Pub Date : 2023-04-03DOI: 10.5194/gchron-5-127-2023
A. McKanna, Isabel Koran, B. Schoene, R. Ketcham
Abstract. Chemical abrasion is a technique that combines thermal annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U–Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. Acid frequently reaches crystal interiors via fractures spatially associated with radiation damage zoning and inclusions to dissolve soluble high-U zones, some inclusions, and material around fractures, leaving behind a more crystalline zircon residue. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically controlled with dissolution proceeding almost exclusively along the c axis. Increasing the leaching temperature from 180 to 210 ∘C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. How a grain dissolves strongly depends on its initial radiation damage content and defect distribution as well as the size and position of inclusions. As such, the effectiveness of any chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely sample-dependent. We also briefly discuss the implications of our findings for deep-time (U-Th)/He thermochronology.
{"title":"Chemical abrasion: the mechanics of zircon dissolution","authors":"A. McKanna, Isabel Koran, B. Schoene, R. Ketcham","doi":"10.5194/gchron-5-127-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-127-2023","url":null,"abstract":"Abstract. Chemical abrasion is a technique that combines thermal annealing and partial\u0000dissolution in hydrofluoric acid (HF) to selectively remove\u0000radiation-damaged portions of zircon crystals prior to U–Pb isotopic\u0000analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope\u0000dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of\u0000zircon dissolution in HF and the impact of different leaching conditions on\u0000the zircon structure, however, are poorly resolved. We present a\u0000microstructural investigation that integrates microscale X-ray computed\u0000tomography (µCT), scanning electron microscopy, and Raman\u0000spectroscopy to evaluate zircon dissolution in HF. We show that µCT\u0000is an effective tool for imaging metamictization and complex dissolution\u0000networks in three dimensions. Acid frequently reaches crystal interiors via\u0000fractures spatially associated with radiation damage zoning and inclusions\u0000to dissolve soluble high-U zones, some inclusions, and material around\u0000fractures, leaving behind a more crystalline zircon residue. Other acid paths\u0000to crystal cores include the dissolution of surface-reaching inclusions and\u0000the percolation of acid across zones with high defect densities. In highly\u0000crystalline samples dissolution is crystallographically controlled with\u0000dissolution proceeding almost exclusively along the c axis. Increasing the\u0000leaching temperature from 180 to 210 ∘C results in\u0000deeper etching textures, wider acid paths, more complex internal dissolution\u0000networks, and greater volume losses. How a grain dissolves strongly depends\u0000on its initial radiation damage content and defect distribution as well as\u0000the size and position of inclusions. As such, the effectiveness of any\u0000chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely\u0000sample-dependent. We also briefly discuss the implications of our findings\u0000for deep-time (U-Th)/He thermochronology.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"106 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75663878","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-03-10DOI: 10.5194/gchron-5-109-2023
Michael C. Sitar, R. Leary
Abstract. Collecting grain measurements for large detrital zircon age datasets is a time-consuming task, but a growing number of studies suggest such data are essential to understanding the complex roles of grain size and morphology in grain transport and as indicators for grain provenance. We developed the colab_zirc_dims Python package to automate deep-learning-based segmentation and measurement of mineral grains from scaled images captured during laser ablation at facilities that use Chromium targeting software. The colab_zirc_dims package is implemented in a collection of highly interactive Jupyter notebooks that can be run either on a local computer or installation-free via Google Colab. These notebooks also provide additional functionalities for dataset preparation and for semi-automated grain segmentation and measurement using a simple graphical user interface. Our automated grain measurement algorithm approaches human measurement accuracy when applied to a manually measured n=5004 detrital zircon dataset. Errors and uncertainty related to variable grain exposure necessitate semi-automated measurement for production of publication-quality measurements, but we estimate that our semi-automated grain segmentation workflow will enable users to collect grain measurement datasets for large (n≥5000) applicable image datasets in under a day of work. We hope that the colab_zirc_dims toolset allows more researchers to augment their detrital geochronology datasets with grain measurements.
