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Earth’s mantle is a complex region of mixing and dynamic flow.  Within the uppermost 410 km of Earth’s mantle, distinct domains have been identified from basalt geochemistry.  The precise locations and long-term stability of these mantle domains provide clues on Earth evolution over time including paleoclimate, the Deep Carbon cycle, and how Earth’s mantle is mixed by deep convective flows and subduction.  Now in a paper published in Nature Communications Earth and Environment, Qian et al. analyzes magma on a fast-moving tectonic plate to understand its tectonic history and map mantle domains in more detail than previously recognized.

Like a distant galaxy, Earth’s mantle is beyond our reach and direct exploration is not possible.  However, one of the clearest ‘windows’ into the mantle is from basalts that erupt and cool into rocks on the seafloor, along mid-ocean ridges.  Mid-ocean ridge basalts are almost entirely generated by partial melting of the mantle rather than the crust.  Thus, the geochemistry of mid-ocean ridge basalts provides one of the most pristine glimpses of upper mantle rocks that lie hundreds of kilometers beneath our feet.

Past studies of mid-ocean ridge basalts found two distinct geochemical mantle domains named ‘Indian’ and ‘Pacific’ that were recognized from cross-plotting lead isotopic ratios.  The mantle domains appear to be divided along a boundary under the western Pacific subduction zones around 150°E±20° latitudes (Fig. 1).  Reconstructions suggest this mantle domain boundary has been established for millions of years, since at least the Cretaceous (65 Myrs ago) or older.  More recently, a third distinct mantle domain named ‘Zealandia-Antarctica’ was recognized in a remote region under the Southern Ocean, south and west of Australia, that was possibly linked to deep mantle upwelling and volcanism.  However, the northern extent of the Zealandia-Australia domain and its boundaries with the Pacific and Indian domains have not been determined. 

Qian et al. re-evaluated basalt geochemistry from the region around and within Philippine Sea plate, Earth’s fastest-moving tectonic plate in the past 65 million years, and found signatures from at least two mantle domains named ‘Indian’ and ‘Zealandia-Antarctica’ (Fig. 2).  The signatures were divided into two parts of the plate known as West Philippine basin and Shikoku-Parece Vela (Fig. 3).  New molybdenum isotopes and water content analyses of magmas from older Cretaceous portions of the Philippine Sea established new details on the lengthy subduction history for the plate.  These lavas were collected using the remotely-operated vehicle (ROV) Haixing 6000 at ~2000 m below the ocean surface using a ‘PacMan’ grabbing arm (Fig. 4).

Qian et al. restored the Philippine Sea plate magmatic rocks to their original eruption locations using plate tectonic reconstructions.  These maps reveal that as the Philippine Sea Plate moved rapidly northwards over the past 50 million years, the plate traversed the Zealandia-Antarctic and Indian mantle domains (Fig. 5).  Mid-ocean ridge spreading within the Philippine Sea plate actively accrued melted mantle from these distinct domains to form new crust.  The present Philippine Sea oceanic crust shows a mosaic of at least two mantle domains that preserves a record of its journey across the mantle underworld boundary (Fig. 3).

The newly recognized domain boundaries of Qian et al. suggests the Zealandia-Antarctic domain stretched north of Australia and west of New Zealand and might be three times larger than previously believed.  The distinctive geochemical signature of the newly delineated Zealandia-Antarctic realm is likely a result of its geodynamic history.  Zealandia-Antarctica is an enclave that remained under ancient oceans for the past 400 million years.  Having experienced only minimal contact with continental plates, Zealandia-Antarctica has been isolated from the introduction of radiogenic continental material into the upper mantle during ocean-continent subduction or continental lithosphere loss.

References

Miyazaki, T., Kimura, J.-I., Senda, R., Vaglarov, B. S., Chang, Q., Takahashi, T., . . . Yoshida, T. (2015). Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles. Geochemistry, Geophysics, Geosystems, 16(9), 3309-3332. doi:https://doi.org/10.1002/2015GC005911

Park, S.-H., Langmuir, C. H., Sims, K. W. W., Blichert-Toft, J., Kim, S.-S., Scott, S. R., . . . Michael, P. J. (2019). An isotopically distinct Zealandia–Antarctic mantle domain in the Southern Ocean. Nature Geoscience, 12, 206-214. https://doi.org/10.1038/s41561-018-0292-4

Qian, S., Wu, J.-T., Wu, J., Philippine Sea plate and surrounding magmatism reveal the Antarctic-Zealandia, Pacific, and Indian mantle domain boundaries, 2024, Nature Communications Earth and Environment, https://doi.org/10.1038/s43247-024-01326-6

Tatsumoto, M. (1978). Isotopic composition of lead in oceanic basalt and its implication to mantle evolution. Earth and Planetary Science Letters, 38(1), 63-87. doi:https://doi.org/10.1016/0012-821X(78)90126-7