸盐地球的W同位素组成,也可能是^(182)Hf未完全灭绝时形成的区域性Hf/W比值差异经^(182)Hf衰变形成的结果。这些结论为探索类地行星形成与演化提供了重要制约。

As one of the short-lived radioactive systems, 182Hf-182W system can well constrain the accretion and differentiation processes in the evolution of the early Earth. Based on the review of Hf-W isotopic evolution characteristics and W isotopic compositions of the lunar-and terrestrial-mantle, we suggest that the continuity and openness of the Earth＇s core-mantle differentiation process induce the uncertainties of dating it using the 182Hf-182W chronology. To acquire temporal information of the lunar-forming giant impact depends on the knowledge of Hf/W ratios of the bulk silicate Moon and Earth. We also discuss the mechanisms of W isotopic heterogeneity in the Earth＇s mantle and suggest that W isotopic anomalies in terrestrial rocks may be inherited from the Earth＇s mantle prior to the late veneer, or be the radiogenic results of source regions obtaining diverse Hf/W ratios during the lifetime of 182Hf. These conclusions provide critical constraints on the formation and evolution of terrestrial planets.