{"title":"Technical note: colab_zirc_dims: a Google Colab-compatible toolset for automated and semi-automated measurement of mineral grains in laser ablation–inductively coupled plasma–mass spectrometry images using deep learning models","authors":"Michael C. Sitar, R. Leary","doi":"10.5194/gchron-5-109-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-109-2023","url":null,"abstract":"Abstract. Collecting grain measurements for large detrital zircon age datasets is a\u0000time-consuming task, but a growing number of studies suggest such data are\u0000essential to understanding the complex roles of grain size and morphology in\u0000grain transport and as indicators for grain provenance. We developed the\u0000colab_zirc_dims Python package to automate\u0000deep-learning-based segmentation and measurement of mineral grains from\u0000scaled images captured during laser ablation at facilities that use Chromium\u0000targeting software. The colab_zirc_dims\u0000package is implemented in a collection of highly interactive Jupyter\u0000notebooks that can be run either on a local computer or installation-free\u0000via Google Colab. These notebooks also provide additional functionalities\u0000for dataset preparation and for semi-automated grain segmentation and\u0000measurement using a simple graphical user interface. Our automated grain\u0000measurement algorithm approaches human measurement accuracy when applied to\u0000a manually measured n=5004 detrital zircon dataset. Errors and\u0000uncertainty related to variable grain exposure necessitate semi-automated\u0000measurement for production of publication-quality measurements, but we\u0000estimate that our semi-automated grain segmentation workflow will enable\u0000users to collect grain measurement datasets for large (n≥5000)\u0000applicable image datasets in under a day of work. We hope that the\u0000colab_zirc_dims toolset allows more\u0000researchers to augment their detrital geochronology datasets with grain\u0000measurements.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89550127","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-02-07DOI: 10.5194/gchron-5-91-2023
Peter E. Martin, James R. Metcalf, Rebecca M. Flowers
Abstract. Although rigorous uncertainty reporting on (U–Th) / He dates is key for interpreting the expected distributions of dates within individual samples and for comparing dates generated by different labs, the methods and formulae for calculating single-grain uncertainty have never been fully described and published. Here we publish two procedures to derive (U–Th) / He single-grain date uncertainty (linear and Monte Carlo uncertainty propagation) based on input 4He, radionuclide, and isotope-specific FT (alpha-ejection correction) values and uncertainties. We also describe a newly released software package, HeCalc, that performs date calculation and uncertainty propagation for (U–Th) / He data. Propagating uncertainties in 4He and radionuclides using a compilation of real (U–Th) / He data (N=1978 apatites and 1753 zircons) reveals that the uncertainty budget in this dataset is dominated by uncertainty stemming from the radionuclides, yielding median relative uncertainty values of 2.9 % for apatite dates and 1.7 % for zircon dates (1 s equivalent). When uncertainties in FT of 2 % or 5 % are assumed and additionally propagated, the median relative uncertainty values increase to 3.5 % and 5.8 % for apatite dates and 2.6 % and 5.2 % for zircon dates. The potentially strong influence of FT on the uncertainty budget underscores the importance of ongoing efforts to better quantify and routinely propagate FT uncertainty into (U–Th) / He dates. Skew is generally positive and can be significant, with ∼ 17 % of apatite dates and ∼ 6 % of zircon dates in the data compilation characterized by skewness of 0.25 or greater assuming 2 % uncertainty in FT. This outcome indicates the value of applying Monte Carlo uncertainty propagation to identify samples with substantially asymmetric uncertainties that should be considered during data interpretation. The formulae published here and the associated HeCalc software can aid in more consistent and rigorous (U–Th) / He uncertainty reporting, which is also a key first step in quantifying whether multiple aliquots from a sample are over-dispersed, with dates that differ beyond what is expected from analytical and FT uncertainties.
{"title":"Calculation of uncertainty in the (U–Th) ∕ He system","authors":"Peter E. Martin, James R. Metcalf, Rebecca M. Flowers","doi":"10.5194/gchron-5-91-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-91-2023","url":null,"abstract":"Abstract. Although rigorous uncertainty reporting on (U–Th) / He dates is key for interpreting the expected distributions of dates within individual samples and for comparing dates generated by different labs, the methods and formulae for calculating single-grain uncertainty have never been fully described and published. Here we publish two procedures to derive (U–Th) / He single-grain date uncertainty (linear and Monte Carlo uncertainty propagation) based on input 4He, radionuclide, and isotope-specific FT (alpha-ejection correction) values and uncertainties. We also describe a newly released software package, HeCalc, that performs date calculation and uncertainty propagation for (U–Th) / He data. Propagating uncertainties in 4He and radionuclides using a compilation of real (U–Th) / He data (N=1978 apatites and 1753 zircons) reveals that the uncertainty budget in this dataset is dominated by uncertainty stemming from the radionuclides, yielding median relative uncertainty values of 2.9 % for apatite dates and 1.7 % for zircon dates (1 s equivalent). When uncertainties in FT of 2 % or 5 % are assumed and additionally propagated, the median relative uncertainty values increase to 3.5 % and 5.8 % for apatite dates and 2.6 % and 5.2 % for zircon dates. The potentially strong influence of FT on the uncertainty budget underscores the importance of ongoing efforts to better quantify and routinely propagate FT uncertainty into (U–Th) / He dates. Skew is generally positive and can be significant, with ∼ 17 % of apatite dates and ∼ 6 % of zircon dates in the data compilation characterized by skewness of 0.25 or greater assuming 2 % uncertainty in FT. This outcome indicates the value of applying Monte Carlo uncertainty propagation to identify samples with substantially asymmetric uncertainties that should be considered during data interpretation. The formulae published here and the associated HeCalc software can aid in more consistent and rigorous (U–Th) / He uncertainty reporting, which is also a key first step in quantifying whether multiple aliquots from a sample are over-dispersed, with dates that differ beyond what is expected from analytical and FT uncertainties.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"152 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-02-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136185685","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-02-03DOI: 10.5194/gchron-5-65-2023
