Pub Date : 2022-12-06DOI: 10.5194/gchron-4-683-2022
R. Tartèse, I. Lyon
Abstract. In situ carbonate U–Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young terrestrial calcite up to Carboniferous in age. To assess the robustness of the carbonate U–Pb chronometer in deep time, we carried out in situ U–Pb analyses in magnesite–ankerite–calcite carbonates in the martian meteorite Allan Hills (ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga, and there is little evidence that much happened to this rock since then, making it an ideal sample to test the robustness of the U–Pb system in old carbonates. We obtained a concordant date of 3941 ± 49/110 Ma (n=14, MSWD = 2.0), which is identical to the step-leaching Rb/Sr date determined previously. These results thus confirm that old carbonates are amenable to U–Pb dating in samples that have had a relatively simple history post-carbonate formation.
摘要近年来，原位碳酸盐U-Pb测年研究急剧增加。几乎所有这些研究的目标都是相对年轻的陆地方解石，年龄一直到石炭纪。为了评估碳酸盐U-Pb时计在深部时间的稳健性，我们对火星陨石Allan Hills(ALH) 84001中的菱镁石-铁白云石-方解石碳酸盐进行了原位U-Pb分析。ALH 84001中的碳酸盐形成于约3.94 Ga，几乎没有证据表明这块岩石在那之后发生了什么变化，这使得它成为测试古老碳酸盐中U-Pb系统坚固性的理想样本。我们得到了一个一致的日期3941±49/110 Ma (n=14, MSWD = 2.0)，这与之前确定的阶梯浸出Rb/Sr日期相同。因此，这些结果证实，在具有相对简单的后碳酸盐岩形成历史的样品中，古老的碳酸盐适合于U-Pb定年。
{"title":"In situ U–Pb dating of 4 billion-year-old carbonates in the martian meteorite Allan Hills 84001","authors":"R. Tartèse, I. Lyon","doi":"10.5194/gchron-4-683-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-683-2022","url":null,"abstract":"Abstract. In situ carbonate U–Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young terrestrial calcite up to Carboniferous in age. To assess the robustness of the carbonate U–Pb chronometer in deep time, we carried out in situ U–Pb analyses in magnesite–ankerite–calcite carbonates in the martian meteorite Allan Hills\u0000(ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga, and there is little evidence that much happened to this rock since then, making\u0000it an ideal sample to test the robustness of the U–Pb system in old carbonates. We obtained a concordant date of\u00003941 ± 49/110 Ma (n=14, MSWD = 2.0), which is identical to the step-leaching Rb/Sr date determined\u0000previously. These results thus confirm that old carbonates are amenable to U–Pb dating in samples that have had a relatively simple\u0000history post-carbonate formation.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76569746","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-11-22DOI: 10.5194/gchron-4-665-2022
M. Corre, A. Agranier, M. Lanson, C. Gautheron, F. Brunet, S. Schwartz
Abstract. Magnetite and spinel thermochronological (U–Th) / He dates often display significantly dispersed values. In the present study, we investigated the contribution of analytical (and standardization) errors to this dispersion. U and Th content of magnetite (natural and synthetic) and natural Al spinel samples with U and Th concentrations between 0.02 and 116 µg g−1 were analyzed using both wet chemistry and in situ laser ablation sampling methods. New magnetite reference samples (NMA and NMB) were synthesized, consisting of U- and Th-doped nano-magnetite powders, whose U and Th concentrations were determined using a wet chemistry method (U and Th of NMA and NMB are ∼40 µg g−1 and ∼0.1 µg g−1, respectively). We show that, for both U and Th analyses, the reproducibility obtained with the wet chemistry protocol depends on the U and Th concentration. It is below 11 % for U–Th values higher than 0.4 µg g−1 and reaches 22 % for U–Th content lower than 0.1 µg g−1. This result implies that (U–Th) / He thermochronological ages cannot be more reproducible than 24 % for magnetite containing less than 0.1 µg g−1 of U and Th, thus explaining part of the natural ages variability. U and Th data obtained by laser ablation ICP-MS on natural magnetite and Al spinel samples were calibrated using both silicate glass standards and synthetic magnetite samples. The U and Th contents determined using NMA are consistent with those obtained by means of the wet chemistry method, but they are overestimated by 30 % when using the glass standard samples only. These results highlight the impact of the matrix effect on the determination of the U–Th content in magnetite. We thus recommend the use of a well-characterized magnetite reference for the calibration of the U–Th signals obtained by laser ablation. The scatter in the (U–Th) / He magnetite ages can be expected to be ∼20 % if the U and Th contents are determined by laser ablation. This level of precision is actually not significantly different from that obtained using the wet chemistry method, which paves the way for the use of laser ablation for determining (U–Th) / He ages. In the absence of a spinel reference for U and Th calibration using LA-ICP-MS, silicate glass references, along with NMA, were used. U and Th contents were found to be ∼30 % lower than the values obtained using wet chemistry. This discrepancy underlines the importance of using a standard with a composition close to that of the mineral of interest. Although magnetite and Al spinel have related crystal structures, the magnetite standard is not appropriate for U and Th analysis in Al spinel using LA-ICP-MS.
