Integrating thermochronology with numerical plate-tectonic models: A case study for Central Asia

Glorie, Stijn1; Zahirovic, Sabin2; Kohlmann, Fabian1,3

1The University of Adelaide, Department of Earth Sciences, Adelaide, Australia, 2The University of Sydney, School of Geosciences, Sydney, Australia, 3Lithodat Pty. Ltd., Melbourne, Australia

The low-temperature thermal history of Central Asia has been extensively studied over the last decade. The exhumation history of this intracontinental deformation zone, derived from thermochronological studies, is often linked to far-field effects associated with discrete tectonic events at the former (Meso-Cenozoic) continental margins. While these links are often speculative, the development of numerical plate-tectonic models, with deformable plate-margins, now allows a more detailed evaluation of how tectonic processes at the margins might have propagated into the Eurasian interior. In this contribution, we present a comprehensive dataset of apatite fission track thermal history models for Central Asia. For over 400 sample locations, published thermal history models have been digitised and standardised, and the time-integrated cooling gradient for each sample has been calculated for each 1 Ma increment between 250 Ma and present day. These data are plotted on the latest GPlates model to reveal how the Eurasian interior responds to modelled plate-tectonic processes. The results show how cooling related with intracontinental deformation propagates from the Tian Shan to the Altai during the Mesozoic in response to roll-back processes in the Tethys Ocean. Cenozoic cooling in the Tian Shan starts at ~55 Ma and accelerates at ~30-25 Ma, which provides constraints on the timing of strain propagation from the India-Eurasia collision.


Biography

Stijn Glorie completed a PhD in Geology in 2012, followed by a short post-doctoral fellowship at Ghent University, on intracontinental deformation within Central Asia. In March 2013, he was appointed as a Lecturer and in 2015 he was promoted to Senior Lecturer at The University of Adelaide.

Thermal Annealing of Implanted 252Cf Fission-Tracks in Monazite

Jones, Sean1, Gleadow, Professor Andy1, Kohn, Professor Barry1

1University Of Melbourne, , Australia

Monazite ((Ce, La, Nd, Sm)PO4), a rare-earth element (REE) phosphate mineral, is found as an accessory mineral in a variety of rock types. Suitable uranium and thorium content make it a useful mineral for isotopic and chemical dating using the (U-Th)/He and U-Th-Pb methods. However, unlike other uranium-bearing minerals such as apatite, zircon and titanite, apart from a few reconnaissance studies, its potential for fission-track dating has not been systematically investigated. These earlier studies produced very young ages suggesting that fission tracks may be annealed at very low temperatures. This study further assesses its potential for thermochronology studies by determining its thermal annealing properties via a series of isochronal heating experiments.

252Cf fission-tracks were implanted into Harcourt Granodiorite (Victoria, Australia) monazite crystals on polished surfaces oriented parallel and perpendicular to {100} prismatic faces. Tracks were annealed over 1, 10, 100 and 1000 hour schedules at temperatures between 30°C and 400°C. Track length measurements were made on captured digital image stacks, and then converted to calculate mean lengths of equivalent confined fission tracks. In all annealed samples, the mean equivalent confined track length was always less than that in unannealed control samples. As annealing progresses, the mean track length is reduced and monazite fission-track lengths also appear to be anisotropic, as is the case for apatite, with tracks oriented perpendicular to the crystallographic c-axis annealing faster than those oriented parallel. To investigate how the mean track lengths decreased as a function of annealing time and temperature, one parallel and two fanning models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for this study. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are -44 to 101°C for the parallel model and -71 to 143°C (both ± 6 – 21°C, 2 standard errors) for the best fitting linear fanning model (T0 = ¥). If a monazite fission-track closure temperature is approximated as the mid-point of the MPAZ, then these results, for tracks with similar mass and energy distributions to those involved in spontaneous fission of 238U, are consistent with previously estimated closure temperatures (calculated from substantially higher energy particles) of <50°C and perhaps not much above ambient surface temperatures. Based on our findings it is estimated that the closure temperature (Tc) for fission tracks in monazite ranges between ~45 and 25°C over geological timescales of 106 – 107 years, making this system potentially useful as an ultralow temperature thermochronometer.


Biography

Sean is a Phd Student in the Thermochronology Research Group, University of Melbourne. His research is on developing monazite fission track thermochronology through a series of developmental experiments and application studies.

