A new approach to (U-Th)/He thermochronometry: exploiting the natural dispersion of single grain ages to obtain robust thermal history information

Lead Research Organisation: University of Glasgow
Department Name: School of Geographical & Earth Sciences


Geochronology, the science of dating rocks and fossils and determining the time sequence of events in the history of the Earth, underpins modern, quantitative geology. Traditional paleontological and biostratigraphic correlation methods are the most common relative dating methods used by geologists. But, to establish quantitative dates and rates of processes an absolute time frame is required. Absolute, or numeric, dating involves methods of determining the geologic age of a fossil, rock, or geologic event in units of time, usually years. These absolute methods establish the ages of samples by measuring the amount of a specific radioactive isotope (the parent) within the sample as well as the amount of the stable product arising from the radioactive decay (the daughter isotope). For example, the U-Pb technique measures how much 238-U (the parent) is present and how much 206-Pb (the daughter) has been formed by radioactive decay of 238-U, and by knowing the rate of radioactive decay of 238-U we can calculate the age of the sample.

The science of thermochronometry extends the practice of geochronology by determining the temperature a rock sample experienced at a particular time, or times, in the past, i.e. the rock's thermal history. Because subsurface temperatures increase systematically with depth within the Earth the thermal history of a sample collected at the surface records the sample's trajectory from depth to the surface. Thermochronometry thus enables geologists to study and quantify a whole range of processes that are important to understanding how the Earth evolved, such as when and how fast mountain ranges are created and destroyed or quantifying the mass of solid and chemical material transported from continents to the oceans by rivers. One of the most widely used thermochronometry techniques is based on measuring the amount of Helium (the daughter product) resulting from the radioactive decay of 238-U and 232-Th in a mineral called apatite and it is known as the (U-Th)/He technique. This technique is now commonly used to determine thermal histories of rocks in geoscience investigations across an extremely wide range of geological settings because of it is sensitive to relatively low temperatures (c. 40-70 C).

For a range of reasons it is now standard practice to analyse individual grains of apatite extracted from a sample, and an analysis is deemed reliable when 2 or 3 (or more) grain ages from the same sample are statistically equivalent, i.e. the individual grain ages are the same (once size and composition are accounted for). In many studies though it has been shown that single grain ages from the same sample are commonly not the same, and in fact are highly dispersed. This is thought to arise from heterogeneous U and Th distribution within grains, He being added to the grain for from outside grain boundaries (so called excess He) or from differences in the size of grains or in the rate that He diffuses out of the grains at any given temperature caused by accumulation of radiation damage to the apatite crystals. However, in many cases, the observed dispersion is shown to be unrelated to these known effects.

We believe we have discovered the underlying reason for why this kind of dispersion occurs, and our initial experiments indicate it is a natural consequence of analysing crystals that have been broken during the rock crushing and mineral separation process. The exciting consequence of our discovery is that, rather than a hindrance to the technique, this dispersion contains valuable information about a sample's thermal history. In this project we aim to demonstrate and fully test a novel new approach to extracting the thermal history information and applying this widely used thermochronometry technique which we believe will resolve decades of uncertainty about the origin of the 'cryptic' dispersion of single grain ages, and will vastly extend the applicability of this powerful analytical method.

Planned Impact

1. Specific users this work might be of interest to and how they will benefit.

The immediate beneficiaries of knowledge arising from this research is anticipated to be the academic research and teaching community within the general field of Earth sciences, and especially practioners and users of thermochronometry information. The main benefit will accrue to researchers interested in understanding how the Earth's surface responds to and interacts with subsurface and surface processes over geological time scales.

The specific results of this project and the generic implications for models of continental erosion and sediment supply, will also be of interest to the wider offshore minerals (diamond) and hydrocarbon exploration industries. Both are interested in being able to make quantitative measures of the timing, amount, and the source of continental erosion and consequent sediment yield which influences quality of reservoir horizons and diamond prospectivity in the resulting sedimentary deposits offshore.

Other potential beneficiaries include organizations promoting teaching and learning of natural sciences and global environmental change in the primary and secondary education. The specific links highlighted in this project between how the study of diffusion of Helium within minerals (a fundamental physical process) and its impact on the evolution of the landscape we live on provides ideal content and an exemplar for such material.

2. Techniques, methods or activities with which you will engage with these groups.

Publications: The scientific results of the project will be published in a timely manner in high impact, international.

Conferences: The scientific results will be presented at several high profile conferences such as EGU and AGU. The project will also be promoted and initial results presented at the 13th Biennial International Thermochronology Conference which is scheduled for China in 2012.

Commercial: Ongoing interaction with industry (Tsodilo Resources Ltd.) will be pursued.

3. Wider user interest.
Other users who will benefit from this are members of the general public with an interest in the natural environment. A dedicated project web site will be developed as a vehicle for disseminating both the scientific results of the research and specific materials aimed at educators, and the general. We will also proactively seek out opportunities to offer, promote and deliver accessible lectures on this.
Description Geologists have been using radiometric dating methods (methods that utilise the radioactive decay of a parent isotope to a stable daughter isotope) to determine the age of rocks for over half a century now. In numerous cases though it is common for analytical techniques to yield different ages for different mineral grains from the same rock sample. This seems impossible because each grain cannot have a different age if thet all come from the same sample. The results of this research demonstrated in a quantitative manner why some grains yield different ages from each other...and the reason is that many crystals extracted from rocks are broken during the separation process (the process of extracting the mineral grains from the host rock). The reason that broken crystals yield different ages from each other is because the ratio of parent to daughter isotopes within the fragments varies because the geometry of crystal fragments varies. This simple observation had never been properly quantified, and this research has shown that it is an important source of dispersion in ages determined on different grains.

The key finding though, in addition to showing why this dispersion happens, is the research showed that the pattern of age dispersion contains useful information about the thermal history of the rock sample. So, rather than adding unwanted noise, added dispersion is beneficial and can be interpreted to obtain additional information other than simply "an age" for a sample.
Exploitation Route In the academic/scientific domain of geochronology the observation and quantitative modelling that showed why broken crystals yield different ages will be relevant to a wide range of dating techniques (any technique where the diffusion domain is the physical grain).

In industry, such as the hydrocarbon exploration industry that uses thermochronometry, the research findings will have relevance to understanding data sets that were previously thought to be poor quality.
Sectors Energy,Environment