Photo by F B Lu Li-Shih
“Boldly going where few have gone before”
In late February and March 2019 a US-China-German collaborative research cruise will visit the Marion Rise on the relatively unexplored SW Indian Ridge (39° to 47°E) using the RV Thomas G Thompson. It will focus on two previously unexplored regions and will map the topography of the sea floor using sonar from the ship with a 60m resolution and using the Sentry autonomous submarine to map the seafloor at a 1-2m resolution. The cruise will also collect rock samples by dredging and geophysical information (gravity/magnetics) from the ship. The cruise will also be hoping to discover new hydrothermal vent systems. This US cruise will be led by Henry Dick of the Woods Hole Oceanographic Institution, Huaiyang Zhou of the State Key Laboratory for Marine Geology of Tongji University, and Dr. Juergen Koepke of Leibniz University Hannover. The outreach will be run by Michael Cheadle and Theresa Williams from the University of Wyoming. .
Twice Melted Mantle
Why might the mantle beneath Marion Rise be different? We will try to answer this question by measuring chemical isotopes in the mantle peridotites we will collect.
The mantle leftover when most of the continental crust was extracted and formed (the melting event at T1 in the figure) is called the depleted mantle, and this mantle has a present day isotope composition of (A”/A)Ocean Crust. This is the mantle from which the ocean crust normally forms at mid-ocean ridges.
If this depleted mantle is melted again, at time T2, the evolution line of the depleted mantle (A”/A)Ocean Crust gets interrupted again and the mantle becomes even more depleted (green line), and importantly this melting event leads to it becoming less dense.
This is what we think happened for the Marion Rise mantle; that mantle has been melted a second time, perhaps during the formation of Gondwana (the southern supercontinent of Africa, South America, India, and Antarctic) and this more buoyant, doubly depleted mantle is now rising up beneath the Marion Rise, resulting over time in an anomalously high A”/A Isotope ratio (A”/A Marion Rise)
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. Some isotopes are unstable and radioactively decay to another element. In the model presented here, isotope A is stable and is not produced by another isotope; A” is continuously produced by decay of another, radioactive, isotope B; A” is radiogenic A. As time passes A”/A always increases as A” is always produced by B and A doesn’t change. At some point in Earths’ history, T1, part of the Earth melts and forms the continental crust (A”/A)Cont) and the depleted mantle (A”/A)OceanCrust.
Vincent Salters/Mike Cheadle
What We've Achieved
At least six newly discovered oceanic core complexes! (How is a core complex defined?)
Discovered a 125km long mylonite bounded, domal, ridge of peridotite.
Carried out 33 dredges
Carried out 5 Sentry dives
We’ve talked to nearly 1100 people via the 2-way ship to shore broadcasts during the survey
Collected ~26,000 square kilometres of multibeam, magnetic and backscatter lines either side of the South-west Indian Ridge
Mapped ~ 10,000 square kilometres of mantle exposed at the sea floor