Stella Birlo, W. Tylmann, B. Zolitschka
Abstract. This study gives an overview of different methods to integrate information from a varve chronology and radiometric measurements in the Bayesian tool Bacon. These techniques will become important for the future as technologies evolve with more sites being revisited for the application of new and high-resolution scanning methods. Thus, the transfer of existing chronologies will become necessary because the recounting of varves will be too time consuming and expensive to be funded. We introduce new sediment cores from Holzmaar (West Eifel Volcanic Field, Germany), a volcanic maar lake with a well-studied varve record. Four different age–depth models have been calculated for the new composite sediment profile (HZM19) using Bayesian modelling with Bacon. All models incorporate new Pb-210 and Cs-137 dates for the top of the record, the latest calibration curve (IntCal20) for radiocarbon ages as well as the new age estimation for the Laacher See Tephra. Model A is based on previously published radiocarbon measurements only, while Models B–D integrate the previously published varve chronology (VT-99) with different approaches. Model B rests upon radiocarbon data, while parameter settings are obtained from sedimentation rates derived from VT-99. Model C is based on radiocarbon dates and on VT-99 as several normal distributed tie points, while Model D is segmented into four sections: sections 1 and 3 are based on VT-99 only, whereas sections 2 and 4 rely on Bacon age–depth models including additional information from VT-99. In terms of accuracy, the parameter-based integration Model B shows little improvement over the non-integrated approach, whereas the tie-point-based integration Model C reflects the complex accumulation history of Holzmaar much better. Only the segmented and parameter-based age integration approach of Model D adapts and improves VT-99 by replacing sections of higher counting errors with Bayesian modelling of radiocarbon ages and thus efficiently makes available the best possible and most precise age–depth model for HZM19. This approach will value all ongoing high-resolution investigations for a better understanding of decadal-scale Holocene environmental and climatic variations.
摘要本研究概述了在贝叶斯工具bacon中整合阀门年表和辐射测量信息的不同方法。随着技术的发展，这些技术将变得越来越重要，因为更多的地点将被重新访问，以应用新的高分辨率扫描方法。因此，现有年表的转移将是必要的，因为阀门的重新计算将是耗时和昂贵的资金。我们介绍了来自Holzmaar(德国西艾菲尔火山场)的新沉积物岩心，这是一个火山maar湖，具有充分研究的阀门记录。利用贝叶斯模型和Bacon对新的复合沉积物剖面(HZM19)计算了四种不同的年龄深度模型。所有模型都采用新的Pb-210和Cs-137日期作为记录的顶部，放射性碳年龄的最新校准曲线(IntCal20)以及Laacher See Tephra的新年龄估计。模型A是基于以前发表的放射性碳测量仅，而模型B-D整合以前发表的阀门年表(VT-99)与不同的方法。模型B基于放射性碳数据，而参数设置则根据VT-99得出的沉降速率获得。模型C基于放射性碳酸盐和VT-99作为几个正态分布的联系点，而模型Dis分为四个部分:第1和3部分仅基于VT-99，而第2和4部分依赖于培根年龄深度模型，包括VT-99的附加信息。在精度方面，基于参数的积分模型B与非积分方法相比几乎没有提高，而基于结合点的积分模型C更能反映Holzmaar复杂的积累历史。只有模型D的分段和基于参数的年龄整合方法适应和改进了vt -99，用放射性碳年龄的贝叶斯建模取代了较高计数误差的部分，从而有效地为HZM19提供了最佳和最精确的年龄深度模型。这种方法将对所有正在进行的高分辨率研究有价值，有助于更好地了解十年尺度的全新世环境和气候变化。
{"title":"Bayesian age–depth modelling applied to varve and radiometric dating to optimize the transfer of an existing high-resolution chronology to a new composite sediment profile from Holzmaar (West Eifel Volcanic Field, Germany)","authors":"Stella Birlo, W. Tylmann, B. Zolitschka","doi":"10.5194/gchron-5-65-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-65-2023","url":null,"abstract":"Abstract. This study gives an overview of different methods to integrate information\u0000from a varve chronology and radiometric measurements in the Bayesian tool\u0000Bacon. These techniques will become important for the future as technologies\u0000evolve with more sites being revisited for the application of new and\u0000high-resolution scanning methods. Thus, the transfer of existing\u0000chronologies will become necessary because the recounting of varves will be\u0000too time consuming and expensive to be funded. We introduce new sediment cores from Holzmaar (West Eifel Volcanic Field,\u0000Germany), a volcanic maar lake with a well-studied varve record. Four\u0000different age–depth models have been calculated for the new composite\u0000sediment profile (HZM19) using Bayesian modelling with Bacon. All models\u0000incorporate new Pb-210 and Cs-137 dates for the top of the record, the\u0000latest calibration curve (IntCal20) for radiocarbon ages as well as the new\u0000age estimation for the Laacher See Tephra. Model A is based on previously\u0000published radiocarbon measurements only, while Models B–D integrate the\u0000previously published varve chronology (VT-99) with different approaches.\u0000Model B rests upon radiocarbon data, while parameter settings are obtained\u0000from sedimentation rates derived from VT-99. Model C is based on radiocarbon\u0000dates and on VT-99 as several normal distributed tie points, while Model D\u0000is segmented into four sections: sections 1 and 3 are based on VT-99 only,\u0000whereas sections 2 and 4 rely on Bacon age–depth models including additional\u0000information from VT-99. In terms of accuracy, the parameter-based\u0000integration Model B shows little improvement over the non-integrated\u0000approach, whereas the tie-point-based integration Model C reflects the\u0000complex accumulation history of Holzmaar much better. Only the segmented and\u0000parameter-based age integration approach of Model D adapts and improves\u0000VT-99 by replacing sections of higher counting errors with Bayesian\u0000modelling of radiocarbon ages and thus efficiently makes available the best\u0000possible and most precise age–depth model for HZM19. This approach will\u0000value all ongoing high-resolution investigations for a better understanding\u0000of decadal-scale Holocene environmental and climatic variations.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"18 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80429444","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-02-02DOI: 10.5194/gchron-5-51-2023
