Archive for Virtual Thermo2020/1
Starting in the fall of 2020, the Organizing Committee for the Thermo2020/2021 meeting began hosting an interim virtual talk and discussion series to promote early career scientists and students, and provide opportunities to share our science.
Talk and discussion events will were ~one hour long, and held at times to allow participation in two of three geographical regions (Americas, Asia/Pacific, Europe) simultaneously.
Talks are 15 minutes, with 5-minute discussion periods afterward. Questions and comments could be entered via chat during the talks, and via chat or live during the discussion periods.
Discussions were intended to generate active dialogue and interchange on areas of interest to the thermochronology community. Participants were encouraged to read the preparatory materials and come with questions, critiques, and ideas. The format was a brief introduction followed by a moderated open floor.
To date there have been seven sessions. Recordings of all talks, include Q&A perriods and chat logs, are provided here.
We thank all who participated by giving talks and tuning in and participating, and especially our student moderators, Megan Flansburg, Cat Ross, and Peter Martin.
Session 1: Tuesday November 10, 2020, 10:00 AM (CST); 5:00 PM (CET)
|Remi Rateau||Trinity College, Dublin||A geo-thermochronology study (LAFT-U/Pb and AHe) offshore West of Ireland – Age and exhumation of the North Porcupine High||Video|
|Ian Hillenbrand||University of Massachusetts, Amherst||The rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in New England, USA||Video|
|Audrey Margirier||GFZ Potsdam||New constraints on the timing of Carnegie Ridge subduction from low-temperature thermochronology (Ecuadorian Andes)||Video|
Session 2: Tuesday November 10, 2020, 5:00 PM (CST); Wednesday November 11, 10:00 AM (AEDT)
|Honcheng Guo||Lehigh University||Using continuous ramped heating to assess dispersion in (U-Th)/He ages from Transantarctic Mountain apatites||Video|
|Murat Tamer||University of Texas at Austin||On step-etching experiments on fission tracks in apatite and effective track etch times||Video|
|Rich Ketcham||University of Texas at Austin||TOPIC: Quantifying fission-track etching, and how it affects efficiency and reproducibility.|
Session 3: Tuesday December 15, 2020, 10:00 AM (CST); 5:00 PM (CET)
|Birk Härtel||TU Bergakademie Freiberg||Zircon Raman dating: Age calculation and closure temperature(s)||Video|
|Chris Mark||University College Dublin||Isotopic forward modelling: A new tool to interpret U-Pb thermochronometry data||Video|
|Catherine Ross||University of Texas at Austin||Zircon (U-Th)/He impact crater thermochronometry and the effects of shock microstructures on helium diffusion kinetics||Video|
Session 4: Tuesday December 15, 2020, 5:00 PM (CST); Wednesday December 16, 10:00 AM (AEDT)
|Li Yang||China University of Geosciences, Beijing||Application of fission track thermochronology to porphyry belts in Eastern Kunlun Mountains, Qinghai-Tibet Plateau||Video|
|Samuel Boone||University of Melbourne||A global platform solution for big data in low-temperature thermochronology: AusGeochem and LithoSurfer||Video|
|Ling Chung||University of Melbourne||The FastTracks digital fission-track analysis training module||Video|
|Murat Tamer||University of Texas at Austin||Proposal for a new inter-laboratory experiment||Video|
Session 5: Tuesday February 23, 2021, 10:00 AM (CST); 5:00 PM (CET)
|Daan Beelen||Colorado School of Mines||Evidence for a Late Cretaceous impact structure in Black Mesa, Arizona||Video|
|Rachel Hoar||Texas A&M University||High-resolution fission track analysis of detrital apatite grains grouped using UPb ages and REE concentrations||Video|
|Andrew Donelick||Gone Fission Mining LLC||A mobile fission track microscope system for STEM education in remote communities||Video|
Session 6: Tuesday March 23, 10:00 AM (CDT); 4:00 PM (CET)
|Mark Coleman||University of Ottawa||Hanging our interpretations on the footwall? Thermochronology of the hanging wall of a metamorphic core complex||Video|
|Chiara Amadori||University of Pavia||High geothermal gradient in the Alps-Apennine transition zone induced by mantle upwelling||Video|
|Alexandra A. DiMonte||Utah State University||Hematite (U-Th)/He thermochronometry and microstructural analysis constrain past slow slip events adjacent to the southern San Andreas Fault, California||Video|
Session 7: Tuesday April 27, 10:00 AM (CDT); 5:00 PM (CET)
|Nicolás Pérez-Consuegra||Syracuse University||Late Miocene–Pliocene onset of fluvial incision of the Cauca River Canyon in the Northern Andes||Video|
|Megan Flansburg||University of Texas at Austin||Coeval Miocene exhumation of the Cycladic Basement and the Cycladic Blueschist Unit in the southern Cyclades (Ios and Sikinos Islands, Greece)||Video|
|Peter E. Martin||University of Colorado||Evaluating (U-Th-Sm)/He data variability of the Durango apatite standard using a Monte Carlo error modeling approach||Video|
November 10, 2020, Session 1
A geo-thermochronology study (LAFT-U/Pb and AHe) offshore West of Ireland – Age and exhumation of the North Porcupine High
Rémi Rateau1*, David Chew1
1 Department of Geology, Trinity College Dublin – Irish Centre for Research in Applied Geosciences
The offshore basins and basement highs west of Ireland are part of the northeast Atlantic Margin and are the result of the protracted break-up of Pangea and opening of the North Atlantic during the Mesozoic to Cenozoic. We acquired new geo-thermochronological data (dual-dating LAFT-U/Pb, AHe and zircon U/Pb) from borehole cuttings and seabed dredges and modelled both new and legacy data (QTQt) to better understand the thermal history and evolution of this part of the margin.