{"title":"U and Th content in magnetite and Al spinel obtained by wet chemistry and laser ablation methods: implication for (U–Th) ∕ He thermochronometer","authors":"M. Corre, A. Agranier, M. Lanson, C. Gautheron, F. Brunet, S. Schwartz","doi":"10.5194/gchron-4-665-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-665-2022","url":null,"abstract":"Abstract. Magnetite and spinel thermochronological (U–Th) / He dates\u0000often display significantly dispersed values. In the present study, we\u0000investigated the contribution of analytical (and standardization) errors to\u0000this dispersion. U and Th content of magnetite (natural and synthetic) and\u0000natural Al spinel samples with U and Th concentrations between 0.02 and 116 µg g−1 were analyzed using both wet chemistry and in situ laser\u0000ablation sampling methods. New magnetite reference samples (NMA and NMB)\u0000were synthesized, consisting of U- and Th-doped nano-magnetite powders, whose U and Th concentrations were determined using a wet chemistry method (U and Th\u0000of NMA and NMB are ∼40 µg g−1 and ∼0.1 µg g−1, respectively). We show that, for both U and Th analyses, the reproducibility obtained\u0000with the wet chemistry protocol depends on the U and Th concentration. It is below 11 % for U–Th values higher than 0.4 µg g−1 and reaches 22 % for U–Th content lower than 0.1 µg g−1. This result implies that (U–Th) / He thermochronological ages cannot be more reproducible than 24 % for magnetite containing less than 0.1 µg g−1 of U and Th, thus\u0000explaining part of the natural ages variability. U and Th data obtained by\u0000laser ablation ICP-MS on natural magnetite and Al spinel samples were\u0000calibrated using both silicate glass standards and synthetic magnetite\u0000samples. The U and Th contents determined using NMA are consistent with\u0000those obtained by means of the wet chemistry method, but they are overestimated by 30 %\u0000when using the glass standard samples only. These results highlight the\u0000impact of the matrix effect on the determination of the U–Th content in\u0000magnetite. We thus recommend the use of a well-characterized magnetite reference\u0000for the calibration of the U–Th signals obtained by laser ablation. The\u0000scatter in the (U–Th) / He magnetite ages can be expected to be\u0000∼20 % if the U and Th contents are determined by laser\u0000ablation. This level of precision is actually not significantly different\u0000from that obtained using the wet chemistry method, which paves the way for the\u0000use of laser ablation for determining (U–Th) / He ages. In the absence of a\u0000spinel reference for U and Th calibration using LA-ICP-MS, silicate glass\u0000references, along with NMA, were used. U and Th contents were found to be\u0000∼30 % lower than the values obtained using wet chemistry.\u0000This discrepancy underlines the importance of using a standard with a\u0000composition close to that of the mineral of interest. Although magnetite and\u0000Al spinel have related crystal structures, the magnetite standard is not\u0000appropriate for U and Th analysis in Al spinel using LA-ICP-MS.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"84 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-11-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73962391","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-10-27DOI: 10.5194/gchron-4-629-2022
John He, P. Reiners
Abstract. Accurate corrections for the effects of alpha ejection (the loss of daughter He near grain or crystal surfaces due to long alpha stopping distances) are central to (U-Th)/He thermochronometry. In the case of apatite (U-Th)/He dating, alpha-ejection correction is complicated by the fact that crystals are often broken perpendicular to the c axis. In such cases, the correction should account for the fact that only some parts of the crystal are affected by alpha ejection. A common current practice to account for such broken crystals is to modify measured lengths of broken crystals missing one termination by a factor of 1.5, and those missing both terminations by a factor of 2. This alpha-ejection “correction correction” systematically overestimates the actual fraction of helium lost to alpha ejection, and thus overcorrects the measured date relative to that determined for an otherwise equivalent unbroken crystal. The ratio of the alpha-ejection-affected surface area to the volume of a fragmented crystal is equivalent to the surface-area-to-volume ratio of an unbroken crystal that is either twice as long (for fragments with one termination) or infinitely long (for fragments with no termination). We suggest that it is appropriate to revise the fragmentation correction to multiply the lengths of crystals missing one c-axis termination by 2, and those missing both c-axis terminations by some large number ≳20. We examine the effect of this revised correction and demonstrate the accuracy of the new method using synthetic datasets. Taking into account alpha ejection, the rounding of the He concentration profile due to diffusive loss, and the accumulation of radiation damage over a range of thermal histories, we show that the revised fragmentation alpha-ejection correction proposed here accurately approximates the corrected date of an unbroken crystal (“true” date) to within <0.7 % on average (±4.2 %, 1σ), whereas the former method overcorrects dates to be ∼3 % older than the “true” date on average. For individual grains, the former method can result in dates that are older by a few percent in most cases, and by as much as 12 % for grains with aspect ratios of up to 1:1. The revised alpha-ejection correction proposed here is both more accurate and more precise than the previous correction, and does not introduce any significant systematic bias into the apparent dates from a sample.
