Pacific drilling program reports some discoveries

Drilling at the Hess Deep in the Pacific uncovered major advances in understanding what goes on inside the earth.

The University of Houston reported Wednesday that a geoscientist there, Jonathan Snow, revealed some of the discoveries in an academic article.

The information comes from the Integrated Ocean Drilling Program Expedition 345 by the vessel JOIDES Resolution, which made port in Costa Rica twice as it probed the deep rock under the Pacific.

Snow and Kathryn Gillis from University of Victoria in Canada led a team of 30 researchers from around the world on the $10 million expedition, finding a few surprises upon penetrating the lower crust of the Pacific, said the university. Their findings are described in the Dec. 1 issue of Nature in a paper titled “Primitive Layered Gabbros from Fast-Spreading Lower Oceanic Crust.”

Gabbro is the rock deep  under the sea floor off Costa Rica’s Pacific coast.

The expedition that generated the information in the nature article came from a drill hole put at the floor of an undersea canyon, Hess Deep. That way the Resolution’s drilling crew did not have to penetrate as much overlying rock.

“The ocean crust makes up two-thirds of the Earth’s surface and forms from volcanic magma at mid-ocean ridge spreading centers,” Snow said in a university release. “The deepest levels of this process are hidden from view due to the miles of upper volcanic crust on top. So, until now we had to make educated guesses about the formation of the lower crust based on seismic evidence and the study of analogous rocks found on land.”

The scientific voyagers recovered core sections of lower crustal rocks, the gabbros, that formed more than two miles beneath the sea floor. The Hess Deep is like an onion sliced and pulled apart, revealing its deeper layers, the university said.

The university release gave this summary of the discoveries:

The expedition confirmed for the first time the widespread existence of layered gabbros in the lower crust. This observation had been predicted by plate tectonic theory and analogies made to fragments of ocean crust found on land, called  ophiolites, but only rarely had actual layered rocks been recovered from the ocean floor.

A second surprise was when the scientists identified substantial amounts of the mineral orthopyroxene, a magnesium silicate that was thought to be absent from the lower crust. But it was in the thin slices of the gabbros.

“Orthopyroxene by itself is nothing special. Traces of it are often found at late stages of crystallization higher in the crust, but we never in our wildest dreams expected a lot of it in the lower crust,” Snow said. “Although this mineral is not economically valuable, the discovery means that basic chemical reactions forming the lower crust will now have to be re-studied.”

A third surprise, Snow says, casts doubt on one of the main theories of the construction of the lower ocean crust. It involved the mineral olivine, also a magnesium silicate. This mineral is known to grow in delicate crystals sometimes found in layered intrusions on land, but never expected in the ocean crust. This is because the separation of the tectonic plates was thought to deform the magma like play dough in a partially molten state that would have broken them up. However, Snow says, the last word isn’t written on this, because the researchers just cored a small section of the crust in one place on this expedition. To know for sure, they will have to explore the lower crust more, which will require drilling.

They will be back. The fourth phase of ocean drilling was approved in late November by the National Science Board, the governing board of the National Science Foundation, that is responsible for guiding the pursuit of national policies for promoting research and education in science and engineering, the university said.

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