Development of a digital apatite fission-track analysis training module

Chung, Ling1, Boone, Samuel C1, Gleadow, Andrew1, McMillan, Malcolm1, Kohn, Barry1

1School of Earth Sciences, the University of Melbourne, Melbourne, Victoria, Australia

We report the development of a digital fission-track analysis training module that delivers a traditionally labour-intensive laboratory training routine to the analyst’s computer, making it more practical, accessible and efficient. The module is made possible by Fission Track Studio, a cross-platform dual software suite that is specialized for microscope control and image acquisition (TrackWorks) and analysis (FastTracks), developed by the Melbourne Thermochronology Research Group. Using high-resolution photomicrograph stacks of fission tracks in a range of mica and apatite samples pre-captured in TrackWorks, the module aims to equip researchers with the confidence and skill to produce reliable and reproducible External Detector Method (EDM) and LA-ICP-MS/Digital Fission Track (LAFT) analyses using FastTracks.

The module comprises a series of step-by-step training exercises focused on acquiring the various skills involved in digital fission track analysis, including identifying fission tracks, choosing appropriate grains for analysis, selecting intragrain regions of interest, using FastTracks’ semi-automated counting, c-axis and Dpar functions and measuring confined track lengths. An additional sub-module teaches trainees how to employ FastTracks’ built-in EDM function for the split-screen analysis of apatite-mica sample pairs, as well as allow them to calculate their own user-specific zeta-calibration through analysis of co-irradiated external detectors from standard glasses. The training image sets include the two most commonly used apatite reference materials, Fish Canyon Tuff and Durango, as well as a further six apatites with distinct chemical compositions and track length distributions obtained from a variety of geological settings. Trainees are able to evaluate their progress by comparing their data with expert-reviewed solution files on a grain-by-grain and track-by-track basis.

The digital fission track analysis training module is cloud-stored, allowing for easy access worldwide. Module material includes fission track age and confined track length image sets of well-characterized apatites, expert determined analytical solutions, and a list of recommended reading material and online resources. In collaboration with two international laboratories, the module is being tested on both experienced conventional fission track analysts and untrained students and augmented for improved usability.

Development of this novel training module will empower geoscientists to become remotely trained to perform digital fission track analysis at low cost without face-to-face tutelage or specialised equipment. This enables a new coordinated digital fission track analysis stream, whereby researchers can outsource sample preparation and image capture to laboratories equipped with suitable equipment. Captured image stacks and parent isotope concentrations, in the case of the LA-ICP-MS technique, would then be returned electronically to the newly trained researcher for digital fission track analysis and interpretation. This advance will enhance the accessibility and affordability of this powerful technique and make digital fission track analysis achievable for geoscientists globally.


Biography

Her research focuses on training and development of fission track analytical methods, and applying thermochronological techniques, which provide  temporal and spatial constraints towards reconstructing plate movements, to study the evolution of continental margins and their landscape development.

AusGeochem and the Future of Big Data in Low-Temperature Thermochronology

Boone, Samuel C1, Kohlmann, Fabian2, Theile, Moritz2, Noble, Wayne2, Kohn, Barry1, Glorie, Stijn3, Danišík, Martin4 and Zhou, Renjie5

1University of Melbourne, School of Earth Sciences, Melbourne, Australia, 2Lithodat Pty. Ltd., Melbourne, Australia, 3University of Adelaide, Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, School of Physical Sciences, Adelaide, Australia, 4Curtin University, John de Laeter Centre, Perth, Australia, 5University of Queensland, School of Earth and Environmental Sciences, Brisbane, Australia

The AuScope Geochemistry Network (AGN) and Lithodat are developing AusGeochem, a novel cloud-based platform for Australian-produced geochemistry data from around the globe. The open platform will allow laboratories to upload, archive, disseminate and publish their datasets, as well as perform statistical analyses and data synthesis within the context of large volumes of publicly funded geochemical data aggregated by the AGN. As part of this endeavour, representatives from four Australian low-temperature thermochronology laboratories (University of Melbourne, University of Adelaide, Curtin University and University of Queensland) are advising the AGN and Lithodat on the development of low-temperature thermochronology (LTT)-specific data models for the relational AusGeochem database and its international counterpart, LithoSurfer.

Adopting established international data reporting best practices, the LTT expert advisory group has designed database schemas for the fission track and (U-Th-Sm)/He techniques, as well as for thermal history modelling results and metadata. In addition to recording the parameters required for LTT analyses, the schemas include fields for reference material results and error reporting, allowing AusGeochem users to independently perform QA/QC on data archived in the database. Development of scripts for the automated upload of data directly from analytical instruments into AusGeochem using its open-source Application Programming Interface are currently under way.

