The Ontong Java Plateau in the western Pacific Ocean is the largest oceanic plateau on Earth, and its formation mechanism has not been well understood.

Now, researchers from the South China Sea Institute of Oceanology (SCSIO) of the Chinese Academy of Sciences have provided a novel explanation after modeling previous formation hypotheses.

Oceanic plateaus are large igneous provinces in the ocean that are characterized by unusually large areas of elevated topography and extraordinarily thick crust. Most are below sea level and thus avoid subaerial erosion.

Two popular hypotheses have been debated regarding their origin: mantle plume versus seafloor spreading.

The first mechanism involves large plumes of hot material rising from the deep mantle to the base of an oceanic plate over tens of millions of years. At that point, the hot mantle spreads out beneath the oceanic plate. As pressure decreases, part of the mantle melts to generate a large volume of magma that erupts as lava and builds a broad volcanic plateau.

The second mechanism involves unusually rapid seafloor spreading, which can generate large volumes of magma through decompression melting of mantle rock beneath mid-ocean ridges.

In contrast, the new study, published in Nature Geoscience on June 11, proposes a thermochemical plume formation process for the Ontong Java Plateau.

This plateau, which was mostly formed during the Early Cretaceous period on the Pacific Plate, is the largest extant oceanic plateau on Earth. Originally, researchers widely believed it was formed through the first mechanism. However, a purely thermal plume would predict uplift above sea level, which is inconsistent with the plateau's mostly submarine emplacement.

The alternate mechanism doesn't provide a good explanation either since the radiometric ages of drilled basaltic rocks from the Ontong Java Plateau are not close to the ages of adjacent magnetic lineations. This implies that the plateau was probably formed in an intraplate setting rather than near mid-ocean ridges.

To evaluate the competing hypotheses, the researchers used thermodynamic modeling to reconstruct the thermal and chemical state of the mantle at the time of the Ontong Java Plateau's formation under the mantle plume and seafloor spreading scenarios.

Their simulations showed that the seafloor-spreading model requires an unreasonably high mantle potential temperature or a high proportion of dense fusible pyroxenite. By contrast, a thermochemical plume, with a temperature anomaly of 135–200 ℃ and containing up to 13% dense fusible pyroxenite, can explain the spatial variations in crustal thickness and lava composition of the Ontong Java Plateau and its mostly submarine emplacement.

Based on crustal thickness and lava composition variations across the Ontong Java Plateau, the researchers proposed a spatial evolution model of a thermochemical mantle plume head.

These findings suggest that thermochemical plumes may play a broader role in the formation of oceanic plateaus than previously recognized.

"In addition to the Ontong Java Plateau, many other oceanic plateaus also occur with heterogeneous mantle sources, suggesting the common involvement of thermochemical plumes in oceanic plateau formation," said Prof. ZHANG Jinchang, first author of the study. "This mechanism differs substantially from the purely thermal plume model that has long been assumed."