Abstracts for Virtual Thermo2020

December 15, 2020, Session 3

Talk 1

Zircon Raman dating: Age calculation and closure temperature(s)

Birk Härtel1*, Raymond Jonckheere1, Bastian Wauschkuhn1, Lothar Ratschbacher1

1 Institute of Geology, TU Bergakademie Freiberg

* birkhaertel@gmx.de

Zircon Raman dating based on radiation damage accumulation and annealing is a debated concept but not yet an established thermochronological method. The α-disintegration of U, Th and their decay products creates lattice disorder in zircon (ZrSiO4) that can be quantified using the broadening of the Raman bands. Measuring the radiation damage and the U and Th content allows to calculate a zircon Raman date. Radiation damage is annealed at high enough temperatures.

We present a new empirical calibration of Raman bandwidth on radiation damage dose based on fission-track age standards. We derived an age equation to calculate zircon Raman dates of single grains. For multi-grain dates, standard procedures such as weighted mean, pooled, and isochron dates are applicable. We carried out isothermal annealing experiments, and fitted a Johnson-Mehl-Avrami-Kolmogorov and a distributed activation energy model to the annealed fraction of the bandwidth. From the kinetic parameters of the models, we estimated closure temperatures of zircon Raman thermochronometry at ~330-370 °C for internal ν2(SiO4) and ν3(SiO4) bands and ~250-300 °C for the external rotation band with a Raman shift of ~356 cm-1. This difference in closure temperatures offers the prospect of multi-temperature zircon Raman dating as a part of multi-method zircon dating.

Talk 2

Isotopic forward modelling: A new tool to interpret U-Pb thermochronometry data

Chris Mark1*, David Chew2, Nathan Cogné3, Andrew Smye4, Pieter Vermeesch5, and J. Stephen Daly1

1 School of Earth Sciences, University College Dublin, Dublin, Ireland;
2 Department of Geology, Trinity College Dublin, Dublin, Ireland;
3 Géosciences Rennes, Université Rennes 1, Rennes, France;
4 Department of Geosciences, Pennsylvania State University, University Park, USA;
5 Department of Earth Sciences, University College London, London, UK.

* chris.mark@ucd.ie

The availability of high-temperature thermochronometers suitable for generation of continuous thermal histories at mid- to lower-crustal temperatures (i.e., ≥ 400 °C) is limited. Available thermochronometers include the recently developed apatite and rutile U-Pb thermochronometers ( ≤ 550 and 640 °C; Kooijman et al., 2010; Cochrane et al., 2014) and arguably also the K-Ar system in white mica (sensitive to temperatures ≤ 500 °C.

Recent work has focussed on U-Pb analysis of apatite and rutile by sector-field and multi-collector LA-ICPMS to generate single-crystal U-Pb age profiles. Such profiles can be inverted to yield continuous thermal histories for high-temperature processes (e.g., Smye et al., 2018). Both rutile and especially apatite routinely incorporate non-trivial amounts of common-Pb during crystallisation (as opposed to radiogenic Pb generated by in-situ radionuclide decay), rendering them discordant in U-Pb isotope space. This common-Pb must be corrected for during age calculation. Such a correction necessitates critical assumptions regarding the isotopic composition of the common-Pb, and also the diffusive response of common- and radiogenic-Pb to heating. Additionally, both apatite and rutile can exhibit crystal growth and dissolution-reprecipitation reactions in the same temperature ranges at which measurable Pb diffusion occurs: neither behaves as a pure thermochronometer in all circumstances (e.g., Chambers and Kohn, 2012; Harlov et al., 2005).

Here, we exploit the observation that common-Pb is isotopically distinct from radiogenic Pb to predict the U-Pb isotopic composition of a given crystal arising from a proposed thermal history by forward modelling. We show that common-Pb can be exploited to validate the assumption of Pb-loss by volume diffusion; the thermal history predicted by age profile inversion; and age profiles arising from volume diffusion (as opposed to (re)crystallisation). We present a case study from the Eastern Alps.

