The widespread presence of Mesoproterozoic detrital zircon in western Laurentia has long been interpreted as evidence for transcontinental river systems transporting sediment from the Grenville orogen. This model asserts that Grenville-age zircon were carried across Laurentia during the Neoproterozoic and Paleozoic, but recent isotopic and provenance data challenge this interpretation. Here, we show that detrital zircon age distributions and Hf isotopic compositions east and west of the Transcontinental Arch remained distinct until the late Paleozoic, inconsistent with repeated sediment mixing by a long-lived transcontinental river system. Instead, multiple Stenian magmatic sources in western Laurentia, including the Llano Uplift, Pikes Peak batholith, and the Southwestern Laurentia Large Igneous Province, provide plausible proximal sources of zircon. Additionally, the Pearya terrane, the northern Yukon, and formerly adjacent Antarctic crust may have contributed detrital zircon to western Laurentia’s sedimentary record. Post-Devonian sediment redistribution may have been accomplished by a combination of fluvial, shallow marine, and eolian processes, or indirect sediment routing. These findings suggest that late Paleozoic processes, rather than a Proterozoic transcontinental river(s), were responsible for distributing Grenville-age zircon across Laurentia. This revised model fundamentally alters the prevailing understanding of sediment routing in deep time and highlights the importance of reevaluating long-assumed geodynamic and provenance frameworks.
{"title":"Rethinking the “Grenville Flood” of Laurentian detrital zircon: Proximal sources, not continental rivers","authors":"Christopher J. Spencer, Mark E. Holland","doi":"10.1130/g53720.1","DOIUrl":"https://doi.org/10.1130/g53720.1","url":null,"abstract":"The widespread presence of Mesoproterozoic detrital zircon in western Laurentia has long been interpreted as evidence for transcontinental river systems transporting sediment from the Grenville orogen. This model asserts that Grenville-age zircon were carried across Laurentia during the Neoproterozoic and Paleozoic, but recent isotopic and provenance data challenge this interpretation. Here, we show that detrital zircon age distributions and Hf isotopic compositions east and west of the Transcontinental Arch remained distinct until the late Paleozoic, inconsistent with repeated sediment mixing by a long-lived transcontinental river system. Instead, multiple Stenian magmatic sources in western Laurentia, including the Llano Uplift, Pikes Peak batholith, and the Southwestern Laurentia Large Igneous Province, provide plausible proximal sources of zircon. Additionally, the Pearya terrane, the northern Yukon, and formerly adjacent Antarctic crust may have contributed detrital zircon to western Laurentia’s sedimentary record. Post-Devonian sediment redistribution may have been accomplished by a combination of fluvial, shallow marine, and eolian processes, or indirect sediment routing. These findings suggest that late Paleozoic processes, rather than a Proterozoic transcontinental river(s), were responsible for distributing Grenville-age zircon across Laurentia. This revised model fundamentally alters the prevailing understanding of sediment routing in deep time and highlights the importance of reevaluating long-assumed geodynamic and provenance frameworks.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"714 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145035558","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Zsuzsanna P. Allerton, Liam Courtney-Davies, Martin Danišík, George J. Hudak, Christian Teyssier, Jennifer T. Mitchell, Phillip Larson
The age and origin of hematite deposits in the Vermilion District of Minnesota (USA), Lake Superior region, has been debated for over a century and inferred to be Neoarchean or Mesoproterozoic. Using a new geochronological approach combining U-Pb and (U-Th)/He double-dating of hematite, we present the first direct dates for hematite deposits at the Soudan iron mine, revealing a previously unknown Paleoproterozoic mineralization event and a thermal history recording the emplacement of the proximal Mesoproterozoic Midcontinent Rift System. Hematite phases yield U-Pb crystallization dates ranging between 1.8 Ga and 1.6 Ga and (U-Th)/He dates in the range of 1.63−0.53 Ga, with a distinct cluster at ca. 1.1 Ga. We propose that replacement-style hematite mineralization was generated during Paleoproterozoic orogenic events, including the Yavapai (1.71−1.68 Ga) and/or Mazatzal (1.65−1.60 Ga) accretionary orogenies and associated magmatism related to the assembly of Laurentia that reactivated shear zones and facilitated hydrothermal alteration deep into the Archean craton. (U-Th)/He data suggest that hematite ore experienced a thermal overprint that did not reset the U-Pb system, with the most consistent dates coinciding with the establishment of the Midcontinent Rift System at ca. 1.1 Ga. Double-dating of hematite is demonstrated to directly link iron mineralization to thermal and tectonic events in Precambrian cratons and to place constraints on genesis not available from coexisting accessory minerals.
