Pub Date : 2021-12-13DOI: 10.5194/gchron-3-545-2021
L. Richards, F. Jourdan, A. Collins, R. King
Abstract. The Salt Range Formation is an extensive evaporite sequence in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra Mine, where it accommodates approximately 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4⚫2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step-heating laser and furnace methods. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1. 40Ar/39Ar age results for both samples did not return any reliable crystallisation age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of ∼514 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured step ages for samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Otherwise, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.
{"title":"Deformation recorded in polyhalite from evaporite detachments revealed by 40Ar ∕ 39Ar dating","authors":"L. Richards, F. Jourdan, A. Collins, R. King","doi":"10.5194/gchron-3-545-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-545-2021","url":null,"abstract":"Abstract. The Salt Range Formation is an extensive evaporite sequence in northern\u0000Pakistan that has acted as the primary detachment accommodating Himalayan\u0000orogenic deformation from the north. This rheologically weak formation forms\u0000a mylonite in the Khewra Mine, where it accommodates approximately 40 km\u0000displacement and is comprised of intercalated halite and potash salts and\u0000gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4⚫2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step-heating laser and furnace methods. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of\u0000ca. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1.\u000040Ar/39Ar age results for both samples did not return any reliable\u0000crystallisation age. This is not unexpected as polyhalite is prone to\u000040Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of ∼514 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured step ages for samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Otherwise, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"36 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80485089","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 : 2021-12-13DOI: 10.5194/gchron-2021-43-supplement
B. Goehring, B. Menounos, Gerald Osbron, Adam C. Hawkins, B. Ward
Abstract. We present a new in situ produced cosmogenic beryllium-10 and carbon-14 nuclide chronology from two sets (outer and inner) of alpine glacier moraines from the Grey Hunter massif of southern Yukon Territory, Canada. The chronology potential of moraines deposited by alpine glaciers outside the limits of the Last Glacial Maximum (LGM) ice sheets potentially provide a less-ambiguous archive of mass balance, and hence climate than can be inferred from the extents of ice sheets themselves. Results for both nuclides are inconclusive for the outer moraines, with evidence for pre-LGM deposition (beryllium-10) and Holocene deposition (carbon-14). Beryllium-10 results from the inner moraine are suggestive of canonical LGM deposition, but with relatively high scatter. Conversely, in situ carbon-14 results from the inner moraines are tightly clustered and suggestive of terminal Younger Dryas deposition. We explore plausible scenarios leading to the observed differences between nuclides and find that the most parsimonious explanation for the outer moraines is that of pre-LGM deposition, but many of the sampled boulder surfaces were not exhumed from within the moraine until the Holocene. Our results thus imply that the inner and outer moraines sampled pre- and post-date the canonical LGM and that moraines dating to the LGM are lacking likely due to overriding by the subsequent Late Glacial/earliest Holocene advance.
