Pub Date : 2021-06-30DOI: 10.5194/gchron-3-383-2021
Simon Nachtergaele, J. De Grave
Abstract. A new method for automatic counting of etched fission tracks in minerals is described and presented in this article. Artificial intelligence techniques such as deep neural networks and computer vision were trained to detect fission surface semi-tracks on images. The deep neural networks can be used in an open-source computer program for semi-automated fission track dating called “AI-Track-tive”. Our custom-trained deep neural networks use the YOLOv3 object detection algorithm, which is currently one of the most powerful and fastest object recognition algorithms. The developed program successfully finds most of the fission tracks in the microscope images; however, the user still needs to supervise the automatic counting. The presented deep neural networks have high precision for apatite (97 %) and mica (98 %). Recall values are lower for apatite (86 %) than for mica (91 %). The application can be used online at https://ai-track-tive.ugent.be (last access: 29 June 2021), or it can be downloaded as an offline application for Windows.
{"title":"AI-Track-tive: open-source software for automated recognition and counting of surface semi-tracks using computer vision (artificial intelligence)","authors":"Simon Nachtergaele, J. De Grave","doi":"10.5194/gchron-3-383-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-383-2021","url":null,"abstract":"Abstract. A new method for automatic counting of etched fission tracks in minerals is\u0000described and presented in this article. Artificial intelligence techniques\u0000such as deep neural networks and computer vision were trained to detect\u0000fission surface semi-tracks on images. The deep neural networks can be used\u0000in an open-source computer program for semi-automated fission track dating\u0000called “AI-Track-tive”. Our custom-trained deep neural networks use the YOLOv3\u0000object detection algorithm, which is currently one of the most powerful and\u0000fastest object recognition algorithms. The developed program successfully\u0000finds most of the fission tracks in the microscope images; however, the user\u0000still needs to supervise the automatic counting. The presented deep neural\u0000networks have high precision for apatite (97 %) and mica (98 %). Recall\u0000values are lower for apatite (86 %) than for mica (91 %). The\u0000application can be used online at https://ai-track-tive.ugent.be (last access: 29 June 2021), or it can be downloaded as an offline application\u0000for Windows.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"27 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77557047","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 : 2021-06-23DOI: 10.5194/GCHRON-3-371-2021
B. Mauz, Loïc A. Martin, M. Discher, C. Tribolo, S. Kreutzer, Chiara Bahl, A. Lang, N. Mercier
Abstract. The dose rate of the 90Sr / 90Y beta source used in most luminescence readers is a laboratory key parameter. There is a well-established body of knowledge about parameters controlling accuracy and precision of the calibration value but some hard-to-explain inconsistencies still exist. Here, we have investigated the impact of grain size, aliquot size and irradiation geometry on the resulting calibration value through experiments and simulations. The resulting data indicate that the dose rate of an individual beta source results from the interplay of a number of parameters, most of which are well established by previous studies. Our study provides evidence for the impact of aliquot size on the absorbed dose in particular for grain sizes of 50–200 µm. For this grain-size fraction, the absorbed dose is enhanced by ∼ 10 %–20 % as aliquot size decreases due to the radial increase of dose rate towards the centre of the aliquot. The enhancement is most variable for 50–100 µm grains mounted as aliquots of < 8 mm size. The enhancement is reversed when large grains are mounted as small aliquots due to the edge effect by which the dose induced by backscattered electrons is reduced. While the build-up of charge dictates the increase of absorbed dose with the increase of grain size, this principle becomes more variable with changing irradiation geometry. We conclude that future calibration samples should consist of subsamples composed of small, medium, large and very large quartz grains, each obtaining several gamma doses. The calibration value measured with small, medium and large aliquots is then obtained from the inverse slope of the fitted line, not from a single data point. In this way, all possible irradiation geometries of an individual beta source are covered, and the precision of the calibration is improved.
