New Theory about Formation of the Oldest Continents

The Earth's structure can be compared to an orange, that is, its crust is the peel supported by the earth's heavy mantle. That peel is made up of a continental crust with 30 to 40 kilometers thick, it is much lighter than the thinner oceanic crust and protrudes from the Earth's mantle because of its lower density, like an iceberg in the sea.

Fig. 1 - Scheme of the Earth's chemical and physical composition/structure (from right to left) (Portal São Francisco - http://www.portalsaofrancisco.com.br/alfa/placas-tectonicas/litosfera.php).

"According to the current theory, the first continental crusts were formed when tectonic plates would collide, submerging oceanic crusts into the Earth's mantle, where they would partially melt at a depth of approximately 100 kilometers. That molten rock then ascended to the Earth's surface and formed the first continents," says adjunct professor Dr. Thorsten Nagel of the Steinmann Institute of Geosciences at the University of Bonn, lead author of the study. The theory has been supported by the oldest known continental rocks, approximately 3.8 billion years old, found in western Greenland.

Fig. 2 - Earth's surface with the oldest continents (Terra Daily, News About Planet Earth - http://www.terradaily.com/reports/A_new_theory_on_the_formation_of_the_oldest_continents_999.html).

Following trace elements

The composition of the continental crust corresponds to a semiliquid version of the oceanic

crust melted by 10 to 30 percent of its original state. Unfortunately, the concentrations of the main chemical components in the re-solidified rock do not provide much information about what depth the fusion occurred at.

Prof. Dr. Carsten Munker of the Institute of Geology and Mineralogy at the University of Cologne explains, "In order to find that out, you have to know what minerals the remaining 70 to 90 percent of the oceanic crust consisted of". Researchers from Bonn and Cologne have now analyzed the Greenlandic rocks for different elements occurring at various high concentrations, also know as trace elements.

"Trace elements provide geologists with a window to the origin of continental crust," says Prof. Munker. "With their help, we can identify minerals in the residual rock that were deposited in the depths by the molten rock." Before the magma separated from the bedrock, the semifluid rock and the leftover solid minerals actively exchanged trace elements.

Dr. Elis Hoffmann from Bonn, coauthor of the study, explains, "Different minerals have characteristic ways of separating when trace elements are smelted. In other words, the concentration of trace elements in the molten rock provide a fingerprint of the residual bedrock". The concentration of trace elements in the oldest continental rock allows geoscientists to reconstruct possible bedrock based on their minerals and thus determine at what depth the continental crust originated.

The oceanic crust did not have to descend

Using computers, the scientists simulated the composition of bedrock and molten rock that would emerge from partially melting the oceanic crust at various depths and temperatures. 

They then compared the data calculated for the molten rock with the actual concentration of trace elements in the oldest continental rocks. "Our results paint a surprising picture," Dr. Nagel reports. "The oceanic crust did not have to descend to a depth of 100 kilometers to create the molten rock that makes up the rocks of the first continents." According to the calculations, a depth of 30 to 40 kilometers is much more probable.

The primeval oceanic crust could have "oozed" continents

...it could definitely have had the power to do so in the Archean Eon. Four billion years ago, the gradually cooling earth was still significantly warmer than it is today. The oceanic crust could have simply "oozed" continents at the same time that other geological processes were occurring, like volcanism, orogeny, and the influx of water.

The geologist from Bonn says, "We think it is unlikely that the contents were formed into subduction zones. Whether or not tectonic plates of the primordial earth had such zones of subsidence is still a matter of debate".

Adapted:

Terra Daily, News About Planet Earth - http://www.terradaily.com/reports/A_new_theory_on_the_formation_of_the_oldest_continents_999.html

How Diamonds are Formed

Drop a Mentos candy in a bottle of Diet Coke, and carbon dioxide will bubble violently out of the soda. Similar chemical reactions may send certain kinds of magma frothing up from deep within the Earth, carrying diamonds along the way.

The discovery, reported in the Jan. 19 Nature, solves several mysteries about why and how diamond-bearing rocks appear where they do. As gem-laden magma rises, the theory goes, it gobbles a mineral called orthopyroxene, changing the magma’s chemical composition and belching carbon dioxide gas that drives its continued ascent.

