The W isotope evolution of the bulk silicate Earth:: constraints on the timing and mechanisms of core formation and accretion
The W isotope evolution of the bulk silicate Earth:: constraints on the timing and mechanisms of core formation and accretion
复制标题
DOI：
10.1016/j.epsl.2004.09.023
复制
发表时间：
2004-11-30
影响因子：
5.3
通讯作者：
Münker, C
中科院分区：
文献类型：
--
作者：
Kleine, T;Mezger, K;Münker, C
地球整体硅酸盐的钨（W）同位素组成显示，相对于球粒陨石，W - 182的丰度有少量但可分辨的过剩，这表明地球的核形成发生在现已灭绝的铪 - 182的半衰期内。这种W - 182过剩为地球核形成时间的下限提供了一个严格的限制。金属向地球核心的分离和聚集不可能在太阳系形成后早于约3000万年就停止。然而，确定核形成的确切时间需要了解新吸积物质与地球地幔的平衡程度。相反，如果有地球核心形成的独立年龄限制，那么地球地幔相对于球粒陨石的W - 182过剩可用于限制地球吸积过程中金属 - 硅酸盐的平衡程度。假设月球形成事件是最后一次大撞击，地球核形成可能停止的最晚时间由最古老的月球样本年龄给出，大约是在太阳系形成后7000万 - 1亿年。如果像看起来很可能的那样，撞击体的核心没有与原始硅酸盐地球广泛地重新平衡，那么月球不可能在约4000万年前形成，这意味着地球和月球的年龄是在太阳系形成后4000万 - 7000万年。地球整体硅酸盐相对于球粒陨石约2ε单位的W - 182过剩，远小于如果地球核心是由早期分化的小行星金属核合并形成所预期的15 - 20ε（W）范围，这表明新吸积的小行星与地球地幔有显著的重新均一化。在假设地球以指数递减速率增长的连续核形成模型中，超过约70%的新吸积物质必须与地球地幔达到平衡。将月球形成撞击纳入这些模型，使得在撞击时地球已吸积89%，撞击体增加了10%，这意味着在原始硅酸盐地球中金属 - 硅酸盐平衡超过约50%。如此高程度的金属 - 硅酸盐平衡只有在核形成是通过液态金属从大部分熔融的硅酸盐中物理分离而发生时才能实现，这为地球存在岩浆洋的假说提供了有力支持。模型计算表明，由晚期月球形成撞击形成的岩浆洋不足以去除因早期核形成而在地幔中积累的放射性W - 182。在月球形成之前必须已经建立了至少50%的高程度金属 - 硅酸盐平衡，这意味着在原始地球中金属的分离已经在岩浆洋中发生。因此，月球形成撞击不是导致岩浆洋形成的唯一撞击，这表明岩浆洋有多次形成或岩浆洋的存在时间较长。（C）2004爱思唯尔公司。保留所有权利。
The W isotope composition of the bulk silicate Earth exhibits a small but resolvable excess in the abundance of W-182 relative to that found in chondrites, indicating that core formation in Earth took place within the life-time of now extinct Hf-182. This W-182 excess provides a film constraint for the lower limit of the time of core formation in Earth. Separation and segregation of metal into Earth's core cannot have ceased earlier than similar to30 Myr after the start of the solar system. Determining the exact timing of core formation, however, requires knowledge of the degree of equilibration of newly accreted material with Earth's mantle. Conversely, if independent age constraints for the formation of Earth's core are available, the W-182 excess of Earth's mantle relative to chondrites can be used to constrain the degree of metal-silicate equilibration during Earth's accretion. Provided that the Moon-forming event is the last large impact, the latest time core formation can have ceased in Earth is provided by the age of the oldest lunar samples and is similar to70-100 Myr after the start of the solar system. If, as seems likely, the impactor's core did not re-equilibrate extensively with the silicate proto-Earth, the Moon cannot have formed before similar to40 Myr resulting in an age of the Earth and the Moon of 40-70 Myr after the start of the solar system. The W-182 excess of the bulk silicate Earth relative to chondrites of similar to2epsilon units is substantially smaller than the 15-20epsilon(W) range expected if Earth's core formed by merging of metal cores of early differentiated planetesimals, indicating significant re-homogenization of newly accreted planetesimals with Earth's mantle. In continuous core formation models that assume growth of Earth at an exponentially decreasing rate, more than similar to70% of the newly accreted material must have equilibrated with Earth's mantle. Including the Moon-forming impact into these models such that Earth was 89% accreted at the time of the impact and 10% was added by the impactor implies more than similar to50% metal-silicate equilibration in the silicate proto-Earth. Such high degrees of metal-silicate equilibration can only be achieved if core formation occurred by the physical separation of liquid metal from mostly molten silicate providing strong support for the hypothesis of a terrestrial magma ocean. Model calculations show that formation of a magma ocean by a late Moon-forming impact is not sufficient in removing radiogenic W-182 from Earth's mantle that would have accumulated as a result of early core formation. A high degree of metal-silicate equilibration of at least 50% must have been established prior to Moon formation, implying that metal segregation in the proto-Earth already occurred in a magma ocean. Therefore, the Moon-forming impact is not the only impact that led to the formation of a magma ocean, indicating multiple formations of magma oceans or a protracted life-time of the magma ocean. (C) 2004 Elsevier B.V. All rights reserved.