{"title":"Technical note: On the reliability of laboratory beta-source calibration for luminescence dating","authors":"B. Mauz, Loïc A. Martin, M. Discher, C. Tribolo, S. Kreutzer, Chiara Bahl, A. Lang, N. Mercier","doi":"10.5194/GCHRON-3-371-2021","DOIUrl":"https://doi.org/10.5194/GCHRON-3-371-2021","url":null,"abstract":"Abstract. The dose rate of the 90Sr / 90Y beta source used in most\u0000luminescence readers is a laboratory key parameter. There is a\u0000well-established body of knowledge about parameters controlling accuracy and\u0000precision of the calibration value but some hard-to-explain inconsistencies\u0000still exist. Here, we have investigated the impact of grain size, aliquot\u0000size and irradiation geometry on the resulting calibration value through\u0000experiments and simulations. The resulting data indicate that the dose rate\u0000of an individual beta source results from the interplay of a number of\u0000parameters, most of which are well established by previous studies. Our\u0000study provides evidence for the impact of aliquot size on the absorbed dose\u0000in particular for grain sizes of 50–200 µm. For this grain-size\u0000fraction, the absorbed dose is enhanced by ∼ 10 %–20 % as\u0000aliquot size decreases due to the radial increase of dose rate towards\u0000the centre of the aliquot. The enhancement is most variable for 50–100 µm\u0000grains mounted as aliquots of < 8 mm size. The enhancement is\u0000reversed when large grains are mounted as small aliquots due to the edge\u0000effect by which the dose induced by backscattered electrons is reduced.\u0000While the build-up of charge dictates the increase of absorbed dose with the\u0000increase of grain size, this principle becomes more variable with changing\u0000irradiation geometry. We conclude that future calibration samples should\u0000consist of subsamples composed of small, medium, large and very large quartz\u0000grains, each obtaining several gamma doses. The calibration value measured\u0000with small, medium and large aliquots is then obtained from the inverse\u0000slope of the fitted line, not from a single data point. In this way, all\u0000possible irradiation geometries of an individual beta source are covered,\u0000and the precision of the calibration is improved.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"20 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80714673","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}
S. Scheidt, M. Lenz, R. Egli, D. Brill, M. Klug, K. Fabian, M. M. Lenz, R. Gromig, J. Rethemeyer, B. Wagner, Grigory Federov, M. Melles
Abstract. This work represents the first palaeomagnetic study carried out on the sedimentary record of lake Levinson-Lessing, which is the deepest lake in northern Central Siberia. Palaeomagnetic analyses were carried out on 730 discrete samples from the upper 38 m of the 46 m-long core Co1401, which was recovered from the central part of the lake. Alternating field demagnetisation experiments were carried out to obtain the characteristic remanent magnetisation. The relative palaeointensity is determined using the magnetic susceptibility, the anhysteretic remanent magnetization and the isothermal remanent magnetization for normalization of the partial natural remanent magnetization. The chronology of Co1401 derives from accelerated mass spectrometer radiocarbon ages, optically stimulated luminescence dating, and correlation of the relative palaeointensity of 642 discrete samples with the GLOPIS-75 reference curve. This study focuses on the part >10 ka but although includes preliminary results for the upper part of the core. The record includes the geomagnetic excursions Laschamps and Mono Lake, and resolves sufficient geomagnetic features to establish a chronology that continuously covers ~62 ka. The results reveal continuous sedimentation and high sedimentation rate between 45 and 95 cm ka−1. High lock-in depths are suggested from the low variability of the magnetic record compared to data sets of reference records with lower sedimentation rate. Although the horizontal component of the characteristic remanent magnetization can only be used with caution because Co1401 was cored without core segment overlap, the magnetic record of Co1401 is the only high-resolution record of relative palaeointensity and palaeosecular variations from the Arctic tangent cylinder going back to ~62 ka.