Melanie Bartz, Jasquelin Peña, Stéphanie Grand, Georgina E. King
Abstract. Chemical weathering alters the chemical composition of mineral grains. As a result, trapped-charge dating signals of primary silicates may be progressively modified. In this study, we treated three feldspar specimens to understand the effect of proton- and ligand-promoted dissolution on their luminescence properties. We conducted kinetic experiments over 720 h using two solutions: (1) oxalic acid (pH 3, 20 ∘C), an organic acid with chelating abilities, and (2) aqua regia (pH < 1, 40 ∘C), a mixture of strong acids creating aggressive acid hydrolysis conditions. These two solutions were chosen to provoke, on laboratory timescales, some of the changes that may occur on geological timescales as minerals weather in nature. The effect of the extracting solutions on mineral dissolution was investigated by monitoring the concentration of dissolved elements, while changes in feldspar surface morphology were assessed by scanning electron microscopy (SEM). Subsequent changes in feldspar luminescence in the near-UV (∼ 340 nm) and blue (∼ 410 nm) thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission bands were assessed at the multi- and/or single-grain levels to gain insight into the emission spectra, dose response, saturation, and anomalous fading characteristics of the feldspars. In all experiments, only minor feldspar dissolution was observed after 720 h. In general, aqua regia, the more chemically aggressive solution, had a larger effect on feldspar dissolution compared to that of oxalic acid. Additionally, our results showed that although the TL and IRSL intensities changed slightly with increasing artificial weathering time, the feldspar luminescence properties were otherwise unmodified. This suggests that chemical alteration of feldspar surfaces may not affect luminescence dating signals obtained from natural samples.
{"title":"Potential impacts of chemical weathering on feldspar luminescence dating properties","authors":"Melanie Bartz, Jasquelin Peña, Stéphanie Grand, Georgina E. King","doi":"10.5194/gchron-5-51-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-51-2023","url":null,"abstract":"Abstract. Chemical weathering alters the chemical composition of mineral grains. As a result, trapped-charge dating signals of primary silicates may be progressively modified. In this study, we treated three feldspar specimens to understand the effect of proton- and ligand-promoted dissolution on their luminescence properties. We conducted kinetic experiments over 720 h using two solutions: (1) oxalic acid (pH 3, 20 ∘C), an organic acid with chelating abilities, and (2) aqua regia (pH < 1, 40 ∘C), a mixture of strong acids creating aggressive acid hydrolysis conditions. These two solutions were chosen to provoke, on laboratory timescales, some of the changes that may occur on geological timescales as minerals weather in nature. The effect of the extracting solutions on mineral dissolution was investigated by monitoring the concentration of dissolved elements, while changes in feldspar surface morphology were assessed by scanning electron microscopy (SEM). Subsequent changes in feldspar luminescence in the near-UV (∼ 340 nm) and blue (∼ 410 nm) thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission bands were assessed at the multi- and/or single-grain levels to gain insight into the emission spectra, dose response, saturation, and anomalous fading characteristics of the feldspars. In all experiments, only minor feldspar dissolution was observed after 720 h. In general, aqua regia, the more chemically aggressive solution, had a larger effect on feldspar dissolution compared to that of oxalic acid. Additionally, our results showed that although the TL and IRSL intensities changed slightly with increasing artificial weathering time, the feldspar luminescence properties were otherwise unmodified. This suggests that chemical alteration of feldspar surfaces may not affect luminescence dating signals obtained from natural samples.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"60 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-02-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135313059","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-01-16DOI: 10.5194/gchron-5-35-2023
P. A. van der Beek, T. Schildgen
Abstract. Interpreting cooling ages from multiple thermochronometric systems and/or from steep elevation transects with the help of a thermal model can provide unique insights into the spatial and temporal patterns of rock exhumation. Although several well-established thermal models allow for a detailed exploration of how cooling or exhumation rates evolved in a limited area or along a transect, integrating large, regional datasets in such models remains challenging. Here, we present age2exhume, a thermal model in the form of a MATLAB or Python script, which can be used to rapidly obtain a synoptic overview of exhumation rates from large, regional thermochronometric datasets. The model incorporates surface temperature based on a defined lapse rate and a local relief correction that is dependent on the thermochronometric system of interest. Other inputs include sample cooling age, uncertainty, and an initial (unperturbed) geothermal gradient. The model is simplified in that it assumes steady, vertical rock uplift and unchanging topography when calculating exhumation rates. For this reason, it does not replace more powerful and versatile thermal–kinematic models, but it has the advantage of simple implementation and rapidly calculated results. We also provide plots of predicted exhumation rates as a function of thermochronometric age and the local relief correction, which can be used to simply look up a first-order estimate of exhumation rate. In our example dataset, we show exhumation rates calculated from 1785 cooling ages from the Himalaya associated with five different thermochronometric systems. Despite the synoptic nature of the results, they reflect known segmentation patterns and changing exhumation rates in areas that have undergone structural reorganization. Moreover, the rapid calculations enable an exploration of the sensitivity of the results to various input parameters and an illustration of the importance of explicit modeling of thermal fields when calculating exhumation rates from thermochronometric data.
