Abstract. Geologic dating methods for the most part do not directly measure ages. Instead, interpreting a geochemical observation as a geologically useful parameter – an age or a rate – requires an interpretive middle layer of calculations and supporting data sets. These are the subject of active research and evolve rapidly, so any synoptic analysis requires repeated recalculation of large numbers of ages from a growing data set of raw observations, using a constantly improving calculation method. Many important applications of geochronology involve regional or global analyses of large and growing data sets, so this characteristic is an obstacle to progress in these applications. This paper describes the ICE-D (Informal Cosmogenic-Nuclide Exposure-age Database) database project, a prototype computational infrastructure for dealing with this obstacle in one geochronological application – cosmogenic-nuclide exposure dating – that aims to enable visualization or analysis of diverse data sets by making middle-layer calculations dynamic and transparent to the user. An important aspect of this concept is that it is designed as a forward-looking research tool rather than a backward-looking archive: only observational data (which do not become obsolete) are stored, and derived data (which become obsolete as soon as the middle-layer calculations are improved) are not stored but instead calculated dynamically at the time data are needed by an analysis application. This minimizes “lock-in” effects associated with archiving derived results subject to rapid obsolescence and allows assimilation of both new observational data and improvements to middle-layer calculations without creating additional overhead at the level of the analysis application.
{"title":"Technical note: A prototype transparent-middle-layer data management and analysis infrastructure for cosmogenic-nuclide exposure dating","authors":"G. Balco","doi":"10.5194/gchron-2020-6","DOIUrl":"https://doi.org/10.5194/gchron-2020-6","url":null,"abstract":"Abstract. Geologic dating methods for the most part do not directly measure ages. Instead, interpreting a geochemical observation as a geologically useful parameter – an age or a rate – requires an interpretive middle layer of calculations and supporting data sets. These are the subject of active research and evolve rapidly, so any synoptic analysis requires repeated recalculation of large numbers of ages from a growing data set of raw observations, using a constantly improving calculation method. Many important applications of geochronology involve regional or global analyses of large and growing data sets, so this characteristic is an obstacle to progress in these applications. This paper describes the ICE-D (Informal Cosmogenic-Nuclide Exposure-age Database) database project, a prototype computational infrastructure for dealing with this obstacle in one geochronological application – cosmogenic-nuclide exposure dating – that aims to enable visualization or analysis of diverse data sets by making middle-layer calculations dynamic and transparent to the user. An important aspect of this concept is that it is designed as a forward-looking research tool rather than a backward-looking archive: only observational data (which do not become obsolete) are stored, and derived data (which become obsolete as soon as the middle-layer calculations are improved) are not stored but instead calculated dynamically at the time data are needed by an analysis application. This minimizes “lock-in” effects associated with archiving derived results subject to rapid obsolescence and allows assimilation of both new observational data and improvements to middle-layer calculations without creating additional overhead at the level of the analysis application.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74837169","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}
R. Powell, E. Green, Estephany Marillo Sialer, J. Woodhead
Abstract. The standard classical statistics approach to isochron calculation assumes that the distribution of uncertainties on the data arising from isotopic analysis is strictly Gaussian. This effectively excludes datasets that have more scatter from consideration, even though many appear to have age significance. A new approach to isochron calculations is developed in order to circumvent this problem, requiring only that the central part of the uncertainty distribution of the data defines a “spine” in the trend of the data. This central spine can be Gaussian but this is not a requirement. This approach significantly increases the range of datasets from which age information can be extracted but also provides seamless integration with well-behaved datasets and thus all legacy age determinations. The approach is built on the robust statistics of Huber (1981) but using the data uncertainties for the scale of data scatter around the spine rather than a scale derived from the scatter itself, ignoring the data uncertainties. This robust data fitting reliably determines the position of the spine when applied to data with outliers but converges on the classical statistics approach for datasets without outliers. The spine width is determined by a robust measure, the normalised median absolute deviation of the distances of the data points to the centre of the spine, divided by the uncertainties on the distances. A test is provided to ascertain that there is a spine in the data, requiring that the spine width is consistent with the uncertainties expected for Gaussian-distributed data. An iteratively reweighted least squares algorithm is presented to calculate the position of the robust line and its uncertainty, accompanied by an implementation in Python.
