The prevalence of kilometer-scale heterogeneity in the source region of MORB upper mantle.
The prevalence of kilometer-scale heterogeneity in the source region of MORB upper mantle.
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DOI：
10.1126/sciadv.1701872
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发表时间：
2017-11
期刊：
影响因子：
13.6
通讯作者：
Liang Y
中科院分区：
文献类型：
--
作者：
Liu B;Liang Y
自然样品中观测到的Nd-Hf分系统可以通过熔化具有千米尺度非均质性的地幔来重现。洋中脊玄武岩源区是不均匀的，由不同大小的化学和岩性域组成。这种非均质地幔源的部分熔融作用导致了MORB和深海橄榄岩同位素组成的多样性。MORB中放射性成因同位素比值的变化归因于源自富集和衰竭地幔组分的熔体的混合。然而，仅靠熔体混合不能完全解释深海橄榄岩中143Nd/144Nd的平均值及其空间相关MORB之间的差异。研究表明，深海橄榄岩中贫Nd同位素组成是熔融柱中熔体迁移引起的混合或涂抹的自然结果。亚公里尺度富集的地幔成分或非均质性在其通过熔融区的过程中，在残余和喷发熔体中均受到明显的阻尼或均质化。粒径越大、不相容微量元素丰度越高的非均质对混合过程的阻力越大。这种对粒度敏感的混合依赖于一个称为富集强度的参数，该参数是非均质大小与富集和衰竭地幔源中不相容微量元素丰度之比的乘积。当富集强度为20 ~ 60 km时，可以再现MORB和深海橄榄岩中观察到的Nd-Hf同位素变化。这些非均质性可能在千米尺度上，与循环海洋地壳具有相似的同位素比率，但不相容的微量元素丰度较小。
Observed Nd-Hf systematics in natural samples can be reproduced by melting a mantle with heterogeneities on kilometer scale. The source regions of mid-ocean ridge basalts (MORB) are heterogeneous, consisting of chemically and lithologically distinct domains of variable size. Partial melting of such heterogeneous mantle sources gives rise to diverse isotopic compositions of MORB and abyssal peridotites. Variations in radiogenic isotope ratios in MORB are attributed to mixing of melts derived from enriched and depleted mantle components. However, melt mixing alone cannot fully account for the difference between the average 143Nd/144Nd in abyssal peridotites and their spatially associated MORB. We show that the more depleted Nd isotope composition in abyssal peridotites is a natural consequence of melt migration–induced mixing or smearing in the melting column. Sub-kilometer scale enriched mantle components or heterogeneities are significantly damped or homogenized in both the residue and erupted melt during their transit through the melting region. Heterogeneities with larger size and higher incompatible trace element abundance are more resistive to the mixing processes. The size-sensitive mixing depends on a parameter called the enrichment strength, which is the product of the heterogeneity size and the ratio between incompatible trace element abundance in the enriched and depleted mantle sources. Observed Nd-Hf isotope variations in MORB and abyssal peridotites can be reproduced if the enrichment strength is 20 to 60 km. These heterogeneities could be on the kilometer scale and have similar isotope ratios to but less incompatible trace element abundances than recycled oceanic crust.
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