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

超大陆的热覆盖效应对元古代斜长岩形成的影响

• 批准号: 2330810
• 负责人: Michael Gurnis
• 金额: $ 36.23万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2024
• 资助国家: 美国
• 起止时间: 2024-02-01 至 2026-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2330810&HistoricalAwards=false
• 关键词: thermal; blanketing; effect; supercontinents; formation

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.

在26亿至5亿年前的元古代，地球见证了努纳和罗迪尼亚两个“超级大陆”的聚集，当时所有大陆汇聚成一个巨大的大陆。正是在这段时间里，“元古宙古陆斜长岩”或PMA--类似于月球表面发现的巨大的不寻常岩石--在这些大陆下面的深处形成。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是因为超级大陆将地球内部与大气层隔离开来。这个想法是，这种绝缘效应使温度足够高，使PMA在整个漫长的元古代逐渐结晶，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规模的地幔对流、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关联且相互依赖的）。 用于这一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（沿着说明和输入数据文件）。元古宙古生界类斜长岩（PMAs）是广泛分布的、神秘的深成岩基形成岩石，其年龄限于2.6至0.5 Ga之间。尽管它们的矿物学很简单（超过90%的斜长石），但这些岩石的起源、岩浆来源和地球动力学背景已经成为世纪以来激烈争论的主题。在元古代，地壳演化主要由两个超级大陆主导，努纳（也称为哥伦比亚）和罗迪尼亚。超大陆旋回被认为在岩石记录的形成和保存以及地幔动力学演化中起着重要作用。在这个项目中，研究人员提出了PMAs的形成与超大陆循环之间的联系。他们将通过进行包含熔融过程的先进地幔对流模型，直接评估超大陆的热覆盖效应。这将使他们能够全面调查超大陆隔热效应的分布，持续时间和程度及其表面表现。研究人员将首先限制母岩浆成分和熔融条件，然后根据现有的岩石学和地球化学观测结果重建全球PMAs的就位环境。接下来，他们将把全球热化学对流模型与热力学计算相结合，以量化超大陆的覆盖效应，并将计算结果与他们汇编的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。地球动力学代码将采用一个现实的粘弹塑性岩石圈流变学模型的底辟上升和侵位的PMA的过程。他们将查明斜长岩浆上升的有利环境，这将对未来在PMA地区勘探高品位Fe-Ti-P矿产资源具有重要意义。通过这三种方法，研究人员的目标是更全面地建立超大陆循环和地幔对流之间的动态联系，以及它们如何转化为壳幔相互作用，导致PMA的时间限制。该奖项反映了NSF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。