{"title":"Short communication: age2exhume – a MATLAB/Python script to calculate steady-state vertical exhumation rates from thermochronometric ages and application to the Himalaya","authors":"P. A. van der Beek, T. Schildgen","doi":"10.5194/gchron-5-35-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-35-2023","url":null,"abstract":"Abstract. Interpreting cooling ages from multiple thermochronometric systems and/or\u0000from steep elevation transects with the help of a thermal model can provide\u0000unique insights into the spatial and temporal patterns of rock exhumation.\u0000Although several well-established thermal models allow for a detailed\u0000exploration of how cooling or exhumation rates evolved in a limited area or\u0000along a transect, integrating large, regional datasets in such models\u0000remains challenging. Here, we present age2exhume, a thermal model in the\u0000form of a MATLAB or Python script, which can be used to rapidly obtain a\u0000synoptic overview of exhumation rates from large, regional\u0000thermochronometric datasets. The model incorporates surface temperature\u0000based on a defined lapse rate and a local relief correction that is\u0000dependent on the thermochronometric system of interest. Other inputs include\u0000sample cooling age, uncertainty, and an initial (unperturbed) geothermal\u0000gradient. The model is simplified in that it assumes steady, vertical\u0000rock uplift and unchanging topography when calculating exhumation rates. For\u0000this reason, it does not replace more powerful and versatile\u0000thermal–kinematic models, but it has the advantage of simple implementation\u0000and rapidly calculated results. We also provide plots of predicted\u0000exhumation rates as a function of thermochronometric age and the local\u0000relief correction, which can be used to simply look up a first-order\u0000estimate of exhumation rate. In our example dataset, we show exhumation\u0000rates calculated from 1785 cooling ages from the Himalaya associated with\u0000five different thermochronometric systems. Despite the synoptic nature of\u0000the results, they reflect known segmentation patterns and changing\u0000exhumation rates in areas that have undergone structural reorganization.\u0000Moreover, the rapid calculations enable an exploration of the sensitivity of\u0000the results to various input parameters and an illustration of the\u0000importance of explicit modeling of thermal fields when calculating\u0000exhumation rates from thermochronometric data.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"208 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-01-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85871096","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-01-13DOI: 10.5194/gchron-5-21-2023
Alexandria J. Koester, N. Lifton
Abstract. Over the last 30 years, in situ cosmogenic nuclides (CNs) have revolutionized surficial processes and Quaternary geologic studies. Commonly measured CNs extracted from common mineral quartz have long half-lives (e.g., 10Be, 26Al) and have been applied over timescales from a few hundred years to millions of years. However, their long half-lives also render them largely insensitive to complex histories of burial and exposure of less than ca. 100 kyr. On the other hand, in situ cosmogenic 14C (in situ 14C) is also produced in quartz, yet its 5.7 kyr half-life renders it very sensitive to complex exposure histories during the last ∼25 ka, a particularly unique and powerful tool when analyzed in concert with long-lived nuclides. In situ 14C measurements are currently limited to relatively coarse-grained (typically sand-sized or larger, crushed or sieved to sand) quartz-bearing rock types, but while such rocks are common, they are not ubiquitous. The ability to extract and interpret in situ 14C from quartz-poor and fine-grained rocks would thus open its unique applications to a broader array of landscape elements and environments. As a first step toward this goal, a robust means of interpreting in situ 14C concentrations derived from rocks and minerals spanning wider compositional and textural ranges will be crucial. We have thus developed a MATLAB®-based software framework to quantify spallogenic production of in situ 14C from a broad range of silicate rock and mineral compositions, including rocks too fine grained to achieve pure quartz separates. As expected from prior work, production from oxygen dominates the overall in situ 14C signal, accounting for >90 % of production for common silicate minerals and six different rock types at sea level and high latitudes (SLHL). This work confirms that Si, Al, and Mg are important targets but also predicts greater production from Na than from those elements. The compositionally dependent production rates for rock and mineral compositions investigated here are typically lower than that of quartz, although that predicted for albite is comparable to quartz, reflecting the significance of production from Na. Predicted production rates drop as compositions become more mafic (particularly Fe-rich). This framework should thus be a useful tool in efforts to broaden the utility of in situ 14C to quartz-poor and fine-grained rock types, but future improvements in measured and modeled excitation functions would be beneficial.
