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

Lead Research Organisation: Open University
Department Name: Environment, Earth & Ecosystems

Abstract

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.

Publications

10 25 50
 
Description Discovered new informatiion on how the noble gas Argon behaves in the Earth's crust during processes such as granite formation and deformation.
Exploitation Route We have used our information to enable us to better understand complex age data derived from commercial projects relating to industry and environment
Sectors Energy,Environment

 
Description Leverhulme Trust Fellowship
Amount £45,000 (GBP)
Funding ID RF-2012-573 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2012 
End 03/2015