A rock from the deep | Curator of Petrology

A rare and special rock, a piece of the upper mantle was donated to the NHM by researchers of Macquarie University, Sydney, Australia. This specimen is one piece of a large collection of mantle xenoliths that is being donated to the NHM.

Read on to learn about this rock and the message it sends from the deep. 

What are mantle xenoliths and how do they form?

Xenolith, from ancient Greek, means “foreign rock” and describes a fragment of the Earth’s upper mantle that gets embedded into a completely different type of rock.

Some magma that is sourced in the upper mantle can tear off pieces of the local rock whilst rising to the surface, which get incorporated into the magma and are then erupted at the Earth’s surface.

What is this particular specimen made of and where is it from?

The sample of mantle xenolith donated to the NHM

This specimen is a piece of the Earth’s upper mantle, made of olivine and pyroxene, which are the most abundant minerals making up the xenoliths. This specific sample is characterised by the presence of diopside in bright green, and garnet with a stunning purple colour which is given by the presence of chromium. The sample was collected in South Africa near Kimberly in 2006.

Why are mantle xenoliths important?

Studying the internal structure and composition of our own planet is very challenging as we do not have the capability to sample the Earth’s interior.

The mantle is in fact located at 20 to 90 kilometres under the typical continental crust, and 5 to 10 kilometres below the ocean. The boundary between the Earth’s crust and the upper mantle is called the Moho.

The pressure and temperature at these depths make it impossible for humans to drill down to the mantle, therefore we are not in the position of being able to collect samples directly from the Earth’s interior.

Luckily for us, xenoliths are brought up to the surface naturally by geological processes providing us with samples directly from the upper mantle; this allows us to learn about the composition of the otherwise inaccessible mantle.

Why do we want to acquire this collection?

Professors Sue O’Reilly and Bill Griffin from Macquarie University in Sydney, Australia with the NHM Petrology curator Epifanio Vaccaro.

The NHM’s mantle xenolith collection is one of the most important and comprehensive among the NHM Petrology collections, providing excellent worldwide coverage. The presence of such important collection in our institution has attracted the attention of Professors Sue O’Reilly and Bill Griffin, who suggested we are the most appropriate institution to appropriately house and manage their collection following their imminent retirement from Macquarie University in Sydney, Australia.

Lherzolite, the largest mantle xenolith ever found, Liverpool rangers, upper hunter valley N.S.W.

This acquisition would be an important and valuable addition to the existing NHM collection, as it features xenoliths that can no longer be collected, from places such as Kerguelen Island, and the largest mantle xenolith ever found!

This acquisition will make the NHM mantle xenolith collection the finest in the world.

Our mantle xenolith collection is available on request. Details of specimens can be found on our data portal and we hope to provide further blogs when the rest of the O’Reilly and Griffin collection arrives at the museum.

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