As of today, the world might have changed forever.
A fundamental assumption underpinning much of modern geochemistry is that the earth has the same composition as a class of meteorites called chondrites. These are small fragments of rock-like, primeval material that have survived from the birth of the sun with few subsequent changes.
So ingrained is this chondritic assumption in geochemical thinking that, as recently as three years ago, to write a paper questioning the chondritic theory would have been regarded as an act of scientific heresy. It’s unlikely any reputable scientific journal would have published it.
Every professor and every textbook has been telling students for more than 40 years that the composition of Earth is chondritic. Consequently, all geochemists assume the chondritic hypothesis forms an unshakable foundation upon which we can build future advances in geochemistry.
But today, with a paper published in Nature, we’re challenging this fundamental assumption, arguing Earth’s composition isn’t chondritic.
While our paper could be a turning point, geochemists have been questioning aspects of the “chondritic” hypothesis for three or four years now.
We know that the argon content of the atmosphere is only about half that predicted by the chondritic hypothesis. This insight has led to the suggestion that the earth’s mantle – the rocky part between the iron-nickel core at the centre of the earth and the surface – is divided into two layers, and only the outer layer has lost argon.
Our study shows this can’t be the case, but more on that in a moment.
Our challenge to the chondritic paradigm comes from studies of neodymium isotopes in volcanic rocks and meteorites. Our studies show that the ratio of samarium to neodymium (both “rare-earth” metals) in Earth’s volcanic rocks is higher than it is in chondritic meteorites.
This means rare-earth elements abundant in the upper part of the earth, as seen in volcanoes, are not chondritic. The simplest explanation for this observation? The composition of the Earth is not chondritic.
But there are other theories being proposed as an explanation of the neodymium paradox. One is that there must be a complementary hidden reservoir of material near the core-mantle boundary with a low samarium-to-neodymium ratio. This would balance out the high samarium-to-neodymium ratio of upper Earth, thereby maintaining the chondritic hypothesis.
Many geochemists have found this hidden reservoir a convenient place to hide excesses or deficiencies of other elements that do not conform to the chondritic Earth hypothesis.
But the “hidden reservoir” hypothesis has a flaw. It requires about 40% of the mantle’s heat-producing elements – uranium, thorium and potassium – to be concentrated near the core-mantle boundary. The problem with the hypothesis is while you can hide elements that don’t fit your theory, you can’t hide the heat they produce.
The only mechanism by which the heat produced by the putative hidden layer of low-samarium-to-neodymium-ratio material can be removed is through mantle plumes. These are columns of hot rock that rise from the core-mantle boundary, almost 3,000km below the surface, and give rise to volcanoes such as those in Hawaii.
The hidden reservoir hypothesis therefore requires 40% of the mantle’s heat-loss to come from mantle plumes. This is inconsistent with the observation that plumes carry less than 20% of the mantle’s heat loss. Consequently the hidden layer hypothesis cannot be correct.
So what’s the alternative? Well, in conjunction with Professor Herbert Palme, my co-author Professor Hugh O'Neill developed an alternative hypothesis for the composition of the earth.
The prevailing theory holds Earth was formed by collisions of planetary bodies of ever-increasing size. Our suggestion is that by the time the planetary bodies reached moderate size (a few hundred kilometres across), they developed an outer shell rich in heat-producing elements and with a samarium-to-neodymium ratio below the chondritic value.
We suggest that during the final stages of formation of the earth, the outer shell was lost by a process called collisional erosion.
This erosion produced an Earth that is depleted in heat-producing elements compared with the value predicted by the chondritic hypothesis and with a higher samarium-to-neodymium ratio.
Our new theory explains why the samarium-to-neodymium ratio of Earth is above the chondritic value and why the atmosphere has less argon.
(The collisional erosion hypothesis predicts the earth will have less potassium, and argon comes from the decay of potassium.)
Many of the paradoxes that have puzzled geochemists for the last 40 years are predicated on the assumption that the composition of Earth’s mantle is chondritic. If we abandon the chondritic hypothesis many of the problems that have been puzzling geochemists for years disappear.
If our theory is correct and Earth isn’t chondritic, then this necessitates a dramatic rethink of the way we understand the formation of Earth.
We might even have spent the past 40 years developing ingenious solutions to problems that didn’t even exist – problems that stemmed from the chondritic hypothesis.