{"title":"A revised alpha-ejection correction calculation for (U–Th) ∕ He thermochronology dates of broken apatite crystals","authors":"John He, P. Reiners","doi":"10.5194/gchron-4-629-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-629-2022","url":null,"abstract":"Abstract. Accurate corrections for the effects of alpha ejection (the loss of daughter He near grain or crystal surfaces due to long alpha stopping distances) are central to (U-Th)/He thermochronometry. In the case of apatite (U-Th)/He dating, alpha-ejection correction is complicated by the fact that crystals are often broken perpendicular to the c axis. In such cases, the correction should account for\u0000the fact that only some parts of the crystal are affected by alpha ejection. A common current practice to account for such broken crystals is to\u0000modify measured lengths of broken crystals missing one termination by a factor of 1.5, and those missing both terminations by a factor of 2. This\u0000alpha-ejection “correction correction” systematically overestimates the actual fraction of helium lost to alpha ejection, and thus overcorrects\u0000the measured date relative to that determined for an otherwise equivalent unbroken crystal. The ratio of the alpha-ejection-affected surface area to the volume of a fragmented crystal is equivalent to the surface-area-to-volume ratio of an unbroken crystal that is either twice as long (for fragments with one termination) or infinitely long (for fragments with no termination). We suggest that it is appropriate to revise the fragmentation correction to multiply the lengths of crystals missing one c-axis termination by 2, and those missing both c-axis terminations by\u0000some large number ≳20. We examine the effect of this revised correction and demonstrate the accuracy of the new method\u0000using synthetic datasets. Taking into account alpha ejection, the rounding of the He concentration profile due to diffusive loss, and the accumulation of radiation damage over a range of thermal histories, we show that the revised fragmentation alpha-ejection correction proposed here accurately\u0000approximates the corrected date of an unbroken crystal (“true” date) to within <0.7 % on average (±4.2 %, 1σ), whereas\u0000the former method overcorrects dates to be ∼3 % older than the “true” date on average. For individual grains, the former method can\u0000result in dates that are older by a few percent in most cases, and by as much as 12 % for grains with aspect ratios of up to 1:1. The revised\u0000alpha-ejection correction proposed here is both more accurate and more precise than the previous correction, and does not introduce any significant systematic bias into the apparent dates from a sample.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"55 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80795404","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-09-15DOI: 10.5194/gchron-4-617-2022
Paul Töchterle, Simon D. Steidle, R. Edwards, Y. Dublyansky, C. Spötl, Xianglei Li, J. Gunn, G. Moseley
Abstract. Cryogenic cave carbonates (CCCs) are a type of speleothem, typically dated with 230Th/U disequilibrium methods, that provide evidence of palaeo-permafrost conditions. In the field, CCCs occur as distinct patches of millimetre- to centimetre-sized loose crystals and crystal aggregates on the floors of cave chambers, so they lack a framework that would allow ages to be validated by stratigraphic order. Correction factors for the initial 230Th (230Th0) are often based on the bulk-earth-derived initial 230Th/232Th activity ratio ((230Th/232Th)0), which is a well-established approach when 230Th0 is moderately low. For samples with elevated levels of 230Th0, however, accuracy can be improved by constraining (230Th/232Th)0 independently. Here, we combine detailed morphological observations from three CCC patches found in Water Icicle Close Cavern in the Peak District (UK) with 230Th/U analyses. We find that individual CCC crystals show a range of morphologies that arise from non-crystallographic branching in response to the chemical evolution of the freezing solution. Results of 230Th/U dating indicate that samples with a large surface area relative to the sample volume are systematically more affected by contamination with 230Th0. By fitting isochrons to these results, we test whether the CCCs in a patch formed during the same freezing event, and demonstrate that (230Th/232Th)0 can deviate substantially from the bulk-earth-derived value and can also vary between the different CCC patches. Where CCCs display elevated 230Th0, isochrons are a useful tool to constrain (230Th/232Th)0 and obtain ages with improved accuracy. Detritus absorbed to the crystal surface is shown to be the most likely source of 230Th0. Our results suggest that some previously published CCC ages may merit re-assessment, and we provide suggestions on how to approach future dating efforts.
