Expedition 345: Hess Deep Plutonic Crust

Christmas at Hess Deep Rift  :

Drill the lower oceanic crust in the Pacific

12 December 2012 to 12 February 2013
Puntarenas, Costa Rica to Panama City, Panama

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IODP Expedition 345 (Hess Deep Plutonic Crust) will take place from 11 December 2012 to 12 February 2013 aboard the scientific drillship JOIDES Resolution. The objective of this project is to sample for the first time, primitive magmatic rocks of the lower crust in the oceanic Pacific. These samples will help scientists seek to understand the manufacturing process of the oceanic crust at a fast-spreading rift, but also to document the effect of cooling the young crust by seawater, and thus the importance of chemical exchanges between the crust and the ocean. They control the chemical evolution of the oceanic crust before recycling into the mantle via subduction zones, and play a fundamental role in geochemical cycles across the globe.

Why are we drilling the crust formed at a fast-spreading rift ?

Scientific ocean drilling allows geologists to reach and study the rocks that make up the deep ocean floor. The scientific community is particularly interested in the oceanic crust formed quickly (this is the case in the Pacific Ocean) because it is more uniform and homogeneous than the slow-spreading ridges formed (located in the Atlantic, for example). Approximately 20% of the rifts are in rapid expansion (> 8 cm / year). They generated almost half of the oceanic crust present, 30% of the surface of the Earth. It is important to characterize this type of crust to better understand the geochemical cycles (such as carbon cycle) across the planet.  This type of crust is recycled in the mantle via subduction zones, especially around the edges of the Pacific Ocean.

Recall that the oceanic crust is formed at the axis of the rift by cooling magma derived from partial melting of mantle rocks. If the magma reaches the surface, on the ocean floor, it cools rapidly and freezes in basalt. Basalt makes up the upper oceanic crust. At deeper depths, the magma crystallizes more slowly, a new igneous rock appears: gabbro. Thus the oceanic crust in a fast-spreading rift is an overlay of deep gabbro and basalt more superficially, a total thickness of about 6 km.

Sampling of the lower crust at Hess Deep:

To reach the deeper parts of the oceanic crust, gabbro in the lower crust, two strategies are possible and complementary. One is to penetrate as deeply as possible a portion of ocean crust. This was the aim of Expedition 335, which took place last year. It is also part of the objective of the project "Mohole to the Mantle," to sample a complete section of oceanic crust and to penetrate in the mantle (www.mohole.org). The alternative strategy, and a complementary one, is to drill in the few areas of the Pacific where the oceanic crust is opened by faults and where lower crustal rocks are available on the seafloor. The objective of Expedition 345, which will begin in mid-December, is to sample for the first time in the Pacific gabbros in the lower crust at Hess Deep. Hess Deep Rift is located at the triple point between the Pacific tectonic plates, Cocos and Nazca. This region is one of the few "tectonic windows" in the Pacific; the young Pacific crust (1 million years old) is opened in the deep part of the crust and the mantle.

The research team will be led by Jonathan Snow (University of Houston, Texas) and Kathy Gillis (University of Victoria, Canada.


  Retrouvez l'expédition 345 et le JOIDES sur http://www.ac-nice.fr/svt/hdc

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