Pub Date : 2023-02-05DOI: 10.1080/08120099.2023.2170466
H. Nie, Q. Chen, P. Li, W. Dang, J. C. Zhang
Abstract The assessment of shale gas potential for the Ordovician Pingliang Formation and Carboniferous–Permian Taiyuan and Shanxi formations in the northwest margin of Ordos Basin, China provides insight into how fluctuation in depositional environments has a significant role on lithofacies and shale gas potential. To investigate the shale gas potential, a series of measurements (i.e. Rock-Eval pyrolysis, maceral composition analyses and X-ray powder diffraction, etc.) on representative outcrop samples were conducted to characterise shale properties. The organic matter from marine Pingliang shale is predominantly type I with a strong predominance of sapropelinite, whereas the transitional Taiyuan-Shanxi shales are dominated by types II to III kerogen. Furthermore, the Pingliang shale is characterised as a ‘poor’ source rock mainly owing to the lower total organic carbon (TOC) content (average 0.79 wt%) and higher maturity [average 1.78% in vitrinite reflectance (R o)], while the transitional Taiyuan-Shanxi shales are mostly characterised as ‘fair’ source rocks, and some samples with high TOC content (more than 2.0 wt%) present good source rocks. It is also found that the sedimentary environment, as a key factor determining the organic matter and TOC content, inevitably influences the type and content of minerals in shale, and controls the shale gas potential. For example, the transitional argillaceous Taiyuan-Shanxi shales are significantly different from the siliceous Pingliang shales, specifically, total clay content for the former is more than 50 wt%, while the latter is rich in quartz content (more than 70 wt%). Additionally, the quartz and clay contents of the Taiyuan shale range widely, especially the smectite content of I–S ML. The barrier coastal facies in the Taiyuan Formation are more conducive to the enrichment and preservation of organic matter because the Shanxi shale was deposited in shallow delta facies with a greater terrestrial influence. Conclusively, the Taiyuan and Shanxi formations have relatively good exploitation potential for shale gas, especially the relatively high TOC content (average 2.45 wt%) and moderate R o value (average 1.25%). For future exploration, selecting areas with relatively large shale thickness, high brittle mineral content, stable tectonics and better preservation conditions are key to optimising favourable exploration areas for shale gas. KEY POINTS The shale gas potentials of the argillaceous Taiyuan-Shanxi shales and siliceous Pingliang shale are compared. The influence of sedimentary facies on reservoir parameters of marine and transitional shales is established. This is a first detailed comparison of the marine and transitional shale gas potential in the northwest margin of Ordos Basin, China.
{"title":"Shale gas potential of Ordovician marine Pingliang shale and Carboniferous–Permian transitional Taiyuan-Shanxi shales in the Ordos Basin, China","authors":"H. Nie, Q. Chen, P. Li, W. Dang, J. C. Zhang","doi":"10.1080/08120099.2023.2170466","DOIUrl":"https://doi.org/10.1080/08120099.2023.2170466","url":null,"abstract":"Abstract The assessment of shale gas potential for the Ordovician Pingliang Formation and Carboniferous–Permian Taiyuan and Shanxi formations in the northwest margin of Ordos Basin, China provides insight into how fluctuation in depositional environments has a significant role on lithofacies and shale gas potential. To investigate the shale gas potential, a series of measurements (i.e. Rock-Eval pyrolysis, maceral composition analyses and X-ray powder diffraction, etc.) on representative outcrop samples were conducted to characterise shale properties. The organic matter from marine Pingliang shale is predominantly type I with a strong predominance of sapropelinite, whereas the transitional Taiyuan-Shanxi shales are dominated by types II to III kerogen. Furthermore, the Pingliang shale is characterised as a ‘poor’ source rock mainly owing to the lower total organic carbon (TOC) content (average 0.79 wt%) and higher maturity [average 1.78% in vitrinite reflectance (R o)], while the transitional Taiyuan-Shanxi shales are mostly characterised as ‘fair’ source rocks, and some samples with high TOC content (more than 2.0 wt%) present good source rocks. It is also found that the sedimentary environment, as a key factor determining the organic matter and TOC content, inevitably influences the type and content of minerals in shale, and controls the shale gas potential. For example, the transitional argillaceous Taiyuan-Shanxi shales are significantly different from the siliceous Pingliang shales, specifically, total clay content for the former is more than 50 wt%, while the latter is rich in quartz content (more than 70 wt%). Additionally, the quartz and clay contents of the Taiyuan shale range widely, especially the smectite content of I–S ML. The barrier coastal facies in the Taiyuan Formation are more conducive to the enrichment and preservation of organic matter because the Shanxi shale was deposited in shallow delta facies with a greater terrestrial influence. Conclusively, the Taiyuan and Shanxi formations have relatively good exploitation potential for shale gas, especially the relatively high TOC content (average 2.45 wt%) and moderate R o value (average 1.25%). For future exploration, selecting areas with relatively large shale thickness, high brittle mineral content, stable tectonics and better preservation conditions are key to optimising favourable exploration areas for shale gas. KEY POINTS The shale gas potentials of the argillaceous Taiyuan-Shanxi shales and siliceous Pingliang shale are compared. The influence of sedimentary facies on reservoir parameters of marine and transitional shales is established. This is a first detailed comparison of the marine and transitional shale gas potential in the northwest margin of Ordos Basin, China.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"411 - 422"},"PeriodicalIF":1.2,"publicationDate":"2023-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46093348","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-02-05DOI: 10.1080/08120099.2023.2171124
A. Brown, C. Spandler, T. Blenkinsop, C. Fergusson
