The tungsten isotopic composition of Eoarchean rocks: Implications for early silicate differentiation and core-mantle interaction on Earth
The tungsten isotopic composition of Eoarchean rocks: Implications for early silicate differentiation and core-mantle interaction on Earth
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DOI：
10.1016/j.epsl.2010.01.012
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发表时间：
2010-03
影响因子：
5.3
通讯作者：
T. Iizuka;S. Nakai;Y. Sahoo;A. Takamasa;T. Hirata;S. Maruyama
中科院分区：
文献类型：
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作者：
T. Iizuka;S. Nakai;Y. Sahoo;A. Takamasa;T. Hirata;S. Maruyama
我们测量了来自 Itsaq 片麻岩杂岩（3.8-3.7Ga 枕状变玄武岩、变英英岩和变沉积物）和 Acasta 片麻岩杂岩（4.0-3.6Ga 长英质正片麻岩）的太太古代岩石的 182W/184W，以评估硅酸盐地球内可能的 W 同位素异质性并限制硅酸盐地球内部 W 同位素演化。 地幔。数据显示，太太古代样品中的 182W/184W 值在分析误差范围内是一致的，与现代可接近的地幔特征无法区分，这表明上地幔的 W 同位素组成自始太古代以来没有发生显着变化。结果表明，要么在后冥宙时代，地幔与地核之间的化学通讯微不足道，要么具有独特W同位素特征的最低地幔已从地幔对流循环中分离出来。大多数陆地岩石样本的 0.2ε142Nd/144Nd 比球粒陨石的平均值高。这需要在太阳系的前 30Ma 内形成隐藏的富集储层，或者地球的 Sm/Nd 含量比球粒陨石平均值高约 5%。我们探索了地球早期硅酸盐分异事件期间 182Hf–182W 同位素系统与 146Sm–142Nd 同位素系统的相关性。在这种情况下，我们证明，尽管与现代可及的地幔相比，Itsaq 岩石中缺乏可解析的 182W 过量，但与地球具有非球粒状 Sm/Nd 的观点更加一致。在非球粒状Sm/Nd地球模型中，182W-142Nd测时法将Itsaq样本的源地幔损耗年龄限制在太阳系起源之后~40Ma以上。我们的结果无法证实之前关于太太古代Itsaq沉积物中182W异常的报告，该异常被解释为反映了撞击产生的陨石成分。
We have measured182W/184W for Eoarchean rocks from the Itsaq Gneiss Complex (3.8–3.7Ga pillow meta-basalts, a meta-tonalite, and meta-sediments) and Acasta Gneiss Complex (4.0–3.6Ga felsic orthogneisses) to assess possible W isotopic heterogeneity within the silicate Earth and to constrain W isotopic evolution of the mantle. The data reveal that182W/184W values in the Eoarchean samples are uniform within the analytical error and indistinguishable from the modern accessible mantle signature, suggesting that the W isotopic composition of the upper mantle has not changed significantly since the Eoarchean era. The results imply either that chemical communication between the mantle and core has been insignificant in post-Hadean times, or that a lowermost mantle with a distinctive W isotope signature has been isolated from mantle convective cycling. Most terrestrial rock samples have a 0.2ε142Nd/144Nd higher than the chondrite average. This requires either the presence of a hidden enriched reservoir formed within the first 30Ma of the Solar System, or the bulk Earth having a ∼5% higher Sm/Nd than the chondrite average. We explored the relevance of the182Hf–182W isotope system to the146Sm–142Nd isotope system during early silicate differentiation events on Earth. In this context, we demonstrate that the lack of resolvable182W excesses in the Itsaq rocks, despite142Nd excesses compared to the modern accessible mantle, is more consistent with the view that the bulk Earth has a non-chondritic Sm/Nd. In the non-chondritic Sm/Nd Earth model, the182W–142Nd chronometry constrains the age of the source mantle depletion for the Itsaq samples to more than ∼40Ma after the Solar System origin. Our results cannot confirm the previous report of182W anomalies in the Eoarchean Itsaq meta-sediments, which were interpreted as reflecting an impact-derived meteoritic component.