One area of interest is the Northern Porcupine High (NPH) which is a source of sediments for both the hyper-extended Porcupine Basin and Rockall Basin. The new results show that protracted exhumation of the high was associated with pulsed Mesozoic rifting (1 to 3 km of total erosion in total) and that the SE margin of the NPH started to be buried during the Early Cretaceous while the summit was still being exhumed and intruded by rift-related magmatism with an associated geothermal gradient increase. However, the new data fail to constrain a base Eocene exhumation event (visible on seismic) because of conflicting track length and AHe ages and the presence of coeval magmatism that may have affected some samples.
Zircon and apatite LA-ICP-MS U/Pb dating of basement samples on the SE margin of the NPH revealed Neoproterozoic (Lower Dalradian Supergroup) metasediments affected by a Late Caledonian thermal event. This new basement geochronology data, combined with previous published ages for the basement samples at the summit of the NPH, confirms that the low magnetic anomaly associated with the NPH represents Dalradian basement and that the sharp and straight magnetic boundary to the south probably represents the offshore extension of the Fair Head-Clew Bay lineament (the Irish extension of the Scottish Highland Boundary Fault, a major Caledonian suture).
The rise and fall of the Acadian altiplano: Evidence for a Paleozoic orogenic plateau in New England, USA
Ian Hillebrand1*, Michael Williams1, Cong Li1, Haiying Gao1
1 Department of Geosciences, University of Massachusetts at Amherst
High elevation orogenic plateaus are formed by a complex interplay of deep and surficial processes and influence a variety of Earth systems. However, limited exposures of plateau mid-crust are presently recognized, hindering understanding of these deeper processes. We present evidence for the existence of an orogenic plateau during and after the Devonian Acadian orogeny whose mid-crustal roots are exposed in the New England Appalachians. The four-dimensional crustal evolution of this paleo-plateau is constrained by the integration of new petrologic and geochronologic databases with geophysical imaging. Doubly thickened crust, widespread amphibolite to granulite-facies metamorphic conditions, a paleo-isobaric surface, and protracted mid-crustal anatexis indicate the development of a high elevation, low relief plateau by 380 Ma.40Ar/39Ar thermochronology shows a distinct thermochronologic signature with very slow cooling rates of 2-4°C/m.y. following peak metamorphic conditions. Thermochronologic data, trace element and Nd isotope geochemistry, and monazite petrochronology suggest a 50 m.y. lifespan of the plateau. Orogen parallel ductile flow and extrusion of gneiss domes resulted in plateau collapse, crustal thinning, and block-like exhumation at ca. 330-310 Ma. Thinning of the plateau crust may have led to the sharp 12-15 km step in Moho depth in western New England, possibly by reactivating the suture between Laurentia and accreted Gondwanan-derived terranes.