{"title":"Hematite double-dating defines Proterozoic mineralization and thermal history of Archean banded iron formations in northeastern Minnesota, USA","authors":"Zsuzsanna P. Allerton, Liam Courtney-Davies, Martin Danišík, George J. Hudak, Christian Teyssier, Jennifer T. Mitchell, Phillip Larson","doi":"10.1130/g53517.1","DOIUrl":"https://doi.org/10.1130/g53517.1","url":null,"abstract":"The age and origin of hematite deposits in the Vermilion District of Minnesota (USA), Lake Superior region, has been debated for over a century and inferred to be Neoarchean or Mesoproterozoic. Using a new geochronological approach combining U-Pb and (U-Th)/He double-dating of hematite, we present the first direct dates for hematite deposits at the Soudan iron mine, revealing a previously unknown Paleoproterozoic mineralization event and a thermal history recording the emplacement of the proximal Mesoproterozoic Midcontinent Rift System. Hematite phases yield U-Pb crystallization dates ranging between 1.8 Ga and 1.6 Ga and (U-Th)/He dates in the range of 1.63−0.53 Ga, with a distinct cluster at ca. 1.1 Ga. We propose that replacement-style hematite mineralization was generated during Paleoproterozoic orogenic events, including the Yavapai (1.71−1.68 Ga) and/or Mazatzal (1.65−1.60 Ga) accretionary orogenies and associated magmatism related to the assembly of Laurentia that reactivated shear zones and facilitated hydrothermal alteration deep into the Archean craton. (U-Th)/He data suggest that hematite ore experienced a thermal overprint that did not reset the U-Pb system, with the most consistent dates coinciding with the establishment of the Midcontinent Rift System at ca. 1.1 Ga. Double-dating of hematite is demonstrated to directly link iron mineralization to thermal and tectonic events in Precambrian cratons and to place constraints on genesis not available from coexisting accessory minerals.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"54 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145035561","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We use phase-equilibrium and trace-element modeling to address the effects of magmatic H2O, oxygen fugacity (fO2), and differentiation depth of mantle-derived magmas on the generation of porphyry copper deposit (PCD)−forming magmas. We explore how these factors control first-order compositional trends indicative of high PCD-forming potential [Sr/Y and (Eu/Eu*)/Yb versus SiO2 and fO2] and test different models for the ideal amount of H2O and fO2 of PCD-forming magmas. Our findings suggest that initial oxidized conditions (i.e., magmas derived from an oxidized source) are necessary for the generation of PCD-forming magmas. We show that wet to super-wet magmatism (H2O >4−8 wt%) combined with an initially oxidized state better matches the required conditions for PCD generation. Importantly, we find that garnet is not necessary for PCD formation; amphibole-bearing but garnet-free assemblages can also produce PCD-forming magmas.