{"title":"Robust isochron calculation","authors":"R. Powell, E. Green, Estephany Marillo Sialer, J. Woodhead","doi":"10.5194/gchron-2020-4","DOIUrl":"https://doi.org/10.5194/gchron-2020-4","url":null,"abstract":"Abstract. The standard classical statistics approach to isochron calculation assumes that the distribution of uncertainties on the data arising from isotopic analysis is strictly Gaussian. This effectively excludes datasets that have more scatter from consideration, even though many appear to have age significance. A new approach to isochron calculations is developed in order to circumvent this problem, requiring only that the central part of the uncertainty distribution of the data defines a “spine” in the trend of the data. This central spine can be Gaussian but this is not a requirement. This approach significantly increases the range of datasets from which age information can be extracted but also provides seamless integration with well-behaved datasets and thus all legacy age determinations. The approach is built on the robust statistics of Huber (1981) but using the data uncertainties for the scale of data scatter around the spine rather than a scale derived from the scatter itself, ignoring the data uncertainties. This robust data fitting reliably determines the position of the spine when applied to data with outliers but converges on the classical statistics approach for datasets without outliers. The spine width is determined by a robust measure, the normalised median absolute deviation of the distances of the data points to the centre of the spine, divided by the uncertainties on the distances. A test is provided to ascertain that there is a spine in the data, requiring that the spine width is consistent with the uncertainties expected for Gaussian-distributed data. An iteratively reweighted least squares algorithm is presented to calculate the position of the robust line and its uncertainty, accompanied by an implementation in Python.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"34 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81058277","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 : 2019-09-26DOI: 10.5194/gchron-1-17-2019
E. Cooperdock, R. Ketcham, D. Stockli
Abstract. (U–Th) ∕ He thermochronometry relies on the accurate and precise quantification of individual grain volume and surface area, which are used to calculate mass, alpha ejection (FT) correction, equivalent sphere radius (ESR), and ultimately isotope concentrations and age. The vast majority of studies use 2-D or 3-D microscope dimension measurements and an idealized grain shape to calculate these parameters, and a long-standing question is how much uncertainty these assumptions contribute to observed intra-sample age dispersion and accuracy. Here we compare the results for volume, surface area, grain mass, ESR, and FT correction derived from 2-D microscope and 3-D X-ray computed tomography (CT) length and width data for > 100 apatite grains. We analyzed apatite grains from two samples that exhibited a variety of crystal habits, some with inclusions. We also present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium (eU). The data illustrate that the 2-D approach systematically overestimates grain volumes and surface areas by 20 %–25 %, impacting the estimates for mass, eU, and ESR – important parameters with implications for interpreting age scatter and inverse modeling. FT factors calculated from 2-D and 3-D measurements differ by ∼2 %. This variation, however, has effectively no impact on reducing intra-sample age reproducibility, even on small aliquot samples (e.g., four grains). We also present a grain-mounting procedure for X-ray CT scanning that can allow hundreds of grains to be scanned in a single session and new software capabilities for 3-D FT and FT-based ESR calculations that are robust for relatively low-resolution CT data, which together enable efficient and cost-effective CT-based characterization.
