A Geological Mystery

Back in 2011, a group of researchers at the University of Bristol performed some ultra high precision analyses on some of the Earth's oldest rocks from Greenland. These were four billion years old—almost as old as the Earth itself at 4.5 billion years. Using tungsten as a benchmark, the studies concluded that tungsten along with our precious metals in the Earth’s crust originated from a global bombardment of “about 20 billion billion tonnes of asteroidal material” that occurred about 200 million years after the Earth had solidified sufficiently to retain any asteroids falling upon it within the crust and upper mantle rather than sinking down into the core, where most of our iron and other heavy metals had sunk when the Earth was still molten. This idea could certainly explain the relative abundance of iron in the crust (5.63%) along with many iron-loving elements known as siderophiles. These include nickel, cobalt, tungsten, and others, along with the platinum metals—ruthenium, rhodium, palladium, osmium, iridium, and platinum.

This theory seems very obvious. But if one does a deeper investigation, he can find a serious discrepancy.

Iron meteorites always contain substantial percentages of nickel—from a minimum of 5% up to 16% (Hoba and Chinga), 19.8% (Gebil Kamil), and up to 32% (Tishomengo). The overall average is 7%. Stony and stony-iron meteorites also contain these nickel percentages in proportion to their iron contents.

The Earth’s crust contains 5.63% iron. But it only contains 0.009% nickel and only 0.003% cobalt. If we compare nickel to iron in the Earth’s crust, the nickel content is only 0.143%--35 TIMES short of a meteorite's minimum nickel content of 5% and 49 times short of the 7% average. The crust's cobalt content is less extreme—0.0476% compared to an iron meteorite's average content of .5%. The discrepency here is less extreme—10.6 times less abundant. But it is still far short.

The other siderophiles, including all six PGMs, are also sparse. The crustal iron content of platinum is only 0.0888 parts per million compared to 10 ppm for meteorite iron content—113 times more sparse. Ruthenium drops from 7 ppm down to 0.0176--a deficiency of 398 times. Palladium falls from 3 ppm down to 0.266—a deficiency of 11.3 times. Rhodium drops from 1 ppm down to 0.001—a deficiency of 1000 times. Iridium drops from 2 ppm down to 0.0177--a deficiency of 113 times. Osmium in meteorites averages 1 ppm but has a wide range from 0.01 ppm up to an extreme 65 ppm. It is the Earth’s rarest stable element—only 50 parts per TRILLION (.00005 PPM) in the entire crust and .000888 ppm in the crust’s iron content. The deficiency is 1126 times.

Not only are these discrepencies extreme, but the variations between them are also extreme—from a little over 10 times more scarce up to over1000 times.

If our iron came from a meteorite bombardment, we would expect our siderophiles relative to crustal iron to be at least roughly proportional to what we find in ALL meteorites. Thousands and thousands of meteorites have been collected all over the world, and not even one has been found with iron anywhere close to the “purity” of that found in the Earth’s crust.

The siderophile contents of meteorites do often vary considerably. Iridium contents can vary from .01 up to 50 parts per million. Tungsten varies from .07 up to 5 ppm. Osmium can range from .01 all the way up to 65 ppm. Variations like these can make calculating averages difficult.

Other siderophiles tend to be more consistent. Nickel and cobalt occur in quantities that are great enough to generate more accurate percentages and therefore more accurate data.

But with the irregularities included, even a cursory examination of the results can reveal a huge difference between the siderophile abundances in all iron meteorites and the iron content in the Earth’s crust.

This is a sharp contrast to the Sudbury Basin, a Chixulub-sized impact structure in Ontario that does contain nickel and other siderophile quantities that are reasonably proportional to the estimated iron contained in the impactor. That includes the platinum metals and the only mine in the world sufficiently rich for the specific mining of palladium.

The conclusion is obvious. We can easily assume that our precious metals came from a global meteorite bombardment. But their quantities are far short of what we would expect.

That is the mystery. The Earth’s crustal iron contains only a fraction of the more abundant siderophiles we find in ALL iron meteorites.

SOURCES

1. Where Does All Earth's Gold Come From?

http://www.geologypage.com/2011/09/where-does-all-earths-gold-come-from.html#ixzz46BjbNjS3

2. The Earth’s Crust.

https://elements.visualcapitalist.com/elements-in-the-earths-crust-abundance/

3. Sudbury Basin, Wikipedia.

https://en.wikipedia.org/wiki/Sudbury_Basin

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