4He/3He laser microprobe analysis: a disruptive new technology for in-situ U-Th-He thermochronology

Ernest Rutherford received the 1908 Nobel Prize for the discovery that radioactivity is a product of the spontaneous disintegration of a radioactive element (the parent) into another element (the daughter). He soon realised that if the decay rate is known, this system can be used to determine the age of geological materials. This is analogous with an hourglass in which sand in the top is the amount of radioactive parent and sand in the bottom is the amount of radiogenic daughter. Rutherford's idea was first applied to the radioactive decay of U and Th, which produces Pb and He, resulting in two chronometers. The proposed method aims to develop and apply a radically new technology in which both chronometers are combined on microscopic samples in a fraction of the time required by conventional techniques.

U-Th-He ages are currently measured by (1) heating entire grains of U,Th-bearing minerals such as apatite or zircon with a long wavelength (IR or visible light) laser, (2) analysing the released He with a noble gas mass spectrometer, (3) dissolving the degassed grains in acid and (4) measuring their U,Th-content on a separate (ICP-MS) mass spectrometer. One problem with this method is that it is time consuming, especially for zircon which is an extremely hard to dissolve mineral. A second problem is that it assumes minerals to have a uniform U and Th composition, because He is spatially separated from its parent due to the energetic nature of the radioactive decay process, and this spatial separation can only be corrected for by making assumptions regarding the location of the parent isotopes.

To solve these problems, the proposed research will develop a radically new technology in which He is not released by degassing entire grains with a long wavelength laser, but is extracted from a small pit in a polished surface by ablation with a short wavelength (UV) laser, and ultimately measured on a noble gas mass spectrometer. U, Th (and Pb) are then analysed by exactly the same means using an ICP-MS. The key innovation of the proposed method over previous attempts at 'in-situ' U-Th-He dating is that all the measurements are done relative to a standard of know U-Th-He age, which is irradiated alongside the sample with high energy protons to produce an internally uniform background signal of 3He (which is a rare isotope of helium).

The new method will deliver a two orders of magnitude increase in sample throughput while enabling us to map out the spatial distribution of U, Th and He within individual grains and producing (U-Th)/(Pb-He) 'double-dates' by default, thus revealing a wealth of geological information previously invisible to the conventional method. We anticipate in-situ U-Th-He dating of proton-irradiated samples to replace conventional whole-grain degassing and dissolution in many (albeit not all) applications, warranting the label of a 'disruptive technology'. We are seeking funds to develop this new dating method as a continuation of Rutherford's groundbreaking research, at a modest cost to the tax payer.

This proposal is directed at establishing an innovative new method for U-Th-He chronometry and has impacts that benefit the Earth science research community and technology companies specialised in noble gas extraction for thermochronometry. The proposal impacts fit well with NERC Earth System and Technology science themes. Amongst the academic community the principal impacts will be improved sample throughput and enhanced data quality resulting in higher-resolution datasets than ever before. These are essential for high spatial and temporal resolution studies required by the next generation of models to understand the speed and magnitude over which landscapes are linked to climate, tectonics and mantle dynamics. For impact on the wider Earth Science community we will convene sessions, at AGU (2014) and EGU (2015). For the noble gas end-user community we will target Goldschmidt (2015) and the bi-annual international thermochronology workshop (2014).

Commercial partners affected by the proposed research are the technology companies that develop and sell extraction-measurement instruments for thermochronometry, and the proton-irradiation facilities that produce the 3He signal which underpins the proposed method. Adoption of our new method will reduce demand for quadrupole mass spectrometers and increase demand for sector-type mass spectrometers, thus causing some disruption in the world of noble gas mass spectrometry. The new methodology will provide a new selling point based on increased sample throughput and enhanced data quality. As explained in the Case for Support, the method hinges on the production of proton-induced 3He in the samples and standards. The Harvard Cyclotron Laboratory in the United States is currently the only facility providing this service to the geological community on a routine basis. One of the aims of the proposed research is to develop a European alternative. Initially, we will use the proton beam at the Paul Scherrer Institute in Switzerland (letter of support attached), in anticipation of a NHS-funded proton beam therapy facility to be completed (at UCL) in 2017. As proton beam therapy is seen by many as a promising new way to treat cancer, an increasing number of proton-irradiation facilities are expected to come online over the next years, bringing down the cost and facilitating the adoption of the proposed method not only the UK but worldwide. Demonstrating the success of the 4He/3He microprobe dating method at this early stage will give us a strong case to make the future NHS facility accessible to academic users for scientific research, and firmly putting the UK at the forefront of thermochronological research.