摘要这项工作是对西伯利亚北部最深的湖泊列文森-莱辛湖的沉积记录进行的第一次古地磁研究。对从湖中部回收的Co1401号46 m长岩心上38 m处的730个离散样品进行了古地磁分析。进行了交变磁场退磁实验，得到了特征剩磁。利用磁化率、非滞后剩磁和等温剩磁对部分自然剩磁进行归一化，确定了相对古强度。Co1401的年代学来自加速质谱仪放射性碳年龄、光学激发发光定年以及642个离散样品的相对古强度与GLOPIS-75参考曲线的相关性。这项研究主要集中在bbb10ka部分，但也包括了岩心上部的初步结果。该记录包括Laschamps和Mono湖的地磁漂移，并解析了足够的地磁特征，以建立一个连续覆盖~62 ka的年代学。结果表明，该区在45 ~ 95 cm ka−1范围内具有连续沉降和高沉降速率。与沉积速率较低的参考记录数据集相比，磁记录的低变异性表明了高锁深。虽然Co1401的特征剩余磁化强度的水平分量只能谨慎使用，因为Co1401是在没有岩心段重叠的情况下取芯的，但Co1401的磁记录是北极切线柱上~62 ka以来相对古强度和古长期变化的唯一高分辨率记录。
{"title":"A 62-ka geomagnetic palaeointensity record from the Taymyr Peninsula, Russian Arctic","authors":"S. Scheidt, M. Lenz, R. Egli, D. Brill, M. Klug, K. Fabian, M. M. Lenz, R. Gromig, J. Rethemeyer, B. Wagner, Grigory Federov, M. Melles","doi":"10.5194/gchron-2021-12","DOIUrl":"https://doi.org/10.5194/gchron-2021-12","url":null,"abstract":"Abstract. This work represents the first palaeomagnetic study carried out on the sedimentary record of lake Levinson-Lessing, which is the deepest lake in northern Central Siberia. Palaeomagnetic analyses were carried out on 730 discrete samples from the upper 38 m of the 46 m-long core Co1401, which was recovered from the central part of the lake. Alternating field demagnetisation experiments were carried out to obtain the characteristic remanent magnetisation. The relative palaeointensity is determined using the magnetic susceptibility, the anhysteretic remanent magnetization and the isothermal remanent magnetization for normalization of the partial natural remanent magnetization. The chronology of Co1401 derives from accelerated mass spectrometer radiocarbon ages, optically stimulated luminescence dating, and correlation of the relative palaeointensity of 642 discrete samples with the GLOPIS-75 reference curve. This study focuses on the part >10 ka but although includes preliminary results for the upper part of the core. The record includes the geomagnetic excursions Laschamps and Mono Lake, and resolves sufficient geomagnetic features to establish a chronology that continuously covers ~62 ka. The results reveal continuous sedimentation and high sedimentation rate between 45 and 95 cm ka−1. High lock-in depths are suggested from the low variability of the magnetic record compared to data sets of reference records with lower sedimentation rate. Although the horizontal component of the characteristic remanent magnetization can only be used with caution because Co1401 was cored without core segment overlap, the magnetic record of Co1401 is the only high-resolution record of relative palaeointensity and palaeosecular variations from the Arctic tangent cylinder going back to ~62 ka.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"25 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88645538","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 : 2021-06-02DOI: 10.5194/gchron-2021-17-supplement
A. Cunningham, J. Buylaert, A. Murray
Abstract. Mineral grains within sediment or rock absorb a radiation dose from the decay of radionuclides in the host matrix. For the beta dose component, the estimated dose rate must be adjusted for the attenuation of beta particles within the mineral grains. Standard calculations, originally designed for thermoluminescence dating of pottery, assume that the grain is embedded in a homogenous medium. However, most current applications of trapped-charge dating concern sand- or silt-sized dosimeters embedded in granular sediment. In such cases, the radionuclide sources are not homogeneous, but are localized in discrete grains or held on grain surfaces. We show here that the mean dose rate to dosimeter grains in a granular matrix is dependent on the grain-size distributions of the source grains, and of the bulk sediment, as well as on the grain size of the dosimeters. We further argue that U and Th sources are likely to be held primarily on grain surfaces, which causes the dose rate to dosimeter grains to be significantly higher than for sources distributed uniformly throughout grains. For a typical well-sorted medium sand, the beta dose rates derived from surface U and Th sources are higher by 9 % and 14 %, respectively, compared to a homogenous distribution of sources. We account for these effects using an expanded model of beta attenuation, and validate the model against Monte Carlo radiation transport simulations within a geometry of packed spheres.
