Potato Creek Johnny's famous gold nugget (left) and the Icebox Nugget.

Precious metals the result of meteorite bombardment, rock analysis finds

The idea that gold came from outer space sounds like science fiction, but it has become well-established - it's pretty much received opinion in the field of earth sciences.

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 Earth was formed.

During the formation of 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 Earth with a four-metre thick layer.

The removal of gold to the core should leave the outer portion of Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in Earth's silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit 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 composed 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 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 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."

This research was funded by the Natural Environment Research Council (NERC), the Science and Technology Facilities Council (STFC) and the Deutsche Forschungsgemeinschaft (DFG).

The above post is reprinted from materials provided by University of Bristol.

Post a Comment

GatorALLin said... December 10, 2016 at 7:12 AM

So why don't we find traces of gold in any of the meteorites we find today?

Ben D. said... January 3, 2017 at 11:01 PM

I don't entirely believe this article, and no, I'm not a scientist. But when you observe, you see things. God laying on top of the ground in Prescott, AZ. Big nuggets, not in any rocks at all. Deep in the ground in Quartz. However, there is abundant quartz on earth that is not gold bearing and not from outer space. And the gold I personally have found in hard rock, not quartz. Then there's the gold in so many parts of the globe. The old timers could read the ground and contours and tell you if there was gold or not. No sir, I do not believe that all gold comes from outer space.

Lately, there's this, if gold came from outer space, where did silver, diamonds, rubies, etc all come from? You see, when you put your hat on one peg, you have to wonder where the other pegs came from and what they are for.

Unknown said... January 4, 2017 at 5:57 PM

Ben, you assume that the gold was always just sitting there on the surfac. Plate tectonics work where ever you go. The ground we now walk on may once have been deep in the crust, uplifted, and then eroded until it is no longer under ground.

Matthew Dullenkopf said... April 28, 2017 at 8:50 PM

A little research on how things for will give you the answers you're looking for. Anything heavier than iron was made in a super nova explosion. Since this doesn't happen on Earth and only in massive stars then obviously gold, silvet, and platinum formed in space, not earth. As far as minerals are concerned, all you need is the right chemicals, heat and pressure. All of which are found here on Earth.

Beau Zar Satori said... July 13, 2017 at 9:44 AM

This remarkable story about the meteoric arrival of gold to our planet and redistribution through geophysical activity, failed to address the origin of gold and other precious metal during the supernovae of stars. Which I now read in the comment of Matthew Dullenkopf, above.

leon said... July 16, 2017 at 4:56 AM

I think this argument may need refining.

Given the continual rain of meteoric dust (estimated at 30 000 tonnes per year, we may be continually accumulating gold. Once it is here, I am sure that geochemical processes take over and dominate/control the pattern of distribution, so of course knowing those will tell you where to find it on earth (hint: don't go looking for craters!).

Interesting article!