📄 earth science

Carbonatite melt pathways connect the deep mantle to Archean craton margins

This study utilizes long-period magnetotelluric data to identify electrically conductive, anisotropic structures beneath the Gawler Craton as evidence of deep mantle carbonatite melts that transport volatiles and rare earth elements to craton margins, where exsolved graphite creates low-resistivity signatures at the Moho.

Original authors: Graham Heinson, David Baker

Published 2026-10-16

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CC BY 4.0

Original authors: Graham Heinson, David Baker

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, a hidden cycle moves carbon from the planet's interior back to the surface, a process that has been active for billions of years. This cycle is crucial because it regulates the chemistry of our atmosphere and oceans, but the mechanism by which carbon travels from the deep mantle up to the crust has remained elusive. Scientists know that carbon-rich rocks are pulled down into the Earth at subduction zones, where one tectonic plate slides beneath another. Some of this carbon is released early in the process, but a significant portion is thought to travel much deeper, carried by a special type of molten rock called carbonatite. These melts are unique because they are rich in carbon and can dissolve and transport rare elements and gases. However, seeing these tiny pockets of melt deep underground is incredibly difficult. They are too small to be seen directly, and the signals they send to the surface can easily be confused with other materials, such as water-rich minerals.

A team of researchers from Adelaide University has now used a powerful method called magnetotellurics to peer into this hidden world beneath the Gawler Craton in southern Australia. This ancient block of rock, known as a craton, forms the stable core of the continent. The scientists deployed a network of sensors across the landscape to measure natural electrical currents flowing through the Earth. By analyzing how these currents change over time, they were able to build a detailed three-dimensional map of electrical conductivity deep underground. Their goal was to find the signature of those elusive carbonatite melts and understand how they move through the Earth's crust and mantle.

The researchers discovered a striking anomaly deep beneath the center of the Gawler Craton. At a depth of approximately 180 kilometers, right at the boundary where the rigid outer shell of the Earth meets the flowing mantle below, they found a layer of rock that is exceptionally conductive. This layer is about 1,000 times more conductive than the rock immediately above it. The team found that this conductive layer is not uniform; it is highly directional, conducting electricity much more easily in one specific direction than in others. This directional property, known as anisotropy, suggests that the conductive material is not just a random blob of liquid but is organized into thin, aligned channels.

The authors argue that the most likely explanation for this conductive, directional layer is a tiny amount of carbonatite melt. They estimate that a volume of melt as small as 0.05 percent—roughly the size of a few drops of water in a large bucket—is sufficient to create the electrical signal they observed. This small fraction of melt is enough to drastically lower the electrical resistance of the rock because the melt forms a connected network along the boundaries of the mineral grains. The researchers explicitly ruled out the idea that water-rich minerals alone could be responsible. While water can make rock more conductive, the level of conductivity they measured is far too high to be explained by water alone, even if the rock were saturated with it. They also considered the possibility of fluorine-rich minerals, but the evidence points more strongly toward the presence of carbon-rich melt.

The study also revealed a second, related phenomenon near the edges of the ancient craton. At the boundary between the crust and the mantle, roughly 40 kilometers deep, the researchers found another zone of low electrical resistance. Unlike the deep mantle conductor, this zone is less directional. The team interprets this as a place where carbon, once carried up by the deep melts, has been released and transformed into graphite. As the carbon-rich fluids moved upward and cooled, the carbon precipitated out of the solution and formed solid graphite, which is an excellent conductor of electricity. This process created a persistent, conductive layer in the lower crust that has remained stable for billions of years.

This discovery suggests a continuous link between the deep mantle and the Earth's surface. The researchers propose that ancient tectonic events, specifically the subduction of oceanic plates around 1.6 billion years ago, pushed carbon-rich material deep into the mantle. This material generated small amounts of carbonatite melt that became trapped in channels at the base of the craton. Over time, some of this melt migrated toward the edges of the craton, where it rose into the crust, leaving behind graphite deposits. This pathway not only explains the electrical signals but also provides a mechanism for how carbon and other valuable elements, such as those found in major mineral deposits like Olympic Dam, were transported and concentrated in the Earth's crust. The findings offer a rare glimpse into a deep Earth process that has been shaping the planet's chemistry and mineral wealth since the dawn of complex life.

Technical Summary: Carbonatite Melt Pathways Connect the Deep Mantle to Archean Craton Margins

Problem Statement
The deep carbon cycle, active since the Paleoproterozoic, involves the transport of carbon-rich oceanic crust into the mantle and its subsequent return to the surface. Petrological evidence suggests carbonatite melts are a significant component of the mantle carbon reservoir, acting as efficient agents for transporting volatiles, rare earth elements, and sulfur. However, direct geophysical imaging of these melts remains ambiguous. Low electrical resistivity and reduced shear-wave velocity in the deep mantle can be attributed to either small fractions of carbonatite melt or hydrous silicates (such as phlogopite) in solid or small melt fractions. Previous studies of the Gawler Craton in southern Australia identified a low-resistivity region beneath the Archean core but lacked the resolution to determine its depth, vertical extent, or anisotropic properties, leaving the causal mechanism (hydration vs. carbonatite melt) unconstrained.