{"title":"230Th ∕ U isochron dating of cryogenic cave carbonates","authors":"Paul Töchterle, Simon D. Steidle, R. Edwards, Y. Dublyansky, C. Spötl, Xianglei Li, J. Gunn, G. Moseley","doi":"10.5194/gchron-4-617-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-617-2022","url":null,"abstract":"Abstract. Cryogenic cave carbonates (CCCs) are a type of\u0000speleothem, typically dated with 230Th/U disequilibrium methods, that provide evidence of palaeo-permafrost conditions. In the field, CCCs occur as distinct patches of millimetre- to centimetre-sized loose crystals and crystal aggregates on the floors of cave chambers, so they lack a framework that would allow ages to be validated by stratigraphic order. Correction factors for the initial\u0000230Th (230Th0) are often based on the bulk-earth-derived\u0000initial 230Th/232Th activity ratio ((230Th/232Th)0), which is a well-established approach when\u0000230Th0 is moderately low. For samples with elevated levels of\u0000230Th0, however, accuracy can be improved by constraining\u0000(230Th/232Th)0 independently. Here, we combine detailed\u0000morphological observations from three CCC patches found in Water Icicle\u0000Close Cavern in the Peak District (UK) with 230Th/U analyses. We find that individual CCC crystals show a range of morphologies that arise from non-crystallographic branching in response to the chemical evolution of the freezing solution. Results of 230Th/U dating indicate that samples with a large surface area relative to the sample volume are systematically more affected by contamination with 230Th0. By fitting isochrons to these results, we test whether the CCCs in a patch formed during the same freezing event, and demonstrate that (230Th/232Th)0 can deviate substantially from the bulk-earth-derived value and can also vary between the different CCC patches. Where CCCs display elevated 230Th0, isochrons are a useful tool to constrain\u0000(230Th/232Th)0 and obtain ages with improved accuracy. Detritus absorbed to the crystal surface is shown to be the most likely\u0000source of 230Th0. Our results suggest that some previously published CCC ages may merit re-assessment, and we provide suggestions on how to approach future dating efforts.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"60 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-09-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78142432","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-09-06DOI: 10.5194/gchron-4-577-2022
D. Subarkah, A. Nixon, Monica Jimenez, A. Collins, M. Blades, J. Farkas, Sarah E. Gilbert, S. Holford, A. Jarrett
Abstract. Recent developments in tandem laser ablation mass spectrometer technology have demonstrated the capacity for separating parent and daughter isotopes of the same mass online. As a result, beta-decay chronometers can now be applied to the geological archive in situ as opposed to through traditional whole-rock digestions. One novel application of this technique is the in situ Rb–Sr dating of Proterozoic shales that are dominated by authigenic clays such as illite. This method can provide a depositional window for shales by differentiating signatures of early diagenetic processes versus late-stage secondary alteration. However, the hydrothermal sensitivity of the Rb–Sr isotopic system across geological timescales in shale-hosted clay minerals is not well understood. As such, we dated the Mesoproterozoic Velkerri Formation from the Altree 2 well in the Beetaloo Sub-basin (greater McArthur Basin), northern Australia, using this approach. We then constrained the thermal history of these units using common hydrocarbon maturity indicators and modelled effects of contact heating due to the intrusion of the Derim Derim Dolerite. In situ Rb–Sr dating of mature, oil-prone shales in the diagenetic zone from the Velkerri Formation yielded ages of 1448 ± 81, 1434 ± 19, and 1421 ± 139 Ma. These results agree with previous Re–Os dating of the unit and are interpreted as recording the timing of an early diagenetic event soon after deposition. Conversely, overmature, gas-prone shales in the anchizone sourced from deeper within the borehole were dated at 1322 ± 93 and 1336 ± 40 Ma. These ages are younger than the expected depositional window for the Velkerri Formation. Instead, they are consistent with the age of the Derim Derim Dolerite mafic intrusion intersected 800 m below the Velkerri Formation. Thermal modelling suggests that a single intrusion of 75 m thickness would have been capable of producing a significant hydrothermal perturbation radiating from the sill top. The intrusion width proposed by this model is consistent with similar Derim Derim Dolerite sill thicknesses found elsewhere in the McArthur Basin. The extent of the hydrothermal aureole induced by this intrusion coincides with the window in which kerogen from the Velkerri Formation becomes overmature. As a result, the mafic intrusion intersected here is interpreted to have caused kerogen in these shales to enter the gas window, induced fluids that mobilize trace elements, and reset the Rb–Sr chronometer. Consequently, we propose that the Rb–Sr chronometer in shales may be sensitive to temperatures of ca. 120 ∘C in hydrothermal reactions but can withstand temperatures of more than 190 ∘C in thermal systems not dominated by fluids. Importantly, this study demonstrates a framework for the combined use of in situ Rb–Sr dating and kerogen maturation indicators to help reveal the thermochronological history of Proterozoic sedimentary basins. As such, this approach can be a powerful tool for iden
{"title":"Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and their subsequent applications","authors":"D. Subarkah, A. Nixon, Monica Jimenez, A. Collins, M. Blades, J. Farkas, Sarah E. Gilbert, S. Holford, A. Jarrett","doi":"10.5194/gchron-4-577-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-577-2022","url":null,"abstract":"Abstract. Recent developments in tandem laser ablation mass spectrometer\u0000technology have demonstrated the capacity for separating parent and daughter\u0000isotopes of the same mass online. As a result, beta-decay chronometers can\u0000now be applied to the geological archive in situ as opposed to through traditional whole-rock digestions. One novel application of this technique is the in situ Rb–Sr\u0000dating of Proterozoic shales that are dominated by authigenic clays such as\u0000illite. This method can provide a depositional window for shales by\u0000differentiating signatures of early diagenetic processes versus late-stage\u0000secondary alteration. However, the hydrothermal sensitivity of the Rb–Sr\u0000isotopic system across geological timescales in shale-hosted clay minerals\u0000is not well understood. As