The advent of a LTT relational database heralds the beginning of a new era of structured Big Data in the field of low-temperature thermochronology. By methodically archiving detailed LTT (meta-)data in structured schemas, intractably large datasets comprising 1000s of analyses produced by numerous laboratories can be readily interrogated in new and powerful ways. These include rapid derivation of inter-data relationships, facilitating on-the-fly age computation, statistical analysis and data visualisation. With the detailed LTT data stored in relational schemas, measurements can then be re-calculated and re-modelled using user-defined constants and kinetic algorithms. This enables analyses determined using different parameters to be equated and compared across regional- to global scales. Indeed, Australian thermochronologists are already using the new AusGeochem LTT data model as a novel research tool to perform intra- and inter-laboratory experiments and continental-scale tectono-thermal imaging of the upper crust.


Biography

Dr. Samuel C Boone is a postdoctoral research fellow in the School of Earth Sciences, University of Melbourne and a Data Scientist within the AuScope Geochemistry Network.

His research concerns improving our understanding of the thermal and tectonic evolution of Earth’s crust through low-temperature thermochronology, geochemistry and structural geology.

Thermochronology Frontiers in Australia 

McInnes, Brent I.A.

1John de Laeter Centre, Curtin University, Perth, Australia

The field of thermochronology in Australia has seen a significant increase in both capability and capacity development over the last decade. New labs have sprung up at University of Adelaide, the University of Queensland and Curtin University, which augment the University of Melbourne lab which has been a research powerhouse for almost half a century. These lab developments are one of many positive outcomes of informal meetings organised by geochemistry labs around the country via TANG3O (Thermochronology and Noble Gas Geochronology and Geochemistry Organisation).

Most labs now take an integrative approach using multiple radiometric dating techniques (e.g., U-Pb, Ar-Ar, U-He, fission-track) to generate geothermochronology data sets which provide a complete cooling history for any given rock sample. Repeating this process for multiple samples at scale allows researchers to detect differences in thermal history models that reflect major tectonic events in crustal evolution (e.g., continental breakup and collision, mountain-building and basin formation). Computational inversion of geothermochronology datasets are also becoming more sophisticated and allow the 4D thermal evolution of the crust to be imaged, providing a more detailed understanding of tectonic processes as well as predictive capability in the search for mineral and energy resources.

Another promising development is the increasing collaboration between research labs and geological surveys across Australia to address significant geoscience questions, such as: (1) mapping out thermal events across the continent (e.g., National Argon Map project led by Geoscience Australia), (2) demarcation of the end of orogenic events (Hall et al., 2016; Quentin de Gromard et al., 2020), and (3) regolith geochronology (Wells et al., 2019). Continued cooperation will lead to the training of a cadre of young geoscientists skilled in being able to provide a “biography” of a geological unit rather than just its “birth date”.   

Challenges remain however in understanding the crystal chemistry factors that produce inaccurate or irreproducible thermochronology ages in Archean and Proterozoic lithologies. The in situ U-Th/Pb-He microanalysis approach (Danisik et al., 2017), which generates grain-scale zircon He maps and quantifies intragrain He distribution, can be used by researchers to exclude problem areas in grains with anomalous He concentrations due to crystal defects or inclusions. The potential adoption of in situ microanalysis in thermochronology can be viewed similarly to the paradigm shift experienced by the geoscience community when SHRIMP became available in the 1990’s, an event which led to orders of magnitude increase in zircon U-Pb data production and fundamental changes to the design of geological maps and our understanding of the planet.

References

Danišík, M et al (2017) Seeing is believing: Visualization of He distribution in zircon and implications for thermal history reconstruction on single crystals. Science Advances 3:2, e1601121. DOI: 10.1126/sciadv.1601121.

Hall, JW et al (2016) Exhumation history of the Peake and Denison Inliers: insights from low-temperature thermochronology. AJES 63:7, 805-820. DOI: 10.1080/08120099.2016.1253615

Quentin de Gromard, R et al (2019) When will it end? Long-lived intracontinental reactivation in central Australia. Geoscience Frontiers, 10, 149-164. DOI: 10.1016/j.gsf.2018.09.003 

Wells, MA et al (2019) (U-Th)/He-dating of ferruginous duricrust: Insight into laterite formation at Boddington, WA. Chemical Geology 522, 148-161. DOI: 10.1016/j.chemgeo.2019.05.030


Biography

Professor Brent McInnes is a Research Professor at Curtin University and Director of the John De Laeter Centre, WA. Previous to this he was a Chief Research Scientist at CSIRO. Educated in Canada and trained at Caltech, he has 28 years of experience in the geoscience and resources research sector. 

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.