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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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}
B. Härtel, R. Jonckheere, B. Wauschkuhn, L. Ratschbacher
Abstract. Zircon Raman dating based on irradiation damage is a debated concept but not an established geo-/thermochronological method. One issue is the temperature range of radiation-damage annealing over geological timescales. We conducted isochronal and isothermal annealing experiments on radiation-damaged zircons between 500 and 1000 ∘C for durations between 10 min and 5 d to describe the annealing kinetics. We measured the widths (Γ) and positions (ω) of the ν1(SiO4), ν2(SiO4), and ν3(SiO4) internal Raman bands, and the external rotation Raman band at ∼974, 438, 1008, and 356 cm−1 after each annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed activation energy model to the fractional annealing data, calculated from the widths of the ν2(SiO4), ν3(SiO4), and external rotation bands. From the kinetic models, we determined closure temperatures Tc for damage accumulation for each Raman band. Tc ranges from 330 to 370 ∘C for the internal ν2(SiO4) and ν3(SiO4) bands; the external rotation band is more sensitive to thermal annealing (Tc∼260 to 310 ∘C). Our estimates are in general agreement with previous ones, but more geological evidence is needed to validate the results. The Tc difference for the different Raman bands offers the prospect of a multi-closure-temperature zircon Raman thermochronometer.
{"title":"The closure temperature(s) of zircon Raman dating","authors":"B. Härtel, R. Jonckheere, B. Wauschkuhn, L. Ratschbacher","doi":"10.5194/gchron-2020-39","DOIUrl":"https://doi.org/10.5194/gchron-2020-39","url":null,"abstract":"Abstract. Zircon Raman dating based on irradiation damage\u0000is a debated concept but not an established geo-/thermochronological\u0000method. One issue is the temperature range of radiation-damage annealing\u0000over geological timescales. We conducted isochronal and isothermal annealing\u0000experiments on radiation-damaged zircons between 500 and 1000 ∘C\u0000for durations between 10 min and 5 d to describe the annealing\u0000kinetics. We measured the widths (Γ) and positions (ω) of\u0000the ν1(SiO4), ν2(SiO4), and ν3(SiO4) internal Raman bands, and the external rotation Raman\u0000band at ∼974, 438, 1008, and 356 cm−1 after each\u0000annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed\u0000activation energy model to the fractional annealing data, calculated from\u0000the widths of the ν2(SiO4), ν3(SiO4), and\u0000external rotation bands. From the kinetic models, we determined closure\u0000temperatures Tc for damage accumulation for each Raman band. Tc\u0000ranges from 330 to 370 ∘C for the internal ν2(SiO4)\u0000and ν3(SiO4) bands; the external rotation band is more\u0000sensitive to thermal annealing (Tc∼260 to\u0000310 ∘C). Our estimates are in general agreement with previous\u0000ones, but more geological evidence is needed to validate the results. The\u0000Tc difference for the different Raman bands offers the prospect of a\u0000multi-closure-temperature zircon Raman thermochronometer.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2020-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82387726","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}
Abstract. Zircon U–Pb geochronology is a staple of crustal evolution studies and sedimentary provenance analysis. Constructing (detrital) U–Pb age spectra is straightforward for concordant 206Pb/238U and 207Pb/206Pb compositions. But unfortunately, many U–Pb datasets contain a significant proportion of discordant analyses. This paper investigates two decisions that must be made when analysing such discordant U–Pb data. First, the analyst must choose whether to use the 206Pb/238U or the 207Pb/206Pb date. The 206Pb/238U method is more precise for young samples, whereas the 207Pb/206Pb method is better suited for old samples. However there is no agreement which “cutoff” should be used to switch between the two. This subjective decision can be avoided by using single-grain concordia ages. These represent a kind of weighted mean between the 206Pb/238U and 207Pb/206Pb methods, which offers better precision than either of the latter two methods. A second subjective decision is how to define the discordance cutoff between “good” and “bad” data. Discordance is usually defined as (1) the relative age difference between the 206Pb/238U and 207Pb/206Pb dates. However, this paper shows that several other definitions are possible as well, including (2) the absolute age difference; (3) the common-Pb fraction according to the Stacey–Kramers mantle evolution model; (4) the p value of concordance; (5) the perpendicular log ratio (or “Aitchison”) distance to the concordia line; and (6) the log ratio distance to the maximum likelihood composition on the concordia line. Applying these six discordance filters to a 70 869-grain dataset of zircon U–Pb compositions reveals that (i) the relative age discordance filter tends to suppress the young age components in U–Pb age spectra, whilst inflating the older age components; (ii) the Stacey–Kramers discordance filter is more likely to reject old grains and less likely to reject young ones; (iii) the p-value-based discordance filter has the undesirable effect of biasing the results towards the least precise measurements; (iv) the log-ratio-based discordance filters are strictest for Proterozoic grains and more lenient for Phanerozoic and Archaean age components; (v) of all the methods, the log ratio distance to the concordia composition produces the best results, in the sense that it produces age spectra that most closely match those of the unfiltered data: it sharpens age spectra but does not change their shape. The popular relative age definition fares the worst according to this criterion. All the methods presented in this paper have been implemented in the IsoplotR toolbox for geochronology.
{"title":"On the treatment of discordant detrital zircon U–Pb data","authors":"P. Vermeesch","doi":"10.5194/gchron-2020-38","DOIUrl":"https://doi.org/10.5194/gchron-2020-38","url":null,"abstract":"Abstract. Zircon U–Pb geochronology is a staple of crustal evolution studies and sedimentary provenance analysis. Constructing (detrital) U–Pb age spectra is straightforward for concordant 206Pb/238U and 207Pb/206Pb compositions. But unfortunately, many U–Pb datasets contain a significant proportion of discordant analyses. This paper investigates two decisions that must be made when analysing such discordant U–Pb data. First, the analyst must choose whether to use the 206Pb/238U or the 207Pb/206Pb date. The 206Pb/238U method is more precise for young samples, whereas the 207Pb/206Pb method is better suited for old samples. However there is no agreement which “cutoff” should be used to switch between the two. This subjective decision can be avoided by using single-grain concordia ages. These represent a kind of weighted mean between the 206Pb/238U and 207Pb/206Pb methods, which offers better precision than either of the latter two methods. A second subjective decision is how to define the discordance cutoff between “good” and “bad” data. Discordance is usually defined as (1) the relative age difference between the 206Pb/238U and 207Pb/206Pb dates. However, this paper shows that several other definitions are possible as well, including (2) the absolute age difference; (3) the common-Pb fraction according to the Stacey–Kramers mantle evolution model; (4) the p value of concordance; (5) the perpendicular log ratio (or “Aitchison”) distance to the concordia line; and (6) the log ratio distance to the maximum likelihood composition on the concordia line. Applying these six discordance filters to a 70 869-grain dataset of zircon U–Pb compositions reveals that (i) the relative age discordance filter tends to suppress the young age components in U–Pb age spectra, whilst inflating the older age components; (ii) the Stacey–Kramers discordance filter is more likely to reject old grains and less likely to reject young ones; (iii) the p-value-based discordance filter has the undesirable effect of biasing the results towards the least precise measurements; (iv) the log-ratio-based discordance filters are strictest for Proterozoic grains and more lenient for Phanerozoic and Archaean age components; (v) of all the methods, the log ratio distance to the concordia composition produces the best results, in the sense that it produces age spectra that most closely match those of the unfiltered data: it sharpens age spectra but does not change their shape. The popular relative age definition fares the worst according to this criterion. All the methods presented in this paper have been implemented in the IsoplotR toolbox for geochronology.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2020-12-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72672528","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 : 2020-11-04DOI: 10.5194/gchron-2-305-2020
D. Mueller, F. Preusser, Marius W. Buechi, Lukas Gegg, G. Deplazes
Abstract. Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading and partial bleaching being some of the obstacles to overcome for the establishment of decisive chronologies. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar, and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld in northern Switzerland, are systematically assessed. Standard performance tests are conducted on all four fractions. Deconvolution of luminescence signals of the quartz fractions is implemented and shows the dominance of stable fast components. Reader-specific low preheat temperatures are investigated on the infrared stimulated luminescence (IRSL) signal of feldspar. Thermal stability of this signal is found for low preheats, and thermal quenching could be excluded for higher preheats. However, anomalous fading is observed in the feldspar and polymineral IRSL signals and two correction approaches are applied. For one approach, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar, as well as the fine-grained polymineral fraction of one sample, are in chrono-stratigraphic agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Hence, the impact of alpha efficiency and water content on the dose rate and thus the ages are assessed. A finite explanation for the observed discrepancies remains lacking, but this systematic investigation of different luminescence signals allows for the establishment of a chronology for the palaeovalley fill dating back to at least Marine Isotope Stage 6 (MIS 6).
