Two-thirds of the Earth’s crust is formed at ocean spreading centres from the melting of the mantle, but how the lower part of the crust is formed remains a matter of debate. Using advanced analysis methods applied to seismic data from the Atlantic Ocean, scientists from the Institut de physique du globe de Paris (IPGP) and the Commonwealth Scientific and Industrial Research Organisation of Australia, have discovered seismic layering in the lower crust, suggesting that the lower oceanic crust is formed by cooling and crystallisation of magma in situ.
Oceanic spreading centres lie in the middle of ocean, forming a chain of volcanoes that extend more than 60,000 km enveloping the Earth’s surface. As the oceanic plates move apart, the mantle upwells and the melting initiates at ~70 km depth and the melt rises to the surface; a part of the melt erupts on the seafloor, but a significant part of the melt remains in the crust, possibly in melt lenses, cools and crystallises forming the lower crust. The Moho (for Mohorovičić discontinuity) is the boundary separating this newly formed crust with the underlying mantle.
In 2017, a team of scientists from IPGP (Université Paris Cité, CNRS) and GEOMAR Helmholtz Centre for Ocean Research in Germany led a marine expedition in the equatorial Atlantic Ocean aboard the German Research Vessel Maria S. Merian. They deployed sensors, called ocean bottom seismometers, on the seafloor, which recorded sound energy source generated from the vessel, and their propagation in the crust. Analysis of the recorded signals using an advanced method developed by IPGP, called seismic full waveform inversion, reveals the presence of 400-500 m thick layers of different rock types below 2 km from the seafloor. These results indicate that sea water penetrates down 2-3 km below the seafloor, which cools the magma and, circulate to brings mineral and nutrient to the seafloor, at hydrothermal sites called black smokers, that feeds millions of organisms in the deep ocean. The cooled magma crystallises in situ and forms rocks of different compositions like a layered cake. These layers extend up to 5 -15 km distance along the profile, covering crust that has been formed up to 300,000-800,000 years, suggesting that this form of lower crustal accretion is a stable process.
These results suggest that oceanic crust at slow spreading centres is also dominantly formed by magmatic process, similar to fast spreading centres, and the magma plays fundamental role in the formation of oceanic crust. These results have been published in Nature Geoscience on June 13, 2022.
> Sources :
Guo, P., Singh, S.C., Vaddineni, V., Grevemeyer, I., and Saygin, E. Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering, Nature Geoscience, 15, https://doi.org/10.1038/s41561-022-00963-w
Satish Singh, marine gesocience group at IPGP