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Introduction

Equipment on board

Cruise science

Cruise science

What are we doing here?

The aims of the cruise are to investigate the workings of the mid ocean ridge in this area. This part of the ridge system is known as the Carlsberg Ridge, but it has never been scientifically investigated. In fact, it is difficult to even know exactly where it is located because no-one has ever been to look for it before!

Mid ocean ridges are the place where new ocean crust is born. Molten rock (magma) from the mantle rises up to the surface, splitting the tectonic plates apart and spilling out along a huge crack, known as a mid ocean ridge. The Carlsberg Ridge is special because scientists are not sure where the magma is coming from. Bramley's team aboard RRS Charles Darwin think that it might be coming from one of two places: either from the melting of old mantle directly below the ridge, or from a plume of mantle material rising under the Afar Rift in east Africa. The schematic sketches below illustrate the two theories that the scientific team will be putting to the test during the cruise.

Theory 1: The Carlsberg Ridge is fed by mantle material brought up by a mantle plume located under the Afar Rift in east Africa (click on image to enlarge).
Theory 2: The Carlsberg Ridge is fed by magma sourced from the melting of old mantle material directly below the ridge itself (click on image to enlarge).


How can you tell the difference between the two magma types?

Magmas generated by melting different rock types have different geochemistry - in other words, they are made up of different components, or different amounts of the same components. Imagine melting different types of chocolate - dark chocolate is different to white chocolate because they contain different ingredients.

Magma sourced from mantle material welling up from deep inside the Earth has a different geochemistry to magma produced by melting of mantle material nearer the surface. Therefore, if Theory 1 is correct, magma from both the Afar Rift and the Carlsberg Ridge should have similar chemical characteristics. However, if Theory 2 is correct, then magma from the Carlsberg Ridge will be chemically different to that from the Afar Rift because they are from different sources.


How do you measure the chemical differences between magma types?

The best way to tell different magmas apart is to look at their isotope geochemistry. [Definition of isotope].

Different isotope systems are used to help solve different geochemical puzzles; in this case, the most suitable isotopes to use are helium (3He / 4He) and the lead (xPb/xPb) isotopes. The relative abundances of these isotopes allow scientists to distinguish mantle sourced from deep within the earth from mantle material which has come from shallower depths. By measuring the amounts of each isotope and then analysing the ratio of one isotope to another, scientists can work out the source of the mantle material.

In order to be able to make these measurements, Bramley's team need to get samples of the rocks that make up the Carlsberg Ridge. These rocks are formed from the magma welling up through the central mid ocean ridge - as soon as the hot magma comes into contact with the cold seawater it solidifies into a rock known as basalt. When the magma solidifies the geochemical characteristics are preserved in the basalt, so a rock sample is rather like having a frozen sample of the magma itself.

Another technique that the team will use to investigate the evolution of the Carlsberg Ridge is analysing microscopic bubbles of magma trapped in crystals within basalt. When magma cools and solidifies, mineral crystals grow and interlock to form the framework of the rock. If the crystals grow rapidly, as is usually the case when the lava cools quickly, tiny bubbles of magma become trapped within crystals (see pictures below). These bubbles, known as melt inclusions, give scientists the opportunity to look at the magma in its original, unaltered state.

Photgraph (taken down microscope) of melt inclusions
trapped within a plagioclase crystal in basalt.
Image courtesy of Laura Font-Morales, SOC.


Why is it important to know all this?

By taking measurements of the isotopes in rocks along the Carlsberg Ridge, scientists can get an idea of how long the Afar Plume has been feeding the Carlsberg Ridge, and how the composition of the plume material has changed over time. Knowledge of how mantle plumes work, what they are caused by and how they behave is still fairly limited. By studying how a mantle plume affects the way a mid ocean ridge works will contribute towards our understanding of plate tectonics, and the break-up of continental masses in particular.


Also in this section...

Introduction

Equipment on board


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