Abstract The Tommy Creek Domain is a complex, yet little studied, terrane in the Eastern Subprovince of the Mount Isa Province, northwest Queensland Australia. In this study, we take advantage of modern low-cost and rapid geochronology techniques to undertake an iterative dating approach integrated with detailed fieldwork to define the ages and extents of numerous lithologies and units of the Tommy Creek Domain. This includes some units not previously identified, lithologies previously grouped together based on field observations but now shown to have multiple distinct ages and dates not commonly represented in Mount Isan time–space plots. We identify an episode of felsic magmatism at ca 1640 Ma, and multimodal intrusions (ca 1615 Ma) immediately preceding the onset of the Isan Orogeny. A major rock package of the Tommy Creek Domain, the Milo beds, are characterised here as the youngest pre-Isan Orogeny sedimentary unit in the Eastern Subprovince (1660–1620 Ma), confirming that sedimentation and possibly rifting continued after deposition of the Soldiers Cap, Mount Albert and Kuridala groups (ca 1690–1650 Ma) before the onset of the Isan Orogeny (ca 1600 Ma). The Milo beds are thus age equivalent to the Mount Isa and McNamara groups of the Western Succession. There is evidence of a compositional shift in sedimentation coincident with the ca 1640 Ma Riversleigh Inversion event, previously only observed in the Western Subprovince in the Lawn Hill Platform. The application of geochronology as part of the mapping workflow can assist with differentiating geological units in terranes where field evidence is ambiguous and can aid in the focusing of objectives for field campaigns to enable the best possible interpretations to be made. KEY POINTS New ages constrain the Milo beds in the Tommy Creek Domain as the youngest stratigraphy in the Eastern Subprovince of the Mount Isa Province. The Milo beds are age equivalents of the McNamara and Mount Isa groups of the Western Subprovince of the inlier. Recognition of felsic and mafic intrusions with ca 1640 Ma and ca 1615 Ma ages. Evidence for Riversleigh Inversion event ca 1640 in the Eastern Subprovince.
{"title":"New age constraints for the Tommy Creek Domain of the Mount Isa Inlier, Australia","authors":"A. Brown, C. Spandler, T. Blenkinsop, C. Fergusson","doi":"10.1080/08120099.2023.2171124","DOIUrl":"https://doi.org/10.1080/08120099.2023.2171124","url":null,"abstract":"Abstract The Tommy Creek Domain is a complex, yet little studied, terrane in the Eastern Subprovince of the Mount Isa Province, northwest Queensland Australia. In this study, we take advantage of modern low-cost and rapid geochronology techniques to undertake an iterative dating approach integrated with detailed fieldwork to define the ages and extents of numerous lithologies and units of the Tommy Creek Domain. This includes some units not previously identified, lithologies previously grouped together based on field observations but now shown to have multiple distinct ages and dates not commonly represented in Mount Isan time–space plots. We identify an episode of felsic magmatism at ca 1640 Ma, and multimodal intrusions (ca 1615 Ma) immediately preceding the onset of the Isan Orogeny. A major rock package of the Tommy Creek Domain, the Milo beds, are characterised here as the youngest pre-Isan Orogeny sedimentary unit in the Eastern Subprovince (1660–1620 Ma), confirming that sedimentation and possibly rifting continued after deposition of the Soldiers Cap, Mount Albert and Kuridala groups (ca 1690–1650 Ma) before the onset of the Isan Orogeny (ca 1600 Ma). The Milo beds are thus age equivalent to the Mount Isa and McNamara groups of the Western Succession. There is evidence of a compositional shift in sedimentation coincident with the ca 1640 Ma Riversleigh Inversion event, previously only observed in the Western Subprovince in the Lawn Hill Platform. The application of geochronology as part of the mapping workflow can assist with differentiating geological units in terranes where field evidence is ambiguous and can aid in the focusing of objectives for field campaigns to enable the best possible interpretations to be made. KEY POINTS New ages constrain the Milo beds in the Tommy Creek Domain as the youngest stratigraphy in the Eastern Subprovince of the Mount Isa Province. The Milo beds are age equivalents of the McNamara and Mount Isa groups of the Western Subprovince of the inlier. Recognition of felsic and mafic intrusions with ca 1640 Ma and ca 1615 Ma ages. Evidence for Riversleigh Inversion event ca 1640 in the Eastern Subprovince.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"358 - 374"},"PeriodicalIF":1.2,"publicationDate":"2023-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44437182","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-01-30DOI: 10.1080/08120099.2022.2153384
B. J. Williams, T. Blenkinsop, R. Lilly, M. Thompson, P. Ava, C. Fergusson
Abstract Small-scale foliation boudinage structures occur in rocks that were sampled in drill core from the Mount Isa Cu deposit, northwest Queensland. The necks of foliation boudinage structures plunge gently to the north and south as a result of layer normal shortening and layer parallel extension of the steeply west-dipping Urquhart Shale. Detailed petrographic analysis of the foliation boudinage structures has identified an initial rim of quartz and dolomite, followed by infill and replacement by pyrrhotite and minor chalcopyrite. Foliation boudinage structures formed after dolomitisation and silicification of the shale. They occur most commonly in the unaltered Urquhart Shale where the anisotropy and homogeneity provided by the shale layering is still intact. Infilling of the structures occurred during protracted silica-dolomite alteration, pyrrhotite and chalcopyrite mineralisation. The paragenesis of the foliation boudinage structures is consistent with the established paragenesis of the main Cu mineralisation. Foliation boudinage structures formed over the period from shortening during D4a through to the main Cu mineralisation during D4b west-northwest–east-southeast sinistral-reverse shortening. The timing of foliation boudinage is consistent with a current kinematic model for the Mount Isa system. KEY POINTS First record of foliation boudinage structures at Mount Isa. Foliation boudinage structures with sulfide-dominated infills. Foliation boudinage structures formed as a result of progressive deformation from a D4a dextral-reverse through to D4b sinistral-reverse slip. Foliation boudinage structures are associated with the timing and kinematics of Cu mineralisation at Mount Isa.