“We’ve provided a simple, chemically reasonable process to have dissolved gas at depth,” says Kelly Russell, lead author of the new paper and a volcanologist at the University of British Columbia in Vancouver.

Diamond mines tap volcanic rocks called kimberlites, which contain many kinds of crystals that must have formed at high pressures 150 kilometers or more deep, in the planetary layer known as the mantle. How those mantle crystals make it to the surface has been a puzzle, since magma gets denser the more crystals it picks up. Most geologists have assumed that the magma must bubble gases to keep it moving up, but no one has been able to explain exactly how.

Russell and his colleagues realized that gas could do the trick if the magma starts out relatively poor in silicon dioxide, a major component of the Earth’s crust also known as silica. As magma rises through cracks it begins to dissolve the surrounding rock — especially that containing lots of orthopyroxene, a mineral rich in magnesium, iron and silica. The orthopyroxene releases its silica into the magma, and as the silica content rises the magma’s ability to hold dissolved carbon dioxide drops. The gas bubbles out and by the time the kimberlite gets to the surface, it erupts at supersonic speeds.

Working in a high-temperature laboratory at the University of Munich, Russell melted sodium carbonate as a stand-in for silica-poor magma. He then added orthopyroxene and watched as the mixture furiously bubbled carbon dioxide.

The research could explain why the gem-laden kimberlites appear only in ancient parts of continents, known as cratons, like those in northwestern Canada and southern Africa. Cratons contain lots of orthopyroxene, allowing the magma to gobble it and ascend. “We’ve always wondered, how do the kimberlites find the craton?” Russell says. “They don’t. Their passage through the craton converts them.”

Russell’s team is now working to see how quickly orthopyroxene and other minerals dissolve in the magma, to better estimate the speeds at which kimberlites rise.

_________________________________

Source:http://www.sciencenews.org

The Boundary Between Magma and Water

Scientists and drillers recovered a remarkable suite of heat-tempered basalts that provide a detailed picture of the rarely seen boundary between magma and seawater. These samples were collected during a return to ODP Hole 1256D, one of the deepest “hard r

ock” penetration sites of scientific ocean drilling. ODP Hole 1256D has been stabilized, cleared to its full depth, and primed for further deepening.

Panama City, Panama – Integrated Ocean Drilling Program (IODP) Expedition 335 Superfast Spreading Rate Crust 4 recently completed operations in Ocean Drilling Program (ODP) Hole 1256D, a deep scientific borehole that extends more than 1500 meters below the seafloor into the Pacific Ocean’s igneous crust – rocks that formed through the cooling and crystallization of magma, and form the basement of the ocean floor.
An international team of scientists led by co-chief scientists Damon Teagle (National Oceanographic Center Southampton, University of Southampton in the UK) and Benoît Ildefonse (CNRS, Université Montpellier 2 in France) returned to ODP Hole 1256D aboard the scientific research vessel, JOIDES Resolution, to sample a complete section of intact oceanic crust down into gabbros.

A granoblastic basalt (the hardest material ever drilled in ocean drilling) viewed under the microscope (picture is 2.3 mm across).

This expedition was the fourth in a series and builds on the efforts of three expeditions in 2002 and 2005.

Gabbros are coarse-grained intrusive rocks formed by the slow cooling of basaltic magmas. They make up the lower two-thirds of the ocean crust. The intrusion of gabbros at the mid-ocean ridges is the largest igneous process active on our planet with more than 12 cubic kilometers of new magma from the mantle intruded into the crust each year. The minerals, chemistry, and textures of gabbroic rocks preserve records of the processes that occur deep within the Earth’s mid-ocean ridges, where new ocean crust is formed.

“The formation of new crust is the first step in Earth's plate tectonic cycle,” explained Teagle. “This is the principal mechanism by which heat and material rise from within the Earth to the surface of the planet. And it’s the motion and interactions of Earth’s tectonic plates that drive the formation of mountains and volcanoes, the initiation of earthquakes, and the exchange of elements (such as carbon) between the Earth's interior, oceans, and atmosphere.”