such, we dated the Mesoproterozoic Velkerri\u0000Formation from the Altree 2 well in the Beetaloo Sub-basin (greater McArthur Basin), northern Australia, using this approach. We then constrained the thermal history of these units using common hydrocarbon maturity indicators and modelled effects of contact heating due to the intrusion of the Derim Derim Dolerite. In situ Rb–Sr dating of mature, oil-prone shales in the diagenetic zone from the Velkerri Formation yielded ages of 1448 ± 81, 1434 ± 19, and 1421 ± 139 Ma. These results agree with previous Re–Os dating of the unit and are interpreted as recording the timing of an early diagenetic event soon after deposition. Conversely, overmature, gas-prone shales in the\u0000anchizone sourced from deeper within the borehole were dated at 1322 ± 93 and 1336 ± 40 Ma. These ages are younger than the expected depositional window for the Velkerri Formation. Instead, they are consistent\u0000with the age of the Derim Derim Dolerite mafic intrusion intersected 800 m\u0000below the Velkerri Formation. Thermal modelling suggests that a single\u0000intrusion of 75 m thickness would have been capable of producing a\u0000significant hydrothermal perturbation radiating from the sill top. The\u0000intrusion width proposed by this model is consistent with similar Derim\u0000Derim Dolerite sill thicknesses found elsewhere in the McArthur Basin. The\u0000extent of the hydrothermal aureole induced by this intrusion coincides with\u0000the window in which kerogen from the Velkerri Formation becomes overmature.\u0000As a result, the mafic intrusion intersected here is interpreted to have\u0000caused kerogen in these shales to enter the gas window, induced fluids that\u0000mobilize trace elements, and reset the Rb–Sr chronometer. Consequently, we\u0000propose that the Rb–Sr chronometer in shales may be sensitive to\u0000temperatures of ca. 120 ∘C in hydrothermal reactions but can\u0000withstand temperatures of more than 190 ∘C in thermal systems not\u0000dominated by fluids. Importantly, this study demonstrates a framework for\u0000the combined use of in situ Rb–Sr dating and kerogen maturation indicators to help reveal the thermochronological history of Proterozoic sedimentary basins. As such, this approach can be a powerful tool for iden","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"98 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88975096","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-09-06DOI: 10.5194/gchron-4-601-2022
A. Beranoaguirre, I. Vasiliev, A. Gerdes
Abstract. Recent developments in analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U–Pb geochronometers in low-U minerals such as carbonates or garnets. The rapid evolution of the technique relies on well-characterized matrix-matched reference materials. In this article, we explore the suitability of using carbonate as an “almost-matrix-matched reference material” for in situ U–Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U–Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238U/204Pb ratios, five of the samples showed a higher dispersion on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages, while another gave an unexpected younger age, each of them with 6 %–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the bias between the U–Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty, and thus the results obtained here suggest that carbonate reference material is currently the best option for standardization of in situ U–Pb sulfate analyses.
{"title":"In situ LA-ICPMS U–Pb dating of sulfates: applicability of carbonate reference materials as matrix-matched standards","authors":"A. Beranoaguirre, I. Vasiliev, A. Gerdes","doi":"10.5194/gchron-4-601-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-601-2022","url":null,"abstract":"Abstract. Recent developments in analytical capabilities in the\u0000field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded\u0000the applications of U–Pb geochronometers in low-U minerals such as\u0000carbonates or garnets. The rapid evolution of the technique relies on\u0000well-characterized matrix-matched reference materials. In this article, we\u0000explore the suitability of using carbonate as an “almost-matrix-matched\u0000reference material” for in situ U–Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U–Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238U/204Pb ratios, five of the samples showed a higher dispersion\u0000on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages, while another gave an unexpected younger age, each of them with 6 %–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the bias between the U–Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty, and thus the results obtained here suggest that carbonate reference material is currently the best option for standardization of in situ U–Pb sulfate analyses.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86459619","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-08-19DOI: 10.5194/gchron-4-561-2022
P. Vermeesch
Abstract. Secondary ion mass spectrometry (SIMS) is a widely used technique for in situ U–Pb geochronology of accessory minerals. Existing algorithms for SIMS data reduction and error propagation make a number of simplifying assumptions that degrade the precision and accuracy of the resulting U–Pb dates. This paper uses an entirely new approach to SIMS data processing that introduces the following improvements over previous algorithms. First, it treats SIMS measurements as compositional data using log-ratio statistics. This means that, unlike existing algorithms, (a) its isotopic ratio estimates are guaranteed to be strictly positive numbers, (b) identical results are obtained regardless of whether data are processed as normal ratios (e.g. 206Pb / 238U) or reciprocal ratios (e.g. 238U / 206Pb), and (c) its uncertainty estimates account for the positive skewness of measured isotopic ratio distributions. Second, the new algorithm accounts for the Poisson noise that characterises secondary electron multiplier (SEM) detectors. By fitting the SEM signals using the method of maximum likelihood, it naturally handles low-intensity ion beams, in which zero-count signals are common. Third, the new algorithm casts the data reduction process in a matrix format and thereby captures all sources of systematic uncertainty. These include significant inter-spot error correlations that arise from the Pb / U–UO(2) / U calibration curve. The new algorithm has been implemented in a new software package called simplex. The simplex package was written in R and can be used either online, offline, or from the command line. The programme can handle SIMS data from both Cameca and SHRIMP instruments.