{"title":"Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility","authors":"E. Cooperdock, R. Ketcham, D. Stockli","doi":"10.5194/gchron-1-17-2019","DOIUrl":"https://doi.org/10.5194/gchron-1-17-2019","url":null,"abstract":"Abstract. (U–Th) ∕ He thermochronometry relies on the accurate and\u0000precise quantification of individual grain volume and surface area, which\u0000are used to calculate mass, alpha ejection (FT) correction, equivalent\u0000sphere radius (ESR), and ultimately isotope concentrations and age. The vast\u0000majority of studies use 2-D or 3-D microscope dimension measurements and an\u0000idealized grain shape to calculate these parameters, and a long-standing\u0000question is how much uncertainty these assumptions contribute to observed\u0000intra-sample age dispersion and accuracy. Here we compare the results for\u0000volume, surface area, grain mass, ESR, and FT correction derived from\u00002-D microscope and 3-D X-ray computed tomography (CT) length and width data\u0000for > 100 apatite grains. We analyzed apatite grains from two\u0000samples that exhibited a variety of crystal habits, some with inclusions. We\u0000also present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium\u0000(eU). The data illustrate that the 2-D approach systematically overestimates\u0000grain volumes and surface areas by 20 %–25 %, impacting the estimates for\u0000mass, eU, and ESR – important parameters with implications for interpreting\u0000age scatter and inverse modeling. FT factors calculated from 2-D and 3-D\u0000measurements differ by ∼2 %. This variation, however, has\u0000effectively no impact on reducing intra-sample age reproducibility, even on\u0000small aliquot samples (e.g., four grains). We also present a grain-mounting\u0000procedure for X-ray CT scanning that can allow hundreds of grains to be scanned\u0000in a single session and new software capabilities for 3-D FT and\u0000FT-based ESR calculations that are robust for relatively low-resolution\u0000CT data, which together enable efficient and cost-effective CT-based\u0000characterization.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"16 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86643162","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. Lougheed, P. Ascough, A. Dolman, L. Löwemark, B. Metcalfe
Abstract. The current geochronological state of the art for applying the radiocarbon (14C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic 14C activities (i.e. from a 14C calibration curve) are assigned to each single foraminifera based on its simulation time step. We find that the normal distribution of 14C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory 14C age and measurement error) is unlikely to be a faithful reflection of the actual 14C age distribution for a specific depth interval. We also find that this deviation from the actual 14C age distribution is greatly amplified during the calibration process. Specifically, we find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age–depth artefacts in downcore calibrated median age. Even in the case of “perfect” simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the 14C measurement and calibration processes generate temporally dynamic median age–depth artefacts on the order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ14C history, when single foraminifera of varying 14C activity can be incorporated into single discrete-depth sediment intervals. For certain lower-SAR scenarios, we find that downcore discrete-depth true median age can systematically fall outside the calibrated age range predicted by the 14C measurement and calibration processes, thus leading to systematically inaccurate age estimations. In short, our findings suggest the possibility of 14C-derived age–depth artefacts in the literature. Furthermore, since such age–depth artefacts are likely to coincide with large-scale changes in global Δ14C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), 14C-derived age–depth artefacts may have been previously incorrectly attributed to changes in SAR coinciding with global climate. Our study highlights the need for the development of improved deep-sea sediment 14C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.
