Constraints on mantle evolution from Ce-Nd-Hf isotope systematics
Constraints on mantle evolution from Ce-Nd-Hf isotope systematics
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DOI：
10.1016/j.gca.2019.12.029
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发表时间：
2020-03
影响因子：
5
通讯作者：
M. Willig;A. Stracke;C. Beier;V. Salters
中科院分区：
文献类型：
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作者：
M. Willig;A. Stracke;C. Beier;V. Salters
地幔演化受部分熔融的持续消耗和洋壳和陆壳再循环的补充控制。然而，仍有几个重要的未知数，如残余贫化地幔的成分变化程度、时间尺度、质量通量以及再循环的洋壳和陆壳的成分。在这里，我们调查的Ce-Nd-Hf同位素系统在全球有代表性的光谱洋中脊和洋岛玄武岩。用Monte Carlo方法再现所观察到的Ce-Nd-Hf同位素的变化表明，类型和年龄的亏损地幔和再生地壳的斜率，分散，和范围的建模Ce-Nd-Hf同位素阵列的主要影响。模型结果表明，亏损地幔的平均亏损年龄相对年轻（<1.5Ga），与深海橄榄岩的Nd和Os同位素模式年龄一致，不相容元素亏损程度明显中等。然而，后者是具有欺骗性的，因为它反映了一种自然的取样偏差，这是由于熔化了一个固有的异质亏损地幔造成的。在原则上，洋壳的再循环可以解释大多数的同位素范围的同位素富集端的Ce-Nd-Hf地幔阵列，但只有当整个组成的变化，再循环地壳被保存在回收，居住在地幔中，和重熔。然而，后者是不可能的，因为许多内部化学变化的来源在地幔中的居住期间，以及随后通过部分熔融取样时，在大块洋壳的尺度上达到了平均值。此外，所观察到的Ce-Nd-Hf地幔阵列的斜率和有限的散射表明，大块洋壳的再循环，即下洋壳的喷出玄武岩和侵入辉长岩都必须考虑，并且是更好地理解壳幔循环的关键。蒙特-卡罗模拟还表明，从大陆地壳到地幔的返回通量主要来自下大陆地壳，与目前的大陆地壳演化模型一致，这些模型都要求大量的镁铁质下大陆地壳必须再循环到地幔中，以保持大陆地壳的平均安山岩组成。
Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (<1.5 Ga) average depletion age of the depleted mantle, consistent with Nd and Os isotope model ages of abyssal peridotites, and an apparent moderate extent of incompatible element depletion. The latter, however, is deceiving, because it reflects a natural sampling bias, resulting from melting an inherently heterogeneous depleted mantle. In principal, recycling of oceanic crust can explain most of the isotopic range of the isotopically enriched end of the Ce-Nd-Hf mantle array, but only if the entire compositional variability of the recycled crust is preserved during recycling, residence in the mantle, and re-melting. The latter is unlikely, however, because many sources of internal chemical variance average out on the scale of the bulk oceanic crust, during residence in the mantle, and subsequent sampling by partial melting. Moreover, both the slope and limited scatter of the observed Ce-Nd-Hf mantle array show that recycling of bulk oceanic crust, that is, both the extrusive basalts and intrusive gabbros of the lower oceanic crust must be considered, and are key to better understand crust-mantle cycling in general. The Monte-Carlo simulation also indicates that the return flux from the continental crust into the mantle mainly derives from the lower continental crust, consistent with current models of continental crust evolution, which all require that a substantial amount of the mafic lower continental crust must be recycled into the mantle to maintain the average andesitic composition of the continental crust.