我们利用相平衡和微量元素模型研究了岩浆H2O、氧逸度（fO2）和幔源岩浆分异深度对斑岩型铜矿床（PCD）形成岩浆的影响。我们探讨了这些因素如何控制指示高形成潜力的一级成分趋势[Sr/Y和(Eu/Eu*)/Yb vs . SiO2和fO2]，并测试了形成pcd岩浆的理想H2O和fO2量的不同模型。我们的研究结果表明，初始氧化条件（即来自氧化源的岩浆）是形成pcd的岩浆的必要条件。研究表明，湿至超湿岩浆作用（H2O >4 - 8 wt%）与初始氧化态相结合，更符合PCD生成所需的条件。重要的是，我们发现石榴石不是形成PCD所必需的；含角闪石但不含石榴石的组合也可产生形成pdc的岩浆。
{"title":"Source-oxidized and (super-)wet magmas explain porphyry copper fertility vectors: A thermodynamic modeling approach","authors":"David Hernández-Uribe, Santiago Tassara","doi":"10.1130/g53405.1","DOIUrl":"https://doi.org/10.1130/g53405.1","url":null,"abstract":"We use phase-equilibrium and trace-element modeling to address the effects of magmatic H2O, oxygen fugacity (fO2), and differentiation depth of mantle-derived magmas on the generation of porphyry copper deposit (PCD)−forming magmas. We explore how these factors control first-order compositional trends indicative of high PCD-forming potential [Sr/Y and (Eu/Eu*)/Yb versus SiO2 and fO2] and test different models for the ideal amount of H2O and fO2 of PCD-forming magmas. Our findings suggest that initial oxidized conditions (i.e., magmas derived from an oxidized source) are necessary for the generation of PCD-forming magmas. We show that wet to super-wet magmatism (H2O >4−8 wt%) combined with an initially oxidized state better matches the required conditions for PCD generation. Importantly, we find that garnet is not necessary for PCD formation; amphibole-bearing but garnet-free assemblages can also produce PCD-forming magmas.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"21 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145035674","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jonathan Obrist-Farner, Jeremy Maurer, Derek Gibson, Trenton McEnaney, Andreas Eckert, William F. Kenney, Jeffery Beeson, Nigel Wattrus, Quin Stangeland, Fatima Reyes
On 4 February 1976, a Mw 7.5 earthquake along the Motagua fault, Guatemala, ruptured ∼230 km of the North American and Caribbean plate boundary. Today, the plate boundary remains poorly monitored, and the 1976 earthquake is still not fully understood. Here, we present seismic reflection profiles and radiometrically dated sediment core data from six lakes around the Motagua fault, together with reports of destruction and a quasi-dynamic rupture model, which show that the 1976 earthquake experienced strong directivity that impacted the distribution of shaking. The earthquake left behind a detailed record of event deposits (EDs) in five of the six study lakes. Thicker EDs are present in Lake Atitlán, near the terminus of the earthquake rupture, whereas thinner EDs were found in lakes off-axis of the rupture direction. We argue that EDs can be utilized to constrain asymmetrical distribution of shaking during earthquakes and that paleoseismic studies should consider directivity as a factor controlling the thickness of EDs.
{"title":"Paleoseismic evidence of directivity for the 1976 Mw 7.5 Motagua earthquake, Guatemala","authors":"Jonathan Obrist-Farner, Jeremy Maurer, Derek Gibson, Trenton McEnaney, Andreas Eckert, William F. Kenney, Jeffery Beeson, Nigel Wattrus, Quin Stangeland, Fatima Reyes","doi":"10.1130/g53449.1","DOIUrl":"https://doi.org/10.1130/g53449.1","url":null,"abstract":"On 4 February 1976, a Mw 7.5 earthquake along the Motagua fault, Guatemala, ruptured ∼230 km of the North American and Caribbean plate boundary. Today, the plate boundary remains poorly monitored, and the 1976 earthquake is still not fully understood. Here, we present seismic reflection profiles and radiometrically dated sediment core data from six lakes around the Motagua fault, together with reports of destruction and a quasi-dynamic rupture model, which show that the 1976 earthquake experienced strong directivity that impacted the distribution of shaking. The earthquake left behind a detailed record of event deposits (EDs) in five of the six study lakes. Thicker EDs are present in Lake Atitlán, near the terminus of the earthquake rupture, whereas thinner EDs were found in lakes off-axis of the rupture direction. We argue that EDs can be utilized to constrain asymmetrical distribution of shaking during earthquakes and that paleoseismic studies should consider directivity as a factor controlling the thickness of EDs.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"12 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145035560","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Oskar Schramm, Patrick J. Frings, Tommaso di Rocco, Andreas Pack, Michael Tatzel
Sedimentary cherts form from amorphous silica precursors that progressively crystallize into opal-CT and quartz during burial diagenesis. Recent studies have shown that the transformation kinetics of the silica polymorphs strongly depend on the prograde thermal history, suggesting a key role of basal heat flow in setting the 18O/16O and 17O/16O 18O/16O oxygen isotope ratios in chert (δ18Ochert, Δ′17Ochert). To investigate the relationship between paleo−heat flow and chert oxygen isotopes, we used Cretaceous to Neogene cherts that formed in the large igneous province Shatsky Rise in the western Pacific Ocean and a compilation of marine chert oxygen isotope ratios from previous studies. Using a reaction-advection-diffusion model, we demonstrate that the relationship between δ18Ochert, Δ′17Ochert, and the age of the underlying crust results from declining heat flow through sediment as the oceanic crust cools. Our reconstruction of heat flow at Shatsky Rise aligns with established geothermal background values. We propose that the Archean chert record indicates a diagenetic environment dominated by high heat flow that transitioned on billion-year time scales toward conditions resembling modern diagenesis.
