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Advancing Apatite Thermochronology: Novel analytical approaches for high eU apatites.

Periodic Reporting for period 1 - AAT-NAA (Advancing Apatite Thermochronology: Novel analytical approaches for high eU apatites.)

Reporting period: 2016-09-01 to 2018-08-31

Rocks in the Earth’s crust are cooled as they are exhumed to the surface and heated as they are buried beneath overlying rock. Using low temperature thermochronometry (LTT) dating on accessory minerals, we can constrain rock thermal histories and resolve the timing and rate of rock exhumation or burial. This information is important for understanding the evolution of topography and the development of potential hydrocarbon rich sedimentary basins. Knowledge of the properties that govern the temperature sensitivity of different LTTs is crucial to their successful application. The rate of fission track thermal annealing over c. 120–60°C defines the sensitivity of the apatite fission-track (AFT) thermochronometer. The rate of He diffusion from apatite over 90–30°C defines the sensitivity of the apatite (U-Th)/He (AHe) thermochronometer. The temperature sensitivity of both systems is also influenced by mineral composition and radiation damage accumulation and annealing (RDAA); however, our understanding of these additional influences remains incomplete. This lack of understanding has resulted in AFT and AHe data that are over-dispersed and challenging to explain using existing models, particularly from rocks that have spent long periods (10s-100s of Myrs) at temperatures of c. 120–30°C. The objectives of this project were to advance AFT and AHe methodology, establish analytical capabilities at the Université de Rennes 1 (UR1) and provide new insights into the thermal history of the crust at geological settings presumed to be stable.
To achieve the objectives, three areas of research were pursued: (i) development of analytical techniques; (ii) novel application of techniques to geological case studies; and (iii) development of numerical modelling techniques.

During the project, analytical capabilities for AFT and AHe analysis at the UR1 were advanced. This included establishing LA-ICP-MS analysis for AFT analysis. The Fellow transferred expertise gained using the technique at partner organisation Trinity College Dublin (TCD) and worked closely with a post-doc (N. Cogne) at UR1 (formerly of TCD) to ensure robust analytical protocols wer implemented. LA-ICP-MS allows U to be measured in-situ by ablating a spot on the grain and yields a complimentary U-Pb age and trace element compositions for the AFT single grain age.

A He extraction line attached to a quadrupole mass spectrometer was also set-up at UR1. As He is accumulated through the decay of U, Th, and Sm accurate measurements of He are required for (U-Th)/He dating. During this project, knowledge exchange with Universite de Paris Sud and technical staff at UR1 enabled significant advancements to be made in the design and construction of the line.

Case studies for the application of AFT and AHe techniques, included borehole and outcrop samples from Fennoscandia and southern Africa. A borehole from south Sweden was analysed to assess the compatibility of multiple LTTs and to what extent multiple LTTs can reveal more detailed thermal history information than is possible when used individually. This project provided new LA-ICP-MS AFT and U-Pb data to existing zircon and apatite (U-Th)/He data. Thermal history modelling of the (U-Th)/He LTTs had previously revealed a monotonic, slow cooling history for the borehole. The new data and joint thermal history modelling of the entire dataset revealed multiple cooling events coeval with the break-up of larger continents in the geological past.

Apatite (U-Th)/He data acquired from outcrop samples from Sweden, Finland and South Africa, complimented an existing AFT dataset and were used to investigate the causes of overdispersion in AHe datasets. New data revealed that current RDAA diffusion models cannot explain the complexity of many datasets and their effective uranium (eU)-age relationships. Modelling techniques were adapted to allow the parameter governing the energy He requires to escape defect traps to be treated as an unknown and sampled as thermal histories are being modelled.

A novel analytical approach also involved breaking whole apatite crystals and performing AHe analysis to exploit lateral variations in the He concentration profile. Refinements were made to the numerical modelling approach to better predict the broken crystal effect. Results from this study show that while many broken grains conform to the expected behaviour, accounting for parent zonation and achieving a better understanding of RDAA effects is of utmost importance as RDAA has a more dominant control over AHe ages.

Borehole samples through a sedimentary basin in the Namibian offshore were analysed using LA-ICP-MS AFT. This approach allowed AFT ages, U-Pb ages and apatite composition to be obtained from a large number of single grains. This data provided information on the grain’s pre and post-depositional thermal history. Thermal history modelling revealed that multiple episodes of erosion during the pre, syn and post break-up of Africa from South America produced the detritus, which was delivered to the basin and was then buried and re-exhumed in the post-break up phase.

Results have been disseminated at international conferences including AGU, Goldschmidt, Thermo2016 and Thermo2018. Research activities conducted by the Fellow will directly lead to 4 publishable manuscripts. Results from the studies on borehole LTT data from Sweden and the offshore Namibian passive margin are in the final stages of preparation for publication and are due to be submitted to international journals in early 2019. Two additional publications are in early stages of preparation and require some additional modelling experiments to fully interpret the datasets. The Fellow aims to submit both publications by the end of 2019.
The project has advanced LTT analytical capabilities of the UR1 and established a state-of-the-art approach for performing LA-ICP-MS AFT and U-Pb double dating. New data acquired using this approach has provided robust and novel insights into geological problems. The wider implications of establishing these analytical capabilities are that data can be acquired more efficiently, with greater accuracy and more cost-effectively. As students will be trained in the state-of-the-art techniques and be suitably skilled for future research careers in thermochronology, the project will have a lasting legacy.

New data has implications for our understanding of LTT methodology, strategies for modelling LTT data and for our understanding of the geological settings investigated. It has been shown that current RDAA diffusion models cannot explain the overdispersion and age-eU relationships observed for AHe datasets, which has major implications for the use and interpretation of AHe data. Modelling approaches have been refined to formally treat the uncertainty in the RDAA models and to handle AHe data from broken apatite's. The project has also shown the challenges chemical zonation presents for analysing broken apatite's.

Results from geological case studies, particularly from borehole profiles, have revealed that the ‘stable’ cratonic interior of Fennoscandia and the offshore ‘passive’ margin of Namibia have experienced multiple episodes of exhumation and burial linked to regional tectonic and deep Earth processes. These results will have implications for our wider understanding of how the Earth’s crust responds to geodynamic processes over long periods of time.