📄 earth science

An ancient, deep, and ultra-depleted mantle domain beneath the northern Atlantic Ocean

This study identifies an ancient, ultra-depleted mantle domain beneath the northern Atlantic Ocean, characterized by high-degree melt extraction at great depths, which the authors propose represents preserved residues of ancient mantle plumes that contribute to the Earth's large-scale compositional structure.

Original authors: Feng Xu, Piero A.B. de Sampaio, Alessio Sanfilippo, Xubo Zhang, Luc Doucet, Zheng-Xiang Li

Published 2026-10-07

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CC BY 4.0

Original authors: Feng Xu, Piero A.B. de Sampaio, Alessio Sanfilippo, Xubo Zhang, Luc Doucet, Zheng-Xiang Li

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep beneath the Earth's surface lies a churning ocean of rock, hotter than the surface of the sun, yet solid enough to carry the continents on its back. This is the mantle, a layer that has been slowly separating itself into different chemical families for billions of years. As the planet cooled and differentiated, it pushed lighter materials up to form the crust we walk on, leaving behind a heavier, more exhausted residue deep below. For decades, scientists have known that this leftover mantle is not uniform. They have mapped out a broad divide: the mantle under the Pacific Ocean tends to be chemically "depleted," meaning it has already given up much of its easy-to-melt ingredients, while the mantle under Africa and the Indian Ocean is "enriched," holding onto more of those volatile components. But a crucial piece of the puzzle remained missing. While the enriched side was well-studied, the nature and origin of the most extreme, most exhausted pockets of mantle remained a mystery.

A new study by Feng Xu and colleagues has finally located one of these extreme pockets, revealing a vast, ancient domain hidden beneath the northern Atlantic Ocean. By analyzing the chemical fingerprints of rocks brought to the surface by volcanoes and mid-ocean ridges, the researchers identified a region that is far more depleted than anything previously seen. This is not just a slightly tired patch of rock; it is an ultra-depleted domain, a place where the mantle has been stripped of its minerals to an extreme degree. The team found that the rocks from this region carry a unique chemical signature, specifically in the way their hafnium and neodymium isotopes behave, which sets them apart from the depleted mantle of the Pacific and the enriched mantle of the south.

To understand how this happened, the researchers had to look back in time. They built a statistical model to simulate how the mantle melts and changes over billions of years. Their work suggests that the ultra-depleted rocks under the northern Atlantic were formed by a massive, high-temperature melting event that occurred at least 800 million years ago. Crucially, this melting happened much deeper than the normal melting that occurs at mid-ocean ridges today. It likely took place in the presence of a giant mantle plume, a column of hot rock rising from the deep Earth, which stripped the mantle of its minerals so thoroughly that the remaining rock became incredibly light and buoyant.

The study explicitly rules out several other possibilities for the origin of these rocks. The researchers argue that this domain was not formed by the normal, shallow melting processes that create the Pacific depleted mantle, nor was it simply a result of the breakup of continents or the recycling of surface crust. The chemical evidence shows that the rocks from the northern Atlantic are distinct from those formed during the opening of the ocean basin in the last 55 million years. Instead, the data points to a much older history. The model indicates that after this ancient, deep melting event, these ultra-depleted blocks sank down into the lower mantle, where they remained preserved for hundreds of millions of years.

The story of how these rocks returned to the surface is as dramatic as their formation. The study proposes that during the last 50 million years, new mantle plumes rose from the deep Earth, specifically beneath Iceland and the Azores. As these plumes rose, they scooped up the ancient, ultra-depleted blocks that had been waiting in the lower mantle. These ancient fragments were then carried upward and erupted as lava, bringing the deep-time history of the Earth to the surface. This discovery changes our understanding of the planet's internal structure, suggesting that the mantle is not just a mix of recycled surface materials and standard residues, but a complex archive containing ancient, deep-seated domains that have survived for nearly a billion years. The presence of these ultra-depleted components helps explain why some volcanic regions have such unusual chemical signatures and suggests that the deep Earth holds onto remnants of its earliest, most violent history, waiting to be brought back to light.

Technical Summary: An Ancient, Deep, and Ultra-Depleted Mantle Domain Beneath the Northern Atlantic Ocean

Problem Statement
The Earth's mantle is chemically heterogeneous, characterized by a first-order east-west dichotomy: an enriched African mantle domain and a depleted Pacific mantle domain. While crustal recycling is widely accepted as the driver for the enriched African domain, the nature, distribution, and origin of depleted mantle residues remain poorly understood. Traditional models treat the depleted upper mantle as a relatively homogeneous reservoir, yet evidence from abyssal peridotites suggests the existence of "ultra-depleted" components with distinct isotopic signatures (e.g., high $^{176}\text{Hf}/^{177}\text{Hf}$ ). However, the global distribution of these ultra-depleted domains is constrained by sparse peridotite sampling, and the mechanisms generating the systematic Hf-isotope discrepancies between oceanic basalts and peridotites remain enigmatic.

