Transition between the Pacific and African mantle domains beneath southwest Pacific marginal basins

Deep beneath the Earth's surface, the planet's interior is not a uniform, churning soup. Instead, it is divided into vast, distinct provinces that have remained separate for hundreds of millions of years. Imagine the mantle as a giant, slow-moving ocean of rock. For a long time, scientists have known that the rock beneath the Pacific Ocean is chemically different from the rock beneath the Atlantic and Indian Oceans. This difference is a legacy of how the Earth's tectonic plates have moved and collided over eons. As plates dive deep into the mantle, they carry pieces of the Earth's crust with them, altering the chemical makeup of the rock below. Over time, this process created a sharp divide: one side of the planet holds a "Pacific" style of mantle, while the other holds an "African" style. The big question for geologists has been exactly where this invisible line runs and how it has shifted as the continents drifted and the oceans opened and closed.

A team of researchers has now traced this boundary through the rocks of the southwest Pacific, a region where the Earth's crust has been stretching and breaking apart for millions of years. By analyzing the chemical fingerprints of ancient lava samples from the ocean floor, they found that the dividing line between these two mantle worlds is not fixed in place. Instead, it has moved and evolved, shaped by the complex history of subduction zones where one plate slides under another. Their work reveals that the boundary is not just a simple line on a map, but a dynamic zone that has changed its position and depth over time, leaving a clear record in the rocks that formed there.

The scientists focused their study on a series of basins in the southwest Pacific, including the Tasman Sea, the Coral Sea, and the Santa Cruz basin. These areas are special because they formed when the ancient continent of Zealandia and the Australian continent began to pull apart. As the land stretched and the ocean floor spread, magma rose from deep within the Earth to fill the gaps, creating new rock. The researchers collected samples of this rock, specifically looking at mid-ocean ridge basalts, which are the most common type of lava found on the ocean floor, and ocean island basalts, which are similar to the lavas found on volcanic islands like Hawaii. They dated these rocks to determine when they formed, with some samples being as old as 84 million years and others as young as 22 million years.

To understand the origin of these rocks, the team measured the ratios of specific elements within them, such as strontium, neodymium, lead, and hafnium. These elements act like a chemical ID card for the mantle source. The researchers found a clear pattern: the rocks from the older, continental marginal basins, which formed between 84 and 50 million years ago, carried the chemical signature of the "African" mantle domain. This was true even though these basins are located in the Pacific region. In contrast, the younger rocks, formed after 50 million years in the oceanic basins further east, showed the distinct signature of the "Pacific" mantle. This shift suggests that the boundary between the two mantle domains was once located further east, right along the edge of the Zealandia continent, before moving to its current position.

The study also looked at the deeper parts of the mantle by examining rocks that formed from hotspots, which are plumes of hot material rising from the very bottom of the mantle. The researchers found that the hotspots in the north, such as the Tasmantid and Lord Howe chains, produced rocks with a chemical signature known as EM-1, which is associated with the enriched African mantle domain. However, the hotspots in the south, near the Zealandia and Antarctica margins, produced rocks with a different signature called HIMU, which is linked to the deep Pacific mantle. This indicates that while the upper layer of the mantle in this region was dominated by the African style, the deep mantle beneath it was still Pacific in nature. The two domains were stacked on top of each other, creating a complex layering effect.

One of the key findings of the paper is that the boundary between these two mantle worlds is not defined by the modern subduction zones we see today. For a long time, scientists thought the current subduction zone, where the Pacific plate dives beneath the Australian plate, marked the dividing line. However, the chemical evidence from the older rocks shows that this line was once much further east, along the continental margin of Zealandia. The researchers argue that the movement of the subduction zone over time, specifically a reversal in the direction of subduction around 50 million years ago, pushed the boundary westward. This movement allowed the Pacific mantle to flow into the area that was previously dominated by the African mantle.

The study also rules out several other explanations for the chemical differences seen in the rocks. The researchers considered whether the mixing of materials from subducted sediments or the breaking apart of the ancient continental crust could explain the patterns, but the data did not support these ideas. Instead, the evidence points strongly to the movement of the mantle domains themselves as the primary driver. The chemical signatures are too distinct and widespread to be the result of local contamination or small-scale mixing. The patterns suggest a large-scale, global process where the deep mantle has remained separated into two distinct provinces for hundreds of millions of years.

By combining the chemical data with models of how the Earth's plates have moved over time, the researchers were able to reconstruct the history of this boundary. They propose that during the time when the supercontinent Pangea was assembling, the long-term subduction of the Pacific plate created a ring of enriched mantle around the Pacific Ocean. This ring acted as a barrier, keeping the Pacific and African mantle domains separate. As the continents broke apart and the subduction zones shifted, the boundary between these domains moved, but the fundamental chemical difference remained. The study provides a detailed map of how this boundary has evolved, showing that it is a first-order feature of the Earth's mantle that has persisted through multiple cycles of supercontinent formation and breakup.

The implications of this work extend beyond just understanding the history of the southwest Pacific. It offers a new way to look at how the Earth's interior works on a global scale. The fact that these mantle domains have remained distinct for so long suggests that the Earth's interior is more stable and organized than previously thought. The movement of the boundary is not random but is driven by the large-scale forces of plate tectonics. This helps scientists better understand the long-term evolution of the planet and how the deep interior influences the surface. The study also highlights the importance of looking at both the shallow and deep parts of the mantle to get a complete picture, as the two layers can behave differently and tell different parts of the story.

In the end, the research paints a picture of a dynamic Earth where the boundaries between its internal provinces are not fixed but are constantly being reshaped by the movement of the plates above. The chemical fingerprints in the rocks of the southwest Pacific serve as a time capsule, preserving the history of these movements and the shifting of the mantle domains. The findings confirm that the divide between the Pacific and African mantle is a real, physical boundary that has moved over time, leaving a clear record in the rocks that formed there. This work helps to fill in the gaps in our understanding of the Earth's deep interior and provides a clearer view of how the planet has changed over the last several hundred million years.