“Understanding the mechanisms that construct new tectonic plates has been a major, long-standing goal of scientific ocean drilling,” added Ildefonse, “but progress has been inhibited by a dearth of appropriate samples because deep drilling (at depths greater than 1000 meters into the crust) in the rugged lavas and intrusive rocks of the ocean crust continues to pose significant technical challenges.”

ODP Hole 1256D lies in the eastern equatorial Pacific Ocean about 900 kilometers to the west of Costa Rica and 1150 kilometers east of the present day East Pacific Rise. This hole is in 15 million year old crust that formed during an episode of “superfast” spreading at the ancient East Pacific Rise, when the newly formed plates were moving apart by more than 200 millimeters per year (mm/yr).

“Although a spreading rate of 200 mm/yr is significantly faster than the fastest spreading rates on our planet today, superfast-spread crust was an attractive target,” stated Teagle, “because seismic experiments at active mid-ocean ridges indicated that gabbroic rocks should occur at much shallower depths than in crust formed at slower spreading rates. In 2005, we recovered gabbroic rocks at their predicted depth of approximately 1400 meters below the seafloor, vindicating the overall ‘Superfast’ strategy.”

Previous expeditions to Hole 1256D successfully drilled through the erupted lavas and thin (approximately one-meter-wide) intrusive “dikes” of the upper crust, reaching into the gabbroic rocks of the lower crust. The drilling efforts of Expedition 335 were focused just below the 1500-meter mark in the critical transition zone from dikes to gabbros, where magma at 1200°C exchanges heat with super-heated seawater circulating within cracks in the upper crust. This heat exchange occurs across a narrow thermal boundary that is perhaps only a few tens of meters thick.

In this zone, the intrusion of magma causes profound textural changes to the surrounding rocks, a process known as contact metamorphism. In the mid-ocean ridge environment this results in the formation of very fine-grained granular rocks, called granoblastic basalts, whose constituent minerals recrystallize at a microscopic scale and become welded together by magmatic heat. The resulting metamorphic rock is as hard as any formation encountered by ocean drilling and sometimes even tougher than the most resilient of hard formation drilling and coring bits.

Expedition 335 reentered Hole 1256D more than five years after the last expedition to this site. The expedition encountered and overcame a series of significant engineering challenges, each of which was unique, although difficulties were not unexpected when drilling in a deep, uncased, marine borehole into igneous rocks.

The patient, persistent efforts of the drilling crew successfully cleared a major obstruction at a depth of 920 that had initially prevented reentry into the hole to its full depth of 1507 meters. Then at the bottom of the hole the very hard granular rocks that had proved challenging during the previous Superfast expedition were once more encountered. Although there may only be a few tens of meters of these particularly tenacious granoblastic basalts, their extreme toughness once more proved challenging to sample– resulting in the grinding down of one of the hardest formation coring bits into a smooth stump.

A progressive, logical course of action was then undertaken to clear the bottom of the hole of metal debris from the failed coring bit and drilling cuttings. This effort required the innovative use of hole-clearing equipment such as large magnets, and involved over 240 kilometers of drilling pipe deployments (trips) down into the hole and back onto the ship. (The total amount of pipe “tripped” was roughly equivalent to the distance from Paris to the English coast, or from New York City to Philadelphia, or Tokyo to Niigata). These efforts returned hundreds of kilograms of rocks and drill cuttings, including large blocks (up to 5 kilograms) of the culprit granoblastic basalts that hitherto had only been very poorly recovered through coring. A limited number of gabbro boulders were also recovered, indicating that scientists are tantalizingly close to breaking through into the gabbroic layer.

Expedition 335 operations also succeeded in clearing Hole 1256D of drill cuttings, much of which appear to have been circulating in the hole since earlier expeditions.

“We recovered a remarkable sample suite of granoblastic basalts along with minor gabbros, providing a detailed picture of a rarely sampled, yet critical interval of the oceanic crust,” Ildefonse observed. “Most importantly,” he added, “the hole has been stabilized and cleared to its full depth, and is ready for deepening in the near future.”

__________________________

Fonte: geology.com