{"title":"Foliation boudinage structures in the Mount Isa Cu system","authors":"B. J. Williams, T. Blenkinsop, R. Lilly, M. Thompson, P. Ava, C. Fergusson","doi":"10.1080/08120099.2022.2153384","DOIUrl":"https://doi.org/10.1080/08120099.2022.2153384","url":null,"abstract":"Abstract Small-scale foliation boudinage structures occur in rocks that were sampled in drill core from the Mount Isa Cu deposit, northwest Queensland. The necks of foliation boudinage structures plunge gently to the north and south as a result of layer normal shortening and layer parallel extension of the steeply west-dipping Urquhart Shale. Detailed petrographic analysis of the foliation boudinage structures has identified an initial rim of quartz and dolomite, followed by infill and replacement by pyrrhotite and minor chalcopyrite. Foliation boudinage structures formed after dolomitisation and silicification of the shale. They occur most commonly in the unaltered Urquhart Shale where the anisotropy and homogeneity provided by the shale layering is still intact. Infilling of the structures occurred during protracted silica-dolomite alteration, pyrrhotite and chalcopyrite mineralisation. The paragenesis of the foliation boudinage structures is consistent with the established paragenesis of the main Cu mineralisation. Foliation boudinage structures formed over the period from shortening during D4a through to the main Cu mineralisation during D4b west-northwest–east-southeast sinistral-reverse shortening. The timing of foliation boudinage is consistent with a current kinematic model for the Mount Isa system. KEY POINTS First record of foliation boudinage structures at Mount Isa. Foliation boudinage structures with sulfide-dominated infills. Foliation boudinage structures formed as a result of progressive deformation from a D4a dextral-reverse through to D4b sinistral-reverse slip. Foliation boudinage structures are associated with the timing and kinematics of Cu mineralisation at Mount Isa.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"972 - 989"},"PeriodicalIF":1.2,"publicationDate":"2023-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47436439","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-01-29DOI: 10.1080/08120099.2023.2161635
Z. Li, K. Rankenburg, L. Normore, N. Evans, B. McInnes, L. M. Dent, I. Fielding
Abstract In sedimentary basins, the determination of the absolute timing of deposition and diagenetic events is a challenging yet critical parameter necessary in the reconstruction of paleo-fluid evolution and burial histories. Here we demonstrate the practical application of in situ calcite U–Pb geochronology on core samples from the Olympic 1 well in the Canning Basin of Western Australia. Using quantitative mineralogy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analytical techniques, we obtained an authigenic calcite U–Pb age of 469.7 ± 4.3 Ma for limestone in the Samphire Marsh Member of the Lower Ordovician Nambeet Formation. This precise depositional age can be independently verified using zircon thermal ionisation mass spectrometry U–Pb ages of 479.4 to 470.2 Ma determined on adjacent volcanic ash beds. Further geochronology studies on calcite cements from the Lower Ordovician Fly Flat Member sandstone from the Nambeet Formation returned a U–Pb age of 365.3 ± 5.8 Ma. This is the first study to place absolute age constraints on the diagenetic event that occluded the intergranular space in a sandstone reservoir. The calcite cementation age suggests that impairment of reservoir quality in the Fly Flat Member sandstone occurred in the Late Devonian, much earlier than major petroleum charge events in the Canning Basin. The calcite U–Pb geochronometer, when combined with complementary quantitative mineralogical analysis, can build direct temporal constraints on the depositional and diagenetic processes in both carbonate and clastic sedimentary rocks in basins worldwide. Key Points The integration of automated scanning electron microscopy/energy-dispersive X‐ray spectrometry quantitative mineralogical analysis with LA-ICP-MS analysis enables more reliable and efficient i n situ calcite U–Pb dating. An accurate age of 469.7 ± 4.3 Ma is obtained for carbonate sedimentation in the Samphire Marsh Member of the Lower Ordovician Nambeet Formation in the Canning Basin. The calcite cementation age of 365.3 ± 5.8 Ma provides an absolute time constraint on pore-occluding lithification of the Fly Flat Member sandstone reservoir in the Canning Basin. I n situ calcite U–Pb dating can place precise temporal constraints on sediment deposition and paragenetic sequence in sedimentary basins.