{"title":"Supplementary material to \"Reconciling the Apparent Absence of a Last Glacial Maximum Alpine Glacial Advance, Yukon Territory, Canada, through Cosmogenic Beryllium-10 and Carbon-14 Measurements\"","authors":"B. Goehring, B. Menounos, Gerald Osbron, Adam C. Hawkins, B. Ward","doi":"10.5194/gchron-2021-43-supplement","DOIUrl":"https://doi.org/10.5194/gchron-2021-43-supplement","url":null,"abstract":"Abstract. We present a new in situ produced cosmogenic beryllium-10 and carbon-14 nuclide chronology from two sets (outer and inner) of alpine glacier moraines from the Grey Hunter massif of southern Yukon Territory, Canada. The chronology potential of moraines deposited by alpine glaciers outside the limits of the Last Glacial Maximum (LGM) ice sheets potentially provide a less-ambiguous archive of mass balance, and hence climate than can be inferred from the extents of ice sheets themselves. Results for both nuclides are inconclusive for the outer moraines, with evidence for pre-LGM deposition (beryllium-10) and Holocene deposition (carbon-14). Beryllium-10 results from the inner moraine are suggestive of canonical LGM deposition, but with relatively high scatter. Conversely, in situ carbon-14 results from the inner moraines are tightly clustered and suggestive of terminal Younger Dryas deposition. We explore plausible scenarios leading to the observed differences between nuclides and find that the most parsimonious explanation for the outer moraines is that of pre-LGM deposition, but many of the sampled boulder surfaces were not exhumed from within the moraine until the Holocene. Our results thus imply that the inner and outer moraines sampled pre- and post-date the canonical LGM and that moraines dating to the LGM are lacking likely due to overriding by the subsequent Late Glacial/earliest Holocene advance.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"5 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84456028","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. The majority of palaeoenvironmental information is inferred from proxy data contained in accretionary sediments, called geo-archives. The validity of proxy data and analysis workflows are usually assumed implicitly, with systematic tests and uncertainty estimates restricted to modern analogue studies or reduced-complexity case studies. However, a more generic and consistent approach to exploring the validity and variability of proxy functions would be to translate a given geo-archive into a model scenario: a "virtual twin". Here, we introduce a conceptual framework and numerical toolset that allows the definition and analysis of synthetic sediment sections. The R package sandbox describes arbitrary stratigraphically consistent deposits by depth-dependent rules and grain-specific parameters, allowing full scalability and flexibility. Virtual samples can be taken, resulting in discrete grain-mixtures with well-defined parameters. These samples can then be virtually prepared and analysed, for example to test hypotheses. We illustrate the concept of sandbox, explain how a sediment section can be mapped into the model and, by focusing on an exemplary field of application, we explore universal geochronological research questions related to the effects of sample geometry and grain-size specific age inheritance. We summarise further application scenarios of the model framework, relevant for but not restricted to the broader geochronological community.
{"title":"sandbox – Creating and Analysing Synthetic Sediment Sections with R","authors":"M. Dietze, S. Kreutzer, M. Fuchs, Sascha Meszner","doi":"10.5194/gchron-2021-39","DOIUrl":"https://doi.org/10.5194/gchron-2021-39","url":null,"abstract":"Abstract. The majority of palaeoenvironmental information is inferred from proxy data contained in accretionary sediments, called geo-archives. The validity of proxy data and analysis workflows are usually assumed implicitly, with systematic tests and uncertainty estimates restricted to modern analogue studies or reduced-complexity case studies. However, a more generic and consistent approach to exploring the validity and variability of proxy functions would be to translate a given geo-archive into a model scenario: a \"virtual twin\". Here, we introduce a conceptual framework and numerical toolset that allows the definition and analysis of synthetic sediment sections. The R package sandbox describes arbitrary stratigraphically consistent deposits by depth-dependent rules and grain-specific parameters, allowing full scalability and flexibility. Virtual samples can be taken, resulting in discrete grain-mixtures with well-defined parameters. These samples can then be virtually prepared and analysed, for example to test hypotheses. We illustrate the concept of sandbox, explain how a sediment section can be mapped into the model and, by focusing on an exemplary field of application, we explore universal geochronological research questions related to the effects of sample geometry and grain-size specific age inheritance. We summarise further application scenarios of the model framework, relevant for but not restricted to the broader geochronological community.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"94 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86158624","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. We introduce a set of methods for analyzing cosmogenic-nuclide depth profiles that formally integrates surface erosion and muogenic production, while retaining the advantages of the linear inversion. For surfaces with erosion, we present solutions for both erosion rate and total eroded thickness, each with their own advantages. For practical applications, erosion must be constrained from external information, such as soil-profile analysis. By combining linear inversion with Monte Carlo simulation of error propagation, our method jointly assesses uncertainty arising from measurement error and erosion constraints. Using example depth profile data sets from the Beida River, northwest China and Lees Ferry, Arizona, we show that our methods robustly produce comparable ages for surfaces with different erosion rates and inheritance. Through hypothetical examples, we further show that both the erosion rate and eroded-thickness approaches produce reasonable age estimates so long as the total erosion less than twice the nucleon attenuation length. Overall, lack of precise constraints for erosion rate tends to be the largest contributor of age uncertainty, compared to the error from omitting muogenic production or radioactive decay.
