The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation
The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation
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DOI：
10.1016/j.epsl.2015.05.027
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发表时间：
2015-08
影响因子：
5.3
通讯作者：
Jingao Liu;J. Scott;Candace E Martin;D. Pearson
中科院分区：
文献类型：
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作者：
Jingao Liu;J. Scott;Candace E Martin;D. Pearson
古老的熔融枯竭的岩石圈地幔在随时间保存大陆地壳方面所起的作用，对于理解大陆是如何建造、破坏和再循环至关重要。虽然很明显，现存的太古宙地壳大部分是由太古宙地幔根部支撑的，但关于通过显生宙地体喷发的元古界橄榄岩熔融耗尽年龄的报道提出了一种可能性，即古代浮力的岩石圈地幔对地球大部分大陆地壳来说是一艘“救生筏”。本文报道了迄今在地球上观察到的最大的地壳-岩石圈地幔年龄分离(∼2.4Ga)，并探讨了这种极端年龄分离的潜在原因。新西兰西奥塔哥的显生宙(<；300 Ma)大陆地壳受到新生代深成岩类的侵入，喷发出类似克拉通地幔的高度亏损的方辉橄榄岩和英安岩。这些橄榄岩的Re亏损Os模式年龄在0.5~2.7Ga之间，确立了太古宙亏损事件的记录。然而，消融年龄的巨大范围与熔体消减或交代示踪指数无关，这对该地区下方存在大量古代地幔根提供的支持很少。相反，化学和同位素数据可以很好地解释太古代贫化橄榄岩地幔残留物的混合，这些残留物在Ga时间尺度上循环穿过软流圈，以及更富饶的对流地幔。与这些熔体残留物在西兰迪亚大陆年轻的大陆地壳下“对接”有关的广泛熔体枯竭解释了我们今天观察到的脱钩的年龄关系。因此，新形成的岩石圈根部包含了来自对流地幔的古代和现代地幔的混合物，这些地幔在近代冷却和吸积。我们认为，在这种情况下，古老的成分在大陆稳定中没有起到早期作用，但它们的高度贫化性质与年轻的成分一起，现在代表了一个高度粘滞、稳定的大陆龙骨。这一模型可以解释在澳大利亚东南部、南极洲西部和新西兰其他地区的岩石圈地幔以及大洋地幔中观察到的可育到中等亏损的橄榄岩中观察到的大范围年龄谱。我们的数据证实了古老的亏损地幔区域在对流地幔中的寿命和扩散，以及它们在年轻大陆之下的重新出现。
The role played by ancient melt-depleted lithospheric mantle in preserving continental crust through time is critical in understanding how continents are built, disrupted and recycled. While it has become clear that much of the extant Archean crust is underpinned by Archean mantle roots, reports of Proterozoic melt depletion ages for peridotites erupted through Phanerozoic terranes raise the possibility that ancient buoyant lithospheric mantle acts as a “life-raft” for much of the Earth's continental crust. Here we report the largest crust–lithospheric mantle age decoupling (∼2.4 Ga) so far observed on Earth and examine the potential cause for such extreme age decoupling.The Phanerozoic (<300 Ma) continental crust of West Otago, New Zealand, is intruded by Cenozoic diatremes that have erupted cratonic mantle-like highly depleted harzburgites and dunites. These peridotites have rhenium depletion Os model ages that vary from 0.5 to 2.7 Ga, firmly establishing the record of an Archean depletion event. However, the vast range in depletion ages does not correlate with melt depletion or metasomatic tracer indices, providing little support for the presence of a significant volume of ancient mantle root beneath this region. Instead, the chemical and isotopic data are best explained by mixing of relict components of Archean depleted peridotitic mantle residues that have cycled through the asthenosphere over Ga timescales along with more fertile convecting mantle. Extensive melt depletion associated with the “docking” of these melt residues beneath the young continental crust of the Zealandia continent explains the decoupled age relationship that we observe today. Hence, the newly formed lithospheric root incorporates a mixture of ancient and modern mantle derived from the convecting mantle, cooled and accreted in recent times. We argue that in this case, the ancient components played no earlier role in continent stabilization, but their highly depleted nature along with that of their younger counterparts now represents a highly viscous, stable continental keel. This model could account for the large spectrum of ages observed in fertile to moderately depleted peridotites sampled from lithospheric mantle beneath SE Australia, W Antarctica and other locations in Zealandia, as well as the oceanic mantle. Our data confirm the longevity and dispersal of ancient depleted mantle domains in the convecting mantle and their re-appearance beneath young continents.