Chambers, J.A., & Kohn, M.J., Am. Mineral., 97, 543–555 (2012); Cochrane, R., et al., Geochim. Cosmochim. Acta, 127, 39–56, (2014); Harlov, D.E., et al., Contrib. Mineral. Petrol, 150, 268–286 (2005); Kooijman, E., et al., Earth Planet. Sci. Lett, 293, 321–330, (2010); Smye, A.J., et al., Chem. Geol., 494, 1–18 (2018).

Talk 3

Zircon (U-Th)/He impact crater thermochronometry and the effects of shock microstructures on helium diffusion kinetics

Catherine H Ross1,2*, Daniel F. Stockli1, Desmond B. Patterson1, Sean P. S. Gulick1,2, Timmons Erickson3

1 Department of Geological Sciences, The University of Texas at Austin
2 Institute for Geophysics, The University of Texas at Austin
3 Johnson Space Center, NASA

* catherine.ross@utexas.edu

Accurate age determinations of impact and cratering events remains difficult and often controversial and less than half of all known impact craters are regarded as accurately and precisely dated. Besides 40Ar/39Ar and U-Pb methods, zircon (U-Th)/He (ZHe) dating of impactites has become novel technique to date large- to medium-sized impact structures. ZHe ages can be fully reset in minutes at 1000°C, T reached commonly reached in central part of impact targets, whereas complete resetting of ZHe at <300°C, which might be encountered near the crater margins or persist in post-impact hydrothermal systems, may take >103-4 years. However, there is a critical need to test the reliability of (U-Th)/He impact dating, in light of shock-induced microstructures and impact metamorphism, and to quantify helium diffusion kinetics in well-characterized variably shocked zircon. For this purpose, we investigated samples from two impact structures, the Chicxulub multi-ring basin and the Ries crater, to compare zircon diffusion kinetics from impact structures with different size, age, and hydrothermal system longevity. Shock microstructures were characterized by backscattered-electron (BSE) imaging prior to determination of diffusion step-heating fractional release experiments using light-bulb furnace with prograde and retrograde incrementally 10°C steps from 300°C to 600°C. We find that zircon with low-level shock microstructures exhibit no significant deviation from helium diffusion kinetics of undamaged zircon. In contrast, zircon grains with planar deformation features and granular textures classified by SEM are characterized by a dramatic decrease in helium retentivity, similar to radiation damage, due to the reduction in the effective domain size and the introduction of fast diffusion pathways.  This likely renders shocked grains more susceptible to impact-induced hydrothermal resetting. Hence, characterization of impact microstructure is critical for determining accurate impact ages, but also offers the opportunity to determine the magnitude and duration of post-impact hydrothermal circulation.

December 15, 2020, Session 4

Talk 1

Application of fission track thermochronology to porphyry belts in Eastern Kunlun Mountains, Qinghai-Tibet Plateau

Li Yang1,*, Wanming Yuan1, Shujiong Hong1, Zirui Feng1, Yunlei Feng1, Pengfei Tian1, Congmiao Cao1

1 Institute of Earth Sciences, China University of Geosciences, Beijing 100083, China

* yangli0211luck@126.com

Studies on the preservation of ore deposits increasingly raise the concern from researchers in recent years. It is an important component of ore deposit geology. However, the extent to which ore bodies are preserved remains a poorly understood area of ore deposit research. The preservation of ore deposits is usually linked with the processes of uplift and exhumation. Thus, it is a reasonable and feasible method to evaluate the preservation of ore deposits based on the analysis and reconstruction of the exhumation and cooling history of mining area. In this dissertation, the hydrothermal ore deposits in the East Kunlun Mountains are chosen to be worked on. The exhumation and cooling history of selected mining area are reconstructed by the combination of fission track themochronology, apatite (U-Th)/He thermochronology, zircon U-Pb geochronology and fluid inclusions analysis. The relationship between buried depth of ore bodies and the amount of exhumation is clearly revealed. A real model is presented to outline the preservation and exhumation history affected by faulting activities. The conclusions of this dissertation will provide reasonable clues for further exploration.