摘要目前在深海沉积物档案中应用放射性碳(14C)方法的地质年代学水平缺乏沉积物生物扰动的关键信息。在这里，我们应用了一个沉积物堆积模型，模拟了数百万有孔虫的沉积和生物扰动，根据其模拟时间步长，将现实的14C活动(即来自14C校准曲线)分配给每个单个有孔虫。我们发现，通常用于表示离散深度沉积物间隔的14C年龄的正态分布(基于报告的实验室14C年龄和测量误差)不太可能忠实地反映特定深度间隔的实际14C年龄分布。我们还发现，在校准过程中，这种与实际14C年龄分布的偏差被大大放大。具体来说，我们发现在许多情况下，系统地低估了总地质年代学误差(高达数千年)，以及在下核校准的中位年龄中产生的年龄深度伪影。即使在“完美”模拟沉积物档案情景的情况下，沉积累积率(SAR)、生物扰动深度、水库年龄和物种丰度都保持不变，14C测量和校准过程也会产生数百年的时间动态中位年龄深度人工制品，因此即使是高SAR情景(40和60 cm kyr−1)也容易受到影响。在与地球Δ14C历史的动态变化相对应的时期，这种年龄深度的人工制品尤其明显，当时不同的14c活动的单个有孔虫可以合并到单个离散深度的沉积区间中。对于某些低sar情景，我们发现下离散深度的真实年龄中位数可能系统性地落在14C测量和校准过程预测的校准年龄范围之外，从而导致系统地不准确的年龄估计。简而言之，我们的研究结果表明，文献中可能存在14c衍生的年龄深度人工制品。此外，由于这种年龄深度人工制品很可能与全球Δ14C的大尺度变化相吻合，而全球Δ14C的大尺度变化本身也可能与全球气候的大尺度变化相吻合(如最后一次消冰期)，因此，14c衍生的年龄深度人工制品以前可能被错误地归因于与全球气候相吻合的SAR变化。我们的研究强调了开发改进的深海沉积物14C校准技术的必要性，其中包括对多样品样品的生物扰动的优先表示。
{"title":"Re-evaluating 14C dating accuracy in deep-sea sediment archives","authors":"B. Lougheed, P. Ascough, A. Dolman, L. Löwemark, B. Metcalfe","doi":"10.31223/osf.io/cwqsk","DOIUrl":"https://doi.org/10.31223/osf.io/cwqsk","url":null,"abstract":"Abstract. The current geochronological state of the art for applying the radiocarbon\u0000(14C) method to deep-sea sediment archives lacks key information on\u0000sediment bioturbation. Here, we apply a sediment accumulation model that\u0000simulates the sedimentation and bioturbation of millions of foraminifera,\u0000whereby realistic 14C activities (i.e. from a 14C calibration\u0000curve) are assigned to each single foraminifera based on its simulation\u0000time step. We find that the normal distribution of 14C age typically\u0000used to represent discrete-depth sediment intervals (based on the reported\u0000laboratory 14C age and measurement error) is unlikely to be a faithful\u0000reflection of the actual 14C age distribution for a specific depth\u0000interval. We also find that this deviation from the actual 14C age\u0000distribution is greatly amplified during the calibration process.\u0000Specifically, we find a systematic underestimation of total geochronological\u0000error in many cases (by up to thousands of years), as well as the generation\u0000of age–depth artefacts in downcore calibrated median age. Even in the case\u0000of “perfect” simulated sediment archive scenarios, whereby sediment\u0000accumulation rate (SAR), bioturbation depth, reservoir age and species\u0000abundance are all kept constant, the 14C measurement and calibration\u0000processes generate temporally dynamic median age–depth artefacts on the\u0000order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts\u0000can be especially pronounced during periods corresponding to dynamic changes\u0000in the Earth's Δ14C history, when single foraminifera of varying\u000014C activity can be incorporated into single discrete-depth sediment\u0000intervals. For certain lower-SAR scenarios, we find that downcore\u0000discrete-depth true median age can systematically fall outside the calibrated\u0000age range predicted by the 14C measurement and calibration processes,\u0000thus leading to systematically inaccurate age estimations. In short, our\u0000findings suggest the possibility of 14C-derived age–depth artefacts in\u0000the literature. Furthermore, since such age–depth artefacts are likely to\u0000coincide with large-scale changes in global Δ14C, which\u0000themselves can coincide with large-scale changes in global climate (such as\u0000the last deglaciation), 14C-derived age–depth artefacts may have been\u0000previously incorrectly attributed to changes in SAR coinciding with global\u0000climate. Our study highlights the need for the development of improved\u0000deep-sea sediment 14C calibration techniques that include an a priori\u0000representation of bioturbation for multi-specimen samples.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"45 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86586031","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 apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.
