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，来调查和量化当矿物和岩石处于地壳内时，额外数量的Ar是如何进入它们的。了解这一点尤其重要，因为这种额外的Ar对地质年代学家(专注于找出地质过程的时间尺度和速率的研究人员)造成了严重破坏，因为它人为地提高了岩石和矿物的年龄，因此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易受这个问题影响的技术是40Ar/39Ar测年，这是现有的最通用的辐射测年技术之一，也是应用最广泛的。绝对年龄或放射性年龄是基于单个矿物中一种同位素到另一种同位素的放射性衰变；40Ar/39Ar可能是最通用的方法之一，因为它是基于钾到Ar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，我们要了解为什么40Ar/39Ar年龄经常是不准确的，因为这些年龄强烈地影响着我们对地球过程的解释，更广泛地影响到地月系统。在大约两个世纪的时间里，地质学家将岩石和过程按时间顺序排列，但在过去的几十年里，是地质年代学家或测年专家将绝对年龄固定在这些事件的相对时间尺度上，如大型火山喷发、陨石撞击，甚至是油藏中的流体流动时间。40Ar/39Ar测年技术已有近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不同矿物中Ar的行为方式。在某些情况下，我们新发现的知识揭示了为什么许多年龄不符合基于古地磁或沉积物沉积速率的年代学，而随着技术的进步，我们已经能够改进这些年龄。然而，我们和其他许多人仍然发现，40Ar/39Ar年龄往往比它们应有的年龄更老，因为这种令人讨厌的额外成分Ar，我们称之为“过量Ar”。我们对此知之甚少：在一些变质岩中，它往往会渗透到那些含钾较多的岩石中，在地壳岩石变形和软弱层面被流体利用的地方，它往往是丰富的。我们知道，这些流体的组成和温度会影响这些过剩的Ar有多少进入岩石或矿物。然而，我们真正不明白的是，多余的Ar从哪里来，它跑了多远，以及它是如何进入最简单的岩石中的矿物中的。这就是我们建议通过40Ar/39Ar年龄的综合研究，并与另一种同位素测年技术以及一系列分析技术相比较来研究的，以确定简单岩石中的岩石、矿物和流体的组成。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，我们一直致力于开发必要的技术，我们相信我们已经实现了这一点，我们在第6页用试点数据证明了这一点。通过开展有史以来最详细的研究之一，我们的目标是将这一麻烦转化为一种工具，来测量地壳中岩石和矿物内部和之间的流体移动的距离，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.