{"title":"Supplementary material to \"Attenuation of beta radiation in granular matrices: implications for trapped-charge dating\"","authors":"A. Cunningham, J. Buylaert, A. Murray","doi":"10.5194/gchron-2021-17-supplement","DOIUrl":"https://doi.org/10.5194/gchron-2021-17-supplement","url":null,"abstract":"Abstract. Mineral grains within sediment or rock absorb a radiation dose from the decay of radionuclides in the host matrix. For the beta dose component, the estimated dose rate must be adjusted for the attenuation of beta particles within the mineral grains. Standard calculations, originally designed for thermoluminescence dating of pottery, assume that the grain is embedded in a homogenous medium. However, most current applications of trapped-charge dating concern sand- or silt-sized dosimeters embedded in granular sediment. In such cases, the radionuclide sources are not homogeneous, but are localized in discrete grains or held on grain surfaces. We show here that the mean dose rate to dosimeter grains in a granular matrix is dependent on the grain-size distributions of the source grains, and of the bulk sediment, as well as on the grain size of the dosimeters. We further argue that U and Th sources are likely to be held primarily on grain surfaces, which causes the dose rate to dosimeter grains to be significantly higher than for sources distributed uniformly throughout grains. For a typical well-sorted medium sand, the beta dose rates derived from surface U and Th sources are higher by 9 % and 14 %, respectively, compared to a homogenous distribution of sources. We account for these effects using an expanded model of beta attenuation, and validate the model against Monte Carlo radiation transport simulations within a geometry of packed spheres.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"41 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83142244","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 : 2021-05-26DOI: 10.5194/GCHRON-3-337-2021
Bar Elisha, P. Nuriel, A. Kylander‐Clark, R. Weinberger
Abstract. Recent U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has demonstrated that reasonable precision (3 %–10 %, 2σ) can be achieved for high-resolution dating of texturally distinct calcite phases. Absolute dating of dolomite, for which biostratigraphy and traditional dating techniques are very limited, remains challenging, although it may resolve many fundamental questions related to the timing of mineral-rock formation by syngenetic, diagenesis, hydrothermal, and epigenetic processes. In this study we explore the possibility of dating dolomitic rocks via recent LA-ICP-MS dating techniques developed for calcite. The in situ U–Pb dating was tested on a range of dolomitic rocks of various origins from the Cambrian to Pliocene age – all of which are from well-constrained stratigraphic sections in Israel. We present imaging and chemical characterization techniques that provide useful information on interpreting the resulting U–Pb ages and discuss the complexity of in situ dolomite dating in terms of textural features that may affect the results. Textural examinations indicate zonation and mixing of different phases at the sub-millimeter scale (< 1 µm), and thus Tera–Wasserburg ages represent mixed dates of early diagenesis and some later epigenetic dolomitization event(s). We conclude that age mixing at the sub-millimeter scale is a major challenge in dolomite dating that needs to be further studied and note the importance of matrix-matched standards for reducing uncertainties of the dated material.
{"title":"Towards in situ U–Pb dating of dolomite","authors":"Bar Elisha, P. Nuriel, A. Kylander‐Clark, R. Weinberger","doi":"10.5194/GCHRON-3-337-2021","DOIUrl":"https://doi.org/10.5194/GCHRON-3-337-2021","url":null,"abstract":"Abstract. Recent U–Pb dating by laser ablation inductively coupled plasma mass\u0000spectrometry (LA-ICP-MS) has\u0000demonstrated that reasonable precision (3 %–10 %, 2σ) can be\u0000achieved for high-resolution dating of texturally distinct calcite phases.\u0000Absolute dating of dolomite, for which biostratigraphy and traditional\u0000dating techniques are very limited, remains challenging, although it may\u0000resolve many fundamental questions related to the timing of mineral-rock\u0000formation by syngenetic, diagenesis, hydrothermal, and epigenetic processes.\u0000In this study we explore the possibility of dating dolomitic rocks via\u0000recent LA-ICP-MS dating techniques developed for calcite. The in situ U–Pb dating\u0000was tested on a range of dolomitic rocks of various origins from the\u0000Cambrian to Pliocene age – all of which are from well-constrained stratigraphic\u0000sections in Israel. We present imaging and chemical characterization\u0000techniques that provide useful information on interpreting the resulting\u0000U–Pb ages and discuss the complexity of in situ dolomite dating in terms of\u0000textural features that may affect the results. Textural examinations\u0000indicate zonation and mixing of different phases at the sub-millimeter scale\u0000(< 1 µm), and thus Tera–Wasserburg ages represent mixed dates\u0000of early diagenesis and some later epigenetic dolomitization event(s). We\u0000conclude that age mixing at the sub-millimeter scale is a major challenge in\u0000dolomite dating that needs to be further studied and note the importance of\u0000matrix-matched standards for reducing uncertainties of the dated material.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"26 1","pages":"337-349"},"PeriodicalIF":0.0,"publicationDate":"2021-05-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74645108","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 : 2021-04-16DOI: 10.5194/gchron-3-181-2021
B. Schoene, M. Eddy, C. Keller, K. Samperton
