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

当地球年轻的时候，它没有大陆——至少不像我们所熟悉的大陆。我们知道这一点是因为绝大多数岩石比地球的年龄年轻得多。这意味着地球一定是加工了可用的原料，形成了大陆；包括我们赖以生存的土地，我们用来制造东西的矿产资源以及制造这些东西的燃料。许多科学家一致认为，产生这些好处的一个关键过程被称为俯冲作用。俯冲作用影响着海洋下的岩石。2亿多年前形成的洋底很少，因为在这个年龄（或更早），大多数洋底开始下沉——被俯冲到地球上。岩石越过其他岩石下沉到坚硬的地球中并不是一个和平的过程，俯冲导致了世界上一些最具破坏性的地震和大多数爆炸性火山。了解潜没是如何工作的很重要，这样我们才能以负责任的方式管理地球的资源，保护自己免受它可能带来的危险。俯冲带的工作原理很像高压锅。一些成分——来自海底的岩石和沉积物——进入并被加热和挤压，然后另一些东西出来——我们在地球其他地方看到的保存下来的岩石。虽然你无法看到内部发生了什么，但俯冲带上方的火山在工作时有点像压力释放。通过研究这些火山喷发出的熔岩的成分，就有可能了解火山内部的情况。有很多不同的化学成分可以用来做到这一点，但我们的项目将重点放在两组。在俯冲过程中，铂族元素（PGEs）的表现与铜和金等经济上重要的金属类似。通过了解pge的行为，我们可以了解矿产资源是如何形成的，并找出可能发现新矿床的地方。特别是，我们将测量有多少俯冲的pge被并入火山岩中，有多少通过俯冲带继续存在。由于俯冲带中的大型矿床被认为是在火山下面形成的，因此火山岩也是理解矿化过程本身的关键部分。大多数科学家认为高场强元素（hfse）是特殊的，因为火山岩中不包含任何来自俯冲洋底和沉积物的高场强元素，只包含来自俯冲带下方一个称为“地幔楔”的区域的岩石的高场强元素。这使得它们对于估算来自楔体的其他元素的比例和来自俯冲物质的比例特别有用。然而，最近的研究对这种“循环”的缺乏提出了挑战，使科学家们陷入了如何确定俯冲成分的贡献的两难境地。通过回答pge和hfse提出的问题，可以解决许多其他俯冲带问题。为了设法解决这些问题，我们将研究汤加俯冲带的火山岩，该俯冲带特别适合研究这些元素。岩石的成分自从在俯冲带形成以来几乎没有变化。进入俯冲带的成分可以很好地表征，它们对熔岩的贡献在600公里长的火山链上有所不同。到目前为止，从这个链中研究的大多数岩石都来自汤加群岛，但我们已经参与了从几个以前未采样的水下火山中收集新的样本。这也是有用的，因为一些pge可以表现得像气体一样，在陆地上火山爆发时被蒸汽蒸发掉。将这两组元素的数据与澳大利亚、美国和英国其他地方的同事进行的其他研究相结合，将使我们对俯冲作用的理解达到一个新的水平。

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.