New constraints on the timing of Carnegie Ridge subduction from low-temperature thermochronology (Ecuadorian Andes)
Audrey Margirier1,2*, Manfred Strecker1, Peter Reiners3, Ismael Casado1, Stuart Thompson3, Sarah W.M. George3, Alexandra Alvarado4
1 Institut für Erd-und Umweltwissenschaften, Universität Potsdam, Potsdam, Germany
2 Helmholtz Centre Potsdam, GFZ German Research Center for Geosciences, Potsdam, Germany
3 Department of Geosciences, University of Arizona, Tucson, United States of America
4 Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador
The Cenozoic growth of the Andes has been strongly influenced by the subduction dynamics and the superposed effects of climate. Recent studies proposed that the Carnegie ridge subduction controlled the late Cenozoic tectonic activity in Ecuador with the formation of a crustal sliver escaping northward; however, the timing of the arrival of the ridge and its effect on topographic growth remain unclear. We provided new thermochronological data from the Western Cordillera to evaluate the possible role of ridge subduction in prompting the growth of the Ecuadorian Andes and to pin point the timing of the ridge subduction. Time-temperature inverse modeling of this thermochronological dataset evidenced two cooling phases. A first one following intrusions emplacement in the Western Cordillera and a second one initiating at ~6 Ma. We interpret the first cooling phase to be associated with magmatic cooling. The second cooling phase, synchronous with the last cooling phase evidenced in the Eastern Cordillera, is likely to be associated with exhumation of the Western Cordillera. Based on our thermal modeling and geological cross-sections we propose that recent crustal shortening and rock uplift triggered exhumation of Ecuadorian Andes at ~6 Ma. We suggest that the onset of Carnegie Ridge subduction at ~6 Ma increased the coupling at the subduction interface and promoted shortening and rock uplift.
November 10, 2020, Session 2
Using continuous ramped heating to assess dispersion in (U-Th)/He ages from Transantarctic Mountain apatites
Hongcheng Guo1*, Peter K. Zeitler1, Bruce D. Idleman1, Annia K. Fayon2, Paul G. Fitzgerald3
1 Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015
2 Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455
3 Department of Earth Sciences, Syracuse University, Syracuse, NY 13210
We performed continuous ramped heating (CRH) on apatite suites from the Transantarctic Mountains (TAM) that are known to have significant intra-sample dispersion in He ages. Examining > 100 apatite grains from a total of six samples, we confirmed prior results showing that measured AHe ages have at least three-fold intra-sample dispersion with no obvious relationships between ages and effective uranium (eU) concentration or grain size.
Apatites showing simple volume diffusion behavior, characterized by unimodal incremental 4He gas-release curves, have younger He ages. In contrast, older grains generally show anomalous gas release in the form of sharp spikes and / or extended gas-release at high temperatures (i.e., >= 800 °C). Considerable age dispersion still remains in well-behaved apatites and exceeds what grain size, radiation damage, and analytical uncertainty can explain, but this dispersion appears to be related to variations in 4He diffusion kinetics. The screened AHe ages from well-behaved apatites together with kinetic information obtained from these grains suggest that the sampled region experienced slow exhumation prior to rapid exhumation beginning ca. ~35 Ma, an interpretation consistent with studies suggesting that increasing incision rates at this time was related to initiation of glaciation and the Eocene-Oligocene climate transition. We attempted to correct the anomalous apatite ages by simply removing anomalous gas-release components (spikes and high-temperature release), but this approach yields some ages that are too young for the samples’ geologic setting, suggesting that the factors that lead to anomalous laboratory release behavior can impact both the expected radiogenic component as well as those that are anomalous.
From our observations we conclude that: (1) CRH screening can serve as a routine tool for AHe dating and offers opportunities to reveal first-order kinetic variations; (2) model-dependent age correction might still be possible but would require some means of estimating the broad proportions of 4He components incorporated into grains before and after cooling, and (3) interpretation of highly dispersed AHe ages will require assessment of individual-grain diffusion kinetics beyond that predicted by radiation-damage models. We further infer that many apatite grains will contain imperfections of varying kinds that contribute significantly to kinetic variability beyond that associated with radiation damage.
Step-etch experiments on fission tracks in apatite: How much has been done and where are we going?
Murat T. Tamer1*, Richard A. Ketcham1
1 Jackson School of Geosciences, University of Texas at Austin
In step-etch experiments, fission tracks are etched and confined track lengths measured in a series of steps. For apatite, step-etch experiments were used to create the standard single-step etch protocols around three decades ago based the concept of maximum etchable length, by monitoring the increase of mean track lengths to reach an optimum etch time. In these early step-etch studies, different sets of tracks were measured in each etch step, due to practical limitations. Recent developments and innovations in hardware and software for fission track analysis have enabled a new generation of step-etch studies, where individual tracks are monitored throughout an experiment, instead of random track selection in each etching step. These have provided new insights affecting the fundamentals of fission-track dating and modelling, such as the need to revise the linear track etch model, a difference in etching behavior between fossil and induced tracks, and the failure to reach the maximum etchable length over the prescribed etch time in the current standard single-step etch protocols.