{"title":"Chlorine-36∕beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA","authors":"G. Balco, K. Blisniuk, A. Hidy","doi":"10.5194/GCHRON-1-1-2019","DOIUrl":"https://doi.org/10.5194/GCHRON-1-1-2019","url":null,"abstract":"Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"103 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88029728","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 : 2019-06-27DOI: 10.5194/gchron-1-43-2019
K. Nichols, B. Goehring
Abstract. Froth flotation is a commonly used procedure for separating feldspars and micas from quartz for the preparation of quartz mineral separates to carry out cosmogenic nuclide analysis. Whilst extracting carbon from quartz we observed in situ carbon-14 (14C) concentrations which were anomalously high and in excess of theoretical geological maximum concentrations. Further etching of sample material reduced carbon yields and 14C concentrations, yet the latter remained unrealistically high. When quartz from the original whole rock sample was isolated in our laboratory, we observed even lower carbon yields and geologically plausible in situ 14C concentrations. After ruling out unlikely geological scenarios and systematic measurement issues, we decided to investigate the quartz isolation procedure as a potential source of 14C contamination. We hypothesised that laurylamine (dodecylamine), an organic compound used as part of the froth flotation procedure, elevates 14C concentrations if residual laurylamine is present. We demonstrate that laurylamine has a 14C modern carbon source and thus has the potential to influence in situ 14C measurements if present in minute but measurable quantities. Furthermore, we show that insufficient sample etching results in contaminant 14C persisting through the step heating of quartz that is subsequently collected with the in situ component released at 1100 ∘C. We demonstrate that froth flotation contaminates in situ 14C measurements. We provide guidelines for the preparation of quartz based on methods developed in our laboratory and demonstrate that all froth-flotation-derived carbon and 14C is removed when applied. We recommend that the procedures presented be used at a minimum when using froth flotation to isolate quartz for in situ 14C measurements.
{"title":"Isolation of quartz for cosmogenic in situ 14C analysis","authors":"K. Nichols, B. Goehring","doi":"10.5194/gchron-1-43-2019","DOIUrl":"https://doi.org/10.5194/gchron-1-43-2019","url":null,"abstract":"Abstract. Froth flotation is a commonly used procedure for\u0000separating feldspars and micas from quartz for the preparation of quartz\u0000mineral separates to carry out cosmogenic nuclide analysis. Whilst\u0000extracting carbon from quartz we observed in situ carbon-14 (14C)\u0000concentrations which were anomalously high and in excess of theoretical\u0000geological maximum concentrations. Further etching of sample material\u0000reduced carbon yields and 14C concentrations, yet the latter remained\u0000unrealistically high. When quartz from the original whole rock sample was\u0000isolated in our laboratory, we observed even lower carbon yields and\u0000geologically plausible in situ 14C concentrations. After ruling out\u0000unlikely geological scenarios and systematic measurement issues, we decided\u0000to investigate the quartz isolation procedure as a potential source of\u000014C contamination. We hypothesised that laurylamine (dodecylamine), an\u0000organic compound used as part of the froth flotation procedure, elevates\u000014C concentrations if residual laurylamine is present. We demonstrate\u0000that laurylamine has a 14C modern carbon source and thus has the\u0000potential to influence in situ 14C measurements if present in minute\u0000but measurable quantities. Furthermore, we show that insufficient sample\u0000etching results in contaminant 14C persisting through the step heating of\u0000quartz that is subsequently collected with the in situ component released at\u00001100 ∘C. We demonstrate that froth flotation contaminates in situ\u000014C measurements. We provide guidelines for the preparation of quartz\u0000based on methods developed in our laboratory and demonstrate that all froth-flotation-derived carbon and 14C is removed when applied. We recommend\u0000that the procedures presented be used at a minimum when using froth\u0000flotation to isolate quartz for in situ 14C measurements.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"128 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83964979","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 : 2019-06-11DOI: 10.5194/gchron-1-85-2019
C. Brenhin Keller, P. Boehnke, B. Schoene, T. M. Harrison
Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (<0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.
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