{"title":"Combined linear regression and Monte Carlo approach to modelling exposure age depth profiles","authors":"Yiran Wang, M. Oskin","doi":"10.5194/gchron-2021-34","DOIUrl":"https://doi.org/10.5194/gchron-2021-34","url":null,"abstract":"Abstract. We introduce a set of methods for analyzing cosmogenic-nuclide depth profiles that formally integrates surface erosion and muogenic production, while retaining the advantages of the linear inversion. For surfaces with erosion, we present solutions for both erosion rate and total eroded thickness, each with their own advantages. For practical applications, erosion must be constrained from external information, such as soil-profile analysis. By combining linear inversion with Monte Carlo simulation of error propagation, our method jointly assesses uncertainty arising from measurement error and erosion constraints. Using example depth profile data sets from the Beida River, northwest China and Lees Ferry, Arizona, we show that our methods robustly produce comparable ages for surfaces with different erosion rates and inheritance. Through hypothetical examples, we further show that both the erosion rate and eroded-thickness approaches produce reasonable age estimates so long as the total erosion less than twice the nucleon attenuation length. Overall, lack of precise constraints for erosion rate tends to be the largest contributor of age uncertainty, compared to the error from omitting muogenic production or radioactive decay.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"55 2 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-11-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77353746","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. Dosimetric dating techniques rely on accurate and precise determination of environmental radioactivity. Gamma spectrometry is the method of choice for determining the activity of 238U, 232Th and 40K. With the aim to standardise gamma-spectrometric procedures for the purpose of determining accurate parent nuclide activities in natural samples, we outline here basics of gamma spectrometry and practical laboratory procedures. This includes gamma radiation and instrumentation, sample preparation, finding the suitable measurement geometry and sample size for a given detector and using the most suitable energy peaks in a gamma spectrum. The issue of correct efficiency calibration is highlighted. The procedures outlined are required for estimating contemporary parent nuclide activity. For estimating changing activities during burial specific data analyses are required and these are also highlighted.
{"title":"Technical note: Quantifying Uranium-series disequilibrium in natural samples for dosimetric dating – Part 1: gamma spectrometry","authors":"B. Mauz, P. Nolan, P. Appleby","doi":"10.5194/gchron-2021-32","DOIUrl":"https://doi.org/10.5194/gchron-2021-32","url":null,"abstract":"Abstract. Dosimetric dating techniques rely on accurate and precise determination of environmental radioactivity. Gamma spectrometry is the method of choice for determining the activity of 238U, 232Th and 40K. With the aim to standardise gamma-spectrometric procedures for the purpose of determining accurate parent nuclide activities in natural samples, we outline here basics of gamma spectrometry and practical laboratory procedures. This includes gamma radiation and instrumentation, sample preparation, finding the suitable measurement geometry and sample size for a given detector and using the most suitable energy peaks in a gamma spectrum. The issue of correct efficiency calibration is highlighted. The procedures outlined are required for estimating contemporary parent nuclide activity. For estimating changing activities during burial specific data analyses are required and these are also highlighted.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73264943","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 : 2021-10-29DOI: 10.5194/gchron-3-525-2021
R. Smedley, D. Small, Richard S. Jones, S. Brough, J. Bradley, Geraint T. H. Jenkins
