Study of a unique rock collection – and its astonishingly beautiful microscopic crystal structures – could change our understanding of how the Earth works.

The rocks we are interested in are igneous – the frozen remains of magma formed at depths of 100 km or more from the mantle and then spewed out of volcanoes

John Maclennan

If it was possible for us to drill to the centre of the Earth, about 6,500 km below the thin crust on which we live, the largest part to traverse would be the mantle. Although solid, this complex mix of minerals is capable of flowing, albeit over long timescales, as a consequence of the massive variations in pressure and temperature to which it is subjected.

But even the deepest drill cores have so far failed to penetrate the dynamic, flowing parts of the Earth’s mantle. Studying these deep layers is crucial to understanding the inner workings of our planet and the driving force behind movement of tectonic plates. Magma produced in the mantle feeds volcanic eruptions and supplies the cocktail of chemical elements required for the maintenance of a habitable planet.

Now a new study funded by the Natural Environment Research Council in the University’s Department of Earth Sciences has turned to a unique rock collection, amassed since at least the early 1800s and held within its Sedgwick Museum, to provide fresh understanding of the composition of the mantle. 

The collection contains around 160,000 specimens of rock and about 250,000 slide-mounted rock slices that, at half the width of a human hair, are thin enough to let light through. “The rocks we are interested in are igneous – the frozen remains of magma formed at depths of 100 km or more from the mantle and then spewed out of volcanoes,” explained project leader Dr John Maclennan, who is working with Dr Arwen Deuss and Dr Tim Holland. “They carry a message about the composition of the deep Earth that we can decrypt using rock chemistry.”

Alfred Harker, the Cambridge geologist who curated the collection at the beginning of the 20th century, was one of the pioneers behind using thin sections of rocks for interpreting geological composition. But where once Harker would have looked for distinctive crystals under an optical microscope, today Maclennan can probe each crystal with an electron beam, looking for traces of magnesium, iron, nickel, calcium and a host of other metals.

“Our understanding of how the Earth’s deep interior behaves is limited,” said Maclennan. “For decades there has been a consensus of opinion suggesting that the mantle has a consistent composition throughout. Recently, however, doubts have been raised by geochemists looking at igneous samples from a group of five ocean island groups. If true, it means we need to rethink how the Earth is built.”

Harker’s original collection has grown with the addition of samples from rock archives worldwide, and now contains thousands of samples from scores of volcanic island groups. “It’s an incredible resource – fresh sampling of this material from remote island locations would cost a fortune in investigator time and travel. Some of the islands are so remote that it can be months before a ship returns to collect you,” said Maclennan.

At the Sedgwick Museum’s conservation unit, drawer upon drawer are filled with slides corresponding to rocks gathered from islands such as Réunion, Kerguelen, Marion, Ascension, Pitcairn, Hawaii, Tristan da Cunha, The Marquesas and Samoa.

The properties of crystals in these rocks can tell geologists about the pressure and temperature conditions under which they were formed. The recent technological advances have meant that the rocks can be analysed with much greater precision than ever before. “It’s a good example of the benefits of hanging on to these collections,” said Maclennan. “Harker was visionary in understanding the importance of the rocks he was collecting but it’s taken a century since he began collecting rocks to reach a point where their true worth is apparent.”

Among the feldspar, pyroxene, spinel and other volcanic crystals, the pale green olivine is especially important to the team’s analysis. “Olivine is thought to be the most abundant mineral in the shallow mantle. Melting of the mantle is caused by an increase in temperature, a decrease in pressure, or a change in composition. After hot magma rises away from the more dense mantle rock beneath, it eventually stalls, cools and starts to make crystals. Olivine seems to the first to form, so it gives the best view of what’s happening deep down,” explained Maclennan.

Scientists also use seismic data to study the Earth’s interior. By tracking the progress of seismic waves of energy released by large earthquakes, they can indirectly assess the physical structure of the mantle. “However, it can be a bit like having a CAT scan of the body but no means of interpreting exactly what you are looking at,” he added.

The new study will marry together the geochemistry of the rock crystals with the expected response of each composition to the passage of seismic waves. Then, by comparing this expected response to those actually observed under the island groups from which the rocks came, the researchers hope to arrive at the most comprehensive assessment of mantle structure to date.

“This will tell us what the Earth is,” said Maclennan, “Does it have a mantle that is uniform in structure, or is it heterogeneous? Have the long-held beliefs in a homogeneous structure been rightly challenged in recent times?

“The mantle accounts for about two thirds of the Earth’s mass and is an engine of global change, implicated in processes that control the environment at timescales from hundreds to billions of years. The rock collection presents a wonderful opportunity to significantly improve our fundamental knowledge of the mantle's structure, and how this links to the planet’s habitability, and may also provide long-term benefits to the UK economy in terms of better understanding of energy or mineral resources.”

The Sedgwick Museum of Earth Sciences holds fossils, rocks and minerals from around the world that cover more than 550 million years of Earth’s history, as well as almost the entire suite of Harker’s papers spanning over 60 years of geological investigations.

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