The East Kunlun Mountains, located in the north of the Tibetan Plateau, is one of the significant metallogenic belts in China. In the region, we work on four hydrothermal ore deposits, including Harizha copper deposit, Halongxiuma molybdenum (copper) deposit, Qingshuihe copper and molybdenum deposit, Dahongshan iron deposit and Aikengdelesite molybdenum and copper deposit, to evaluate the preservation of the ore deposits. The zircon U-Pb dating data suggests that all of these five hydrothermal deposits formed from 253 Ma to 215 Ma in Triassic. The fluid inclusions from these deposits are characterized of high-medium forming temperature, high salinity and high-medium density. The mean ore forming depths ranging from 4.20 km to 7.42 km. The forming and post-forming processes of these four porphyry deposits are generally consistent.

All deposits experienced rapid cooling after magma emplacement, and the temperature dropped to the temperature of the surrounding rock, then suffered different modes of denudation cooling process. Four deposits underwent similar rapid-stable-rapid cooling pattern in this stage. But the Aikengdelesite deposit in this stage experienced a unique rapid-stable pattern that different from other four deposits. As evaluated in this dissertation, the preservation of these four deposits are different. The ore bodies of Harizha and Halongxiuma deposits are well preserved, while the preservation of Qingshiuhe deposit is not so well but still positive. However, the Aikengdelesite deposit is not well preserved.

Key words: thermochronology, preservation of ore deposits, East Kunlun, cooling and exhumation

Talk 2

A global platform solution for big data in low-temperature thermochronology: AusGeochem and LithoSurfer

Samuel C Boone1,2*, Fabian Kohlmann3, Moritz Theile3, Wayne Noble3, Barry Kohn1,2, Andrew Gleadow1,2, Stijn Glorie4, Martin Danišík5 and Renjie Zhou6

1University of Melbourne, School of Earth Sciences, Melbourne, Australia
2AuScope Geochemistry Network, Curtin University, John de Laeter Centre, Perth, Australia
3Lithodat Pty. Ltd., Melbourne, Australia
4University of Adelaide, Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, School of Physical Sciences, Adelaide, Australia
5Curtin University, John de Laeter Centre, Perth, Australia
6University of Queensland, School of Earth and Environmental Sciences, Brisbane, Australia

* samuel.boone@unimelb.edu.au

The AuScope Geochemistry Network (AGN; https://www.auscope.org.au/agn) have partnered with Lithodat Pty Ltd (https://lithodat.com/) to develop the dual AusGeochem-LithoSurfer geochemistry data platforms. The open and cloud-based dual-platform solution will allow laboratories to upload, archive, disseminate and publicise their datasets, as well as perform statistical analyses and data synthesis within the context of large volumes of global geochemical data aggregated by the AGN, Lithodat and other platform users. While AusGeochem is designed specifically to serve the Australian geochemistry community (https://www.auscope.org.au/ausgeochem), the coupled LithoSurfer platform is designed as a global data repository for laboratories across the globe. Geosample information and geochemistry data can be bulk uploaded into the AusGeochem-LithoSurfer relational database through a specially developed ETL (Extract, Transform and Load), allowing legacy data to be uploaded via an easy drag-and-drop of csv files. The AusGeochem-LithoSurfer platforms also utilise an Open REST API which can be used to automatically upload data from analytical software and in-lab hard drives, enabling a flexible and automated data migration process that accommodates diverse data types while minimising manual data handling. In addition to facilitating data management, the Open REST API is designed such that any developer can build clients to interact with the platform to, for example, automatically retrieve data from its database, add-in enhanced data visualisation and create direct links to analytical equipment.

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-LithoSurfer database. Adopting established international data reporting best practices, the LTT advisory group has designed database schemas for the fission track and (U-Th-Sm)/He techniques. A separate data model enables the archiving of digitised thermal history modelling results and metadata, making time-temperature reconstructions geospatially comparable and quantifiable. In addition to recording the parameters required for LTT analyses, the schemas include fields for reference material results and error reporting, allowing platform users to independently perform QA/QC on data archived in the database.