Abstract. Recent attempts to establish the eruptive history of the Deccan Traps large igneous province have used both U−Pb (Schoene et al., 2019) and 40Ar/39Ar (Sprain et al., 2019) geochronology. Both of these studies report dates with high precision and unprecedented coverage for a large igneous province and agree that the main phase of eruptions began near the C30n–C29r magnetic reversal and waned shortly after the C29r–C29n reversal, totaling ∼ 700–800 kyr duration. These datasets can be analyzed in finer detail to determine eruption rates, which are critical for connecting volcanism, associated volatile emissions, and any potential effects on the Earth's climate before and after the Cretaceous–Paleogene boundary (KPB). It is our observation that the community has frequently misinterpreted how the eruption rates derived from these two datasets vary across the KPB. The U−Pb dataset of Schoene et al. (2019) was interpreted by those authors to indicate four major eruptive pulses before and after the KPB. The 40Ar/39Ar dataset did not identify such pulses and has been largely interpreted by the community to indicate an increase in eruption rates coincident with the Chicxulub impact (Renne et al., 2015; Richards et al., 2015). Although the overall agreement in eruption duration is an achievement for geochronology, it is important to clarify the limitations in comparing the two datasets and to highlight paths toward achieving higher-resolution eruption models for the Deccan Traps and for other large igneous provinces. Here, we generate chronostratigraphic models for both datasets using the same statistical techniques and show that the two datasets agree very well. More specifically, we infer that (1) age modeling of the 40Ar/39Ar dataset results in constant eruption rates with relatively large uncertainties through the duration of the Deccan Traps eruptions and provides no support for (or evidence against) the pulses identified by the U−Pb data, (2) the stratigraphic positions of the Chicxulub impact using the 40Ar/39Ar and U−Pb datasets do not agree within their uncertainties, and (3) neither dataset supports the notion of an increase in eruption rate as a result of the Chicxulub impact. We then discuss the importance of systematic uncertainties between the dating methods that challenge direct comparisons between them, and we highlight the geologic uncertainties, such as regional stratigraphic correlations, that need to be tested to ensure the accuracy of eruption models. While the production of precise and accurate geochronologic data is of course essential to studies of Earth history, our analysis underscores that the accuracy of a final result is also critically dependent on how such data are interpreted and presented to the broader community of geoscientists.
{"title":"An evaluation of Deccan Traps eruption rates using geochronologic data","authors":"B. Schoene, M. Eddy, C. Keller, K. Samperton","doi":"10.5194/gchron-3-181-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-181-2021","url":null,"abstract":"Abstract. Recent attempts to establish the eruptive history of the Deccan Traps large igneous province have used both U−Pb (Schoene et al., 2019) and\u000040Ar/39Ar (Sprain et al., 2019) geochronology. Both of these studies report dates with high precision and unprecedented coverage\u0000for a large igneous province and agree that the main phase of eruptions began near the C30n–C29r magnetic reversal and waned shortly after the\u0000C29r–C29n reversal, totaling ∼ 700–800 kyr duration. These datasets can be analyzed in finer detail to determine eruption rates, which\u0000are critical for connecting volcanism, associated volatile emissions, and any potential effects on the Earth's climate before and after the\u0000Cretaceous–Paleogene boundary (KPB). It is our observation that the community has frequently misinterpreted how the eruption rates derived from\u0000these two datasets vary across the KPB. The U−Pb dataset of Schoene et al. (2019) was interpreted by those authors to indicate four major\u0000eruptive pulses before and after the KPB. The 40Ar/39Ar dataset did not identify such pulses and has been largely interpreted by\u0000the community to indicate an increase in eruption rates coincident with the Chicxulub impact (Renne et al., 2015; Richards et al., 2015). Although\u0000the overall agreement in eruption duration is an achievement for geochronology, it is important to clarify the limitations in comparing the two\u0000datasets and to highlight paths toward achieving higher-resolution eruption models for the Deccan Traps and for other large igneous provinces. Here,\u0000we generate chronostratigraphic models for both datasets using the same statistical techniques and show that the two datasets agree very well. More\u0000specifically, we infer that (1) age modeling of the 40Ar/39Ar dataset results in constant eruption rates with relatively large\u0000uncertainties through the duration of the Deccan Traps eruptions and provides no support for (or evidence against) the pulses identified by the\u0000U−Pb data, (2) the stratigraphic positions of the Chicxulub impact using the 40Ar/39Ar and U−Pb datasets do not\u0000agree within their uncertainties, and (3) neither dataset supports the notion of an increase in eruption rate as a result of the Chicxulub\u0000impact. We then discuss the importance of systematic uncertainties between the dating methods that challenge direct comparisons between them, and we\u0000highlight the geologic uncertainties, such as regional stratigraphic correlations, that need to be tested to ensure the accuracy of eruption\u0000models. While the production of precise and accurate geochronologic data is of course essential to studies of Earth history, our analysis\u0000underscores that the accuracy of a final result is also critically dependent on how such data are interpreted and presented to the broader community\u0000of geoscientists.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":"181-198"},"PeriodicalIF":0.0,"publicationDate":"2021-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88986574","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 : 2021-04-01DOI: 10.5194/GCHRON-3-395-2021
F. Hofmann, E. Cooperdock, A. West, Dominic Hildebrandt, Kathrin Strößner, K. Farley
Abstract. We test whether X-ray micro computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning Strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of expected cosmogenic production. We present Li concentrations, major and trace element analysis, and magnetite (U-Th)/He cooling ages of samples in order to model the contribution from radiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to four times that of the cosmogenic 3He component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ka, we calibrate a magnetite 3He SLHL production rate of 116 ± 13 at g−1 a−1. We suggest that the microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.