Abstract. A new luminescence erosion meter has huge potential for inferring erosion rates on sub-millennial scales for both steady and transient states of erosion, which is not currently possible with any existing techniques capable of measuring erosion. This study applies new rock luminescence techniques to a well-constrained scenario provided by the Beinn Alligin rock avalanche, NW Scotland. Boulders in this deposit are lithologically consistent and have known cosmogenic nuclide ages and independently derived Holocene erosion rates. We find that luminescence-derived exposure ages for the Beinn Alligin rock avalanche were an order of magnitude younger than existing cosmogenic nuclide exposure ages, suggestive of high erosion rates (as supported by field evidence of quartz grain protrusions on the rock surfaces). Erosion rates determined by luminescence were consistent with independently derived rates measured from boulder edge roundness. Inversion modelling indicates a transient state of erosion reflecting the stochastic nature of erosional processes over the last ∼4.5 kyr in the wet, temperate climate of NW Scotland. Erosion was likely modulated by known fluctuations in moisture availability and to a lesser extent temperature, which controlled the extent of chemical weathering of these highly lithified rocks prior to erosion. The use of a multi-elevated temperature, post-infra-red, infra-red stimulated luminescence (MET-pIRIR) protocol (50, 150 and 225 ∘C) was advantageous as it identified samples with complexities that would not have been observed using only the standard infra-red stimulated luminescence (IRSL) signal measured at 50 ∘C, such as that introduced by within-sample variability (e.g. surficial coatings). This study demonstrates that the luminescence erosion meter can infer accurate erosion rates on sub-millennial scales and identify transient states of erosion (i.e. stochastic processes) in agreement with independently derived erosion rates for the same deposit.
{"title":"Erosion rates in a wet, temperate climate derived from rock luminescence techniques","authors":"R. Smedley, D. Small, Richard S. Jones, S. Brough, J. Bradley, Geraint T. H. Jenkins","doi":"10.5194/gchron-3-525-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-525-2021","url":null,"abstract":"Abstract. A new luminescence erosion meter has huge potential for inferring erosion rates on sub-millennial scales for both steady and transient states of erosion, which is not currently possible with any existing techniques capable of measuring erosion. This study applies new rock luminescence techniques to a well-constrained scenario provided by the Beinn Alligin rock avalanche, NW Scotland. Boulders in this deposit are lithologically consistent and have known cosmogenic nuclide ages and independently derived Holocene erosion rates. We find that luminescence-derived exposure ages for the Beinn Alligin rock avalanche were an order of magnitude younger than existing cosmogenic nuclide exposure ages, suggestive of high erosion rates (as supported by field evidence of quartz grain protrusions on the rock surfaces). Erosion rates determined by luminescence were consistent with independently derived rates measured from boulder edge roundness. Inversion modelling indicates a transient state of erosion reflecting the stochastic nature of erosional processes over the last ∼4.5 kyr in the wet, temperate climate of NW Scotland. Erosion was likely modulated by known fluctuations in moisture availability and to a lesser extent temperature, which controlled the extent of chemical weathering of these highly lithified rocks prior to erosion. The use of a multi-elevated temperature, post-infra-red, infra-red stimulated luminescence (MET-pIRIR) protocol (50, 150 and 225 ∘C) was advantageous as it identified samples with complexities that would not have been observed using only the standard infra-red stimulated luminescence (IRSL) signal measured at 50 ∘C, such as that introduced by within-sample variability (e.g. surficial coatings). This study demonstrates that the luminescence erosion meter can infer accurate erosion rates on sub-millennial scales and identify transient states of erosion (i.e. stochastic processes) in agreement with independently derived erosion rates for the same deposit.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"106 3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-10-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78260379","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 : 2021-10-19DOI: 10.5194/gchron-3-505-2021
A. Christ, P. Bierman, J. Lamp, J. Schaefer, G. Winckler
