Tongan test of high field strength - and platinum group element mobility during subduction.

When the Earth was young it didn't have continents - at least not anything like the continents we are used to. We know this because the vast majority of rocks are much younger than the age of the Earth. This means that the planet must have processed the raw ingredients available to it and constructed the continents; including the land we live off, the mineral resources we use to make things and the fuel to make them with. Many scientists agree that a key process in producing these benefits is called subduction. Subduction affects rocks beneath the oceans. There is very little of the ocean floor that formed more than 200 million years ago, because at this age (or sooner) most ocean floor starts to sink - are subducted - into the Earth. Rocks sinking past other rocks into the solid Earth is not a peaceful process and subduction causes some of the world's most devastating earthquakes and most of its explosive volcanoes. Understanding how subduction works is important so that we can manage the planet's resources in a responsible way and protect ourselves from the dangers it can pose. A subduction zone works rather like a pressure cooker. Some ingredients - the rocks and sediments from the ocean floor - go in and get heated and squeezed and something else comes out - rocks that we see preserved in other parts of the Earth. Although you can't look inside to see what is happening the volcanoes above a subduction zone act a bit like a pressure release while it is working. By examining the composition of lavas erupted from these volcanoes it is possible to understand what is going on inside. There are lots of different chemical components that can be used to do this but our project will focus on two groups. During subduction the Platinum Group Elements (PGEs) behave like economically important metals, such as copper and gold. By understanding the behaviour of PGEs we can learn how mineral resources form and figure out where new mineral deposits may be found. In particular we will measure how much of the subducted PGEs get incorporated into the volcanic rocks and how much carry on through the subduction zone. Since large mineral deposits in subduction zones are thought to form underneath volcanoes then the volcanic rocks are also a key part of understanding the mineralization process itself. Most scientists think High Field Strength Elements (HFSEs) are special because the volcanic rocks do not contain any HFSEs from the subducted ocean floor and sediments, only from the rocks already beneath the subduction zone in a zone called the 'mantle wedge'. This makes them especially useful for estimating the fraction of other elements comes from the wedge and the fraction that comes from the subducted material. However, recent research has challenged this lack of 'recycling', leaving scientists with a dilemma of how to determine the contribution of the subducted ingredients. Many other subduction zone problems can be resolved by answering the questions posed by PGEs and HFSEs. To try and resolve these questions we shall study volcanic rocks from the Tonga subduction zone, which is particularly suitable for studying these elements. The composition of the rocks have changed little since they formed in the subduction zone. The ingredients entering the subduction zone can be well characterised and their contribution to the lavas is known to vary along the 600km chain of volcanoes. Until now most of the rocks studied from this chain are from the Tongan islands but we have been involved in collecting new samples from several previously unsampled underwater volcanoes. This is also useful because some PGEs can behave like gases and be lost to the steam that accompanies volcanic eruptions on land. Combining the data for the two groups of elements with other studies conducted by colleagues in Australia, the United States and elsewhere in Britain will allow us to construct a new level of understanding about how subduction works.