{"title":"Exposure dating of detrital magnetite using 3He enabled by microCT and calibration of the cosmogenic 3He production rate in magnetite","authors":"F. Hofmann, E. Cooperdock, A. West, Dominic Hildebrandt, Kathrin Strößner, K. Farley","doi":"10.5194/GCHRON-3-395-2021","DOIUrl":"https://doi.org/10.5194/GCHRON-3-395-2021","url":null,"abstract":"Abstract. We test whether X-ray micro computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning Strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of expected cosmogenic production. We present Li concentrations, major and trace element analysis, and magnetite (U-Th)/He cooling ages of samples in order to model the contribution from radiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to four times that of the cosmogenic 3He component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ka, we calibrate a magnetite 3He SLHL production rate of 116 ± 13 at g−1 a−1. We suggest that the microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"8 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82893719","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}
J. Muston, M. Forster, C. Alderton, Shawn Crispin, G. Lister
Abstract. The Martabe deposits in Sumatra, Indonesia formed in a shallow crustal epithermal environment (200–350 °C) associated with mafic intrusions, usually recognised in domes, adjacent to an active right-lateral wrench system. Ten samples containing alunite were collected for high-resolution 40Ar/39Ar geochronology, to determine if overprinting fluid systems could be recognised. At the same time, ultra-high-vacuum (UHV) furnace step-heating 39Ar diffusion experiments were conducted, to determine the argon retentivity of the mineral grains being analysed. The heating schedule chosen to ensure Arrhenius data uniformly populated the inverse temperature axis, with sufficient detail to allow the application of the Fundamental Asymmetry Principle (FAP) during data analysis. The heating time for each step was chosen to ensure reasonable uniformity in terms of incremental percentage gas release during each step. Results show activation energies between 360–500 kJ/mol, with normalised frequency factor between 1.89e14s−1 and 8.62e18s−1. Closure temperatures range from 390–519 °C for a cooling rates of 20 °C/Ma, giving confidence that the ages represent growth during periods of active fluid movement and alteration. The Martabe deposit formed at temperatures
{"title":"Direct dating of overprinting fluid systems in the Martabe\u0000epithermal gold deposit using highly retentive alunite","authors":"J. Muston, M. Forster, C. Alderton, Shawn Crispin, G. Lister","doi":"10.5194/gchron-2020-41","DOIUrl":"https://doi.org/10.5194/gchron-2020-41","url":null,"abstract":"Abstract. The Martabe deposits in Sumatra, Indonesia formed in a shallow crustal epithermal environment (200–350 °C) associated with mafic intrusions, usually recognised in domes, adjacent to an active right-lateral wrench system. Ten samples containing alunite were collected for high-resolution 40Ar/39Ar geochronology, to determine if overprinting fluid systems could be recognised. At the same time, ultra-high-vacuum (UHV) furnace step-heating 39Ar diffusion experiments were conducted, to determine the argon retentivity of the mineral grains being analysed. The heating schedule chosen to ensure Arrhenius data uniformly populated the inverse temperature axis, with sufficient detail to allow the application of the Fundamental Asymmetry Principle (FAP) during data analysis. The heating time for each step was chosen to ensure reasonable uniformity in terms of incremental percentage gas release during each step. Results show activation energies between 360–500 kJ/mol, with normalised frequency factor between 1.89e14s−1 and 8.62e18s−1. Closure temperatures range from 390–519 °C for a cooling rates of 20 °C/Ma, giving confidence that the ages represent growth during periods of active fluid movement and alteration. The Martabe deposit formed at temperatures \u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"93 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74906584","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 : 2020-12-18DOI: 10.5194/GCHRON-2-425-2020
S. Tapster, J. W. Bright