We conducted a new series of step-etch experiments on unannealed induced, lab-annealed induced, and fossil tracks in Durango apatite to decipher along-track etch rates (VT(x)). Additional etch-anneal-etch (EAE) experiments aimed to determine the bulk etch rate (VB). We estimate the average VB as ~0.02 µm/s in Durango apatite, and 98% of EAE experiment tracks had etching velocities under 0.06 µm/s. Generally, etch rates along unannealed tracks did not fall to VB until the 25-30s etching step, even when considering only tracks that were visible within the first 10 s of etching. Using 0.06 µm/s as a threshold for under-etching, ~78% of induced and ~53% of fossil tracks in our data were under etched in Durango apatite using the 5.5M 20s 21°C etching protocol, once again considering only tracks that were visible after 10 s. This procedure eliminates analyst bias in track selection, as it only considers a group of tracks that have etched for well over 10 seconds, resulting in an overall increase in mean length. Analyst selection (bias) by normal measurement after 20 s lowers this number to ~60% for induced tracks. We expect that different analysts with different track selection criteria would get different values, and that the combination of under-etching and variable criteria underlies the poor level of community reproducibility of these measurements. A quantitative understanding of along-track etching, based on data such as this study, can provide an avenue to addressing this important problem, and optimizing etching procedures.
TOPIC: Quantifying fission-track etching, and how it affects efficiency and reproducibility.
Led by: Rich Ketcham, Jackson School of Geosciences, University of Texas at Austin
Why do we etch apatite fission tracks the way we do? Why do different lab groups etch with different recipes? Does it even matter, if labs using the same etching get different results? What are the advantages we seek, and what are the penalties we suffer?
A contribution, now in its public review period at https://gchron.copernicus.org/preprints/gchron-2020-31/, proposes that differences in how analysts select tracks to measure is actually more important for reproducibility than precisely how we etch. However, it also proposes we don’t etch as much as we should, and that etching can be optimized to reveal more and better-defined confined tracks, while still retaining a quantitative link to experimental annealing data sets used for modeling. Is this credible?
There will be a short presentation to kick things off, and then the floor will be open for discussion. The more different perspectives we get, the better!
December 15, 2020, Session 3
Zircon Raman dating: Age calculation and closure temperature(s)
Birk Härtel1*, Raymond Jonckheere1, Bastian Wauschkuhn1, Lothar Ratschbacher1
1 Institute of Geology, TU Bergakademie Freiberg
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.
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.
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).
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
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
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
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
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, rew Gleadow,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
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 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 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 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.
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
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.
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
February 16, 2021, Session 5
Evidence for a Late Cretaceous impact structure in Black Mesa, Arizona
Daan Beelen1*, Ray Donelick2
1 SAnD Consortium, Colorado School of Mines, 1500 Illinois St, Golden, Colorado, USA
2 Apatite.com Partners LLC, 1075 Matson Road, Viola, Idaho, USA
This study presents evidence for a hitherto undiscovered impact structure in Black Mesa, NE Arizona. The structure’s macrostructure is mapped on satellite data as a series of concentric catchments and landscape geometries approximately 7 kilometers in diameter, with upward tilting strata dipping from the center of the structure. Subsequent field research has yielded observations of meso-scale impact structures in the form of melt breccia’s and titled blocks with abundant pseudotachylite melt. Partially molten and shocked zircon grains inside the molten breccias were identified using a SEM, and X-ray diffraction indicates elevated amounts of ultra-refractory elements within the same samples. The structure lies within strata belonging to the Upper Cretaceous Toreva Member and Wepo Members, Mesa Verde Group, Upper Cretaceous.
Apatite fission track (AFT) data were measured for two samples showing evidence of partial melting. Apatite grains – and their associated fission track data – were sorted into sub-groups using rare earth element concentrations and uranium-lead ages. The pooled AFT ages for all sub-groups studied are shown in the figure below.
Seven apatite grain sub-groups are common to both samples studied. For sample P10014_004, apatite grain sub-groups 2A and 4A yield pooled AFT ages of 70.6 -12.4/+15.1 Ma (95% CI; 14 grains) and 63.2 -8.6/+9.9 (95% CI; 30 grains), respectively, and a weighted mean age of 65.5 ± 7.7 Ma (2σ). The evidence presented here insinuates to the presence of the largest impact structure in the state of Arizona and may have implications to the Cretaceous–Paleogene mass extinction, although more work is necessary to prove these theories.