Abstract. The preservation of cosmogenic nuclides that accumulated during periods of prior exposure but were not subsequently removed by erosion or radioactive decay complicates interpretation of exposure, erosion, and burial ages used for a variety of geomorphological applications. In glacial settings, cold-based, non-erosive glacier ice may fail to remove inventories of inherited nuclides in glacially transported material. As a result, individual exposure ages can vary widely across a single landform (e.g., moraine) and exceed the expected or true depositional age. The surface processes that contribute to inheritance remain poorly understood, thus limiting interpretations of cosmogenic nuclide datasets in glacial environments. Here, we present a compilation of new and previously published exposure ages of multiple lithologies in local Last Glacial Maximum (LGM) and older Pleistocene glacial sediments in the McMurdo Sound region of Antarctica. Unlike most Antarctic exposure chronologies, we are able to compare exposure ages of local LGM sediments directly against an independent radiocarbon chronology of fossil algae from the same sedimentary unit that brackets the age of the local LGM between 12.3 and 19.6 ka. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source and surface processes prior to, during, and after glacial entrainment explain scatter. 10Be exposure ages of quartz in granite, sourced from the base of the stratigraphic section in the Transantarctic Mountains, are scattered but young, suggesting that clasts entrained by sub-glacial plucking can generate reasonable apparent exposure ages. 3He exposure ages of pyroxene in Ferrar Dolerite, which crops out above outlet glaciers in the Transantarctic Mountains, are older, which suggests that clasts initially exposed on cliff faces and glacially entrained by rock fall carry inherited nuclides. 3He exposure ages of olivine in basalt from local volcanic bedrock in the McMurdo Sound region contain many excessively old ages but also have a bimodal distribution with peak probabilities that slightly pre-date and post-date the local LGM; this suggests that glacial clasts from local bedrock record local landscape exposure. With the magnitude and geological processes contributing to age scatter in mind, we examine exposure ages of older glacial sediments deposited by the most extensive ice sheet to inundate McMurdo Sound during the Pleistocene. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide inventories held in Antarctic glacial sediments.
{"title":"Cosmogenic nuclide exposure age scatter records glacial history and processes in McMurdo Sound, Antarctica","authors":"A. Christ, P. Bierman, J. Lamp, J. Schaefer, G. Winckler","doi":"10.5194/gchron-3-505-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-505-2021","url":null,"abstract":"Abstract. The preservation of cosmogenic nuclides that accumulated during periods of prior exposure but were not subsequently removed by erosion or\u0000radioactive decay complicates interpretation of exposure, erosion, and burial ages used for a variety of geomorphological applications. In glacial\u0000settings, cold-based, non-erosive glacier ice may fail to remove inventories of inherited nuclides in glacially transported material. As a result,\u0000individual exposure ages can vary widely across a single landform (e.g., moraine) and exceed the expected or true depositional age. The surface\u0000processes that contribute to inheritance remain poorly understood, thus limiting interpretations of cosmogenic nuclide datasets in glacial\u0000environments. Here, we present a compilation of new and previously published exposure ages of multiple lithologies in local Last Glacial Maximum\u0000(LGM) and older Pleistocene glacial sediments in the McMurdo Sound region of Antarctica. Unlike most Antarctic exposure chronologies, we are able to\u0000compare exposure ages of local LGM sediments directly against an independent radiocarbon chronology of fossil algae from the same sedimentary unit\u0000that brackets the age of the local LGM between 12.3 and 19.6 ka. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source\u0000and surface processes prior to, during, and after glacial entrainment explain scatter. 10Be exposure ages of quartz in granite, sourced\u0000from the base of the stratigraphic section in the Transantarctic Mountains, are scattered but young, suggesting that clasts entrained by sub-glacial\u0000plucking can generate reasonable apparent exposure ages. 3He exposure ages of pyroxene in Ferrar Dolerite, which crops out above outlet\u0000glaciers in the Transantarctic Mountains, are older, which suggests that clasts initially exposed on cliff faces and glacially entrained by rock\u0000fall carry inherited nuclides. 