Abstract. Cassiterite (SnO2) is the most common ore phase of Sn. Typically containing 1–100 µg g−1 of uranium and relatively low concentrations of common Pb, cassiterite has been increasingly targeted for U–Pb geochronology, principally via microbeam methods, to understand the timing and durations of granite-related magmatic–hydrothermal systems throughout geological time. However, due to the extreme resistance of cassiterite to most forms of acid digestion, there has been no published method permitting the complete, closed-system decomposition of cassiterite under conditions in which the basic necessities of measurement by isotope dilution can be met, leading to a paucity of reference and validation materials. To address this a new low blank (< 1 pg Pb) method for the complete acid decomposition of cassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pb purification, and thermal ionisation mass spectrometry (TIMS) analyses has been developed. Decomposition rates have been experimentally evaluated under a range of conditions. A careful balance of time and temperature is required due to competing effects (e.g. HBr oxidation), yet the decomposition of 500 µm diameter fragments of cassiterite is readily achievable over periods comparable to zircon decomposition. Its acid-resistant nature can be turned into an advantage by leaching common Pb-bearing phases (e.g. sulfides, silicates) without disturbing the U–Pb systematics of the cassiterite lattice. The archetypal Sn–W greisen deposit of Cligga Head, SW England, is used to define accuracy relative to chemical abrasion–isotope dilution–thermal ionisation mass spectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential of this new method for resolving high-resolution timescales (<0.1 %) of magmatic–hydrothermal systems. However, data also indicate that the isotopic composition of initial common Pb varies significantly, both between crystals and within a single crystal. This is attributed to significant fluid–rock interactions and the highly F-rich acidic nature of the hydrothermal system. At microbeam precision levels, this issue is largely unresolvable and can result in significant inaccuracy in interpreted ages. The ID-TIMS U–Pb method described herein can, for the first time, be used to properly characterise suitable reference materials for microbeam cassiterite U–Pb analyses, thus improving the accuracy of the U–Pb cassiterite chronometer as a whole.
{"title":"High-precision ID-TIMS cassiterite U–Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology","authors":"S. Tapster, J. W. Bright","doi":"10.5194/GCHRON-2-425-2020","DOIUrl":"https://doi.org/10.5194/GCHRON-2-425-2020","url":null,"abstract":"Abstract. Cassiterite (SnO2) is the most common ore phase of Sn. Typically\u0000containing 1–100 µg g−1 of uranium and relatively low concentrations of common\u0000Pb, cassiterite has been increasingly targeted for U–Pb geochronology,\u0000principally via microbeam methods, to understand the timing and durations\u0000of granite-related magmatic–hydrothermal systems throughout geological time.\u0000However, due to the extreme resistance of cassiterite to most forms of acid\u0000digestion, there has been no published method permitting the complete,\u0000closed-system decomposition of cassiterite under conditions in which the basic\u0000necessities of measurement by isotope dilution can be met, leading to a\u0000paucity of reference and validation materials. To address this a new low\u0000blank (< 1 pg Pb) method for the complete acid decomposition of\u0000cassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pb\u0000purification, and thermal ionisation mass\u0000spectrometry (TIMS) analyses has been developed. Decomposition rates have\u0000been experimentally evaluated under a range of conditions. A careful balance\u0000of time and temperature is required due to competing effects (e.g. HBr\u0000oxidation), yet the decomposition of 500 µm diameter fragments of\u0000cassiterite is readily achievable over periods comparable to zircon\u0000decomposition. Its acid-resistant nature can be turned into an advantage by\u0000leaching common Pb-bearing phases (e.g. sulfides, silicates) without\u0000disturbing the U–Pb systematics of the cassiterite lattice. The archetypal\u0000Sn–W greisen deposit of Cligga Head, SW England, is used to define accuracy\u0000relative to chemical abrasion–isotope dilution–thermal ionisation mass\u0000spectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential of\u0000this new method for resolving high-resolution timescales (<0.1 %) of magmatic–hydrothermal systems. However, data also indicate that the\u0000isotopic composition of initial common Pb varies significantly, both between\u0000crystals and within a single crystal. This is attributed to significant\u0000fluid–rock interactions and the highly F-rich acidic nature of the\u0000hydrothermal system. At microbeam precision levels, this issue is largely\u0000unresolvable and can result in significant inaccuracy in interpreted ages.\u0000The ID-TIMS U–Pb method described herein can, for the first time, be used to\u0000properly characterise suitable reference materials for microbeam\u0000cassiterite U–Pb analyses, thus improving the accuracy of the U–Pb\u0000cassiterite chronometer as a whole.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87110761","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 : 2020-12-16DOI: 10.5194/gchron-2-411-2020
