domingo, 4 de diciembre de 2011
Geology - Plate Tectonics Active 2 Billion Years Ago --A Key to Life on Earth & Beyond
According to scientitsts at the Harvard Smithsonian Center for Astrophysics, habitable worlds are most likely found on large, rocky planets that are up to ten times the size of Earth and contain plate tectonics. Plate tectonics play a critical role in determining the rate of cooling of a potentially habitable planet by creating the optimum temperature ranges for the development of intelligent animal life -as continents grow, planets cool.
Now, a study of two billion year old rocks from African gold mines has now shown that the process of subduction we observe today as a by-product of large-scale continental movements, was present on Earth more than two billion years ago.
Some fifty years ago, plate tectonics, motions of large parts of the Earth's surface against each other, became a generally accepted theory to explain continental drift, along with many volcanic and earthquake zones, and seafloor spreading. The exact origin of the forces driving these large-scale movements of the continental plates is still a matter of scientific debate.
Today, some eight large plates move against each other and experience so-called plate subduction at their boundaries. In these subduction zones, one tectonic plate moves under another, lowering itself into the Earth's mantle. This is a slow process at a rate of a few centimetres per year. The shape of the continents suggests that 250 million years ago, the Earth’s land masses were united in a single continent, called Pangaea, from which today’s plates started to move away.
The high pressure and temperature in subduction zones are the main drivers for chemical elements to accumulate in high concentration in an ore. Metal ore mines therefore are remnants of past subduction events, even if today they are located far away from a plate boundary. The exact composition of the minerals constituting an ore is, like a fingerprint, representative of the pressures and temperatures experienced when the plate sank into the Earth’s mantle. Today, scientists can deduce these values, and their evolution in time, from the crystalline and elemental composition of a given ore.
For this study, the scientists investigated ore mineral samples aged between 2 and 2.2 billion years collected in West Africa in an area rich in gold mines. Using an electron microprobe, they established detailed maps of the spatial distribution of major chemical elements in the samples, notably of iron. However, the iron oxidation state (Fe2+ or Fe3+) cannot be measured with electron microprobes but can vary inside ore-bearing minerals.
Thanks to the use of X-ray absorption near-edge spectroscopy (XANES) with a submicrometric beam at ESRF beamline ID21, important variations of the Fe3+ content in the minerals were shown, which had major repercussions for the pressure and temperature calculations.
These XANES measurements confirmed a theoretical model on the quantity of Fe3+ in the minerals, which implied values for the pressure and temperature at the subduction event 2.2 billion years ago that are very close to those in modern "cold" subduction events. Subduction as we know it today was probably already happening on Earth at the onset of tectonic motion some two billion years ago, despite a much hotter Earth and mantle than today.
The team is already planning to gear up their use of X-rays. In a future experiment at the ESRF, they wish to use a new technique to analyse a large number of even older samples from the Archean age (2.5 — 3.8 billion years ago) formed during several key events in Earth’s history.
"Mega-pixel XANES imaging with a sub-micrometric resolution will make it possible to produce large-scale maps of the oxidation states of iron for many different samples. We hope to benefit from these high resolution maps to identify zones in the samples where very early minerals are present even in minute quantities", says Vincent de Andrade from BNL who developed the new technique at the ESRF, "because the proof of the universality of subduction through the ages of Earth’s history cannot rely on our pioneering measurements alone, and because there are still a lot of grey areas to be elucidated in the early history of our planet."
The study was performed by an international team of scientists led by J. Ganne of the University of Toulouse and included scientists from Brookhaven National Laboratory (BNL), Monash University in Melbourne, the Universities of Cambridge, Grenoble, Lausanne and Ouagadougou, and the ESRF.
In studying the possibility of life beyond Earth, researchers believe that super-Earths, or planets up to ten times the size of the Earth are the best places to find extraterrestrial life. These planets contain a solid inner core that is surrounded by a liquid mantle, and on top a crust. What is seen as critical to life on one of these large extra-solar planets (exoplanets)—or planets circling a star other then the Sun—is the presence of plate tectonics.
Plate tectonics helps to explain large motions of the Earth’s lithosphere -the solid, outermost layer of the Earth which is made up of the rocky crust that form the tectonic plates. The next layer is the asthenosphere, the softer layer of the upper mantle, which steadily but very slowly flows like a liquid over millions of years. The lower layer of the mantle, which is below the asthenosphere, is a fairly rigid section because of extreme pressures around it.
There are eight to twelve major tectonic plates on our planet and twenty or so smaller ones all moving at speeds and in different directions. The constant turmoil prevents the plates from fusing into a single immobile plate.These plates move together at about 0.75 centimeters each year, some moving apart and some moving together, causing earthquakes, volcanoes, and other physical events within the Earth.
The researchers contend that such geological activity is essential for finding life on super-Earths. They have found that activities associated with plate tectonics help to enhance the chance for life, add to the diversity of life forms, and constantly move minerals, chemicals, and other life-giving substances throughout the soil.
Earth has a solid/liquid metal core that includes radioactive material giving off heat. The metal core produces a magnetic field that protects the surface of the planet from radiation from space, and the radioactive heat from the core, mantel and crust fuels plate tectonics. Why the Earth is alone among rocky planets in our Solar System in having plate tectonics is a mystery. Venus, our twin in size and density, has no tectonic activity.
In addition to plate tectonics and the Earth's location in the Milky Way with its relative abundance of elements heavier than helium, the Earth is also fortunate to exist in a spiral galaxy rather than elliptical galaxies, which are nearly as old as the universe and have a low abundance of heavy elements.
Source: The Daily Galaxy - Brookhaven National Laboratory and harvard.edu