Continental life rafts
Unravelling the mysteries of continent formation
The idea of large tectonic plates slowly drifting across the planet, forming and destroying continents, is now well established in science, but exactly how pieces of old mantle and crust interplay with newer freshly melted material is a hot topic in modern geoscience.
Alex McCoy-West is currently completing his PhD at the ANU Research School of Earth Sciences, looking at the processes that formed New Zealand. “There’s been a lot of focus on the processes that formed very old continents like Africa or Australia but what I’ve been doing is looking at the other end of the spectrum, at a very young continental fragment, New Zealand.” Alex says, “And as with all geology one of the first and perhaps trickiest steps is obtaining the right samples.”
Obtaining samples from the Earth’s crust is relatively straight forward. They can be collected from the side of a mountain, if you know what you’re looking for, or from drill cores to approximately 8 km deep. However, to see the whole picture, scientists need to know what’s going on in the deeper portions of the Earth.
The lithosphere consists of the crust and upper most solid part of the underlying mantle. These regions are quite rigid, rocky and brittle, whereas the lower mantle is hotter, more plastic and convects over geologic time. “You can think of sections of the rigid crust and upper mantle moving together, rather like an iceberg with a little piece visible and a vast keel beneath.” Alex says, “and obviously obtaining samples from ‘the keel of the iceberg’ deep within the lithosphere is quite challenging because it’s far deeper than drill cores can reach.”
However, as is often the case, nature offers a solution. Basaltic volcanoes can rapidly bring up hot liquid rock from deep within the Earth, pouring it down their sides in the form of magma. As this magma makes its way up through cracks in the lithosphere, it sheers off fragments of the walls and drags them to the surface, rather like raisins in a cake mix; to the trained eye of a geologist, these mantle xenoliths ‘foreign rocks’ are easy to spot.
“I’ve spent quite a lot of time scouring the slopes of volcanoes around New Zealand looking for suitable samples.” Alex says, “But even once we’ve found these xenoliths from deep in the lithosphere, it’s quite difficult and complicated to derive information from them.”
Much of the information that can be extracted from these xenolith samples relies on the measurement of isotope ratios of different elements.
When rocks melt the various trapped elements have the opportunity to either enter the melt phase and migrate away, or remain in the solid residue of the rock. This partitioning is controlled by the compatibility of the elements in the minerals present. For example, the extremely rare element rhenium strongly enters the melt phase when temperatures get hot enough because it is highly incompatible, whereas osmium is compatible in sulphide phases that are common in the mantle and therefore remains behind in the residue from melting.
One isotope of rhenium is unstable and undergoes radioactive decay into osmium (187Re beta decays to 187Os with a half-life of 41.2 billion years). Over long periods time this creates small variations in the isotopic ratios. By measuring very precisely the ratio of osmium, rhenium and their various isotopes, scientists can calculate how much time has passed since the sample was last melted.
“The concentration of osmium and rhenium in samples from New Zealand is ridiculously low, osmium concentrations are less than 5 parts per billion with rhenium generally 1000 times lower”. Alex explains, “So we have to be careful during the analytical processes not to lose our sample. After I’ve crushed up the available samples and chemically purified the rhenium and osmium there’s so little there that if it wasn’t for the acid it’s suspended in, you’d never be able to see it in the beaker!”
In spite of this, the team were able to deduce geological age information for the samples using some of the most sophisticated diagnostic equipment in the world at ANU and the University of Maryland in the USA, with the results recently published in the journal Geology (Feb 2013).
The picture that emerges is that a very ancient fragment of mantle lithosphere underlies eastern New Zealand and probably served as a nucleation point for the much younger material to form around. “The age difference between the crust and the underlying mantle is 1.5 billion years,” Alex explains, “That’s in stark contrast to older continents like Africa where there’s typically no difference at all. It’s quite exciting because it looks like New Zealand is a classic example of what’s known as a continental life raft.” Additionally, this discovery of ancient lithosphere within New Zealand provides new information on its origins and assembly history, with major tectonic implications for the present-day development of the Australia-Pacific plate boundary cutting through New Zealand.