{"title":"In situ calcite U–Pb geochronology of carbonate and clastic sedimentary rocks from the Canning Basin, Western Australia","authors":"Z. Li, K. Rankenburg, L. Normore, N. Evans, B. McInnes, L. M. Dent, I. Fielding","doi":"10.1080/08120099.2023.2161635","DOIUrl":"https://doi.org/10.1080/08120099.2023.2161635","url":null,"abstract":"Abstract In sedimentary basins, the determination of the absolute timing of deposition and diagenetic events is a challenging yet critical parameter necessary in the reconstruction of paleo-fluid evolution and burial histories. Here we demonstrate the practical application of in situ calcite U–Pb geochronology on core samples from the Olympic 1 well in the Canning Basin of Western Australia. Using quantitative mineralogy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analytical techniques, we obtained an authigenic calcite U–Pb age of 469.7 ± 4.3 Ma for limestone in the Samphire Marsh Member of the Lower Ordovician Nambeet Formation. This precise depositional age can be independently verified using zircon thermal ionisation mass spectrometry U–Pb ages of 479.4 to 470.2 Ma determined on adjacent volcanic ash beds. Further geochronology studies on calcite cements from the Lower Ordovician Fly Flat Member sandstone from the Nambeet Formation returned a U–Pb age of 365.3 ± 5.8 Ma. This is the first study to place absolute age constraints on the diagenetic event that occluded the intergranular space in a sandstone reservoir. The calcite cementation age suggests that impairment of reservoir quality in the Fly Flat Member sandstone occurred in the Late Devonian, much earlier than major petroleum charge events in the Canning Basin. The calcite U–Pb geochronometer, when combined with complementary quantitative mineralogical analysis, can build direct temporal constraints on the depositional and diagenetic processes in both carbonate and clastic sedimentary rocks in basins worldwide. Key Points The integration of automated scanning electron microscopy/energy-dispersive X‐ray spectrometry quantitative mineralogical analysis with LA-ICP-MS analysis enables more reliable and efficient i n situ calcite U–Pb dating. An accurate age of 469.7 ± 4.3 Ma is obtained for carbonate sedimentation in the Samphire Marsh Member of the Lower Ordovician Nambeet Formation in the Canning Basin. The calcite cementation age of 365.3 ± 5.8 Ma provides an absolute time constraint on pore-occluding lithification of the Fly Flat Member sandstone reservoir in the Canning Basin. I n situ calcite U–Pb dating can place precise temporal constraints on sediment deposition and paragenetic sequence in sedimentary basins.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"332 - 343"},"PeriodicalIF":1.2,"publicationDate":"2023-01-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49306330","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-01-23DOI: 10.1080/08120099.2023.2157485
W. Witt, C. Fisher, S. Hagemann, M. Roberts
Abstract Apatite was separated from four samples of syenite porphyry, taken from the Karari gold deposit, in the Kurnalpi Terrane of the Archean Kalgoorlie-Kurnalpi Rift, Eastern Goldfields Superterrane (EGST). The alkalic composition of the syenitic magmas inhibited zircon crystallisation, so apatite provided the best mineral for geochronological investigations. LA-ICP-MS analysis of U, Th and Pb isotopes in the apatite gave a relatively wide range of lower intercept ages, with large errors, ranging from 1 to 3%, using OD-306 apatite as the primary standard. Cathodoluminescent (CL)-darker cores that comprise the major volume of apatite grains are relatively homogeneous in two samples, with one having clear oscillatory zoning. These samples yielded intercept ages of 2701 ± 34 Ma and 2699 ± 25 Ma, respectively. These ages are interpreted to approximate the magmatic crystallisation age of the apatite. Younger intercept ages were generated by apatite from two other samples, which display more complex and heterogeneous patterns of CL brightness. The apatite ages from these two samples are interpreted to have been produced by integrated analysis of apatite that has been heterogeneously modified by younger events. However, the magnitude of the temporal gap between magma emplacement and closure of the U–Pb system in apatite from these two samples remains unknown. Our best estimate of the age of the magmatic apatite from at least two of the syenitic intrusions at Karari is ca 2.70 Ga, which identifies these as the oldest intrusions of the Syenitic Group of magmas yet identified in the EGST. However, if ages are corrected to offset observed in the 401 apatite secondary standard, the two oldest syenitic intrusions are dated at ca 2660 Ma. Key Points Syenite porphyry intrusions are spatially associated with gold mineralisation at Karari, gold deposit, WA. Apatite in the syenite porphyry intrusions has been used to determine the age of the intrusions in these zircon-poor rocks. LA-ICP-MS analysis of U–Pb isotopes from two intrusions produce apatite ages that are interpreted to approximate the age of magmatic crystallisation. The interpreted magmatic dates are older than any previously dated syenitic intrusions in the Kalgoorlie-Kurnalpi Rift.