Field work on the Navajo Nation was conducted under a permit from the Navajo Nation Minerals Department. Any person(s) wishing to conduct geologic investigations on the Navajo Nation must first apply for and receive a permit from the Navajo Nation Minerals Department, P.O. Box 1910, Window Rock, Arizona 86515 and Telephone No. +1 (928) 871-6587
High-resolution fission track analysis of detrital apatite grains grouped using UPb ages and REE concentrations
Ray Donelick1, Cleber Soares2, Liu Zhaoqian3, Rachel Hoar4,*, Daan Beelen5, Andrew Donelick6, and Richard Carlton7
1 Apatite.com Partners LLC, 1075 Matson Road, Viola, Idaho, USA
2 Chronuscamp Research, Rua João Rodrigues dos Santos 60, Itapira, São Paulo, Brazil
3 Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan 430074, China
4 Geology & Geophysics, Texas A&M University, College Station, Texas, USA
5 SAnD Consortium, Colorado School of Mines, 1500 Illinois St, Golden, Colorado, USA
6 Gone Fission Mining LLC, 238 E 5th Street, Walsenburg, Colorado, USA
7 Navajo Nation Minerals Department, PO Box 1910, Window Rock, Arizona, USA
O’Sullivan et al. (2018, G3, v. 19, no. 9; 2020, Earth-Science Reviews, v. 201, no. 103044) demonstrate how UPb age and rare earth element (REE) concentrations can provide useful genetic and provenance information for detrital apatite grains. Here, we use UPb age, REE concentrations, and parameter Rmr0 to bin apatite grains – and their associated fission track data – into kinetic populations for thermal history modeling. Each UPb age is based on a single LA-QICP-MS spot analysis with an assumed common-Pb component (after Chew and Donelick, 2012, MAC Short Course, v. 42). The REEs are C1-chondrite normalized using – as a matrix-matched standard – the same Durango crystal studied by Chew et al. (2016, Chemical Geology, v. 435). Kinetic parameter Rmr0 is determined using LA-QICP-MS data (Na, Mg, Al, Si, P, S, Cl, Ca, Mn, Fe, As, Br Y, 14 REEs, Hg, Pb, Th, U), assuming only chlorine (measured after Chew et al. 2014, Geostandards and Geoanalytical Research, v. 38; US Patent 8,901,485) and fluorine (calculated from chlorine) in the halogen site.
Conventional AFT analysis uses measured values such as Dpar (US Patent 5,267,274) or elemental compositions (e.g., chlorine concentration from EPMA) to distribute apatite fission track data into kinetic populations. The protocol used here offers a higher degree of resolution of the annealing kinetics, because the apatite fission track data are first distributed into provenance groups defined by UPb age and REE concentrations. Within these provenance groups, apatite fission track data can be further distributed into kinetic populations using Rmr0, much like the conventional analysis.
This work was funded in part by The National Science and Technology Major Project of China (No.2016ZX05035-001-003 and No.2016ZX05002-006-007) and National Natural Science Foundation of China (No.41302112).
A mobile fission track microscope system for STEM education in remote communities
Ray Donelick1, Andrew Donelick2,*, Cleber Soares3, Tais Fontes Pinto4, Zachary Dodds5, Daan Beelen6, Murat Tamer7
1 Apatite.com Partners LLC, 1075 Matson Road, Viola, Idaho, USA
2 Gone Fission Mining LLC, 238 E 5th Street, Walsenburg, Colorado, USA
3 Chronuscamp Research, Rua João Rodrigues dos Santos 60, Itapira, São Paulo, Brazil
4 Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil
5 Computer Science Department, Harvey Mudd College, Claremont, California, USA
6 SAnD Consortium, Colorado School of Mines, 1500 Illinois St, Golden, Colorado, USA
7 Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA
We tested and demonstrated components of a mobile fission track microscope system, at American Associate of Petroleum Geologists, Annual Conference and Exhibition, in San Antonio, Texas during May 2019. The microscope system and the technology behind it can be hauled anywhere – such as from Idaho to our AAPG exhibitor’s booth in Texas – using a personal vehicle and requires simple climate-controlled space with electricity.
Fission track thermochronology – its calibration and its application – will benefit from datasets comprised of 1000s of datum points per sample. We envision employing young students (in USA, Grades 6-12 for example) to collect the bulk of these data for select interesting samples using our mobile fission track microscope system. We believe this system can be used to excite and educate young students in a range of science, technology, engineering, and mathematics (STEM) disciplines. The young students can do the heavy lifting of finding and initially characterizing mineral grains for fission track analysis using various AI tools. Results from the young students can be vetted by digitally connected and collaborating professional scientists. This mobile fission track laboratory enables distance learning with a personal touch while applying state-of-the-art science in collaboration with an international network of passionate and skilled scientists.
