Long-term preservation of early formed mantle heterogeneity by mobile lid convection: Importance of grainsize evolution
Long-term preservation of early formed mantle heterogeneity by mobile lid convection: Importance of grainsize evolution
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DOI：
10.1016/j.epsl.2017.07.031
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发表时间：
2017-10-01
影响因子：
5.3
通讯作者：
Rizo, Hanika
中科院分区：
文献类型：
--
作者：
Foley, Bradford J.;Rizo, Hanika
海甸和太古宙地球上的构造样式，特别是板块构造是否在活动，是有争议的。对早期地球构造的一个重要的间接制约来自对早期形成的地球化学不均质性的观察：从不同地点的Hadean到显生宙岩石记录的ND-142和W-182异常表明，在地球历史的第一个类似500年的时间里形成的化学非均质储层，在重新混合进入地幔超过1个旋回后存活了下来。如此长的混合时间很难解释，因为预计地球早期会出现更热的地幔温度，这会降低地幔粘度，增加对流活力。以前的研究发现，在这种条件下，流动的盖子对流通常会在类似100英里的范围内消除不均一性，从而导致了地球早期冰盖对流停滞导致观测到的长混合时间的假设。然而，使用包含粒度演化的二维笛卡尔对流模型，我们发现，流动的盖子对流可以在高地幔温度条件下保持非均质性的时间比之前认为的要长得多，因为更高的地幔温度导致岩石圈中的颗粒更大。这些更大的颗粒导致了更强的板块边界，这些边界减缓了地表和内部的对流运动，与温度对地幔粘度的直接影响形成竞争。我们的模型表明，在早期地幔温度下，当初始非均质性与背景地幔具有相同的粘性时，流动盖子对流可以保持约0.4-1个旋回，而当非均质性比背景地幔粘性大10倍时，活动盖子对流可以保持约1-4个旋回。因此，不需要盖子对流的停滞来解释早期形成的地球化学不均质性的长期存留，尽管这些非均质性与周围地幔相比具有更高的粘度，可能是它们保存所必需的。(C)2017爱思唯尔B.V.保留所有权利。
The style of tectonics on the Hadean and Archean Earth, particularly whether plate tectonics was in operation or not, is debated. One important, albeit indirect, constraint on early Earth tectonics comes from observations of early-formed geochemical heterogeneities: Nd-142 and W-182 anomalies recorded in Hadean to Phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs, formed during the first similar to 500 Myrs of Earth's history, survived their remixing into the mantle for over 1 Gyrs. Such a long mixing time is difficult to explain because hotter mantle temperatures, expected for the early Earth, act to lower mantle viscosity and increase convective vigor. Previous studies found that mobile lid convection typically erases heterogeneity within similar to 100 Myrs under such conditions, leading to the hypothesis that stagnant lid convection on the early Earth was responsible for the observed long mixing times. However, using two-dimensional Cartesian convection models that include grainsize evolution, we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought, because higher mantle temperatures lead to larger grainsizes in the lithosphere. These larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions, in competition with the direct effect temperature has on mantle viscosity. Our models indicate that mobile lid convection can preserve heterogeneity for approximate to 0.4-1 Gyrs at early Earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle, and approximate to 1-4 Gyrs when the heterogeneity is ten times more viscous than the background mantle. Thus, stagnant lid convection is not required to explain long-term survival of early formed geochemical heterogeneities, though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their preservation. (C) 2017 Elsevier B.V. All rights reserved.