{"title":"Oxygen isotopes in cherts record paleo−heat flow on Shatsky Rise (western Pacific Ocean)","authors":"Oskar Schramm, Patrick J. Frings, Tommaso di Rocco, Andreas Pack, Michael Tatzel","doi":"10.1130/g53296.1","DOIUrl":"https://doi.org/10.1130/g53296.1","url":null,"abstract":"Sedimentary cherts form from amorphous silica precursors that progressively crystallize into opal-CT and quartz during burial diagenesis. Recent studies have shown that the transformation kinetics of the silica polymorphs strongly depend on the prograde thermal history, suggesting a key role of basal heat flow in setting the 18O/16O and 17O/16O 18O/16O oxygen isotope ratios in chert (δ18Ochert, Δ′17Ochert). To investigate the relationship between paleo−heat flow and chert oxygen isotopes, we used Cretaceous to Neogene cherts that formed in the large igneous province Shatsky Rise in the western Pacific Ocean and a compilation of marine chert oxygen isotope ratios from previous studies. Using a reaction-advection-diffusion model, we demonstrate that the relationship between δ18Ochert, Δ′17Ochert, and the age of the underlying crust results from declining heat flow through sediment as the oceanic crust cools. Our reconstruction of heat flow at Shatsky Rise aligns with established geothermal background values. We propose that the Archean chert record indicates a diagenetic environment dominated by high heat flow that transitioned on billion-year time scales toward conditions resembling modern diagenesis.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"35 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145017899","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sheree E. Armistead, Bruce M. Eglington, Sally J. Pehrsson, David L. Huston
Understanding Earth’s early crustal architecture is critical to reconstructing the planet’s geodynamic evolution. We present a global analysis of ∼25,000 Pb isotope analyses from ore deposits, focusing on Archean cratons to investigate lithospheric evolution and continental configurations. Spatial patterns in model age and source µ (238U/204Pb) reveal isotopic domains that align with proposed Archean supercratons. Low µ values are characteristic of cratons interpreted to belong to the ca. 2.6 Ga Superia supercraton (e.g., Superior [Canada], Kaapvaal [southern Africa], and Hearne [Canada] cratons), whereas moderate to high µ values are typical of cratons associated with the contemporaneous Sclavia supercraton (e.g., Slave [Canada], Yilgarn [Australia], Dharwar [India], and Zimbabwe [southern Africa] cratons). These long-lived terrane-scale signatures suggest Pb isotopes can resolve lithospheric domains preserved across multiple cratons. Our results offer new geochemical constraints on Archean continental assembly and highlight the utility of ore-deposit Pb isotope data for testing early Earth plate configurations.
