What is the Mantle?
The mantle is that portion of our planet that lies between the Earth’s crust and core. In terms of volume, the mantle makes up 84% of the Earth, making it by far the largest portion of the Earth. The mantle is rocky and solid, largely made up of Mg-Fe silicate minerals, such as olivine and pyroxene. Despite their solidity, mantle minerals can undergo deformation and, over geologic time, large portions of the mantle convect (i.e. flow as a response to variations in temperature and density).
On the basis of mineralogy, the mantle is divided into three main layers, the upper mantle, which extends from the base of the crust to about 410 km depth, the transition zone, which extends down to about 670 km, and the lower mantle, which extends down to the core-mantle boundary at about 2900 km depth. These internal divisions correspond to changes in the structure of mantle minerals that occur due to the increase in pressure with depth. Virtually all samples of mantle rocks on the Earth’s surface have come from the upper mantle. Mantle rocks can be emplaced tectonically in large blocks (called orogenic peridotites) as the result of collision of the Earth’s tectonic plates. These may be large (many square killometers in extent), but tend to be from the shallowest portion of the mantle. Small fragments of mantle rock are commonly brought to the surface entrained within volcanic rocks such as alkaline basalts. Kimberlites are among the most deeply derived volcanic rocks on Earth and these can bring relatively large (up to 1 m diameter) xenoliths (meaning foreign stone) from depths as great as 150-200 km. Kimberlites also bring diamonds to the surface and some diamonds contain inclusions of individual mineral grains with structures and compositions indicating that they were derived from the transition zone and possibly the lower mantle.
Kimberlite volcanism has occurred in southern Africa over at least the past 1.7 billion years, and was particularly intense in the Mesozoic, from about 150 to 75 million years ago, resulting in the emplacement of more than one thousand kimberlite diatremes (pipe-like volcanic features). Because of their economic importance as a source of diamonds, kimberlites and their xenoliths have been the subject of intensive study for the past fifty years. Much of the petrological, mineralogical and geochemical investigation of these important and interesting rocks have been done by researchers at the University of Cape Town, most notably Professors John Gurney and Tony Erlank, and their international collaborators. Mantle researchers at UCT, with the generous cooperation and support of industry partners, have over the past 40 years assembled what is probably the largest research collection of kimberlites, mantle and crustal xenoliths and related materials currently in existence. This collection is housed in a suite of rooms in the basement of the Leslie Social Science Building (adjacent to the Department of Geological Sciences) known collectively as “the Mantle Room”.
Samples from the Mantle Room collection have been instrumental in revealing the physical, chemical, thermal and age structure of the deep continental lithosphere, the formation age of diamonds and the relationship between the composition of mantle samples from kimberlites and their diamond grade. Although the bulk of the collection consists of material from southern Africa, it also includes mantle-related material from Australia, Europe and North America.
Specimens in the Mantle Room collection are available to the global research community and researchers from overseas frequently visit the collection to obtain samples for analysis. Collecting trips continue to be made to add to the collection and mantle research remains a core activity in the Department of Geological Sciences at UCT.