The thermal blanketing effect of supercontinents on the formation of Proterozoic anorthosites

在26亿到5亿年前的元古代，地球见证了两个“超级大陆”——努纳和罗迪尼亚的合并，所有的大陆融合成一个巨大的大陆。正是在这段时间里，“元古宙块状斜长岩”（简称pma）在这些大陆的地下深处形成了，它们是一大片与月球表面相似的不寻常的岩石。科学家们长期以来一直在争论是什么导致了pma的形成。Gurnis， Asimow和他们的团队将开发模型来确定它们的形成是否因为超级大陆将地球内部与大气隔离开来。这个想法是，这种绝缘效应使温度保持在足够高的水平，使pma在漫长的元古代时期逐渐结晶，之后温度下降，pma停止形成。为了验证这一想法，研究人员将开发计算机模型，并在超级计算机上进行模拟。这些模型代表了全球尺度的地幔对流、热流、地幔岩石的化学反应和板块构造运动（因为所有这些过程都是相互联系和依赖的）。用于此建模的计算机程序将通过一个可公开访问的网站与其他科学家免费共享（以及说明和输入数据文件）。元古代块状斜长岩（pma）是广泛分布的、神秘的深部岩基形成岩石，其年龄限制在2.6 - 0.5 Ga之间。尽管它们的矿物学很简单（超过90%是斜长石），但这些岩石的起源、岩浆来源和地球动力学背景在一个多世纪以来一直是激烈争论的主题。在元古代，地壳演化由两个超级大陆主导，努纳（也被称为哥伦比亚）和罗迪尼亚。超大陆旋回被认为对岩石记录的形成和保存以及地幔动力学的演化都起着深远的作用。在这个项目中，研究人员提出了PMAs的形成与超大陆旋回之间的联系。他们将通过进行包含熔融过程的先进地幔对流模型，直接评估超级大陆的热覆盖效应。这将使他们能够全面研究一个超大陆的保温作用的分布、持续时间和程度及其表面表现。研究人员将首先对母岩浆组成和熔融条件进行约束，然后在现有岩石学和地球化学观测的基础上重建全球pma的侵位环境。接下来，他们将把全球热化学对流模型与热力学计算结合起来，量化超大陆的覆盖效应，并将计算结果与他们编制的PMA生成条件记录进行比较。将使用的数值工具包括他们开发的全动态模型和板块构造重建软件GPlates。地球动力学程序将采用一种真实的粘弹塑性岩石圈流变学模型来模拟底辟上升和pma就位的过程。这将为今后寻找高铁、高钛、高磷矿产资源提供重要的指导意义。通过这三种方法，研究人员旨在更全面地建立超大陆循环与地幔对流之间的动态联系，以及它们如何转化为导致pma时间限制的壳幔相互作用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

During the Proterozoic era, spanning from 2.6 to 0.5 billion years ago, the Earth witnessed the assembly of two "supercontinents", Nuna and Rodinia, when all continents converged into a single massive landmass. It was during this time that "Proterozoic massif anorthosites" or PMAs - massive expanses of unusual rock similar to that found on the surface of the Moon - formed at depth beneath these continents. Scientists have long debated what led to the formation of PMAs. Gurnis, Asimow, and their team will develop models to determine whether they formed because the supercontinents insulated the inner Earth from the atmosphere. The idea is that this insulating effect kept temperatures high enough for PMAs to gradually crystallize throughout the long Proterozoic era, after which temperatures dropped and the PMAs stopped forming. To test this idea, the investigators will develop computer models and run simulations on supercomputers. The models represent global-scale mantle convection, heat flow, chemical reactions of mantle rocks, and plate tectonic motions all together (because all of these processes are linked and depend on each other). The computer programs used for this modeling will be shared freely with other scientists (along with instructions and input data files) via a publicly accessible website.Proterozoic massif-type anorthosites (PMAs) are widespread, enigmatic plutonic batholith-forming rocks limited to ages between 2.6 and 0.5 Ga. Despite their simple mineralogy (over 90% plagioclase feldspar), the origin, magma sources, and geodynamic contexts of these rocks have been subjects of vigorous debate for more than a century. During the Proterozoic, the Earth’s crust evolution was dominated by two supercontinents, Nuna (also known as Columbia) and Rodinia. A supercontinent cycle is believed to play a profound role both in the formation and preservation of the rock record and in the evolution of mantle dynamics. In this project, the researchers propose a link between the formation of PMAs and the supercontinent cycle. They will directly assess the thermal blanketing effect of supercontinents by conducting advanced mantle convection models that incorporate melting processes. This will allow them to comprehensively investigate the distribution, duration, and extent of a supercontinent's thermal insulation effect and its surface expression. The researchers will first constrain the parental magma composition and melting conditions, followed by the reconstruction of the emplacement environment of global PMAs based on existing petrological and geochemical observations. Next, they will integrate global thermochemical convection models with thermodynamic calculations to quantify the supercontinent's blanketing effect and compare the computed results with their compiled record of PMA generation conditions. The numerical tools that will be used include fully dynamic models they develop, and plate tectonic reconstruction software GPlates. The geodynamic code will employ a realistic visco-elasto-plastic lithospheric rheology to model the processes of diapiric uprising and emplacement of PMAs. They will pinpoint the advantageous settings for the ascent of anorthositic mushes, which will have important implications for future exploration for high-grade Fe-Ti-P mineral resources in PMA districts. Through these three approaches, the researchers aim to more fully establish the dynamic link between supercontinent cycling and mantle convection and how they translate into crust-mantle interactions that cause the temporal restriction of PMAs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.