{"title":"Archean geodynamics and plate tectonics linked to Pb isotope variability","authors":"Sheree E. Armistead, Bruce M. Eglington, Sally J. Pehrsson, David L. Huston","doi":"10.1130/g53553.1","DOIUrl":"https://doi.org/10.1130/g53553.1","url":null,"abstract":"Understanding Earth’s early crustal architecture is critical to reconstructing the planet’s geodynamic evolution. We present a global analysis of ∼25,000 Pb isotope analyses from ore deposits, focusing on Archean cratons to investigate lithospheric evolution and continental configurations. Spatial patterns in model age and source µ (238U/204Pb) reveal isotopic domains that align with proposed Archean supercratons. Low µ values are characteristic of cratons interpreted to belong to the ca. 2.6 Ga Superia supercraton (e.g., Superior [Canada], Kaapvaal [southern Africa], and Hearne [Canada] cratons), whereas moderate to high µ values are typical of cratons associated with the contemporaneous Sclavia supercraton (e.g., Slave [Canada], Yilgarn [Australia], Dharwar [India], and Zimbabwe [southern Africa] cratons). These long-lived terrane-scale signatures suggest Pb isotopes can resolve lithospheric domains preserved across multiple cratons. Our results offer new geochemical constraints on Archean continental assembly and highlight the utility of ore-deposit Pb isotope data for testing early Earth plate configurations.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"130 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145017900","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Zongqi Zou, Yi-Gang Xu, Zaicong Wang, Yu Wang, Ming Li, Meiling Wang, Yutian Lei
The low-velocity zone (LVZ) in shallow asthenosphere is crucial for Earth’s geodynamics and is widely linked to the presence of partial melts. Incipient melts from mantle peridotite melting are highly mobile and tend to quickly escape from their sources; however, it remains enigmatic how the LVZ can extend to the depths where partial melting initiates. Here, we identify a suite of primitive low-MgO basalts derived from an eclogitized oceanic crust at ∼200 km within the LVZ, providing a compelling scenario for the LVZ formation. These primitive basalts exhibit significantly heavier Fe isotopes and lighter Ca-Mo-O isotopes compared to mid-ocean-ridge basalts, indicating that they originated from eclogitized oceanic crust rather than peridotite. This highlights the fact that oceanic crust recycled into the mantle can melt to form primitive Mg-poor and Si-rich melts. Given their relatively high density and viscosity, these melts show low-mobility and accumulate at depths of ∼150−200 km, forming a low-velocity layer within the asthenosphere.
{"title":"Melting of eclogitic oceanic crust for the low-velocity zone within Earth’s upper asthenosphere","authors":"Zongqi Zou, Yi-Gang Xu, Zaicong Wang, Yu Wang, Ming Li, Meiling Wang, Yutian Lei","doi":"10.1130/g53540.1","DOIUrl":"https://doi.org/10.1130/g53540.1","url":null,"abstract":"The low-velocity zone (LVZ) in shallow asthenosphere is crucial for Earth’s geodynamics and is widely linked to the presence of partial melts. Incipient melts from mantle peridotite melting are highly mobile and tend to quickly escape from their sources; however, it remains enigmatic how the LVZ can extend to the depths where partial melting initiates. Here, we identify a suite of primitive low-MgO basalts derived from an eclogitized oceanic crust at ∼200 km within the LVZ, providing a compelling scenario for the LVZ formation. These primitive basalts exhibit significantly heavier Fe isotopes and lighter Ca-Mo-O isotopes compared to mid-ocean-ridge basalts, indicating that they originated from eclogitized oceanic crust rather than peridotite. This highlights the fact that oceanic crust recycled into the mantle can melt to form primitive Mg-poor and Si-rich melts. Given their relatively high density and viscosity, these melts show low-mobility and accumulate at depths of ∼150−200 km, forming a low-velocity layer within the asthenosphere.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"24 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145003072","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Elizabeth J. Trower, Miquela Ingalls, James R. Gutoski, Virginia T. Wala
Although it is difficult to reconstruct Earth surface temperatures during Neoproterozoic time, sedimentological and paleomagnetic evidence demonstrate a dynamic climate, featuring two global “Snowball Earth” glaciations. The recent observation of petrographic fingerprints of ikaite, a mineral that typically forms in near-freezing sedimentary environments, in late Tonian strata was interpreted as evidence that low-latitude shallow marine environments were cold millions of years prior to the Cryogenian Period. Meanwhile, other recent work has demonstrated that elevated phosphate concentration ([DIP]) can inhibit calcite nucleation (perhaps enabling ikaite to form and persist at warmer temperatures) and that late Tonian carbonates formed in phosphate-rich seawater. So, was late Tonian seawater cold, or was it phosphate-rich? To address this question, we combined measurements of carbonate-associated phosphate and ooid-size-based pH constraints to reconstruct seawater [DIP] values for one snapshot of time in the late Tonian Period. Our seawater [DIP] estimates range from 3.8 µM to 7.8 µM, substantially elevated relative to modern shallow seawater and consistent with inferences from previous approaches. Our estimates are below values at which calcite nucleation inhibition has been observed, suggesting that elevated phosphate is an insufficient explanation for the ikaite forming in warm conditions and supporting the hypothesis that late Tonian climate was cool.