We envision the following value propositions:
- For young students
- introduction to STEM using state-of-the-art science over a range of subjects: natural sciences from isotope geology to tectonics to meteorites, chemistry and physics, computer science
- hands-on experience in a true ‘peer laboratory’ on par with any advanced laboratory anywhere
March 23, 2021, Session 6
Hanging our interpretations on the footwall? Thermochronology of the hanging wall of a metamorphic core complex
Mark Coleman1*, David Schneider2, Konstantinos Soukis2, Bernhard Grasemann3
1 University of Ottawa
2 National and Kapodistrian University of Athens
3 University of Vienna
Within the Attic-Cycladic Complex of Greece, low-angle detachments have accommodated significant extension since the Oligo-Miocene. We present zircon (U-Th)/He thermochronology from the footwall and hanging wall of the northernmost metamorphic core complex within the Cyclades. Bedrock mapping of Mt. Hymittos identified a paired ductile-then-brittle detachment system with top-to-SSW kinematics. The detachments are ~500 metres apart and divide the local tectonostratigraphy into three packages: prehnite-pumpellyite facies phyllites and marbles in the uppermost package (hanging wall; Upper Unit), and high-pressure greenschist-facies schists and marbles in the lower two packages (footwalls; Middle & Lower units). Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores indicating cooling during extension. Geochronology on samples from the Middle & Lower units produce late Oligocene to early Miocene white mica 40Ar/39Ar ages and mid-to-late Miocene zircon (U-Th)/He ages, suggesting synchroneity of the detachments. Dates from the Upper Unit are more dispersed: white mica 40Ar/39Ar ages range from the Early to Late Cretaceous, whereas zircon (U-Th)/He ages span the Early Cretaceous through the early Oligocene with eU values of 53 to 1084 ppm. Since the footwalls exhumed at relatively high rates, cooling was rapid and residence within the partial retention zone was short. In contrast, forward and inverse modelling of dispersed ages in the Upper Unit can produce significant insight. Forward modelling confirms that some hanging wall samples preserve a detrital population of zircon (U-Th)/He ages, but an overall positive then negative age-eU correlation within the data suggests a discernible thermal signature. Inverse modelling indicates that the majority of the hanging wall experienced a greenschist facies metamorphic event that partially to fully reset most samples, correlated to a regionally recognized Late Cretaceous greenschist to amphibolite facies metamorphic event elsewhere within this tectonic unit. The tectonic unit subsequently became the upper plate of the closure of an oceanic basin during a c. 55 Ma high-pressure metamorphic event and the Upper Unit remained sufficiently cool such that the high-pressure event was not recorded in the zircon (U-Th)/He data. Finally, unmetamorphosed sedimentary samples from late Miocene to Pliocene basins within the hanging wall, overlying the Upper Unit, exhibit similar zircon (U-Th)/He ages and models, supporting the conclusion that sedimentation into these basins was locally derived and occurred as a result of uplift and denudation of the Upper Unit perhaps driven by upward flexure of the metamorphic dome in the late Miocene.
High geothermal gradient in the Alps-Apennine transition zone induced by mantle upwelling
C. Amadori1*, M. Maino1,2,M. Marini3, L. Casini 4, A. Langone2, S. Reguzzi3, B. Carrapa5 and A. Di Giulio1
1 University of Pavia, Department of Earth and Environmental Sciences, Pavia, Italy
2 IGG-CNR U.O.S. – Pavia, Italy
3 University of Milan, Department of Earth Science ‘A. Desio’, Milan, Italy
4 University of Sassari, Department of Chemistry and Pharmacy, Sassari, Italy
5 University of Arizona, Geosciences Department, AZ, USA
The Tertiary Piedmont Basin (TPB) is an episutural basin developed since the Late Eocene at the Alps–Apennines transition zone. During Late Eocene-Miocene time the basin was filled by clastic deposits in response to erosion related to the late stages of Alpine collision and progressive NE-migration of the Apenninic orogenic front. The opening of the Liguro-Provençal Basin driving the anticlockwise drift of the Corsica-Sardinia block marginally affected the basin. Here, we present the results of combined detrital apatite low-T thermochronology (fission-track and (U-Th-Sm)/He dating), zircon U-Pb dating, burial history and 2D numerical modeling in order to better constrain the tectonic-thermal history of the region.