T. Clow, J. Willenbring, M. Schaller, J. Blum, M. Christl, P. Kubik, F. von Blanckenburg
Abstract. Meteoric 10Be (10Bemet) concentrations in soil profiles have great potential as a geochronometer and a tracer of Earth surface processes, particularly in fine-grained soils lacking quartz that would preclude the use of in situ produced 10Be (10Bein situ). One prerequisite for using this technique for accurately calculating rates and dates is constraining the delivery, or flux, of 10Bemet to a site. However, few studies to date have quantified long-term (i.e., millennial) delivery rates, and none have determined a delivery rate for an eroding soil. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in eroding soils sampled from the same depth profiles to calibrate a long-term 10Bemet delivery rate. We did so on the Pinedale (∼ 21–25 kyr) and Bull Lake (∼ 140 kyr) glacial moraines at Fremont Lake, Wyoming (USA), where age, grain sizes, weathering indices, and soil properties are known, as are erosion and denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile, solving for the delivery rate of 10Bemet, and normalizing for paleomagnetic and solar intensity variations over the Holocene, we calculate 10Bemet fluxes of 1.46 (±0.20) × 106 atoms cm−2 yr−1 and 1.30 (±0.48) × 106 atoms cm−2 yr−1 to the Pinedale and Bull Lake moraines, respectively, and compare these values to two widely used 10Bemet delivery rate estimation methods that substantially differ for this site. Accurately estimating the 10Bemet flux using these methods requires a consideration of spatial scale and temporally varying parameters (i.e., paleomagnetic field intensity, solar modulation) to ensure the most realistic estimates of 10Bemet-derived erosion rates in future studies.
{"title":"Calibrating a long-term meteoric 10Be delivery rate into eroding western US glacial deposits by comparing meteoric and in situ produced 10Be depth profiles","authors":"T. Clow, J. Willenbring, M. Schaller, J. Blum, M. Christl, P. Kubik, F. von Blanckenburg","doi":"10.5194/gchron-2-411-2020","DOIUrl":"https://doi.org/10.5194/gchron-2-411-2020","url":null,"abstract":"Abstract. Meteoric 10Be (10Bemet) concentrations in\u0000soil profiles have great potential as a geochronometer and a tracer of Earth\u0000surface processes, particularly in fine-grained soils lacking quartz that\u0000would preclude the use of in situ produced 10Be (10Bein situ). One\u0000prerequisite for using this technique for accurately calculating rates and\u0000dates is constraining the delivery, or flux, of 10Bemet to a site.\u0000However, few studies to date have quantified long-term (i.e., millennial)\u0000delivery rates, and none have determined a delivery rate for an eroding\u0000soil. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in eroding soils sampled\u0000from the same depth profiles to calibrate a long-term 10Bemet\u0000delivery rate. We did so on the Pinedale (∼ 21–25 kyr) and Bull\u0000Lake (∼ 140 kyr) glacial moraines at Fremont Lake, Wyoming\u0000(USA), where age, grain sizes, weathering indices, and soil properties are\u0000known, as are erosion and denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile,\u0000solving for the delivery rate of 10Bemet, and normalizing for\u0000paleomagnetic and solar intensity variations over the Holocene, we calculate\u000010Bemet fluxes of 1.46 (±0.20) × 106 atoms cm−2 yr−1 and 1.30 (±0.48) × 106 atoms cm−2 yr−1 to\u0000the Pinedale and Bull Lake moraines, respectively, and compare these values\u0000to two widely used 10Bemet delivery rate estimation methods that\u0000substantially differ for this site. Accurately estimating the 10Bemet\u0000flux using these methods requires a consideration of spatial scale and\u0000temporally varying parameters (i.e., paleomagnetic field intensity, solar\u0000modulation) to ensure the most realistic estimates of\u000010Bemet-derived erosion rates in future studies.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"52 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88897109","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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