{"title":"An algorithm for U–Pb geochronology by secondary ion mass spectrometry","authors":"P. Vermeesch","doi":"10.5194/gchron-4-561-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-561-2022","url":null,"abstract":"Abstract. Secondary ion mass spectrometry (SIMS) is a widely used technique for in situ U–Pb geochronology of accessory minerals. Existing algorithms for SIMS data reduction and error propagation make a number of simplifying assumptions that degrade the precision and accuracy of the resulting U–Pb dates. This paper uses an entirely new approach to SIMS data processing that introduces the following improvements over previous algorithms. First, it treats SIMS measurements as compositional data using log-ratio statistics. This means that, unlike existing algorithms, (a) its isotopic ratio estimates are guaranteed to be strictly positive numbers, (b) identical results are obtained regardless of whether data are processed as normal ratios (e.g. 206Pb / 238U) or reciprocal\u0000ratios (e.g. 238U / 206Pb), and\u0000(c) its uncertainty estimates account for the positive skewness of\u0000measured isotopic ratio distributions. Second, the new algorithm\u0000accounts for the Poisson noise that characterises secondary electron\u0000multiplier (SEM) detectors. By fitting the SEM signals using the\u0000method of maximum likelihood, it naturally handles low-intensity ion\u0000beams, in which zero-count signals are common. Third, the new\u0000algorithm casts the data reduction process in a matrix format and\u0000thereby captures all sources of systematic uncertainty. These\u0000include significant inter-spot error correlations that arise from\u0000the Pb / U–UO(2) / U calibration curve. The new\u0000algorithm has been implemented in a new software package called\u0000simplex. The simplex package was written in R and can\u0000be used either online, offline, or from the command line. The programme\u0000can handle SIMS data from both Cameca and SHRIMP instruments.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"16 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88136409","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-07-21DOI: 10.5194/gchron-4-501-2022
E. Cooperdock, F. Hofmann, Ryley M. C. Tibbetts, Anahi Carrera, Aya Takase, A. Celestian
Abstract. Apatite and zircon are among the best-studied and most widely used accessory minerals for geochronology and thermochronology. Given that apatite and zircon are often present in the same lithologies, distinguishing the two phases in crushed mineral separates is a common task for geochronology, thermochronology, and petrochronology studies. Here we present a method for efficient and accurate apatite and zircon mineral phase identification and verification using X-ray micro-computed tomography (microCT) of grain mounts that provides additional three-dimensional grain size, shape, and inclusion suite information. In this study, we analyze apatite and zircon grains from Fish Canyon Tuff samples that went through methylene iodide (MEI) and lithium heteropolytungstate (LST) heavy liquid density separations. We validate the microCT results using known standards and phase identification with Raman spectroscopy, demonstrating that apatite and zircon are distinguishable from each other and other common phases, e.g., titanite, based on microCT X-ray density. We present recommended microCT scanning protocols after systematically testing the effects of different scanning parameters and sample positions. This methodology can help to reduce time spent performing density separations with highly toxic chemicals and visually inspecting grains under a light microscope, and the improved mineral identification and characterization can make geochronologic data more robust.
