Mantle Xenoliths: A window into the interior of the Earth

Studying the inside of the Earth is incredibly difficult, the pressure and temperature at these depths means it is unlikely that we will ever collect samples directly from the Earth’s interior. Considering how difficult it is to sample the interior of the Earth we have been able to collect a remarkable amount of information on how the earth is put together. Thanks to geophysicists we have been able to image the different layers of the Earth by using seismic waves created by large Earthquakes, a bit like using a massive sonar /radar detector. And geochemists not wanting to feel left out have been analysing rocks brought to the surface by volcanic eruptions to determine the chemistry of the deep Earth. So we have a good understanding of the major components of the Earth but we still struggle to understand some of the intricate workings of the Earth’s mantle (The plastic layer of the Earth between the crust and outer core) . One of the best ways to discover the workings of the upper mantle, which drives the global tectonic cycle is to analyse mantle xenoliths which have been brought to the surface through volcanic eruptions.

Mantle xenolith composed of peridotite within an alkali basalt

Mantle xenoliths represent part of the Earth’s upper mantle which has been incorporated into a rising volcanic melt and then subsequently erupted onto the surface. Unlike most volcanic eruptions which are derived from the melting of the  continental crust, mantle xenoliths provide a sample direct from the upper part of the Earth’s Mantle.  This allows us to learn all about the goings on that are occurring miles below our feet.

The area of rifting between East and West Antarctica (Image courtesy of NASA)

So now we are all clued up on what mantle xenolith are and why we study them I will give a brief overview of my project. I have been studying xenolith from the Western Antarctic Rift Zone (WARZ) which is a massive area of volcanic activity. This period of volcanic activity is associated with the breaking apart of the Antarctic Continental Plate. The continental plates are continually moving and breaking apart over periods of 100s of millions of years, so to be able to witness this break up happening is incredibly rare. By analysing the xenoliths that have been brought to the surface, we have been able to discover the processes which are occurring within the mantle which are literally able to rip a continent apart.

Schematic diagram of the interior of the Earth (Image courtesy of NASA)

It requires huge forces to cause the continent to be pulled and stretched apart. There are only a few geological processes which are able to cause such huge scale movements of the continental plates. Previous studies completed on the WARZ have suggested a massive upwelling of buoyant mantle form near the Earth’s core has risen up beneath Antarctica and is pulling it apart. This type of magma upwelling is known as a volcanic plume and can be seen in places such as Hawaii and Iceland. Plumes carry a unique geochemical signature which differs from that of normal rift related magmatism. So by studying the chemistry of the xenoliths it is possible to determine if the rifting in Antarctica is driven by a rising plume forcing its way up through the continent or by the convecting upper mantle applying a stretching force on the bottom of the continental plate.

For my next blog entry I will discuss in more detail the chemistry of plumes and rifts and how to tell them apart and will present some of my data which should hopefully settle the argument around which type of rifting is occurring beneath Antarctica.

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5 Responses to Mantle Xenoliths: A window into the interior of the Earth

  1. Morgan says:

    I have to do a model on the Western Antarctic Rift in my Earth Space Science class and needed a few ideas. Any suggestions on how to make 3D representation of the rift. If so I would love to hear what your thinking 🙂

    • Michael Broadley says:

      Hi, thanks very much for taking the time to read our blog I hope you enjoyed it. As I am sure you have figured out the Western Antarctic rift has an extremely complex and varied history so i would maybe need to know a bit more about what exactly you are trying to model. If you can give some more information on your project I will be glad to help you come up with a few ideas.
      Thanks Michael

  2. Pingback: Geochemistry Group Research in Progress Meeting 2013 | Earth & Solar System

  3. Pingback: Mantle Processes | UMaineGeodynamics2014

  4. Neil Kumar says:

    Hi Michael,
    I am about to embark on the project which utilizes mantle xenoliths to look at how the earth has evolved chemically over the last 3 billion years. I am going to do this as a comparative study using Archean Xenoliths from South Africa and Recent Mantle Xenoliths from Fiji and then comparing these to chondrites as well. This is in its very early stages at the moment and I would very much appreciate if you had some ideas and or comments on this.

    Best Regards,
    Neil

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