Generation of Archean Granitoids and the Onset of Plate Tectonics.

太古代花岗岩的生成和板块构造的开始。

• 批准号: 404681101
• 负责人: Professor Dr. Boris J.P. Kaus
• 金额: --
• 依托单位: Institut des Sciences de la Terre (ISTerre)
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/404681101?language=en
• 关键词: Generation; Archean; Granitoids; Onset; Plate

The formation of the continental crust had a first order impact on the habitability of the Earth, being directly linked to the emergence of life and later allowing life to migrate onto land. Continents have a high elevation compared to oceans, because continental crustal rocks are granitic in composition and have a relatively low density compared to mantle rocks. It is generally accepted that these rocks must have formed by melting the mantle in a multi-step process that requires basalt and hydrous phases to be present. On the present-day Earth this occurs in subduction zones. Yet, in the Archean Earth, mantle temperatures were higher, more melt was produced in the mantle, and oceanic crust was significantly thicker. Models suggest that if mantle potential temperatures were 150-250K higher than present-day values, subduction could not have operated. Instead, parts of the crust may have dripped down into the mantle, a process known as sagduction. The question of whether plate tectonics operated in the Archean therefore depends on two key points: 1) that temperatures in the Archean upper mantle were so high that oceanic plates were unsubductable; 2) that Archean granitoids were produced by a process independent of subduction. How granites formed is incompletely understood, particularly for a warmer Earth. In this proposal, we will use petrological constraints to build a model of thick oceanic crust; one that is strongly differentiated with ultramafic cumulates in the lower half and hydrated basalt restricted to upper layers. We will also re-evaluate petrological constraints and cooling models that define temperatures in Archean ambient mantle. Then we will use 2D and 3D numerical models to simulate dehydration, melting and deformation within this crust and compare the results with available petrological/ geochemical constraints. By including recently developed thermodynamic melting models and the chemical evolution of melts in the thermomechanical numerical models, we can use model predictions to test whether sagduction or subduction is a more efficient process to create granites. In addition, we will derive parameterizations that describe the effect of both subduction and sagduction on the cooling of the Earth and include these in parameterized cooling models. This will give new insights in how such processes affect the thermal evolution of the Earth, how this affected crustal growth and on the timing of the onset of plate tectonics on our planet.

大陆地壳的形成对地球的可居住性产生了第一级影响，与生命的出现直接相关，后来允许生命迁移到陆地上。与海洋相比，大陆的海拔较高，因为大陆地壳岩石的成分是花岗岩，与地幔岩石相比密度相对较低。人们普遍认为，这些岩石一定是通过熔融地幔在一个多步骤的过程，需要玄武岩和含水相存在。在今天的地球上，这发生在俯冲带。然而，在太古代地球，地幔温度更高，更多的熔体在地幔中产生，洋壳明显更厚。模型表明，如果地幔位温比现在高150- 250 K，俯冲就不可能发生。相反，部分地壳可能已经滴入地幔，这一过程被称为sagduction。因此，关于太古代是否存在板块构造的问题取决于两个关键点：1）太古代上地幔的温度如此之高，以至于海洋板块无法俯冲; 2）太古代花岗岩类是由一个独立于俯冲作用的过程产生的。花岗岩是如何形成的还不完全清楚，特别是在地球变暖的情况下。在这个建议中，我们将使用岩石学的限制，以建立一个模型的厚洋壳，一个是强烈分化的超镁铁堆在下半部分和水合玄武岩局限于上层。我们还将重新评估岩石学的限制和冷却模型，确定太古代环境地幔的温度。然后，我们将使用2D和3D数值模型来模拟该地壳内的脱水，熔融和变形，并将结果与可用的岩石学/地球化学约束进行比较。通过包括最近开发的热力学熔融模型和化学演化的熔体的热力学数值模型，我们可以使用模型预测，以测试是否sagduction或俯冲是一个更有效的过程，创造花岗岩。此外，我们将推导出参数化，描述俯冲和凹陷对地球冷却的影响，并将其纳入参数化冷却模型。这将使人们对这些过程如何影响地球的热演化、这如何影响地壳生长以及我们星球上板块构造开始的时间有新的认识。