Evolution in the News - June 2016
by Do-While Jones

The Tungsten Problem

Tungsten isotopes are inconsistent with the currently accepted theory about how Earth formed.

In addition to the question of when Earth formed, there is the question of how quickly Earth formed. Creationists like to point to polonium halos as evidence that Earth formed rapidly—but we aren’t going to address that argument. Instead, let’s look at recently published discoveries of tungsten isotopes which are inconsistent with the conventional cosmological myth.

Here is the way Science News summarized it:

Earth formed about 4.6 billion to 4.5 billion years ago as planetary bodies collided, disintegrating and melting to accrete into one mass like a hot, rocky lint ball. Geologists have assumed that any relics of this bumpy beginning were mixed beyond recognition.

Instead, Rizo’s team found a surprise: Some modern flood basalts have unusually high concentrations of tungsten-182. That’s significant because that isotope forms only from radioactive decay of hafnium-182. And hafnium-182 only existed during Earth’s first 50 million years. “These isotopes had to be created early,” says Rizo, of the University of Quebec in Montreal.

Her team found that levels of tungsten-182 in the lavas varied, suggesting that the deep sources of these younger rocks were different pieces of Earth’s oldest material, each with their own isotopic signature and history. These results also show that the ancient remnants have somehow escaped being mixed by convection currents. 1

Rizo’s article was published in the journal, Science. The editors said this about her discovery:

On page 809 of this issue, Rizo et al. report W [W is the chemical symbol for tungsten] isotope data from young mantle-derived rocks with µ182W excesses of 10 to 48 ppm. This result is spectacular because the range of µ182W values in mantle-derived rocks is larger than can be accommodated by late accretion; the implication is that remnants of Earth's earliest mantle have been preserved over the entirety of Earth's history. 1

Let’s try to put this in simple terms. Conventional wisdom is that Earth formed when a bunch of rocks orbiting the Sun were pulled together by gravity. When they smashed together at high rates of speed, their kinetic energy was converted to heat, resulting in a large ball of gooey, molten rock. Heavier elements sunk before the rock cooled enough to harden. When they did cool, Earth had an inner core, a mantle, and a crust. It presumably took a long time for Earth to cool because of its large thermal mass.

The recent measurements seem to indicate that Earth cooled quickly enough that elements were frozen in place before they could distribute themselves as expected.

Rizo believes tungsten-182 “forms only from radioactive decay of hafnium-182.” “And hafnium-182 only existed during Earth’s first 50 million years.” Yes, hafnium-182 does have a half-life of only 8.9 million years, so it should be all gone in 50 million years; but traces of it have been found in nature. (There must have been an awful, awful lot of it to begin with for any of it to be left at all! ) And hafnium-182 does decay to tungsten-182 (after existing as tantalum-182 for a few months); but how does anyone know that all tungsten-182 came from the decay of hafnium-182? Whatever process created all matter could have created tungsten-182 directly, could it not?

The Moon

This discovery also has some bearing on one theory about the origin of the Moon. All the theories about the origin of the Moon are controversial. One, however, is that the Moon was created when another (unknown) planet, roughly the size of Earth, smashed into Earth and knocked some of Earth’s mantle into space, where it somehow defied the laws of orbital mechanics and started orbiting Earth. (Remember, the Apollo astronauts had to fire retro rockets on the far side of the moon to achieve lunar orbit.) Since the Moon is really just part of Earth (or so they say) it should have similar isotopic content. But the Science editor said,

The new data from flood basalt lavas that erupted into the North Atlantic Igneous Province (Baffin Bay locale) and the Ontong Java Plateau (western Pacific Ocean) show larger µ182W variability than between the Moon and Earth's mantle. In fact, the mantle source of the Ontong Java and Baffin Bay lavas prior to late accretion probably had a radiogenic W excess of +70 ppm. This exceeds what can be accomplished by late accretion. By implication, these parts of Earth's mantle predate the Moon and did not chemically equilibrate metal and silicate during the giant impact that formed the Moon. 1

We’ve quoted what Science News and the journal, Science, said about the discovery. Here’s what the authors of the study themselves said,

Variability in 182W/184W ratios reflects Hf/W fractionation while 182Hf was extant. Hf/W fractionation has been observed in early solar system materials, so variable W isotopic compositions in terrestrial samples can reflect the imperfect mixing of late additions of such materials. The µ182W value of +48 for Baffin Bay sample Pd-2, however, is larger than can be accounted for by this process, and so this possibility is discounted (supplementary text). Hf/W fractionation can also occur as the result of endogenous Earth differentiation processes, such as magma ocean crystallization and core formation. However, silicate fractionation processes cannot be responsible for the generation of the anomalous 182W in the sources of the Baffin Bay and Ontong Java lavas. If the high µ182W was due to silicate fractionation in a magma ocean while 182Hf was extant, then µ182W should positively correlate with µ142Nd, the decay product of the short-lived 146Sm (t1/2 = 103 million years) isotope system. Instead, the µ142Nd values of the samples are indistinguishable from all other modern basalts measured so far (fig. S3 and table S5). 1

We could not have said it better ourselves! Well, maybe we could. That’s why we quoted the other authors and tried to explain it ourselves.

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Footnotes:

1 Beth Geiger, Science News, June 11, 2016, “Remnants from Earth’s birth linger 4.5 billion years later“, p 13, https://www.sciencenews.org/article/remnants-earth-birth-linger-45-billion-years-later?mode=magazine&context=191919&tgt=nr
2 Dahl, Science, 13 May 2016, “Identifying remnants of early Earth”, pp. 768-769, http://science.sciencemag.org/content/352/6287/ 768.full
3 https://en.wikipedia.org/wiki/Hafnium
4 ibid.
5 Dahl, Science, 13 May 2016, “Identifying remnants of early Earth”, pp. 768-769, http://science.sciencemag.org/content/352/6287/768.full
6 Rizo, et al., Science, 13 May 2016, “Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts”, pp. 809-812, http://science.sciencemag.org/content/352/6287/809.full