{"title":"Technical note: A software framework for calculating compositionally dependent in situ 14C production rates","authors":"Alexandria J. Koester, N. Lifton","doi":"10.5194/gchron-5-21-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-21-2023","url":null,"abstract":"Abstract. Over the last 30 years, in situ cosmogenic nuclides (CNs) have revolutionized\u0000surficial processes and Quaternary geologic studies. Commonly measured CNs\u0000extracted from common mineral quartz have long half-lives (e.g.,\u000010Be, 26Al) and have been applied over timescales from a few\u0000hundred years to millions of years. However, their long half-lives also\u0000render them largely insensitive to complex histories of burial and exposure of\u0000less than ca. 100 kyr. On the other hand, in situ cosmogenic 14C (in situ 14C) is\u0000also produced in quartz, yet its 5.7 kyr half-life renders it very sensitive\u0000to complex exposure histories during the last ∼25 ka, a\u0000particularly unique and powerful tool when analyzed in concert with\u0000long-lived nuclides. In situ 14C measurements are currently limited to\u0000relatively coarse-grained (typically sand-sized or larger, crushed or sieved to\u0000sand) quartz-bearing rock types, but while such rocks are common, they are\u0000not ubiquitous. The ability to extract and interpret in situ 14C from\u0000quartz-poor and fine-grained rocks would thus open its unique applications\u0000to a broader array of landscape elements and environments. As a first step toward this goal, a robust means of interpreting in situ 14C\u0000concentrations derived from rocks and minerals spanning wider compositional\u0000and textural ranges will be crucial. We have thus developed a\u0000MATLAB®-based software framework to quantify\u0000spallogenic production of in situ 14C from a broad range of silicate rock and\u0000mineral compositions, including rocks too fine grained to achieve pure\u0000quartz separates. As expected from prior work, production from oxygen\u0000dominates the overall in situ 14C signal, accounting for >90 %\u0000of production for common silicate minerals and six different rock types at\u0000sea level and high latitudes (SLHL). This work confirms that Si, Al, and Mg\u0000are important targets but also predicts greater production from Na than\u0000from those elements. The compositionally dependent production rates for rock\u0000and mineral compositions investigated here are typically lower than that of\u0000quartz, although that predicted for albite is comparable to quartz,\u0000reflecting the significance of production from Na. Predicted production\u0000rates drop as compositions become more mafic (particularly Fe-rich). This framework should thus be a useful tool in efforts to broaden the utility of\u0000in situ 14C to quartz-poor and fine-grained rock types, but future\u0000improvements in measured and modeled excitation functions would be\u0000beneficial.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90389826","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
C.W. Magee Jr, S. Bodorkos, C. Lewis, J. Crowley, C. Wall, R. Friedman
Abstract. Estimations of the reproducibility of U–Pb ages from SHRIMP (Sensitive High-Resolution Ion MicroProbe) instruments are based on data from studies that are nearly 2 decades old. Since that time, refinement of analytical procedures and operational improvements have reduced the historically identified uncertainties of SHRIMP U–Pb analysis. This paper investigates 36 SHRIMP thermal ionisation mass spectrometry (TIMS) double-dated “real-world” geologic samples from a variety of igneous rock types to better understand both geological and analytical sources of disagreement between the two dating methods. Geoscience Australia's (GA) use of high-precision chemical abrasion thermal ionisation mass spectrometry (CA-TIMS) for chronostratigraphy in Australian sedimentary basins has produced a substantial selection of precisely dated zircons, which we can use to cross-correlate the SHRIMP and CA-TIMS ages throughout the Phanerozoic. A total of 33 of the 36 ages were reported with external SHRIMP uncertainties less than 1 % (95 % confidence). Six of eight cases where the CA-TIMS age was outside the SHRIMP uncertainty envelope were in samples where the 95 % confidence interval of the reported SHRIMP age was below 0.66 % uncertainty, suggesting that SHRIMP analyses of untreated zircon with smaller uncertainties are probably overoptimistic. The mean age offset between SHRIMP and TIMS ages is 0.095 %, but the distribution appears bimodal. Geological explanations for age discrepancies between SHRIMP and CA-TIMS are suggested by considering intrusive and extrusive age results separately. All but one sample where the SHRIMP age is more than 0.25 % older are volcanic. This offset could be explained by the better single-grain age resolution of TIMS, allowing identification and exclusion of antecrysts from the eruptive population, while SHRIMP does not have a sufficient single-grain precision to deconvolve these populations – leading to an apparent older SHRIMP age. In contrast, SHRIMP ages from plutonic rocks – particularly plutonic rocks from the early Paleozoic – are typically younger than the CA-TIMS ages from the same samples, most likely reflecting Pb loss from non-chemically abraded SHRIMP zircons, while chemical abrasion of zircons prior to TIMS analysis destroyed or corrected these areas of Pb loss.
