Understanding excess argon by ultra-high resolution 40Ar/39Ar laserprobe analysis

这个项目使用了我们开发的一种令人印象深刻的新方法，来调查和量化额外数量的氩气如何进入地壳内的矿物和岩石。理解这一点特别重要，因为这些额外的氩会给地质年代学家（专注于找出地质过程的时间尺度和速率的研究人员）带来严重破坏，因为它人为地增加了岩石和矿物的年龄，而且许多岩石和矿物似乎比实际年龄更老。地质年代学家使用的技术是40 Ar/39 Ar测年，这是最通用的放射性测年技术之一，也是应用最广泛的。绝对年龄或放射性年龄是基于个别矿物中一种同位素到另一种同位素的放射性衰变; 40 Ar/39 Ar可能是最通用的年龄之一，因为它是基于钾到氩的衰变，而钾是地球上岩石和矿物中最丰富的元素之一。重要的是，我们要理解为什么40 Ar/39 Ar年龄往往是不准确的，因为年龄强烈影响我们对地球过程的解释，更广泛的是地月系统。两个世纪以来，地质学家一直按照时间顺序排列岩石和过程，但在过去的几十年里，是地质年代学家或年龄测定者将绝对年龄确定在这些事件的相对时间尺度上，如大型火山爆发，陨石撞击，甚至是油藏中流体流动的时间。40 Ar/39 Ar测年技术已经有近40年的历史，但我们发现，随着每一项新技术的发展，我们能够更多地了解地球过程中氩在不同矿物中的行为方式。在某些情况下，我们的新发现的知识揭示了为什么许多年龄与基于古地磁或沉积物沉积速率的年代学不相符，并且随着技术的进步，我们已经能够改进这些年龄。然而，我们和许多其他人仍然发现，40 Ar/39 Ar年龄往往比他们应该的年龄要老，因为这种“讨厌的”额外的氩成分，我们称之为“过量氩”。我们对此了解一点：在一些变质岩中，它往往会渗透到那些含有更多钾的岩石中，在地壳中岩石变形和薄弱面被流体利用的地方，它通常是丰富的，我们知道这些流体的成分和温度会影响这些“多余的氩”进入岩石或矿物的数量。我们真正不明白的是多余的氩来自哪里，以及它如何进入最简单的岩石中的矿物。这就是我们建议通过40 Ar/39 Ar年龄的综合研究，与另一种同位素测年技术进行比较，以及一系列用于确定简单岩石中岩石、矿物和流体成分的分析技术。这是以前没有做过的，因为技术没有跟上理论发展的步伐。自2005年以来，我们一直在努力开发必要的技术，我们相信我们已经实现了这一目标，我们在第6页用试验数据证明了这一点。通过进行有史以来最详细的研究之一，我们的目标是将这种麻烦转化为一种工具，以测量地壳中岩石和矿物内部和之间的流体所行进的距离，并最终提高我们对地质过程的速率和时间尺度的理解。

This project uses an impressive new methodology that we have developed, to investigate and quantify how extra amounts of argon enter minerals and rocks when they are within the Earth's crust. This is particularly important to understand because this extra argon causes havoc for geochronologists (researchers that are focussed on finding out the timescales and rates of geological processes) because it artificially increases the ages of the rocks and minerals, and so many seem to be older than they really are. The technique that the geochronologists use that is susceptible to this problem is 40Ar/39Ar dating, which is one of the most versatile radiometric dating techniques available, and also the most widely applied. Absolute, or radiometric, ages are based on the radioactive decay of one isotope to another within individual minerals; the reason why 40Ar/39Ar is perhaps one of the most versatile is because it is based on the decay of potassium to argon, and potassium is one of the most abundant elements in rocks and minerals on Earth. It is important that we understand why 40Ar/39Ar ages are often inaccurate because the ages strongly influence our interpretation of Earth processes, and wider still the Earth-Moon system. For two centuries or thereabouts, geologists have placed rocks and processes in chronological order, but it is the geochronologists, or age-daters, that in the last few decades have pinned absolute ages on these relative timescales of such events as large volcanic eruptions, meteorite impacts, even times of fluid flow in oil reservoirs. The 40Ar/39Ar dating technique is nearly 40 years old, but we are finding that with each new technological development we are able to learn more about the way argon behaves in different minerals during Earth processes. In some cases our newfound knowledge has shed light on why many ages did not fit with the chronology which was based on palaeomagnetims or rates of sediment deposition, and with technological advances we have been able to improve these ages. However, we and many others still find that 40Ar/39Ar ages are often older than they should be, because of this 'nuisance' additional component of argon, that we term 'excess argon'. We know a little about it: that in some metamorphic rocks it tends to pervade those that have more potassium, it is often abundant where rocks are deformed in the Earth's crust and the planes of weakness have been exploited by fluids, and we know that the composition and temperature of these fluids can affect how much of this 'excess argon' enters the rock or mineral. What we really don't understand though is where excess argon comes from, how far it travels, and how it gets into minerals in the simplest rocks. This is what we are proposing to investigate through an integrated study of 40Ar/39Ar ages, in comparison with another isotopic dating technique, and a series of analytical techniques for establishing the composition of the rocks, minerals and fluids in simple rocks. This has not been done before because the technology has not kept pace with theoretical development. We have been working since 2005 to develop the necessary technology, and we believe that we have achieved this, which we have demonstrated on page 6 with pilot data. By undertaking one of the most detailed studies ever proposed, we aim to transform this nuisance into a tool to measure the distances travelled by fluids within and between rocks and minerals in the Earths crust, and ultimately improve our understanding of rates and timescales of geological processes.