{"title":"Oldest syenitic intrusions of the Yilgarn Craton identified at Karari gold deposit, Carosue Dam camp, Western Australia?","authors":"W. Witt, C. Fisher, S. Hagemann, M. Roberts","doi":"10.1080/08120099.2023.2157485","DOIUrl":"https://doi.org/10.1080/08120099.2023.2157485","url":null,"abstract":"Abstract Apatite was separated from four samples of syenite porphyry, taken from the Karari gold deposit, in the Kurnalpi Terrane of the Archean Kalgoorlie-Kurnalpi Rift, Eastern Goldfields Superterrane (EGST). The alkalic composition of the syenitic magmas inhibited zircon crystallisation, so apatite provided the best mineral for geochronological investigations. LA-ICP-MS analysis of U, Th and Pb isotopes in the apatite gave a relatively wide range of lower intercept ages, with large errors, ranging from 1 to 3%, using OD-306 apatite as the primary standard. Cathodoluminescent (CL)-darker cores that comprise the major volume of apatite grains are relatively homogeneous in two samples, with one having clear oscillatory zoning. These samples yielded intercept ages of 2701 ± 34 Ma and 2699 ± 25 Ma, respectively. These ages are interpreted to approximate the magmatic crystallisation age of the apatite. Younger intercept ages were generated by apatite from two other samples, which display more complex and heterogeneous patterns of CL brightness. The apatite ages from these two samples are interpreted to have been produced by integrated analysis of apatite that has been heterogeneously modified by younger events. However, the magnitude of the temporal gap between magma emplacement and closure of the U–Pb system in apatite from these two samples remains unknown. Our best estimate of the age of the magmatic apatite from at least two of the syenitic intrusions at Karari is ca 2.70 Ga, which identifies these as the oldest intrusions of the Syenitic Group of magmas yet identified in the EGST. However, if ages are corrected to offset observed in the 401 apatite secondary standard, the two oldest syenitic intrusions are dated at ca 2660 Ma. Key Points Syenite porphyry intrusions are spatially associated with gold mineralisation at Karari, gold deposit, WA. Apatite in the syenite porphyry intrusions has been used to determine the age of the intrusions in these zircon-poor rocks. LA-ICP-MS analysis of U–Pb isotopes from two intrusions produce apatite ages that are interpreted to approximate the age of magmatic crystallisation. The interpreted magmatic dates are older than any previously dated syenitic intrusions in the Kalgoorlie-Kurnalpi Rift.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"344 - 357"},"PeriodicalIF":1.2,"publicationDate":"2023-01-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42489125","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-12-07DOI: 10.1080/08120099.2023.2150682
G. Williams
Abstract The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.
{"title":"Cyclic tidalites and seismites at a submarine hydrothermal system for a 2450 Ma banded iron formation, Hamersley Basin, Western Australia","authors":"G. Williams","doi":"10.1080/08120099.2023.2150682","DOIUrl":"https://doi.org/10.1080/08120099.2023.2150682","url":null,"abstract":"Abstract The Hamersley Group in the Hamersley Basin, Western Australia, contains major deposits of banded iron formation of Neoarchean–Paleoproterozoic age (2629–2449 Ma). The 2450 Ma Weeli Wolli Formation near the top of the group includes banded iron formation with microbands (≤0.05–0.5 mm to rarely 1 mm thick) of chert and iron oxide lamina couplets that display distinctive cycles. Some adjacent, thin microbands have merged with compaction and the diagenetic removal of chert laminae, and maximum counts of cycle period by different studies is preferred, suggesting a period of ∼28 ± 2 microbands. Associated cyclic iron formation shows soft-sediment plastic deformation and brittle fracturing. Rare earth element geochemistry indicates that much of the iron in late Archean–early Paleoproterozoic banded iron formations was derived from submarine hydrothermal systems. Monitoring of hydrothermal vents on mid-ocean ridges and seamounts has identified tidal modulation of the discharge, temperature and dispersal of deep-sea hydrothermal plumes principally by semidiurnal variations in tidal loading and earth tides. Microearthquake swarms are associated with deep-sea hydrothermal systems at the crests of mid-ocean ridges, and tidal triggering of microearthquake activity is indicated for a hydrothermal system on the East Pacific Rise. The microband cycles in the Weeli Wolli Formation are interpreted as tidalites, comprising ∼28 ± 2 semidiurnal microbands per lunar (synodic) fortnightly cycle, related to the tidally modulated activity of a submarine hydrothermal system. The associated plastically deformed and brittle-fractured sediments are viewed as seismites resulting from microearthquakes also related to the hydrothermal system. The Weeli Wolli microband cycles and seismites may represent a rarely identified instance of iron-formation deposition near a submarine hydrothermal system. KEY POINTS Submarine hydrothermal venting was a major source of iron in late Archean–early Paleoproterozoic banded iron formations. Semidiurnal variations in tidal loading and earth tides commonly modulate present-day deep-sea hydrothermal activity, and microearthquake swarms are associated with deep-sea hydrothermal systems. Early Paleoproterozoic banded iron formation from the Hamersley Basin displays cycles with an estimated period of ∼28 ± 2 microbands and associated plastic deformation and brittle fracturing. The microband cycles are interpreted as tidalites comprising ∼28 ± 2 semidiurnal increments grouped in fortnightly cycles at a tidally modulated submarine hydrothermal system, and the deformed sediments as seismites caused by related microearthquake activity.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"323 - 331"},"PeriodicalIF":1.2,"publicationDate":"2022-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46003777","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-11-30DOI: 10.1080/08120099.2023.2148187
C. Fergusson, P. Hatherly
Abstract The 95 km-long Lapstone Structural Complex is a segmented structural association consisting of north-trending faults and monoclines in the western Sydney Basin between the Cumberland Basin to the east and lower Blue Mountains to the west. It has developed in a compressional regime. At depth, the Lapstone Structural Complex is most likely a deeply penetrating, west-dipping thrust fault that is seismogenic in the brittle middle crust. This structure has propagated upwards into the overlying Sydney Basin in the top ∼3 km of the crust and formed a suite of distinct fault and fault–monocline structures, including: (a) in the north, inferred imbricate faults at depth in the lower Sydney Basin dying out upwards into a well-displayed, major east-facing monocline (central limb has an overall consistent <20°E dip); (b) in the middle part of the complex, an east-facing monocline with dips increasing from west to east formed as a fault-propagation fold; and (c) as found in the south, a single thrust fault (Bargo and Nepean faults). The complex has a probable late Cenozoic age and has played a role in landscape development as shown by topography, uplifted river gravels and knick points along lower order streams. Therefore, it has formed late in the uplift history of the southeastern Australian highlands postdating uplift in the Cretaceous and uplift associated with basaltic eruptions in the Cenozoic. KEY POINTS The Lapstone Structural Complex is an association of segmented faults and monoclines formed at the tip of a deep-seated thrust fault. The largest structure in the complex is a gently east-dipping monocline formed above an inferred imbricate fault system in the lower Sydney Basin succession. Much of the middle and southern parts of the complex are single faults and a fault-propagation monocline related to a thrust fault at depth. The Lapstone Structural Complex is of late Cenozoic age and is most likely seismogenic.