{"title":"Examination of the accuracy of SHRIMP U–Pb geochronology based on samples dated by both SHRIMP and CA-TIMS","authors":"C.W. Magee Jr, S. Bodorkos, C. Lewis, J. Crowley, C. Wall, R. Friedman","doi":"10.5194/gchron-5-1-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-1-2023","url":null,"abstract":"Abstract. Estimations of the reproducibility of U–Pb ages from SHRIMP (Sensitive High-Resolution Ion MicroProbe)\u0000instruments are based on data from studies that are nearly 2 decades old. Since\u0000that time, refinement of analytical procedures and operational improvements\u0000have reduced the historically identified uncertainties of SHRIMP U–Pb\u0000analysis. This paper investigates 36 SHRIMP thermal\u0000ionisation mass spectrometry (TIMS) double-dated “real-world”\u0000geologic samples from a variety of igneous rock types to better understand\u0000both geological and analytical sources of disagreement between the two\u0000dating methods. Geoscience Australia's (GA) use of high-precision chemical abrasion thermal\u0000ionisation mass spectrometry (CA-TIMS) for chronostratigraphy in Australian\u0000sedimentary basins has produced a substantial selection of precisely dated\u0000zircons, which we can use to cross-correlate the SHRIMP and CA-TIMS ages\u0000throughout the Phanerozoic. A total of 33 of the 36 ages were reported with\u0000external SHRIMP uncertainties less than 1 % (95 % confidence). Six of\u0000eight cases where the CA-TIMS age was outside the SHRIMP uncertainty\u0000envelope were in samples where the 95 % confidence interval of the\u0000reported SHRIMP age was below 0.66 % uncertainty, suggesting that SHRIMP\u0000analyses of untreated zircon with smaller uncertainties are probably\u0000overoptimistic. The mean age offset between SHRIMP and TIMS ages is 0.095 %, but the\u0000distribution appears bimodal. Geological explanations for age discrepancies\u0000between SHRIMP and CA-TIMS are suggested by considering intrusive and\u0000extrusive age results separately. All but one sample where the SHRIMP age is\u0000more than 0.25 % older are volcanic. This offset could be explained by the\u0000better single-grain age resolution of TIMS, allowing identification and\u0000exclusion of antecrysts from the eruptive population, while SHRIMP does not\u0000have a sufficient single-grain precision to deconvolve these populations –\u0000leading to an apparent older SHRIMP age. In contrast, SHRIMP ages from\u0000plutonic rocks – particularly plutonic rocks from the early Paleozoic – are\u0000typically younger than the CA-TIMS ages from the same samples, most likely\u0000reflecting Pb loss from non-chemically abraded SHRIMP zircons, while\u0000chemical abrasion of zircons prior to TIMS analysis destroyed or corrected\u0000these areas of Pb loss.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87685709","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-12-15DOI: 10.5194/gchron-4-713-2022
Nathan Vandermaelen, K. Beerten, François Clapuyt, M. Christl, V. Vanacker
Abstract. Pleistocene braided-river deposits commonly represent long periods of non-deposition or erosion that are interrupted by rapid and short aggradation phases. When dating these sedimentary sequences with in situ-produced cosmic radionuclides (CRNs), simple concentration depth profiling approaches often fall short, as they assume that the alluvial sedimentary sequence has been deposited with a constant and rapid aggradation rate and been exposed to cosmic radiations afterwards. Numerical modelling of the evolution of CRNs in alluvial sequences permits one to account for aggradation, non-deposition and erosion phases and can simulate which scenarios of aggradation and preservation most likely represent the river dynamics. In this study, such a model was developed and applied to a Middle Pleistocene gravel sheet (Zutendaal gravels) exposed in NE Belgium. The model parameters were optimised to the observed 10Be and 26Al concentrations of 17 sediment samples taken over a depth interval of 7 m that constitutes the top of a gravel sheet up to 20 m thick. In the studied sedimentary sequence, (at least) three individual aggradation phases that were interrupted by non-deposition or erosion can be distinguished, each interruption lasting ∼ 40 kyr. The age for the onset of aggradation of the upper 7 m of the gravel sheet was further constrained to 654-62+218 ka. This age, within error limits, does not invalidate previous correlations of the gravel sheet with the Cromerian Glacial B and Marine Isotope Stage (MIS) 16. The deposition of the entire gravel sheet likely represents more than one climatic cycle and demonstrates the importance of accounting for the depositional modes of braided rivers when applying in situ cosmogenic radionuclide techniques.
