New study highlights geology of young rock at fast-spreading mid-ocean ridges
How exactly does molten rock from the Earth’s mantle form new ocean crust in the deep sea? This has long been one of the great puzzles in geochemistry and geophysics. Now, a team of researchers has studied the first significant sample of primitive rock from deep within the crust, retrieved earlier this year on board the JOIDES Resolution. The data is providing some critical answers to some basic questions, and the results are outlined in the December 1 Advance Online Publication of the journal Nature.
Integrated Ocean Drilling Program Expedition 345 (Hess Deep Plutonic Crust) recovered the first continuous core section of rock formed nearly two and a half miles beneath the surface of the crust – mostly layered gabbros. The cores come from Hess Deep, a large rift valley in the eastern equatorial Pacific. Like an onion sliced and pulled apart, revealing its deeper layers, Hess Deep is perhaps the best place in the world to study these young crustal rocks.
“By finding these layers in modern ocean crust, we have confirmed predictions and refined our hypotheses about how the lower crust forms,” says Kathryn Gillis (University of Victoria, Canada), lead author on the Nature study. Together with Jonathan Snow (University of Houston), Gillis co-led the science team on Expedition 345.
In most places, gabbros are buried beneath a thick layer of sediment and other rocks, making them nearly impossible to access. Until now, one of the best opportunities to study gabbro was in ophiolites – sections of ocean crust that had been lifted up on land. However, it is difficult to track the origin of ophiolites, making it hard to draw conclusions about the formation of ocean crust in today’s Earth.
Hess Deep Rift is located about 600 miles west of the Galapagos Islands. It is the best location in the world to observe the geology of the deep seafloor formed at a fast-spreading ridge. The crust exposed at Hess Deep formed at the East Pacific Rise about 1.2 million years ago. The expedition proved technically challenging, requiring the crew and engineers on board the JOIDES Resolution to drill in nearly 3 miles of water into nearly bare rock, with no sediment to keep the drill bit in place.
The effort definitely paid off: Expedition 345 was the first to recover a significant section of primitive, layered oceanic gabbros. These layers, evidenced by differing shades of grey, are defined by changes in the mineralogy and/or texture of the rock.
“These primitive cores provide the last missing rock type from the crustal section,” Gillis explains. “This allowed us to construct the first composite bulk composition of fast-spreading ocean crust. With this information, we should be able to assess the role that layered gabbros play in building the crust, as well as in broader global geochemical cycles.”
Unexpectedly, the team discovered the mineral orthopyroxene as an early crystallizing phase in these rocks. This is very rare in primitive gabbro, and suggests that some of the melts that migrate from the mantle into the lowermost part of the ocean crust can vary in composition. This is surprising, because when melt erupts onto the seafloor as lava, it has a uniform composition. This suggests that, somewhere along the way, the melts mix thoroughly in the lower crust before they exit as lava.
The International Ocean Discovery Program (IODP) is an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring, and monitoring the subseafloor. The US Science Support Program (USSSP) supports the involvement of the US scientific community in IODP and is funded by the US National Science Foundation (NSF). The JOIDES Resolution is a scientific research vessel managed by the US Implementing Organization of IODP (USIO). Together, Texas A&M University, Lamont-Doherty Earth Observatory of Columbia University, and the Consortium for Ocean Leadership compose the USIO. IODP is supported by: the US National Science Foundation (NSF); Japan’s Ministry of Education, Culture, Sports, Science, and Technology (MEXT); the European Consortium for Ocean Research Drilling (ECORD); the Australia-New Zealand IODP Consortium (ANZIC); India’s Ministry of Earth Sciences; the People’s Republic of China (Ministry of Science and Technology); the Korea Institute of Geoscience and Mineral Resources (KIGAM); and Brazil’s Ministry of Education (CAPES). For more information, visit www.iodp.org.
For more information about IODP Expedition 345 (Hess Deep Plutonic Crust), visit http://iodp.tamu.edu/scienceops/expeditions/hess_deep.html
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University of Victoria
Victoria, British Columbia, Canada
University of Houston
Houston, Texas, USA
Consortium for Ocean Leadership
Washington, D.C. USA