{"title":"Technical note: Rapid phase identification of apatite and zircon grains for geochronology using X-ray micro-computed tomography","authors":"E. Cooperdock, F. Hofmann, Ryley M. C. Tibbetts, Anahi Carrera, Aya Takase, A. Celestian","doi":"10.5194/gchron-4-501-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-501-2022","url":null,"abstract":"Abstract. Apatite and zircon are among the best-studied and most widely used accessory\u0000minerals for geochronology and thermochronology. Given that apatite and\u0000zircon are often present in the same lithologies, distinguishing the two\u0000phases in crushed mineral separates is a common task for geochronology,\u0000thermochronology, and petrochronology studies. Here we present a method for\u0000efficient and accurate apatite and zircon mineral phase identification and\u0000verification using X-ray micro-computed tomography (microCT) of grain mounts\u0000that provides additional three-dimensional grain size, shape, and inclusion\u0000suite information. In this study, we analyze apatite and zircon grains from\u0000Fish Canyon Tuff samples that went through methylene iodide (MEI) and lithium\u0000heteropolytungstate (LST) heavy liquid density separations. We validate the\u0000microCT results using known standards and phase identification with Raman\u0000spectroscopy, demonstrating that apatite and zircon are distinguishable from\u0000each other and other common phases, e.g., titanite, based on microCT X-ray\u0000density. We present recommended microCT scanning protocols after\u0000systematically testing the effects of different scanning parameters and\u0000sample positions. This methodology can help to reduce time spent performing\u0000density separations with highly toxic chemicals and visually inspecting\u0000grains under a light microscope, and the improved mineral identification and\u0000characterization can make geochronologic data more robust.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"163 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-07-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77182785","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-07-20DOI: 10.5194/gchron-4-471-2022
A. Reid, M. Forster, W. Preiss, Alicia Caruso, S. Curtis, T. Wise, D. Vasegh, N. Goswami, G. Lister
Abstract. In this study, we provide 40Ar / 39Ar geochronology data from a suite of variably deformed rocks from a region of low-grade metamorphism within the Cambro–Ordovician Delamerian Orogen, South Australia. Low-grade metamorphic rocks such as these can contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. Hence, they typically yield complex 40Ar / 39Ar age spectra that can be difficult to interpret. Therefore, we have undertaken furnace step heating 40Ar / 39Ar geochronology to obtain age spectra with many steps to allow for application of the method of asymptotes and limits and recognition of the effects of mixing. The samples analysed range from siltstone and shale to phyllite and contain muscovite or phengite with minor microcline as determined by hyperspectral mineralogical characterisation. Whole rock 40Ar / 39Ar analyses were undertaken in most samples due to their very fine-grained nature. All samples are dominated by radiogenic 40Ar, and contain minimal evidence for atmospheric Ca- or Cl-derived argon. Chloritisation may have resulted in limited recoil, causing 39Ar argon loss in some samples, which is especially evident within the first few percent of gas released. Most of the age data, however, appear to have some geological significance. Viewed with respect to the known depositional ages of the stratigraphic units, the age spectra from this study do appear to record both detrital mineral ages and ages related to the varying influence of either cooling or deformation-induced recrystallisation. The shape of the age spectra and the degree of deformation in the phyllites suggest the younger ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Given that the younger limit of deformation recorded in the high-metamorphic-grade regions of the Delamerian Orogen is ca. 490 Ma, the ca. 470 to ca. 458 Ma ages obtained in this study suggest deformation in low-grade shear zones within the Delamerian Orogen may have persisted until ca. 20–32 million years after high-temperature ductile deformation in the high-grade regions of the orogen. We suggest that these younger ages for deformation could reflect reactivation of older structures formed both during rift basin formation and during the main peak of the Delamerian orogeny itself. The younger ca. 470 to ca. 458 Ma deformation may have been facilitated by far-field tectonic processes occurring along the eastern paleo-Pacific margin of Gondwana.
{"title":"Complex 40Ar ∕ 39Ar age spectra from low-grade metamorphic rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen","authors":"A. Reid, M. Forster, W. Preiss, Alicia Caruso, S. Curtis, T. Wise, D. Vasegh, N. Goswami, G. Lister","doi":"10.5194/gchron-4-471-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-471-2022","url":null,"abstract":"Abstract. In this study, we provide 40Ar / 39Ar\u0000geochronology data from a suite of variably deformed rocks from a region of\u0000low-grade metamorphism within the Cambro–Ordovician Delamerian Orogen, South Australia. Low-grade metamorphic rocks such as these can contain both\u0000detrital minerals and minerals newly grown or partly recrystallised during\u0000diagenesis and metamorphism. Hence, they typically yield complex\u000040Ar / 39Ar age spectra that can be difficult to interpret.\u0000Therefore, we have undertaken furnace step heating 40Ar / 39Ar\u0000geochronology to obtain age spectra with many steps to allow for\u0000application of the method of asymptotes and limits and recognition of the\u0000effects of mixing. The samples analysed range from siltstone and shale to\u0000phyllite and contain muscovite or phengite with minor microcline as\u0000determined by hyperspectral mineralogical characterisation. Whole rock\u000040Ar / 39Ar analyses were undertaken in most samples due to their very fine-grained nature. All samples are dominated by radiogenic 40Ar, and contain minimal evidence for atmospheric Ca- or Cl-derived argon. Chloritisation may have resulted in limited recoil, causing 39Ar argon loss in some samples, which is especially evident within the first few percent of gas\u0000released. Most of the age data, however, appear to have some geological\u0000significance. Viewed with respect to the known depositional ages of the\u0000stratigraphic units, the age spectra from this study do appear to record\u0000both