UBristol: Where does all the gold come from?

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UBristol: Where does all the gold come from?

Post by bystander » Sun Sep 11, 2011 4:25 pm

Where does all the gold come from?
University of Bristol | 2011 Sept 07
Ultra high precision analyses of some of the oldest rock samples on Earth by researchers at the University of Bristol provides clear evidence that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. The research is published today in Nature.
During the formation of the Earth, molten iron sank to its centre to make the core. This took with it the vast majority of the planet’s precious metals – such as gold and platinum. In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a four metre thick layer.

The removal of gold to the core should leave the outer portion of the Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in the Earth’s silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the Earth after the core formed. The full load of meteorite gold was thus added to the mantle alone and not lost to the deep interior.

To test this theory, Dr Matthias Willbold and Professor Tim Elliott of the Bristol Isotope Group in the School of Earth Sciences analysed rocks from Greenland that are nearly four billion years old, collected by Professor Stephen Moorbath of the University of Oxford. These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.

The researchers determined the tungsten isotopic composition of these rocks. Tungsten (W) is a very rare element (one gram of rock contains only about one ten-millionth of a gram of tungsten) and, like gold and other precious elements, it should have entered the core when it formed. Like most elements, tungsten is comprised of several isotopes, atoms with the same chemical characteristics but slightly different masses. Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to the Earth would leave a diagnostic mark on its W isotope composition.

Dr Willbold observed a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks. This small but significant change is in excellent agreement with that required to explain the excess of accessible gold on Earth as the fortunate by-product of meteorite bombardment.

Dr Willbold said: “Extracting tungsten from the rock samples and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements.”

The impacting meteorites were stirred into the Earth’s mantle by gigantic convection processes. A tantalising target for future work is to study how long this process took. Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today.

Dr Willbold continued: “Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material.”

The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment - M. Willbold,T. Elliott, S. Moorbath
Meteorites Brought Gold to Earth
Science Shot | Richard A. Kerr | 2011 Sept 07

Meteorites Pummeled Earth, Delivering Gold
Discovery News | Jessica Marshall | 2011 Sept 07

Meteorite storm showered planet in gold
New Scientist | Michael Marshall | 2011 Sept 07

Asteroids May Have Brought Precious Metals to Earth
Space.com | Charles Q. Choi | 2011 Sept 07

Meteors Delivered Gold to Baby Earth
National Geographic | Rachel Kaufman | 2011 Sept 07

Earthly riches heaven sen
Science News | Devin Powell | 2011 Sept 07

Meteorites Delivered Earth's Mineable Gold
Scientific American | Karen Hopkin | 2011 Sept 08

Terminal bombardment put the bling in Earth's crust
ars technica | John Timmer | 2011 Sept 08
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— Garrison Keillor

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MPA: Cosmic Crashes Forging Gold

Post by bystander » Sun Sep 11, 2011 5:02 pm

Cosmic Crashes Forging Gold
Max Plank Institute for Astrophysics | 2011 Sept 08
The cosmic site where the heaviest chemical elements such as lead or gold are formed is likely to be identified: Ejected matter from neutron stars merging in a violent collision provides ideal conditions. In detailed numerical simulations, scientists of the Max Planck Institute for Astrophysics (MPA) and affiliated to the Excellence Cluster Universe and of the Free University of Brussels (ULB) have verified that the relevant reactions of atomic nuclei do take place in this environment, producing the heaviest elements in the correct abundances.

Most heavy chemical elements are formed in nuclear fusion reactions in stars. Also in the centre of our Sun, hydrogen is ”burned“ to create helium, thereby releasing energy. Heavier elements are then produced from helium if the star is more massive than our Sun. This process, however, only works up to iron; further fusion reactions do not yield any net energy gain. Therefore heavier elements cannot be produced in this fashion. Instead, they can be assembled when neutrons are captured onto ”seed“-nuclei, which then decay radioactively.

This involves two main processes: the slow neutron capture (s-process), which takes place at low neutron densities inside stars during their late evolution stages, and the rapid neutron capture (r-process), which needs very high neutron densities. Physicists know that the r-process is responsible for producing a large fraction of the elements much heavier than iron (those with nuclear mass numbers A>80), including platinum, gold, thorium, and plutonium (Fig. 1). However, the question of which astrophysical objects can accommodate for this r-process remains to be answered.

”The source of about half of the heaviest elements in the Universe has been a mystery for a long time,“ says Hans-Thomas Janka, senior scientist at the Max Planck Institute for Astrophysics (MPA) and within the Excellence Cluster Universe. ”The most popular idea has been, and may still be, that they originate from supernova explosions that end the lives of massive stars. But newer models do not support this idea.“

Violent mergers of neutron stars in binary systems offer an alternative scenario, when the two stars collide after millions of years of spiralling towards each other. For the first time, scientists at the MPA and the ULB have now simulated all stages of the processes occurring in such mergers by detailed computer models (Fig. 2). This includes both the evolution of the neutron star matter during the relativistic cosmic crashes and the formation of chemical elements in the tiny fraction of the whole matter that gets ejected during such events, involving the nuclear reactions of more than 5000 atomic nuclei (chemical elements and their isotopes).

”In just a few split seconds after the merger of the two neutron stars, tidal and pressure forces eject extremely hot matter equivalent to several Jupiter masses,“ explains Andreas Bauswein, who carried out the simulations at the MPA. Once this so-called plasma has cooled to less than 10 billion degrees, a multitude of nuclear reactions take place, including radioactive decays, and enable the production of heavy elements. ”The heavy elements are `recycled' several times in various reaction chains involving the fission of super-heavy nuclei, which makes the final abundance distribution become largely insensitive to the initial conditions provided by the merger model,“ adds Stephane Goriely, ULB researcher and nuclear astrophysics expert of the team (see also Fig. 3). This agrees well with previous speculations that the reaction properties of the atomic nuclei involved should be the decisive determining factor because this is the most natural explanation for the essentially identical abundance distributions of the heaviest r-process elements observed in many old stars and in our solar system.

The simulations showed that the abundance distribution of the heaviest elements (with mass numbers A>140) agrees very well with the one observed in our solar system. If one combines the results of the simulations and the estimated number of neutron star collisions in the Milky Way in the past, the figures indicate that such events could in fact be the main sources of the heaviest chemical elements in the Universe.

The team plans now to conduct new studies to further improve the theoretical predictions by refined computer simulations that can follow the physical processes in even more detail. On the other hand, observational astronomers look out for detecting the transient celestial sources that should be associated with the ejection of radioactive matter in neutron star mergers. Because of the heating by radioactive decays, the ejecta will shine up with almost the brightness of a supernova explosion — albeit only for a few days. A discovery would mean the first observational hint of freshly produced r-process elements in the source of their origin. The hunt is on!

R-Process Nucleosynthesis in Dynamically Ejected Matter of Neutron Star Mergers - S. Goriely, A. Bauswein, H.-T. Janka
Cosmic Collisions – The Astronomical Alchemist
Universe Today | Tammy Plotner | 2011 Sept 08
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Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor