Kate Jenkins, Karsten Goemann, Ivan Belousov, Maxwell Morissette, Leonid Danyushevsky
{"title":"安德氏石榴石和金红石的剥蚀行为研究及其对U-Pb地质年代学的启示","authors":"Kate Jenkins, Karsten Goemann, Ivan Belousov, Maxwell Morissette, Leonid Danyushevsky","doi":"10.1111/ggr.12478","DOIUrl":null,"url":null,"abstract":"<p>U-Pb dating of andradite-grossular garnet (grandite) and rutile by LA-ICP-MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down-hole fractionation and U-Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U-Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm<sup>-2</sup>, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy-related and making higher fluences, such as 5 J cm<sup>-2</sup>, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm<sup>-2</sup> suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down-hole fractionation patterns of the samples are similar at 5 J cm<sup>-2</sup>, the U-Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U-Pb dating of rutile.</p>","PeriodicalId":12631,"journal":{"name":"Geostandards and Geoanalytical Research","volume":"47 2","pages":"267-295"},"PeriodicalIF":2.7000,"publicationDate":"2023-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Investigation of the Ablation Behaviour of Andradite-Grossular Garnets and Rutile with Implications for U-Pb Geochronology\",\"authors\":\"Kate Jenkins, Karsten Goemann, Ivan Belousov, Maxwell Morissette, Leonid Danyushevsky\",\"doi\":\"10.1111/ggr.12478\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<p>U-Pb dating of andradite-grossular garnet (grandite) and rutile by LA-ICP-MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down-hole fractionation and U-Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U-Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm<sup>-2</sup>, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy-related and making higher fluences, such as 5 J cm<sup>-2</sup>, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm<sup>-2</sup> suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down-hole fractionation patterns of the samples are similar at 5 J cm<sup>-2</sup>, the U-Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U-Pb dating of rutile.</p>\",\"PeriodicalId\":12631,\"journal\":{\"name\":\"Geostandards and Geoanalytical Research\",\"volume\":\"47 2\",\"pages\":\"267-295\"},\"PeriodicalIF\":2.7000,\"publicationDate\":\"2023-01-12\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Geostandards and Geoanalytical Research\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://onlinelibrary.wiley.com/doi/10.1111/ggr.12478\",\"RegionNum\":2,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Geostandards and Geoanalytical Research","FirstCategoryId":"89","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1111/ggr.12478","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
Investigation of the Ablation Behaviour of Andradite-Grossular Garnets and Rutile with Implications for U-Pb Geochronology
U-Pb dating of andradite-grossular garnet (grandite) and rutile by LA-ICP-MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down-hole fractionation and U-Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U-Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm-2, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy-related and making higher fluences, such as 5 J cm-2, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm-2 suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down-hole fractionation patterns of the samples are similar at 5 J cm-2, the U-Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U-Pb dating of rutile.
期刊介绍:
Geostandards & Geoanalytical Research is an international journal dedicated to advancing the science of reference materials, analytical techniques and data quality relevant to the chemical analysis of geological and environmental samples. Papers are accepted for publication following peer review.