摘要发光测年已经成为了解瑞士阿尔卑斯山北部前陆更新世冰川推进的支柱。然而，该地区的石英和长石作为剂量计同样具有挑战性，异常褪色和部分漂白是建立决定性年代学需要克服的一些障碍。在这项研究中，系统地评估了来自瑞士北部Rinikerfeld古山谷的8个样品的粗粒和细粒石英、长石和多矿物组分的发光特性。对所有四个分数进行标准性能测试。对石英组分的荧光信号进行了反卷积，结果表明稳定的快速组分占主导地位。研究了长石红外激发发光(IRSL)信号的阅读器特异性低温预热。该信号在低预热时具有热稳定性，在高预热时可以排除热淬火。但长石和多矿物的红外光谱信号存在异常衰减现象，采用了两种校正方法。一种方法是，褪色校正的粗粒长石年龄与石英的年龄一致。一般来说，一个样品的粗粒石英和长石以及细粒多矿物部分在年代地层上是一致的，可以忽略部分漂白的证据。然而，细粒石英的年龄被发现低估了粗粒石英的年龄。因此，评估了α效率和含水量对剂量率的影响，从而评估了年龄。对所观察到的差异仍然缺乏明确的解释，但是对不同发光信号的系统研究允许建立一个至少可以追溯到海洋同位素阶段6 (MIS 6)的古山谷填充物年表。
{"title":"Luminescence properties and dating of glacial to periglacial sediments from northern Switzerland","authors":"D. Mueller, F. Preusser, Marius W. Buechi, Lukas Gegg, G. Deplazes","doi":"10.5194/gchron-2-305-2020","DOIUrl":"https://doi.org/10.5194/gchron-2-305-2020","url":null,"abstract":"Abstract. Luminescence dating has become a pillar of the understanding of\u0000Pleistocene glacial advances in the northern foreland of the Swiss Alps.\u0000However, both quartz and feldspar from the region are equally challenging as\u0000dosimeters with anomalous fading and partial bleaching being some of the\u0000obstacles to overcome for the establishment of decisive chronologies. In\u0000this study, luminescence properties of coarse- and fine-grained quartz,\u0000feldspar, and polymineral fractions of eight samples from a palaeovalley,\u0000Rinikerfeld in northern Switzerland, are systematically assessed. Standard\u0000performance tests are conducted on all four fractions. Deconvolution of\u0000luminescence signals of the quartz fractions is implemented and shows the\u0000dominance of stable fast components. Reader-specific low preheat\u0000temperatures are investigated on the infrared stimulated luminescence (IRSL)\u0000signal of feldspar. Thermal stability of this signal is found for low\u0000preheats, and thermal quenching could be excluded for higher preheats.\u0000However, anomalous fading is observed in the feldspar and polymineral IRSL\u0000signals and two correction approaches are applied. For one approach, fading\u0000corrected coarse-grained feldspar ages are consistent with those derived\u0000from quartz. In general, coarse-grained quartz and feldspar, as well as the\u0000fine-grained polymineral fraction of one sample, are in chrono-stratigraphic\u0000agreement and present negligible evidence for partial bleaching. However,\u0000ages derived from fine-grained quartz are found to underestimate those of\u0000the coarse-grained quartz fractions. Hence, the impact of alpha efficiency\u0000and water content on the dose rate and thus the ages are assessed. A\u0000finite explanation for the observed discrepancies remains lacking, but this\u0000systematic investigation of different luminescence signals allows for the\u0000establishment of a chronology for the palaeovalley fill dating back to at\u0000least Marine Isotope Stage 6 (MIS 6).\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77706729","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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