{"title":"Segmentation and fault–monocline relationships in the Lapstone Structural Complex, Sydney Basin, New South Wales","authors":"C. Fergusson, P. Hatherly","doi":"10.1080/08120099.2023.2148187","DOIUrl":"https://doi.org/10.1080/08120099.2023.2148187","url":null,"abstract":"Abstract The 95 km-long Lapstone Structural Complex is a segmented structural association consisting of north-trending faults and monoclines in the western Sydney Basin between the Cumberland Basin to the east and lower Blue Mountains to the west. It has developed in a compressional regime. At depth, the Lapstone Structural Complex is most likely a deeply penetrating, west-dipping thrust fault that is seismogenic in the brittle middle crust. This structure has propagated upwards into the overlying Sydney Basin in the top ∼3 km of the crust and formed a suite of distinct fault and fault–monocline structures, including: (a) in the north, inferred imbricate faults at depth in the lower Sydney Basin dying out upwards into a well-displayed, major east-facing monocline (central limb has an overall consistent <20°E dip); (b) in the middle part of the complex, an east-facing monocline with dips increasing from west to east formed as a fault-propagation fold; and (c) as found in the south, a single thrust fault (Bargo and Nepean faults). The complex has a probable late Cenozoic age and has played a role in landscape development as shown by topography, uplifted river gravels and knick points along lower order streams. Therefore, it has formed late in the uplift history of the southeastern Australian highlands postdating uplift in the Cretaceous and uplift associated with basaltic eruptions in the Cenozoic. KEY POINTS The Lapstone Structural Complex is an association of segmented faults and monoclines formed at the tip of a deep-seated thrust fault. The largest structure in the complex is a gently east-dipping monocline formed above an inferred imbricate fault system in the lower Sydney Basin succession. Much of the middle and southern parts of the complex are single faults and a fault-propagation monocline related to a thrust fault at depth. The Lapstone Structural Complex is of late Cenozoic age and is most likely seismogenic.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"375 - 392"},"PeriodicalIF":1.2,"publicationDate":"2022-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44708461","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-11-21DOI: 10.1080/08120099.2023.2145614
Y. B. Sun, Y. F. Zhang, A. Xi, Y. Tang, Bo Zhang, S. Pei, R. R. Li, H. Yin, Q. Zeng, H. Qu, R. Zhou
Abstract Reservoir spaces, such as vesicles, ‘secondary’ amygdales, dissolution caverns and geodes, are widely developed in the Emeishan basaltic lavas in the Zhoudaping section, Leshan, west Sichuan, China. The dissolution characteristics, cementation sequences, hydrothermal activity stages, as well as fluid types, and their effects on the reservoir capacity were investigated for each stage. Macroscopically, the dissolution features present as irregular dissolution zones, which are characterised by a light red colour. Microscopically, in the dissolved zone, the cementation-filling minerals are associated with complex fill sequences, such as quartz/laumontite/chlorite–chlorite/saponite–epidote/celadonite–cryptocrystalline chlorite–laumontite/calcite/quartz. The U–Pb geochronology shows that the age of chlorite fill in amygdales is 235.3 ± 19.6 Ma; the coarse-crystalline quartz inside dissolution caverns/geodes, 124.47 ± 5.63 to 123.84 ± 5.63 Ma; and the siliceous mineral-filled amygdales, 118.34 ± 3.70 to 114.08 ± 3.76 Ma, which correspond to the early Late Triassic and the mid–late Early Cretaceous, respectively. Combined with geochemical characteristics of post-dissolution fill, the amygdales are affected by two stages of hydrothermal activity: chlorite filling of the amygdales corresponds to post-magma hydrothermal fluids during the early Late Triassic, and the siliceous mineral-fill in amygdales corresponds to deep-sourced hydrothermal fluids during the mid–late Early Cretaceous. The geodes/dissolution caverns result from a single stage of hydrothermal activity related to the mid–late Early Cretaceous deep-source low-temperature hydrothermal fluid. The Late Triassic post-magma hydrothermal fluids are generally destructive to pores, and tectonic-related dissolution of deep-sourced hydrothermal fluids has a positive effect on the formation of reservoir spaces, greatly enhancing fluid storage and flow capacities of the volcanic lavas. We recommend the multi-stage hydrothermal dissolution during Late Triassic–Early Cretaceous and faults, fractures and columnar joints be the focus of hydrocarbon exploration. KEY POINTS The reservoir spaces developed in the Zhoudaping section, such as amygdales, dissolution caverns and geodes, were controlled by different stages and types of hydrothermal alteration. Amygdales are the product of two hydrothermal events, which correspond to post-magma hydrothermal fluids during the early Late Triassic and the deep-sourced hydrothermal fluids of the mid–late Early Cretaceous. Geodes/dissolution caverns are affected by deep-source low-temperature hydrothermal fluids in the mid–late Early Cretaceous. The multi-stage hydrothermal dissolution during the Late Triassic–Early Cretaceous and faults, fractures and columnar joints should be the focus of hydrocarbon exploration.