3He exposure ages of olivine in basalt from local volcanic bedrock in the McMurdo Sound region contain many\u0000excessively old ages but also have a bimodal distribution with peak probabilities that slightly pre-date and post-date the local LGM; this suggests\u0000that glacial clasts from local bedrock record local landscape exposure. With the magnitude and geological processes contributing to age scatter in\u0000mind, we examine exposure ages of older glacial sediments deposited by the most extensive ice sheet to inundate McMurdo Sound during the\u0000Pleistocene. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide\u0000inventories held in Antarctic glacial sediments.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"17 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87388658","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 : 2021-09-23DOI: 10.5194/gchron-3-465-2021
J. Hopkins, Janine E. Bidmead, D. Lowe, R. Wysoczanski, B. Pillans, L. Ashworth, A. Rees, F. Tuckett
Abstract. Although analyses of tephra-derived glass shards have been undertaken in New Zealand for nearly four decades (pioneered by Paul Froggatt), our study is the first to systematically develop a formal, comprehensive, open-access reference dataset of glass-shard compositions for New Zealand tephras. These data will provide an important reference tool for future studies to identify and correlate tephra deposits and for associated petrological and magma-related studies within New Zealand and beyond. Here we present the foundation dataset for TephraNZ, an open-access reference dataset for selected tephra deposits in New Zealand. Prominent, rhyolitic, tephra deposits from the Quaternary were identified, with sample collection targeting original type sites or reference locations where the tephra's identification is unequivocally known based on independent dating and/or mineralogical techniques. Glass shards were extracted from the tephra deposits, and major- and trace-element geochemical compositions were determined. We discuss in detail the data reduction process used to obtain the results and propose that future studies follow a similar protocol in order to gain comparable data. The dataset contains analyses of glass shards from 23 proximal and 27 distal tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal yr BP) to the Hikuroa Pumice member (2.0 ± 0.6 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone. We report 1385 major-element analyses obtained by electron microprobe (EMPA), and 590 trace-element analyses obtained by laser ablation (LA)-ICP-MS, on individual glass shards. Using principal component analysis (PCA), Euclidean similarity coefficients, and geochemical investigation, we show that chemical compositions of glass shards from individual eruptions are commonly distinguished by major elements, especially CaO, TiO2, K2O, and FeOtt (Na2O+K2O and SiO2/K2O), but not always. For those tephras with similar glass major-element signatures, some can be distinguished using trace elements (e.g. HFSEs: Zr, Hf, Nb; LILE: Ba, Rb; REE: Eu, Tm, Dy, Y, Tb, Gd, Er, Ho, Yb, Sm) and trace-element ratios (e.g. LILE/HFSE: Ba/Th, Ba/Zr, Rb/Zr; HFSE/HREE: Zr/Y, Zr/Yb, Hf/Y; LREE/HREE: La/Yb, Ce/Yb). Geochemistry alone cannot be used to distinguish between glass shards from the following tephra groups: Taupō (Unit Y in the post-Ōruanui eruption sequence of Taupō volcano) and Waimihia (Unit S); Poronui (Unit C) and Karapiti (Unit B); Rotorua and Rerewhakaaitu; and Kawakawa/Ōruanui, and Okaia. Other characteristics, including stratigraphic relationships and age, can be used to separate and distinguish all of these otherwise-similar tephra deposits except Poronui and Karapiti. Bimodality caused by K2O variability is newly identified in Poihipi and Tahuna tephras. Using glass-shard compositions, tephra sourced from Taupō Volcanic Centre (TVC) and Mangakino Volcanic Centre (MgVC) can be separated using