{"title":"New constraints on phosphate concentration and temperature in shallow late Tonian seawater","authors":"Elizabeth J. Trower, Miquela Ingalls, James R. Gutoski, Virginia T. Wala","doi":"10.1130/g53532.1","DOIUrl":"https://doi.org/10.1130/g53532.1","url":null,"abstract":"Although it is difficult to reconstruct Earth surface temperatures during Neoproterozoic time, sedimentological and paleomagnetic evidence demonstrate a dynamic climate, featuring two global “Snowball Earth” glaciations. The recent observation of petrographic fingerprints of ikaite, a mineral that typically forms in near-freezing sedimentary environments, in late Tonian strata was interpreted as evidence that low-latitude shallow marine environments were cold millions of years prior to the Cryogenian Period. Meanwhile, other recent work has demonstrated that elevated phosphate concentration ([DIP]) can inhibit calcite nucleation (perhaps enabling ikaite to form and persist at warmer temperatures) and that late Tonian carbonates formed in phosphate-rich seawater. So, was late Tonian seawater cold, or was it phosphate-rich? To address this question, we combined measurements of carbonate-associated phosphate and ooid-size-based pH constraints to reconstruct seawater [DIP] values for one snapshot of time in the late Tonian Period. Our seawater [DIP] estimates range from 3.8 µM to 7.8 µM, substantially elevated relative to modern shallow seawater and consistent with inferences from previous approaches. Our estimates are below values at which calcite nucleation inhibition has been observed, suggesting that elevated phosphate is an insufficient explanation for the ikaite forming in warm conditions and supporting the hypothesis that late Tonian climate was cool.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"33 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145003073","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Trishit Ruj, Hanaya Okuda, Goro Komatsu, Hitoshi Hasegawa, James W. Head, Tomohiro Usui, Shun Mihira, Makito Kobayashi
Subsurface ice in the mid-latitudes of Mars represents one of the largest present-day water ice reservoirs. While atmospheric models predict Late Amazonian (during the past hundreds of millions of years) obliquity-driven ice accumulation, its long-term variations, and the factors influencing accumulation remain unclear. Using geomorphological evidence and numerical modeling, we reveal a southwestern depositional trend within northern mid-latitudinal crater walls and floors. Detailed crater-fill deposit analyses indicate multiple glaciation stages, including an earlier, high-intensity stage followed by a later, lower-intensity stage, both exhibiting this southwestern trend (ca. 640−98 Ma). We conclude that persistent multiple-stage Amazonian glaciations were governed by atmospheric water availability and obliquity-driven climate cycles.