detrital mineral ages and ages related to the varying influence of\u0000either cooling or deformation-induced recrystallisation. The shape of the\u0000age spectra and the degree of deformation in the phyllites suggest the\u0000younger ages may record recrystallisation of detrital minerals and/or new\u0000mica growth during deformation. Given that the younger limit of deformation\u0000recorded in the high-metamorphic-grade regions of the Delamerian Orogen is\u0000ca. 490 Ma, the ca. 470 to ca. 458 Ma ages obtained in this study suggest\u0000deformation in low-grade shear zones within the Delamerian Orogen may have\u0000persisted until ca. 20–32 million years after high-temperature ductile\u0000deformation in the high-grade regions of the orogen. We suggest that these\u0000younger ages for deformation could reflect reactivation of older structures\u0000formed both during rift basin formation and during the main peak of the\u0000Delamerian orogeny itself. The younger ca. 470 to ca. 458 Ma deformation may have been facilitated by far-field tectonic processes occurring along the eastern paleo-Pacific margin of Gondwana.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"24 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-07-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90807633","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-07-06DOI: 10.5194/gchron-4-455-2022
Richard F. Ott, S. Gallen, D. Granger
Abstract. Cosmogenic radionuclides (CRNs) are the standard tool to derive centennial-to-millennial timescale denudation rates; however, it has been demonstrated that chemical weathering in some settings can bias CRNs as a proxy for landscape denudation. Currently, studies investigating CRN weathering biases have mostly focused on the largely insoluble target mineral quartz in felsic lithologies. Here, we examine the response of CRN build-up for both soluble and insoluble target minerals under different weathering scenarios. We assume a simple box model in which bedrock is converted to a well-mixed regolith at a constant rate, and denudation occurs by regolith erosion and weathering either in the regolith or along the regolith–bedrock interface, as is common in carbonate bedrock. We show that weathering along the regolith–bedrock interface increases CRN concentrations compared to a no-weathering case and how independently derived weathering rates or degrees can be used to correct for this bias. If weathering is concentrated within the regolith, insoluble target minerals will have a longer regolith residence time and higher nuclide concentration than soluble target minerals. This bias can be identified and corrected using paired-nuclide measurements of minerals with different solubility coupled with knowledge of either the bedrock or regolith mineralogy to derive denudation and long-term weathering rates. Similarly, single-nuclide measurements on soluble or insoluble minerals can be corrected to determine denudation rates if a weathering rate and compositional data are available. Our model highlights that for soluble target minerals, the relationship between nuclide accumulation and denudation is not monotonic. We use this understanding to map the conditions of regolith mass, weathering, and denudation rates at which weathering corrections for cosmogenic nuclides become large and ambiguous, as well as identify environments in which the bias is mostly negligible and CRN concentrations reliably reflect landscape denudation. We highlight how measurements of CRNs from soluble target minerals, coupled with bedrock and regolith mineralogy, can help to expand the range of landscapes for which centennial-to-millennial timescale denudation and weathering rates can be obtained.
{"title":"Cosmogenic nuclide weathering biases: corrections and potential for denudation and weathering rate measurements","authors":"Richard F. Ott, S. Gallen, D. Granger","doi":"10.5194/gchron-4-455-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-455-2022","url":null,"abstract":"Abstract. Cosmogenic radionuclides (CRNs) are the standard tool to derive\u0000centennial-to-millennial timescale denudation rates; however, it has been\u0000demonstrated that chemical weathering in some settings can bias CRNs as a\u0000proxy for landscape denudation. Currently, studies investigating CRN\u0000weathering biases have mostly focused on the largely insoluble target\u0000mineral quartz in felsic lithologies. Here, we examine the response of CRN\u0000build-up for both soluble and insoluble target minerals under different\u0000weathering scenarios. We assume a simple box model in which bedrock is\u0000converted to a well-mixed regolith at a constant rate, and denudation occurs by regolith erosion and weathering either in the regolith or along the regolith–bedrock interface, as is common in carbonate bedrock. We show that weathering along the regolith–bedrock interface increases CRN concentrations compared to a no-weathering case and how independently derived weathering rates or degrees can be used to correct for this bias. If weathering is concentrated within the regolith, insoluble target minerals will have a longer regolith residence time and higher nuclide concentration than soluble\u0000target minerals. This bias can be identified and corrected using paired-nuclide measurements of minerals with different solubility coupled with\u0000knowledge of either the bedrock or regolith mineralogy to derive denudation\u0000and long-term weathering rates. Similarly, single-nuclide measurements on\u0000soluble or insoluble minerals can be corrected to determine denudation rates if a weathering rate and compositional data are available. Our model\u0000highlights that for soluble target minerals, the relationship between\u0000nuclide accumulation and denudation is not monotonic. We use this\u0000understanding to map the conditions of regolith mass, weathering, and\u0000denudation rates at which weathering corrections for cosmogenic nuclides\u0000become large and ambiguous, as well as identify environments in which the\u0000bias is mostly negligible and CRN concentrations reliably reflect landscape denudation. We highlight how measurements of CRNs from soluble target minerals, coupled with bedrock and regolith mineralogy, can help to expand the range of landscapes for which centennial-to-millennial timescale\u0000denudation and weathering rates can be obtained.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-07-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74031208","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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