{"title":"Hydrothermal alteration and corresponding reservoir significance of the Permian Emeishan basaltic lavas, west Sichuan, China","authors":"Y. B. Sun, Y. F. Zhang, A. Xi, Y. Tang, Bo Zhang, S. Pei, R. R. Li, H. Yin, Q. Zeng, H. Qu, R. Zhou","doi":"10.1080/08120099.2023.2145614","DOIUrl":"https://doi.org/10.1080/08120099.2023.2145614","url":null,"abstract":"Abstract Reservoir spaces, such as vesicles, ‘secondary’ amygdales, dissolution caverns and geodes, are widely developed in the Emeishan basaltic lavas in the Zhoudaping section, Leshan, west Sichuan, China. The dissolution characteristics, cementation sequences, hydrothermal activity stages, as well as fluid types, and their effects on the reservoir capacity were investigated for each stage. Macroscopically, the dissolution features present as irregular dissolution zones, which are characterised by a light red colour. Microscopically, in the dissolved zone, the cementation-filling minerals are associated with complex fill sequences, such as quartz/laumontite/chlorite–chlorite/saponite–epidote/celadonite–cryptocrystalline chlorite–laumontite/calcite/quartz. The U–Pb geochronology shows that the age of chlorite fill in amygdales is 235.3 ± 19.6 Ma; the coarse-crystalline quartz inside dissolution caverns/geodes, 124.47 ± 5.63 to 123.84 ± 5.63 Ma; and the siliceous mineral-filled amygdales, 118.34 ± 3.70 to 114.08 ± 3.76 Ma, which correspond to the early Late Triassic and the mid–late Early Cretaceous, respectively. Combined with geochemical characteristics of post-dissolution fill, the amygdales are affected by two stages of hydrothermal activity: chlorite filling of the amygdales corresponds to post-magma hydrothermal fluids during the early Late Triassic, and the siliceous mineral-fill in amygdales corresponds to deep-sourced hydrothermal fluids during the mid–late Early Cretaceous. The geodes/dissolution caverns result from a single stage of hydrothermal activity related to the mid–late Early Cretaceous deep-source low-temperature hydrothermal fluid. The Late Triassic post-magma hydrothermal fluids are generally destructive to pores, and tectonic-related dissolution of deep-sourced hydrothermal fluids has a positive effect on the formation of reservoir spaces, greatly enhancing fluid storage and flow capacities of the volcanic lavas. We recommend the multi-stage hydrothermal dissolution during Late Triassic–Early Cretaceous and faults, fractures and columnar joints be the focus of hydrocarbon exploration. KEY POINTS The reservoir spaces developed in the Zhoudaping section, such as amygdales, dissolution caverns and geodes, were controlled by different stages and types of hydrothermal alteration. Amygdales are the product of two hydrothermal events, which correspond to post-magma hydrothermal fluids during the early Late Triassic and the deep-sourced hydrothermal fluids of the mid–late Early Cretaceous. Geodes/dissolution caverns are affected by deep-source low-temperature hydrothermal fluids in the mid–late Early Cretaceous. The multi-stage hydrothermal dissolution during the Late Triassic–Early Cretaceous and faults, fractures and columnar joints should be the focus of hydrocarbon exploration.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"393 - 410"},"PeriodicalIF":1.2,"publicationDate":"2022-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42263135","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-11-16DOI: 10.1080/08120099.2023.2139757
J. Clemens, G. Stevens, L. Coetzer
Abstract In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance. KEY POINTS Mesoproterozoic sections of the Selwyn Block cannot be the sources for all the Devonian silicic magmas in central Victoria. I-type rocks have 650–500 Ma zircon age peaks, suggesting derivation from arc-related metavolcanic rocks in the upper Selwyn Block. Hornblende-bearing I-type rocks have the strongest zircon inheritance patterns, indicating the largely crustal origins of I-type magmas. Exposed metasedimentary country rocks were not involved in magma genesis.