{"title":"Long-term and multi-stage ice accumulation in the martian mid-latitudes during the Amazonian","authors":"Trishit Ruj, Hanaya Okuda, Goro Komatsu, Hitoshi Hasegawa, James W. Head, Tomohiro Usui, Shun Mihira, Makito Kobayashi","doi":"10.1130/g53418.1","DOIUrl":"https://doi.org/10.1130/g53418.1","url":null,"abstract":"Subsurface ice in the mid-latitudes of Mars represents one of the largest present-day water ice reservoirs. While atmospheric models predict Late Amazonian (during the past hundreds of millions of years) obliquity-driven ice accumulation, its long-term variations, and the factors influencing accumulation remain unclear. Using geomorphological evidence and numerical modeling, we reveal a southwestern depositional trend within northern mid-latitudinal crater walls and floors. Detailed crater-fill deposit analyses indicate multiple glaciation stages, including an earlier, high-intensity stage followed by a later, lower-intensity stage, both exhibiting this southwestern trend (ca. 640−98 Ma). We conclude that persistent multiple-stage Amazonian glaciations were governed by atmospheric water availability and obliquity-driven climate cycles.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"63 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-09-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144931026","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Cody L. Colleps, Peter van der Beek, Julien Amalberti, Edward R. Sobel, Marissa M. Tremblay, Maxime Bernard
Apatite 4He/3He thermochronology has the potential to provide high-resolution low-temperature thermal histories that bring valuable insight into near-surface crustal processes. However, this system has yet to be directly evaluated using single-grain 4He/3He analyses from a natural sample with an established thermal history. We present apatite 4He/3He spectra from the widely used Fish Canyon Tuff (FCT) age standard (San Juan volcanic field, southern Colorado, USA), collected at two localities with contrasting thermal histories: (1) a distal locality (FCT-D) where the early Oligocene eruptive age of the FCT is preserved in the apatite (U-Th)/He (AHe) system and the thermal history is well established; and (2) the classic FCT sampling locality (FCT-C) with younger, Early Miocene AHe dates and an unconstrained posteruptive thermal history. FCT-D apatite shows 4He/3He spectra indicative of no diffusive loss, with relative edge depletion of 4He induced by alpha ejection only, corroborating rapid eruptive cooling. In contrast, FCT-C apatite revealed notably diffusive 4He/3He spectra. Thermal-history inversions highlight the resolving power of apatite 4He/3He thermochronology, demonstrating its ability to (1) independently resolve rapid eruptive cooling at the FCT-D site, and (2) improve the resolution of postemplacement reheating and Early Miocene cooling at the FCT-C site. Refined FCT-C thermal histories reveal a distinctive onset of moderate cooling at ca. 20−19 Ma, likely reflecting footwall topographic development coinciding with regional Rio Grande rifting. This collective assessment of 4He/3He systematics further verifies its ability to substantially improve thermal history resolution, which is crucial to elucidating mechanisms driving crustal cooling.
{"title":"Evaluating the resolving power of apatite 4He/3He thermochronology: Insights from the Fish Canyon Tuff","authors":"Cody L. Colleps, Peter van der Beek, Julien Amalberti, Edward R. Sobel, Marissa M. Tremblay, Maxime Bernard","doi":"10.1130/g53000.1","DOIUrl":"https://doi.org/10.1130/g53000.1","url":null,"abstract":"Apatite 4He/3He thermochronology has the potential to provide high-resolution low-temperature thermal histories that bring valuable insight into near-surface crustal processes. However, this system has yet to be directly evaluated using single-grain 4He/3He analyses from a natural sample with an established thermal history. We present apatite 4He/3He spectra from the widely used Fish Canyon Tuff (FCT) age standard (San Juan volcanic field, southern Colorado, USA), collected at two localities with contrasting thermal histories: (1) a distal locality (FCT-D) where the early Oligocene eruptive age of the FCT is preserved in the apatite (U-Th)/He (AHe) system and the thermal history is well established; and (2) the classic FCT sampling locality (FCT-C) with younger, Early Miocene AHe dates and an unconstrained posteruptive thermal history. FCT-D apatite shows 4He/3He spectra indicative of no diffusive loss, with relative edge depletion of 4He induced by alpha ejection only, corroborating rapid eruptive cooling. In contrast, FCT-C apatite revealed notably diffusive 4He/3He spectra. Thermal-history inversions highlight the resolving power of apatite 4He/3He thermochronology, demonstrating its ability to (1) independently resolve rapid eruptive cooling at the FCT-D site, and (2) improve the resolution of postemplacement reheating and Early Miocene cooling at the FCT-C site. Refined FCT-C thermal histories reveal a distinctive onset of moderate cooling at ca. 20−19 Ma, likely reflecting footwall topographic development coinciding with regional Rio Grande rifting. This collective assessment of 4He/3He systematics further verifies its ability to substantially improve thermal history resolution, which is crucial to elucidating mechanisms driving crustal cooling.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"10 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144919587","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"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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