{"title":"Constraining the aggradation mode of Pleistocene river deposits based on cosmogenic radionuclide depth profiling and numerical modelling","authors":"Nathan Vandermaelen, K. Beerten, François Clapuyt, M. Christl, V. Vanacker","doi":"10.5194/gchron-4-713-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-713-2022","url":null,"abstract":"Abstract. Pleistocene braided-river deposits commonly represent long periods\u0000of non-deposition or erosion that are interrupted by rapid and short\u0000aggradation phases. When dating these sedimentary sequences with in situ-produced cosmic radionuclides (CRNs), simple concentration depth profiling\u0000approaches often fall short, as they assume that the alluvial sedimentary\u0000sequence has been deposited with a constant and rapid aggradation rate and\u0000been exposed to cosmic radiations afterwards. Numerical modelling of the\u0000evolution of CRNs in alluvial sequences permits one to account for aggradation,\u0000non-deposition and erosion phases and can simulate which scenarios of\u0000aggradation and preservation most likely represent the river\u0000dynamics. In this study, such a model was developed and applied to a Middle\u0000Pleistocene gravel sheet (Zutendaal gravels) exposed in NE Belgium. The\u0000model parameters were optimised to the observed 10Be and 26Al\u0000concentrations of 17 sediment samples taken over a depth interval of 7 m\u0000that constitutes the top of a gravel sheet up to 20 m thick. In the studied sedimentary sequence, (at least) three individual aggradation phases that were interrupted by non-deposition or erosion can be distinguished, each interruption lasting ∼ 40 kyr. The age for the onset of aggradation of the upper 7 m of the gravel sheet was further constrained to 654-62+218 ka. This age, within error limits, does not invalidate\u0000previous correlations of the gravel sheet with the Cromerian Glacial B and\u0000Marine Isotope Stage (MIS) 16. The deposition of the entire gravel sheet\u0000likely represents more than one climatic cycle and demonstrates the\u0000importance of accounting for the depositional modes of braided rivers when\u0000applying in situ cosmogenic radionuclide techniques.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"89 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82741126","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-12-14DOI: 10.5194/gchron-4-691-2022
F. Hofmann
Abstract. Cosmic-ray exposure (CRE) dating of boulders on terminal moraines has become a well-established technique to reconstruct glacier chronologies. If topographic obstructions are present in the surroundings of sampling sites, CRE ages need to be corrected for topographic shielding. In recent years, geographical information system (GIS)-based approaches have been developed to compute shielding factors with elevation data, particularly two toolboxes for the ESRI ArcGIS software. So far, the output of the most recent toolbox (Li, 2018) has only been validated with a limited number of field-data-based shielding factors. Additionally, it has not been systematically evaluated how the spatial resolution of the input elevation data affects the output of the toolbox and whether a correction for vegetation leads to considerably more precise shielding factors. This paper addresses these issues by assessing the output of the toolbox with an extensive set of field-data-based shielding factors. Commonly used elevation data with different spatial resolutions were tested as input. To assess the impact of the different methods on CRE ages, ages of boulders with different 10Be concentrations at sites with varying topography and 10Be production rates were first recalculated with GIS-based shielding factors and then with field-data-based shielding factors. For sampling sites in forested low mountainous areas and in high Alpine settings, the shielding factors were independent of the spatial resolution of the input elevation data. Vegetation-corrected elevation data allowed more precise shielding factors to be computed for sites in a forested low mountainous area. In most cases, recalculating CRE ages of the same sampling sites with different shielding factors led to age shifts between 0 % and 2 %. Only one age changed by 5 %. It is shown that the use of elevation data with a very high resolution requires precise x and y coordinates of sampling sites and that there is otherwise a risk that small-scale objects in the vicinity of sampling sites will be misinterpreted as topographic barriers. Overall, the toolbox provides an interesting avenue for the determination of shielding factors. Together with the guidelines presented here, it should be more widely used.
{"title":"Technical note: Evaluating a geographical information system (GIS)-based approach for determining topographic shielding factors in cosmic-ray exposure dating","authors":"F. Hofmann","doi":"10.5194/gchron-4-691-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-691-2022","url":null,"abstract":"Abstract. Cosmic-ray exposure (CRE) dating of boulders on terminal\u0000moraines has become a well-established technique to reconstruct glacier\u0000chronologies. If topographic obstructions are present in the surroundings of\u0000sampling sites, CRE ages need to be corrected for topographic shielding. In\u0000recent years, geographical information system (GIS)-based approaches have\u0000been developed to compute shielding factors with elevation data,\u0000particularly two toolboxes for the ESRI ArcGIS software. So far, the output\u0000of the most recent toolbox (Li, 2018) has only been validated with a limited\u0000number of field-data-based shielding factors. Additionally, it has not been\u0000systematically evaluated how the spatial resolution of the input elevation\u0000data affects the output of the toolbox and whether a correction for\u0000vegetation leads to considerably more precise shielding factors. This paper\u0000addresses these issues by assessing the output of the toolbox with an\u0000extensive set of field-data-based shielding factors. Commonly used elevation\u0000data with different spatial resolutions were tested as input. To assess the\u0000impact of the different methods on CRE ages, ages of boulders with different\u000010Be concentrations at sites with varying topography and 10Be\u0000production rates were first recalculated with GIS-based shielding factors and then with field-data-based shielding factors. For sampling sites in forested low mountainous\u0000areas and in high Alpine settings, the shielding factors were independent of\u0000the spatial resolution of the input elevation data. Vegetation-corrected\u0000elevation data allowed more precise shielding factors to be computed for\u0000sites in a forested low mountainous area. In most cases, recalculating CRE\u0000ages of the same sampling sites with different shielding factors led to age\u0000shifts between 0 % and 2 %. Only one age changed by 5 %. It is shown that\u0000the use of elevation data with a very high resolution requires precise\u0000x and y coordinates of sampling sites and that there is otherwise a risk that small-scale\u0000objects in the vicinity of sampling sites will be misinterpreted as\u0000topographic barriers. Overall, the toolbox provides an interesting avenue\u0000for the determination of shielding factors. Together with the guidelines\u0000presented here, it should be more widely used.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"84 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85702645","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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