Results show that detrital apatite in Late Priabonian deposits (~34 Ma) experienced fission-track thermal reset at ~25 Ma, similarly to the Ligurian Alps. Conversely, apatite grains in Early Rupelian strata (~32) Ma are only partially annealed, and Late Rupelian-early Miocene sediments (28-16 Ma) have no traces of annealing.
These observations show that the bottom of the TPB sequence and its Alpine basement experienced >110 °C heating before the Miocene. In the eastern sector of the basin, the restored maximum sedimentary thickness of less than 3 km indicates an elevated geothermal gradient of about 45°C/Km to account for this result.
Different mechanisms are tested to explain this high heat flow, anomalous for a thickened orogenic lithosphere, and mantle upwelling (linked to Apennine slab rollback and/or Alpine slab break-off), even coupled with exhumation, is the scenario that better explains the data.
Hematite (U-Th)/He thermochronometry and microstructural analysis constrain past slow slip events adjacent to the southern San Andreas Fault, California
Alexandra A. DiMonte1*, Alexis K. Ault1, Kelly K. Bradbury1, Greg Hirth2
1 Dept. of Geosciences, Utah State University, Logan, UT
2 Dept. of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI
Shallow, transient slow slip events are observed via geodesy along major fault segments worldwide. Slow slip events are an important part of the earthquake cycle, but the influence of mineralogy on the mechanics of shallow slow slip is poorly constrained. Outcrop to nanoscale structures and mineral textures observed in exhumed faults record evidence of varying slip rates in the past and may inform ongoing fault deformation mechanisms at depth. Hematite, a common mineral that precipitates in shallow faults, exhibits textures potentially diagnostic of prior slip rate and is amenable to (U-Th)/He thermochronometry. We report field and scanning electron microscopy (SEM) observations of structures and textures with hematite (U-Th)/He dates from minor fault surfaces in the exhumed basement damage zone of the Painted Canyon fault zone (PCF) in Mecca Hills, CA, adjacent and parallel to the southern San Andreas fault, to evaluate the rock record of slow slip events. Hematite slip surfaces are pervasive within the PCF damage zone in association with ‘green schist’ or minor clay-rich faults. SEM reveals multiple hematite morphologies including ubiquitous high-aspect ratio plates with rounded or serrated grain boundaries, as well as stubby and euhedral, hexagonal plates. The absence of recrystallization textures likely precludes high temperatures generated along faults due to frictional heating during earthquakes. Hematite-filled injection veins formed during fluid overpressure events; interlayered hematite, calcite, and phyllosilicate; S-C fabric; and reworked clasts of hematite imply multiple periods of precipitation and deformation at subseismic velocities. New hematite (U-Th)/He dates (n = 37) expand previously published apatite and hematite (U-Th)/He thermochronometry datasets (Moser et al., 2017; Spotila et al., 2020). Data document PCF exhumation in the upper 2 km from ~2.2 to 0.8 Ma and superimposed fluid flow and hematite mineralization events between ~0.8 to 0.5 Ma. Together, these observations support repeated episodes of hematite precipitation and subsequent deformation during shallow slow slip events
April 27, 2021, Session 7
Late Miocene–Pliocene onset of fluvial incision of the Cauca River Canyon in the Northern Andes
N. Pérez-Consuegra1*, G. D. Hoke1, P. Fitzgerald1, A. Mora2, C. Montes3, E. R. Sobel4, J. Glodny5
1 Department of Earth and Environmental Sciences, Syracuse University, Syracuse, NY, USA
2 Ecopetrol Brasil, Rio de Janeiro, Brazil
3 Departamento de Física y Geociencias, Universidad del Norte, Barranquilla, Colombia
4 Institut für Geowissenschaften, Universität Potsdam, Potsdam-Golm, Germany
5 GFZ German Research Centre for Geosciences, Potsdam, Germany
The incision of kilometer-scale canyons into high-standing topography is often used to constrain the surface uplift history of mountain ranges, controlled by tectonic and geodynamic processes. However, changes in climate, such as orographic enhancement of precipitation during mountain growth or periods of time with increased rates of precipitation, may also be responsible for canyon incision. This study deciphers the timing and the role of tectonic/climatic mechanisms on the incision of the ~2.5 km deep Cauca River Canyon in the Central Cordillera of the Northern Andes using the cooling (exhumation) history of rocks from the canyon walls. Ten bedrock samples and one detrital sample were collected on the eastern border of the canyon between 300 m and 2,300 m elevation. Bedrock and detrital AFT data yield ages from 38 to 50 Ma, while two bedrock AHe ages from the valley bottom yield ages of 6–7 Ma. The AHe cooling ages and inverse thermal history models reveal a previously unidentified Late Miocene (ca. 6–7 Ma) pulse of exhumation which corresponds to the onset of carving of the Cauca River Canyon. The absence of younger AFT ages in the Cauca River Canyon demonstrates that the magnitude exhumation during canyon incision was insufficient to reveal younger AFT cooling ages below an exhumed Partial Annealing Zone. The Cauca Canyon was carved because of increased rock uplift rates in the northern Central Cordillera and headward propagation of an erosion wave into the mountain range.