{"title":"TephraNZ: a major- and trace-element reference dataset for glass-shard analyses from prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation","authors":"J. Hopkins, Janine E. Bidmead, D. Lowe, R. Wysoczanski, B. Pillans, L. Ashworth, A. Rees, F. Tuckett","doi":"10.5194/gchron-3-465-2021","DOIUrl":"https://doi.org/10.5194/gchron-3-465-2021","url":null,"abstract":"Abstract. Although analyses of tephra-derived glass shards have been undertaken in New\u0000Zealand for nearly four decades (pioneered by Paul Froggatt), our study is\u0000the first to systematically develop a formal, comprehensive, open-access\u0000reference dataset of glass-shard compositions for New Zealand tephras. These\u0000data will provide an important reference tool for future studies to identify\u0000and correlate tephra deposits and for associated petrological and\u0000magma-related studies within New Zealand and beyond. Here we present the\u0000foundation dataset for TephraNZ, an open-access reference dataset for\u0000selected tephra deposits in New Zealand. Prominent, rhyolitic, tephra deposits from the Quaternary were identified,\u0000with sample collection targeting original type sites or reference locations\u0000where the tephra's identification is unequivocally known based on\u0000independent dating and/or mineralogical techniques. Glass shards were\u0000extracted from the tephra deposits, and major- and trace-element geochemical\u0000compositions were determined. We discuss in detail the data reduction\u0000process used to obtain the results and propose that future studies follow a\u0000similar protocol in order to gain comparable data. The dataset contains\u0000analyses of glass shards from 23 proximal and 27 distal\u0000tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal yr BP) to the Hikuroa Pumice member (2.0 ± 0.6 Ma)\u0000from six or more caldera sources, most from the central Taupō Volcanic\u0000Zone. We report 1385 major-element analyses obtained by electron microprobe\u0000(EMPA), and 590 trace-element analyses obtained by laser ablation\u0000(LA)-ICP-MS, on individual glass shards. Using principal component analysis (PCA), Euclidean similarity coefficients, and geochemical investigation,\u0000we show that chemical compositions of glass shards from individual eruptions\u0000are commonly distinguished by major elements, especially CaO, TiO2,\u0000K2O, and FeOtt (Na2O+K2O and SiO2/K2O), but not\u0000always. For those tephras with similar glass major-element signatures, some\u0000can be distinguished using trace elements (e.g. HFSEs: Zr, Hf, Nb; LILE: Ba,\u0000Rb; REE: Eu, Tm, Dy, Y, Tb, Gd, Er, Ho, Yb, Sm) and trace-element ratios\u0000(e.g. LILE/HFSE: Ba/Th, Ba/Zr, Rb/Zr; HFSE/HREE: Zr/Y, Zr/Yb, Hf/Y;\u0000LREE/HREE: La/Yb, Ce/Yb). Geochemistry alone cannot be used to distinguish between glass shards from\u0000the following tephra groups: Taupō (Unit Y in the post-Ōruanui\u0000eruption sequence of Taupō volcano) and Waimihia (Unit S); Poronui (Unit\u0000C) and Karapiti (Unit B); Rotorua and Rerewhakaaitu; and\u0000Kawakawa/Ōruanui, and Okaia. Other characteristics, including\u0000stratigraphic relationships and age, can be used to separate and distinguish\u0000all of these otherwise-similar tephra deposits except Poronui and Karapiti.\u0000Bimodality caused by K2O variability is newly identified in Poihipi and\u0000Tahuna tephras. Using glass-shard compositions, tephra sourced from\u0000Taupō Volcanic Centre (TVC) and Mangakino Volcanic Centre (MgVC) can be\u0000separated using ","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"102 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-09-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73640381","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 : 2021-08-19DOI: 10.5194/GCHRON-3-421-2021
B. Ritter, A. Vogt, T. Dunai
Abstract. We established a new laboratory for noble gas mass spectrometry that is dedicated for the development and application to cosmogenic nuclides at the University of Cologne (Germany). At the core of the laboratory are a state-of-the-art high mass resolution multicollector Helix MCPlus (Thermo-Fisher) noble gas mass spectrometer and a novel custom-designed automated extraction line. The Mass-spectrometer is equipped with five combined Faraday Multiplier collectors, with 1012 Ω and 1013 Ω pre-amplifiers for faraday collectors. We describe the extraction line and the automized operation procedure for cosmogenic neon and the current performance of the experimental setup. Performance tests were conducted using gas of atmospheric isotopic composition (our primary standard gas); as well as CREU-1 intercomparison material, containing a mixture of neon of atmospheric and cosmogenic composition. We use the results from repeated analysis of CREU-1 to assess the performance of the current experimental setup at Cologne. The precision in determining the abundance of cosmogenic 21Ne is equal or better than those reported for other laboratories. The absolute value we obtain for the concentration of cosmogenic 21Ne in CREU is indistinguishable from the published value.