Methodology
The authors utilized a dataset comprising approximately 200 long-period magnetotelluric (MT) sites (periods 10–10,000 s) covering an area of 800 km by 800 km over the Gawler Craton and Musgrave Province. The data included both impedance tensor and vertical magnetic field transfer functions.

3D Isotropic Inversion: A 3D resistivity model was generated using finite-difference forward modeling and a nonlinear conjugate gradient inversion algorithm. The model grid extended to 800 km depth with specific padding to accommodate the continent-ocean boundary.
3D Anisotropic Inversion: To address the ambiguity of isotropic models, a new anisotropic 3D inversion was performed. This approach solved for three orthogonal resistivity components ( $\rho_x, \rho_y, \rho_z$ ) within each cell, allowing horizontal components to differ from the vertical component by up to a factor of 10. The inversion was initialized using the final isotropic model to treat anisotropy as a perturbation.
Comparative Analysis: The resulting resistivity models were compared against shear-wave velocity anomalies derived from seismic tomography and predicted resistivity profiles for hydrated lherzolite mantle.

Key Results

Deep Lithospheric Conductor: A pronounced conductive anomaly (resistivity ~1–10 $\Omega$ m) was imaged at approximately 180 km depth, near the base of the lithosphere beneath the Archean core of the Gawler Craton. This represents a decrease of two to three orders of magnitude relative to the overlying mantle.
Strong Anisotropy: The anisotropic inversion revealed that this conductor is highly anisotropic. The resistivity in the northwest–southeast direction ( $\rho_y$ ) is approximately 1 $\Omega$ m, while the orthogonal horizontal ( $\rho_x$ ) and vertical ( $\rho_z$ ) components are ~10 $\Omega$ m. This results in an anisotropy ratio of ~10 at 180 km depth. The anisotropic model improved the data fit (RMS misfit) by 20% compared to the isotropic model, specifically reducing systematic phase residuals in long-period off-diagonal impedance components.
Crustal Conductors: Low-resistivity regions (<100 $\Omega$ m) were also imaged at Moho depths (35–45 km) in an arcuate belt around the craton margin. These regions coincide with shear-wave velocity decreases and exhibit less electrical anisotropy than the deep mantle conductor.
Differentiation from Hydration: Resistivity profiles above 150 km depth are consistent with hydrated lherzolite (100–200 ppm H₂O). However, the minimum resistivity of 1 $\Omega$ m at 180 km depth is an order of magnitude lower than predicted for hydrated mantle, suggesting hydration alone is insufficient to explain the observations.

Interpretation and Claims
The authors interpret the deep, highly anisotropic conductor as a metasomatically modified, channelized structure associated with carbon-bearing melts.

Carbonatite Melt: Based on SIGMELTS parametrization, the authors estimate that a very small fraction (~0.05 vol%) of interconnected carbonatite melt is sufficient to reduce bulk mantle resistivity to the observed 1 $\Omega$ m. The strong anisotropy suggests these melts are preferentially interconnected along aligned grain boundaries, fractures, or pathways controlled by lithospheric fabric and regional stress.
Graphite Precipitation: The conductive regions at the crust-mantle boundary (Moho) are interpreted as the signature of carbon exsolved as graphite from upward-migrating deep-mantle carbonatite melts. Under the Gawler Craton's low heat flow conditions, graphite precipitation is favored at these depths, creating persistent conductive zones.
Linked System: The paper proposes a linked mantle-to-crustal carbon system. Small fractions of carbonatite melt remain trapped near the lithosphere-asthenosphere boundary, while earlier, larger volumes of melt migrated laterally and ascended to the craton margins, precipitating graphite and potentially contributing to the formation of Iron-Oxide Copper-Gold (IOCG) deposits.

Significance
This study provides direct geophysical evidence linking deep mantle processes to craton margins. By resolving the anisotropic nature of the deep conductor, the authors distinguish between hydrous silicates and carbonatite melts, arguing that the latter is the primary cause of the observed low resistivity. The findings suggest that carbonatite melts have remained in situ since their emplacement approximately 1.6 Ga, serving as a long-lived conduit for deep carbon cycling and chemical enrichment of the Earth's interior. The spatial association of these conductive pathways with major IOCG deposits implies a genetic link between deep carbonatite magmatism and crustal mineralization.

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