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Earth's heterogeneous mantle: A product of convection-driven interaction between crust and mantle

机译：地球的异质地幔：地壳与地幔之间由对流驱动的相互作用的产物

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Ubiquitous heterogeneity in the Earth's mantle has been documented by numerous chemical and isotopic analyses of oceanic basalts. Despite the ever-increasing amount of data, the way in which compositional heterogeneity is manifest in the Earth's mantle, as well as the processes leading to mantle heterogeneity remain fundamental questions. The large amount of available isotope data in oceanic basalts shows that, statistically, only two principal compositional vectors capture the essential features of the data. Care must be taken, however, when estimating the isotopic composition of mantle from basalt samples. This is because partial melting, and melt mixing during melt extraction leads to a biased representation and subdued compositional variability in the basalts relative to their mantle sources. In both ridge and ocean island settings, for example, erupted lavas are expected to be isotopically less depleted than the most depleted source components. Abyssal peridotites indeed range to much more depleted isotope compositions than mid ocean ridge basalts (MORB). The extent of heterogeneity of the MORB mantle source, the depleted mantle, therefore depends on the proportion, as well as differences in composition, age, and sampling of its various depleted and enriched source components. While MORB data thus do not reflect the full extent of mantle heterogeneity, the large amount of trace element and isotope data in ocean island basalts (OIB) suggests that enriched isotope signatures in OIB closely correspond to those of their average enriched mantle components. OIB can therefore be used to trace the geologic reservoirs that exchange mass with the mantle and to identify the geological processes that introduce enriched material into the Earth's mantle. The generation and subduction of oceanic plates into the deeper mantle, together with small amounts of lower and upper continental crust, appears to be the main process for mantle enrichment. Thereby, erosion and subduction of the lower continental crust accounts for a large part of the enriched isotope signatures in oceanic basalts. Recycling of the upper continental crust, on the other hand, is inferred to be only a minor process, but required to explain the entire spectrum of enriched OIB signatures. Hence a first order geologic process - the generation and subduction of oceanic plates - accounts for the first-order heterogeneity of the Earth's mantle. Moreover, one of the main processes for establishing the composition of the continental crust - erosion and recycling of the lower continental crust - is also one of the main processes for the generation of mantle heterogeneity. Overall, large-scale chemical cycling between Earth's two major lithophile element reservoirs, the mantle and the oceanic and continental crust, is responsible for mantle enrichment. Once introduced into the mantle, the heterogeneous materials become stretched, reduced in size and distributed by mantle convection. The isotopic heterogeneity observed in melt inclusions and abyssal peridotites suggests that eventually, the heterogeneity of the mantle sources of oceanic basalts will exist at relatively small scales, certainly on the kilometer scale of the melting region but perhaps even smaller. The way in which mantle heterogeneity is manifest in the source of oceanic basalts is therefore directly related to the fluid dynamics of mantle convection, whereas the timing, nature, and extent of crust-mantle interaction govern the differentiation and compositional evolution of the silicate Earth.
机译：地球玄武岩中普遍存在的异质性已经通过对海洋玄武岩的大量化学和同位素分析得到了证明。尽管数据量不断增加，但是组成异质性在地球地幔中的表现方式以及导致地幔异质性的过程仍然是基本问题。海洋玄武岩中大量可用的同位素数据表明，从统计学上讲，只有两个主要的成分矢量捕获了数据的基本特征。但是，从玄武岩样品估算地幔的同位素组成时必须小心。这是因为部分熔融以及熔融物提取过程中的熔融混合导致玄武岩相对于地幔源的偏斜表示和柔和的成分变化。例如，在山脊和海洋岛屿环境中，预计喷发的熔岩在同位素上的消耗要少于最贫化的源组分。实际上，深海橄榄岩的同位素组成比中洋脊玄武岩（MORB）丰富得多。因此，MORB地幔源（贫化地幔）的异质程度取决于比例，以及组成，年龄和各种贫化和富集的震源成分采样的差异。尽管MORB数据不能完全反映出地幔的非均质性，但海洋玄武岩（OIB）中大量的痕量元素和同位素数据表明，OIB中富集的同位素特征与其平均富集的地幔成分密切相关。因此，可以将OIB用来追踪与地幔交换质量的地质储层，并确定将富集的物质引入地球地幔的地质过程。海洋板块的形成和俯冲，再加上少量的上，下陆壳，似乎是地幔富集的主要过程。因此，下部大陆壳的侵蚀和俯冲作用是海洋玄武岩富集同位素特征的很大一部分。另一方面，推断上大陆壳的再循环只是一个很小的过程，但需要解释丰富的OIB签名的整个范围。因此，一阶地质过程-海洋板块的产生和俯冲-解释了地球地幔的一阶异质性。此外，建立大陆壳组成的主要过程之一-下大陆壳的侵蚀和再循环-也是形成地幔非均质性的主要过程之一。总体而言，地球上两个主要的嗜石元素储集层，地幔以及海洋和大陆壳之间的大规模化学循环是地幔富集的原因。一旦引入地幔中，异质材料就被拉伸，减小尺寸并通过地幔对流分布。在熔体包裹体和深海橄榄岩中观察到的同位素异质性表明，海洋玄武岩地幔源的异质性最终将以相对较小的规模存在，当然在熔融区的千米范围内可能存在，但可能甚至更小。因此，在海洋玄武岩源中表现出地幔非均质性的方式与地幔对流的流体动力学直接相关，而地壳-地幔相互作用的时间，性质和程度决定着硅酸盐地球的分化和组成演化。

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