A recent study has revealed that “earthly” minerals such as rhodium and platinum did not originate from our beloved blue planet. U of T geology professor James Brenan collaborated with William McDonough at the University of Maryland to outline a new theory explaining the existence of certain metals in the Earth’s crust.
Brenan and McDonough simulated the extreme temperature conditions that occurred during the Earth’s formation. This allowed them to measure the proportion of metals that would have remained after the Earth’s temperature cooled. Through their intensive research, they demonstrated that metals such as platinum, rhodium, and iridium (from the platinum metal group) should have been completely eliminated from the Earth’s outermost layer as a result of high temperatures.
“The only alternative [to this conclusion] is that they must have come in externally in the form of extraterrestrial rain,” says Brenan. “This meteorite [rain shower], which we termed the late veneer, happened about 100 million years ago and brought back a lot of the minerals which were [initially] lost to the core.”
Brenan examined the platinum metal group behaviour to verify the effects of the late veneer. He explains, “[these metals] are the ones which we can detect most readily. [They] are normally present in such low concentrations in the Earth’s crust that we can see the fingerprint of the meteorite.”
Although many types of meteorites existed in the solar system at the time of the late veneer, the chondrite is the meteorite that is most likely to have produced Earth’s current mineral mix. “[A chondrite] has about the same composition of what we think the Earth started as. In fact, it has about the same composition as the sun,” explains Brenan. “[If] just a little bit of that hit the Earth and mixed in [with the rock], it could account for all that we see today.”
The original theory of Earth’s formation posed that a small planet about the size of Mars hit the Earth and provided it enough energy to melt almost completely. This process allowed the metals to separate towards the centre of the Earth while the rock separated out to form the outer crust layer.
In order to explain the presence of minerals on Earth’s outer layer from the traditional theory, a theory emerged in the early ’90s that at high enough temperatures, some of these metals would have been left at the surface rather than concentrated in the core. “The idea was kicked around, yet a lot of people thought it was wrong. It was a theoretical model, but people still held to it because it allowed a slightly different viewpoint,” comments Brenan.
Experimental work soon followed to verify the process and test traditional theory. “[While the theory was] partially verified, our results were much more extensive and comprehensive. There is no way that can happen—there is no internal process through which these elements could be left behind on the rocky part of the earth,” explains Brenan.
While relatively recent geologic events such as the creation of the Chicxulub crater in the Gulf of Mexico (mark of the meteorite believed to have led to the extinction of the dinosaurs) and the Sudbury basin in Ontario have followed a similar pattern, these events are much younger than the late veneer. “[The late veneer] occurred when there was no crust. Sudbury was hit 1.5 billion years ago when there was a significant amount of this very thin layer of crust,” explains Brennan. The crust is what preserved the meteorite’s impact.
Brenan and McDonough’s theory has possible applications in the field of resource extraction. “Certainly there has been talk about mining asteroids, that is simply because an asteroid that was formally a core of planet which would be filled with such metals [compared with the Earth’s crust].”
At the same time, Brenan acknowledges that science is not the sole driving factor for this type of expansion. “The bottom line is how valuable these metals are compared to the cost of trying to get them. Before the economic downturn, rhodium was trading at $10,000 an ounce—there is no substance that is more valuable.”
The study will be extended to create a complete data set for all the platinum group metals to provide a more complete and comprehensive picture. Brenan adds, “In Canada, we have an abundance of these resources, so this research is applicable [to the resource extraction industry].”