Methodology
The study integrates global geochemical datasets of oceanic basalts (MORBs, OIBs, and OPBs) and abyssal peridotites to reconstruct a global mantle depletion framework.

Geochemical Compilation: The authors compiled isotopic (Nd, Hf, Sr, Pb) and elemental data from global mid-ocean ridges and near-ridge hotspots. Special emphasis was placed on Hf isotopes due to their sensitivity to depleted mantle components.
Statistical Modeling: To resolve the "Hf isotopic paradox" (where abyssal peridotites exhibit higher $\varepsilon\text{Hf}$ $ε Hf$ than co-genetic MORBs), the authors developed a Monte Carlo statistical mantle model. This model simulates mantle compositions under variable depletion extents and ages.
- Input Parameters: The model utilizes a Johnson-SU distribution fitted to $^{147}\text{Sm}/^{144}\text{Nd}$ ratios of abyssal peridotites to represent the range of depleted mantle assemblages.
- Scenarios: Three distinct depletion scenarios were tested:
  - Scenario I: Shallow depletion exclusively within the spinel peridotite field.
  - Scenario II: Depletion at greater depths involving variable proportions of spinel- and garnet-peridotite residues.
  - Scenario III: A two-stage history involving ancient oceanic lithosphere recycling followed by shallow melt extraction.
Mixing Models: The simulations incorporate mixing between mantle assemblages (formed at 800 Ma or older) and minor recycled oceanic crust components to reproduce observed isotopic arrays.

Key Results

Identification of a Northern Atlantic Ultra-Depleted (UDM) Domain: The analysis reveals a distinct geochemical domain beneath the northern Atlantic Ocean (from the Azores–Iceland region northward to the Knipovich ridge). This domain is characterized by:
- High $\varepsilon\text{Nd}$ and positive $\Delta\varepsilon\text{Hf}$ (plotting above the terrestrial array).
- Systematically lower $^{87}\text{Sr}/^{86}\text{Sr}$ and $^{208}\text{Pb}^*/^{206}\text{Pb}^*$ compared to enriched domains.
- Depleted major and incompatible element compositions (e.g., lower $\text{TiO}_2$ and $\text{Na}_2\text{O}$ at comparable $\text{MgO}$ ), indicative of clinopyroxene-poor peridotites.
- These signatures persist over distances $>1000$ km from plume centers, suggesting a widespread source rather than localized plume melting.
Distinct Depletion Histories:
- Northern Atlantic (UDM): The model indicates this domain requires an ancient ( $\ge 800$ Ma), ultra-depleted mantle assemblage formed by high-degree melting at depths greater than normal mid-ocean-ridge (MOR) melting (involving garnet-peridotite stability fields).
- Pacific (Depleted): In contrast, the depleted Pacific domain reflects a history of recycled ancient oceanic lithosphere followed by progressive, shallow melt extraction at MOR depths.
Resolution of the Hf Isotopic Paradox: The statistical model demonstrates that the systematic offset between MORBs and peridotites can be explained by distinct depletion histories. The UDM domain's high $\varepsilon\text{Hf}$ is consistent with ancient, deep melting events, whereas the Pacific domain's negative $\Delta\varepsilon\text{Hf}$ is consistent with shallow, progressive depletion of recycled lithosphere.

Significance and Claims
The paper proposes that the ultra-depleted mantle (UDM) domain beneath the northern Atlantic Ocean represents residues of ancient mantle plumes that were recycled into the lower mantle and preserved there. These residues were subsequently remobilized to the surface by Cenozoic mantle plumes (e.g., Iceland, Azores).

The authors claim this discovery has several implications:

Mantle Structure: It challenges the view of the depleted mantle as homogeneous, suggesting instead that distinct depletion histories (deep vs. shallow, ancient vs. progressive) contribute significantly to the mantle's compositional structure.
Geodynamic Evolution: The presence of ancient, buoyant, clinopyroxene-poor peridotites in the lower mantle may help explain the anomalous buoyancy and upward flux of mantle plumes.
Deep Time Preservation: The study suggests that ancient mantle residues (potentially Hadean-Archean in age) can survive in the lower mantle for billions of years, contributing to the long-term chemical heterogeneity of the Earth.

The authors explicitly state that while UDM signatures may exist in other plume systems (e.g., Marion Rise, Sierra Leone, Hawaii), they are often obscured by enriched components, highlighting the need to account for ancient depleted components in models of Earth's geodynamic evolution.

Technical Summary: An Ancient, Deep, and Ultra-Depleted Mantle Domain Beneath the Northern Atlantic Ocean

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