Unveiling the Enigma of Earth’s Lower Mantle: Bridgmanite and the Viscosity Puzzle

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Researchers have unearthed a remarkable phenomenon deep within the Earth’s lower mantle, occurring at depths ranging from 800 to 1,200 kilometers. This discovery centers on the influence of bridgmanite-enriched rocks, which exhibit larger grain sizes and exert a substantial impact on various geophysical and geochemical processes.

The investigation, led by Prof. Dr. Tomoo Katsura and an international research team based at the Bavarian Research Institute of Experimental Geochemistry and Geophysics, University of Bayreuth, has successfully unveiled the underlying reason for the sudden increase in viscosity within the Earth’s interior at these specific depths.

This transformation is attributed to the prevalence of bridgmanite-enriched rocks, constituting the majority of the Earth’s lower mantle below approximately 1,000 kilometers. These rocks possess significantly larger grain sizes compared to those found above, resulting in heightened viscosity. The findings from this study have been documented in the prestigious journal, Nature, with co-authorship credits to Dr. Nicolas Walte, a scientist associated with MLZ.

Enrichment of Bridgmanite in the Lower Mantle

Bridgmanite, named after Nobel Prize in Physics laureate Percy Bridgman, stands as the predominant mineral in the Earth’s lower mantle, spanning depths from 660 kilometers to 2,900 kilometers and encompassing roughly half of the planet’s mass. Collaborative research involving scientists from Germany, China, France, the UK, and the USA has confirmed that bridgmanite’s grain size undergoes a notable augmentation at around the 1,000-kilometer depth mark, corresponding to the increasing enrichment of bridgmanite in lower-mantle compositions as depth escalates.

This process culminates in a pronounced rise in viscosity within the upper portion of the lower mantle, primarily due to the positive correlation between viscosity and grain size. The upper section of the lower mantle predominantly consists of pyrolite, a rock composition comprising 20% secondary minerals that inhibit the growth of bridgmanite grains. In contrast, the bridgmanite-enriched rocks contain a considerably lower proportion of secondary minerals, allowing bridgmanite to grow unhindered, resulting in the development of larger grains.

Significance of the Viscosity Transition in the Lower Mantle

The abrupt increase in viscosity has wide-ranging implications for a multitude of geophysical and geochemical processes. Dr. Hongzhan Fei, the lead author of the study, offers insight into its consequences, stating that while subducted tectonic plates subside relatively smoothly into the lower mantle, their descent is significantly decelerated in the shallow lower mantle. Conversely, the ascent of mantle plumes, responsible for the formation of volcanoes across the Earth’s surface, accelerates beyond the 1,000-kilometer depth threshold. These observations, previously perplexing, can now be comprehensively explained.

Origin and Preservation of Highly Viscous Bridgmanite-Enriched Rocks

The highly viscous bridgmanite-enriched rocks, characterized by their billion-year-old structures, emerged early in the Earth’s history. Their substantial viscosity prevents their integration with other mantle components through mantle convection. Consequently, these rocks have remained preserved in the deep lower mantle for eons.

Correlating Research Findings with Seismic Data

Prof. Dr. Tomoo Katsura relates these groundbreaking research findings to seismic observations. Seismologists have long noted the stagnation of subducted slabs within the layer situated between 600 and 1,500 kilometers in depth. Furthermore, they have observed that plumes, responsible for the formation of volcanoes, rise vertically and become less discernible above the 1,000-kilometer depth threshold.

The newly formulated theory effectively reconciles these observations. The increase in viscosity with depth impedes the penetration of slabs into regions deeper than 1,000 kilometers. Conversely, plumes rise at an accelerated pace at this depth, causing them to become thinner and more challenging to detect.

This groundbreaking study, featured in Nature, materialized through the close collaboration of esteemed scientists, including Prof. Dr. Tomoo Katsura (University of Bayreuth), Prof. Dr. Hongzhan Fei (University of Bayreuth and Zhejiang University, China), Dr. Nicolas Walte (Technical University of Munich), Prof. Dr. Maxim Ballmer (University College London, UK), Dr. Ulrich Faul (Massachusetts Institute of Technology, Cambridge, USA), and Dr. Weiwei Cao (Extreme Conditions and Materials: High Temperature and Irradiation, Orléans, France).

Frequently Asked Questions (FAQs) about Viscosity Transition

More about Viscosity Transition

• Nature: Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump
• University of Bayreuth – Prof. Dr. Tomoo Katsura
• Zhejiang University – Dr. Hongzhan Fei
• Technical University of Munich – Dr. Nicolas Walte
• University College London – Prof. Dr. Maxim Ballmer
• Massachusetts Institute of Technology – Dr. Ulrich Faul
• Extreme Conditions and Materials: High Temperature and Irradiation (CEMHTI) – Dr. Weiwei Cao