Coeval Miocene exhumation of the Cycladic Basement and the Cycladic Blueschist Unit in the southern Cyclades (Ios and Sikinos Islands, Greece)
Megan E. Flansburg1*, Eirini M. Poulaki1, Daniel F. Stockli1, Konstantinos Soukis2
1 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, TX, USA
2 National and Kapodistrian University of Athens, Greece
The North Cycladic, West Cycladic, Naxos-Paros, and Santorini detachment systems accommodated large-scale Oligo-Miocene exhumation and formation of metamorphic core complexes in the backarc of the retreating Hellenic subduction zone. While these detachment systems are either contained within the Cycladic Blueschist Unit (CBU) or at the CBU-Upper Unit interface, the sheared contact between the CBU and the underlying Cycladic Basement (CB) in the southern Cyclades has been debated for over 30 years, due to the coexistence of top-N and top-S shear sense indicators and dispersed ages from previous thermochronology. New zircon and apatite (U-Th)/He ages and HeFTy time-temperature pathways for the CB and CBU on Ios and Sikinos islands show that both units cooled through these low temperature regimes from ~14-6 Ma suggesting that the units were juxtaposed prior to and exhumed together during bivergent Miocene top-S (Santorini) and top-N (Naxos-Paros) detachment faulting not directly exposed on Ios or Sikinos islands.
Evaluating (U-Th-Sm)/He data variability of the Durango apatite standard using a Monte Carlo error modeling approach
Peter E. Martin1*, James R. Metcalf1, Rebecca M. Flowers1
1 Department of Geological Sciences, University of Colorado Boulder
(U-Th-Sm)/He data commonly exhibit intra-sample variability beyond that predicted by analytical uncertainty. Possible contributions to data dispersion include varying diffusion kinetics (e.g., grain size, radiation damage), alpha particle redistribution (implantation, parent nuclide zonation), and parent nuclide-rich inclusions. A complete understanding of intra-sample variability is reliant on quantification of its exact extent, which requires a careful accounting of analytical uncertainty in (U-Th-Sm)/He data. However, derivation of the uncertainty in (U-Th-Sm)/He dates is not straightforward owing to the fact that the helium age equation lacks an analytical solution. We therefore developed a Monte Carlo error propagation code to derive uncertainty in (U-Th-Sm)/He data, which serves the dual purpose of permitting highly accurate uncertainty quantification and direct comparison to analytical methods of uncertainty propagation. These analyses demonstrate that the Monte Carlo and analytical methods of deriving uncertainty are nearly indistinguishable. Uncertainty generated using the Monte Carlo method tends to be very slightly larger (a difference of ~0.1% in relative standard error) and exhibits very minor skew in the probability distribution. These differences are so slight that the more computationally efficient analytical error propagation methods are acceptable in nearly all cases.
We apply these uncertainty derivation methods to archived Durango apatite data from the University of Colorado Thermochronology Research and Instrumentation Laboratory (CU TRaIL). This de facto standard exhibits few of the above physical characteristics that potentially cause intra-sample variability, and the CU TRaIL has a large amount of archived Durango (U-Th)/He results. We analyze a subset of these data (n=234) acquired in a three year period following acquisition of new instrumentation in September of 2017. We find that Durango apatite exhibits intra-sample variability (standard deviation of ±2.6 Ma) beyond that predicted from analytical uncertainty alone (±0.6 Ma, 1σ). However, the shape of the histogram of data is distinctly non-gaussian, suggesting that a kernel density estimate (KDE) is a more accurate descriptor of the true data distribution. The KDE has a 68% confidence interval of [+1.8, -2.2], implying a smaller degree of overdispersion and reflecting a shoulder to the low side of the peak. The KDE also includes relatively long tails, suggesting that the large standard deviation of the data is a result of extreme values. This overall shape is consistent with prior modeling in the literature describing the effects of parent nuclide zonation in Durango apatite, suggesting that such zonation is the cause of overdispersion in these data.