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

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.

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