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 LTT analyses determined using different parameters to be equated and compared across regional- to global scales. Once standardised and normalised, these data can then be compared directly with other datasets, such as thermal conductivity, heat flow and other geochemistry data.

Talk 3

The FastTracks digital fission-track analysis training module

Ling Chung1*, Samuel C Boone1, Andrew Gleadow1, Malcolm McMillan1 and Barry Kohn1

1 School of Earth Sciences, University of Melbourne

* ling.chung@unimelb.edu.au

Training for fission tack analysis is normally a very slow and labour-intensive process.  The development of the digital fission-track training module aims to deliver a guided training routine to the analyst’s computer, equipping researchers with equivalent level of confidence and skill to produce reliable and reproducible External Detector Method (EDM) and LA-ICP-MS/Digital Fission Track (LAFT) analyses using an image-based protocol. Originally, there are five stages of hand-on practice involved in comprehensive training conducted by Melbourne Thermochronology Research Group (MTRG). Chiefly utilising the MTRG-developed Fission Track Studio (FTS), a cross-platform dual software suite that is specialized for microscope control and image acquisition (TrackWorks) and image analysis (FastTracks), trainees are led to complete a series of carefully designed exercises to acquire the various skills in fission track dating. (1) Preparing grain mount for Durango (DUR) apatite provided. (2) Practicing grain selection and track-counting (track identification and region of interest, ROI, determination) by analysing seven sets of pre-imaged mica, DUR and Fish Canyon Tuff (FCT) apatites using FastTracks. (3) Challenging ‘unknowns’ by using the full capacity of FTS, i.e. capturing images of >25 selected grains each from the DUR mount and 20 well-characterized apatite reference mounts from the MTRG collection using TrackWorks and performing semi-automated analysis on grain and length images using FastTracks. (4) Acquiring single grain/spot uranium concentration using LA-ICP-MS. (5) Practicing age calculation. Reproducibility of data obtained from exercise (2) to (5), including single-grain track density, 3D confined track length measurements and average Dpar values, are constantly reviewed throughout the course.

Experience learned from this laboratory-based routine is a powerful tool for efficiently and comfortably examining data quality and evaluating source(s) of analytical discrepancy between analysts. The digital training module is a streamlined version of the laboratory-based practice. It allows trainees remote access to exercises for steps (2) to (5) above through analysing a series of selected grain and length image sets using FastTracks. Steps  (3) and (4) require specialized laboratory equipment but can be practised by providing pre-captured images of 6 ‘unknown’ samples with a mixture of many suitable and unsuitable grains from which an appropriate selection of grains for analysis can be made.  Similarly, provision of LA-ICP-MS data for the grains enables calculation of fission track ages from the analysed grains. In addition, a sub-module allows EDM users calculating their own user-specific zeta-calibration through analysis of images of apatite-mica sample pairs and co-irradiated external detectors from standard glasses. Together with proposed analytical solutions and a list of recommended reading list, trainees are able to evaluate their progress by comparing their data with solution files on a grain-by-grain and track-by-track basis. Images of two samples used in previous inter-laboratory studies (Ketcham et al., 2015 & Ketcham et al., 2018) are part of the exercise for data quality evaluation. As the training package is cloud-stored, researchers, who do not have access to suitable facility to conduct full suite of analysis or cannot physically travel for training due to various restrictions, can become trained without face-to-face tutelage or specialised equipment. 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 content and usability.

Development of the digital training module 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.

References
Ketcham, R. A., Carter, A., & Hurford, A. J. (2015). Inter‐laboratory comparison of fission track confined length and etch figure measurements in apatite. American Mineralogist, 100 (7), 1452– 1468. https://doi.org/10.2138/am‐2015‐5167
Ketcham, R. A., van der Beek, P., Barbarand, J., Bernet, M., & Gautheron, C. (2018). Reproducibility of thermal history reconstruction from apatite fission‐track and (U‐Th)/He data. Geochemistry, Geophysics, Geosystems, 19, 2411– 2436.  https://doi.org/10.1029/2018GC007555