{"title":"Technical Note: Noble gas extraction procedure and performance of the Cologne Helix MC Plus multi-collector noble gas mass spectrometer for cosmogenic neon isotope analysis","authors":"B. Ritter, A. Vogt, T. Dunai","doi":"10.5194/GCHRON-3-421-2021","DOIUrl":"https://doi.org/10.5194/GCHRON-3-421-2021","url":null,"abstract":"Abstract. We established a new laboratory for noble gas mass spectrometry that is dedicated for the development and application to cosmogenic nuclides at the University of Cologne (Germany). At the core of the laboratory are a state-of-the-art high mass resolution multicollector Helix MCPlus (Thermo-Fisher) noble gas mass spectrometer and a novel custom-designed automated extraction line. The Mass-spectrometer is equipped with five combined Faraday Multiplier collectors, with 1012 Ω and 1013 Ω pre-amplifiers for faraday collectors. We describe the extraction line and the automized operation procedure for cosmogenic neon and the current performance of the experimental setup. Performance tests were conducted using gas of atmospheric isotopic composition (our primary standard gas); as well as CREU-1 intercomparison material, containing a mixture of neon of atmospheric and cosmogenic composition. We use the results from repeated analysis of CREU-1 to assess the performance of the current experimental setup at Cologne. The precision in determining the abundance of cosmogenic 21Ne is equal or better than those reported for other laboratories. The absolute value we obtain for the concentration of cosmogenic 21Ne in CREU is indistinguishable from the published value.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"13 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83749167","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 : 2021-08-02DOI: 10.5194/GCHRON-3-415-2021
Yang Li, P. Vermeesch
Abstract. Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re/188Os and 187Os/188Os, which often exhibit strong error correlations that may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar/39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting 188Os/187Os vs. 187Re/187Os produces an “inverse isochron”, defining a binary mixing line between an inherited Os component whose 188Os/187Os ratio is given by the vertical intercept, and the radiogenic 187Re/187Os ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate the identification of outliers and other sources of data dispersion. They can also be applied to other geochronometers such as the K–Ca method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf methods. Conventional and inverse isochron ages are similar for precise datasets but may significantly diverge for imprecise ones. A semi-synthetic data simulation indicates that, in the latter case, the inverse isochron age is more accurate. The generalised inverse isochron method has been added to the IsoplotR toolbox for geochronology, which automatically converts conventional isochron ratios into inverse ratios, and vice versa.
{"title":"Short communication: Inverse isochron regression for Re–Os, K–Ca and other chronometers","authors":"Yang Li, P. Vermeesch","doi":"10.5194/GCHRON-3-415-2021","DOIUrl":"https://doi.org/10.5194/GCHRON-3-415-2021","url":null,"abstract":"Abstract. Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re/188Os and 187Os/188Os, which often exhibit\u0000strong error correlations that may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar/39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting\u0000188Os/187Os vs. 187Re/187Os produces an\u0000“inverse isochron”, defining a binary mixing line between an inherited\u0000Os component whose 188Os/187Os ratio is given by the\u0000vertical intercept, and the radiogenic 187Re/187Os ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate\u0000the identification of outliers and other sources of data dispersion.\u0000They can also be applied to other geochronometers such as the K–Ca\u0000method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf\u0000methods. Conventional and inverse isochron ages are similar for\u0000precise datasets but may significantly diverge for imprecise ones. A\u0000semi-synthetic data simulation indicates that, in the latter case, the\u0000inverse isochron age is more accurate. The generalised inverse\u0000isochron method has been added to the IsoplotR toolbox for\u0000geochronology, which automatically converts conventional isochron\u0000ratios into inverse ratios, and vice versa.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"136 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2021-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80906189","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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