{"title":"Zircon inheritance, sources of Devonian granitic magmas and crustal structure in central Victoria","authors":"J. Clemens, G. Stevens, L. Coetzer","doi":"10.1080/08120099.2023.2139757","DOIUrl":"https://doi.org/10.1080/08120099.2023.2139757","url":null,"abstract":"Abstract In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance. KEY POINTS Mesoproterozoic sections of the Selwyn Block cannot be the sources for all the Devonian silicic magmas in central Victoria. I-type rocks have 650–500 Ma zircon age peaks, suggesting derivation from arc-related metavolcanic rocks in the upper Selwyn Block. Hornblende-bearing I-type rocks have the strongest zircon inheritance patterns, indicating the largely crustal origins of I-type magmas. Exposed metasedimentary country rocks were not involved in magma genesis.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"227 - 259"},"PeriodicalIF":1.2,"publicationDate":"2022-11-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42871040","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2022-11-13DOI: 10.1080/08120099.2023.2137585
N. Sergeev
Abstract Chemical weathering indices (CWIs) based on bulk chemical rock composition can potentially provide an objective tool for estimation of weathering intensity and classification of weathering products. However, despite their long history and the large number of indices that have now been developed, their applicability still has serious limitations. To overcome most of the limitations, this paper proposes a new weathering index based on the review of geochemical and mineral rock evolution during weathering and analysis of the existing CWIs. The new index, the alumina + ferric oxide to bases ratio (AFB), is expressed as AFB = AFBw/AFBp where AFB = (Al2O3+Fe2O3)/(K2O + Na2O + CaO + MgO); w, weathered rock and p, parent rock, with all elements in molecular proportions. The index provides a basis for comparison of weathering intensities between different lithologies by linking the chemical and mineral transformations that characterise the regolith profile. It is sensitive to all stages of weathering, including lateritisation by using the ratio of the stable (relatively immobile) to mobile metals expressed as oxides. An extensive, worldwide chemical database on 40 well-documented regolith profiles developed on felsic, mafic and ultramafic substrate was used for testing the proposed index and its comparison with the most common indices. The rocks were mostly weathered under tropical or subtropical conditions. The results confirmed consistent increase in the AFB value with intensifying weathering. The index is applicable to all silicate rock types including Al-poor ultramafic rocks. Use of parent rock normalisation allows a more accurate comparison of weathering intensities between different lithologies. At the expense of these benefits, the parent normalised index AFB requires data for the unweathered protolith that is commonly difficult to obtain. The index is also sensitive to inhomogeneity of the original rocks. The unreferenced to parent rock AFBu index has potentially broader applications including provenance and the weathering history of sediments, soil and engineering studies, although collection of more data is required for understanding the index constraints for various conditions and rock types. KEY POINTS A new chemical weathering index, expressed as the alumina+ferric oxide to bases ratio normalised to parent rock provides reasonable results for all major types of silicate lithologies. The index is applicable for all types of weathering including the lateritic environment. On the downside, the index is sensitive to inhomogeneity of the original substrate and to later epigenetic modifications of the residual regolith.
{"title":"Quantifying weathering intensity using chemical proxies: a weathering index AFB","authors":"N. Sergeev","doi":"10.1080/08120099.2023.2137585","DOIUrl":"https://doi.org/10.1080/08120099.2023.2137585","url":null,"abstract":"Abstract Chemical weathering indices (CWIs) based on bulk chemical rock composition can potentially provide an objective tool for estimation of weathering intensity and classification of weathering products. However, despite their long history and the large number of indices that have now been developed, their applicability still has serious limitations. To overcome most of the limitations, this paper proposes a new weathering index based on the review of geochemical and mineral rock evolution during weathering and analysis of the existing CWIs. The new index, the alumina + ferric oxide to bases ratio (AFB), is expressed as AFB = AFBw/AFBp where AFB = (Al2O3+Fe2O3)/(K2O + Na2O + CaO + MgO); w, weathered rock and p, parent rock, with all elements in molecular proportions. The index provides a basis for comparison of weathering intensities between different lithologies by linking the chemical and mineral transformations that characterise the regolith profile. It is sensitive to all stages of weathering, including lateritisation by using the ratio of the stable (relatively immobile) to mobile metals expressed as oxides. An extensive, worldwide chemical database on 40 well-documented regolith profiles developed on felsic, mafic and ultramafic substrate was used for testing the proposed index and its comparison with the most common indices. The rocks were mostly weathered under tropical or subtropical conditions. The results confirmed consistent increase in the AFB value with intensifying weathering. The index is applicable to all silicate rock types including Al-poor ultramafic rocks. Use of parent rock normalisation allows a more accurate comparison of weathering intensities between different lithologies. At the expense of these benefits, the parent normalised index AFB requires data for the unweathered protolith that is commonly difficult to obtain. The index is also sensitive to inhomogeneity of the original rocks. The unreferenced to parent rock AFBu index has potentially broader applications including provenance and the weathering history of sediments, soil and engineering studies, although collection of more data is required for understanding the index constraints for various conditions and rock types. KEY POINTS A new chemical weathering index, expressed as the alumina+ferric oxide to bases ratio normalised to parent rock provides reasonable results for all major types of silicate lithologies. The index is applicable for all types of weathering including the lateritic environment. On the downside, the index is sensitive to inhomogeneity of the original substrate and to later epigenetic modifications of the residual regolith.","PeriodicalId":8601,"journal":{"name":"Australian Journal of Earth Sciences","volume":"70 1","pages":"260 - 284"},"PeriodicalIF":1.2,"publicationDate":"2022-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42037066","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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