This project supports the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This laboratory develops methodology for small and difficult sample analysis often at the limits of existing mass spectrometer technology.
Media
Sources/Usage: Public Domain. View Media Details
Science Issue and Relevance
The 40Ar/39Ar Method: 40Ar/39Ar geochronology is an experimentally robust and versatile method for constraining the age and thermal history of rocks. Such information is extremely valuable for understanding a variety of geological processes including the formation of ore deposits, mountain building and history of volcanic events, paleo-seismic events, and paleo-climate. The 40Ar/39Ar isotopic dating method has evolved into the most commonly applied geochronological method, and can be applied to many geological problems that require precise and accurate time and temperature control.
Methodology to Address Issue
This project provides partial support for the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This facility houses necessary equipment for sample preparation and analysis, including high-sensitivity noble gas mass spectrometers and ultraviolet (UV) and infrared (IR) lasers. The versatility of the 40Ar/39Ar method permits determining the timing of processes and events such as igneous intrusions and extrusions, ore mineralization and hydrothermal fluid circulation, metamorphic cooling and exhumation, mineral formation and recrystallization, and shallow crustal faulting. Scientists are dependent on the geochronologist for data and interpretations to determine these parameters. This laboratory develops methodology for small and difficult sample analysis often at the limits of existing mass spectrometer technology.
Below are other science projects associated with this project.
Below are data releases associated with this project. Visit USGS Geochron - a database of geochronologic and thermochronologic dates and data.
Filter Total Items: 30
40Ar/39Ar geochronology of the Tonopah, Divide, and Goldfield districts, Nevada 40Ar/39Ar geochronology of the Tonopah, Divide, and Goldfield districts, Nevada
Ar/Ar data are reported from minerals and rocks that were collected as part of a mineral resource investigation of the Tonopah, Divide, and Goldfield districts in Nevada. Data are reported from 92 samples and associated standards from eight separate neutron irradiations in the Denver USGS TRIGA reactor, including separated mineral grains and whole rocks. Data were collected by infrared...
Argon geochronology data from the Austroalpine-Pennine boundary, Central Alps, Switzerland Argon geochronology data from the Austroalpine-Pennine boundary, Central Alps, Switzerland
Ar/Ar data are reported from eight samples of white mica separated from rock samples collected along the Austroalpine-Pennine boundary in the Central Alps of Switzerland. These samples were collected as part of a Caltech PhD study to examine the tectonic history of this boundary and these argon geochronology data represent one small part of the study. Data are reported from these eight...
Argon data for: Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province, New Mexico Argon data for: Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province, New Mexico
This dataset accompanies publication 'Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province archaeological obsidian, Southwestern New Mexico: Solving issues of inter-source discrimination', published in Geoarchaeology (https://dx.doi.org/10.1002/gea.21672), which studied the ability of isotopic and argon geochronologic analyses to discriminate between
Data release of Geologic Map of the Upper Arkansas River Valley Region, North-Central, Colorado Data release of Geologic Map of the Upper Arkansas River Valley Region, North-Central, Colorado
This 1:50,000-scale geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley, between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This work also incorporates new detailed...
Argon data for Poncha Pass Geologic Map Argon data for Poncha Pass Geologic Map
This dataset was collected by Leah E. Morgan and Michael A. Cosca in the Argon Geochronology Laboratory of the USGS in Denver, Colorado in 2015. The dataset contains full raw argon isotopic data for samples presented in: Geologic Map of the Poncha Map Area, Chaffee, Fremont, and Saguache Counties, Colorado.
40Ar/39Ar data for: Characteristics and 40Ar/39Ar geochronology of the Erdenet Cu-Mo deposit, Mongolia 40Ar/39Ar data for: Characteristics and 40Ar/39Ar geochronology of the Erdenet Cu-Mo deposit, Mongolia
This dataset was collected by Leah E. Morgan in the Argon Geochronology Laboratory of the USGS in Denver, Colorado in 2015. The dataset contains full raw argon isotopic data for samples presented in the manuscript: Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia. Imants Kavalieris, Bat-Erdene Khashgerel, Leah E. Morgan, Alexander Undrakhtamir, Adiya...
Below are publications associated with this project.
Filter Total Items: 87
Detrital record of the late Oligocene – Early Miocene mafic volcanic arc in the southern Patagonian Andes (~51 °S) from single-clast geochronology and trace element geochemistry Detrital record of the late Oligocene – Early Miocene mafic volcanic arc in the southern Patagonian Andes (~51 °S) from single-clast geochronology and trace element geochemistry
Retroarc foreland basins are important archives of continental arc magmatism and upper plate deformational processes that control the evolution of continental lithosphere. However, resolving source areas in foreland basin infill dominated from mixed mafic and recycled sediment using conventional methods such as detrital zircon geochronology poses a challenge to thorough analysis due to...
Authors
Rebecca VanderLeest, Julie C Fosdick, Joel Leonard, Leah Morgan
Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.) Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.)
Integrated, multi-method provenance studies of siliciclastic sedimentary deposits are increasingly used to reconstruct the history of source-to-sink transport, paleogeography, and tectonics. Invariably, analysis of large-scale depositional systems must confront issues regarding how to best sample the system and adequately cope with the details of sediment mixing. Potential biases...
Authors
Matthew A. Malkowski, Glenn Sharman, Samuel Johnstone, Marty Grove, Dave Kimbrough, Stephen Graham
Multiproxy Cretaceous-Paleogene boundary event stratigraphy: An Umbria-Marche basin-wide perspective Multiproxy Cretaceous-Paleogene boundary event stratigraphy: An Umbria-Marche basin-wide perspective
The complete and well-studied pelagic carbonate successions from the Umbria-Marche Basin (Italy) permit the study of the event-rich stratigraphical interval around the Cretaceous-Paleogene (K-Pg) boundary (e.g., Deccan volcanism, boundary impact, Paleocene recovery and climate). To test the robustness of various proxy records (bulk carbonate δ13C, δ18O, 87Sr/86Sr and Ca, Fe, Sr and Mn
Authors
Matthias Sinnesael, Alessandro Montanari, Fabrizio Frontalini, Rodolfo Coccioni, Jerome Gattacceca, Christophe Snoeck, Wencke Wegner, Christian Koeberl, Leah Morgan, Niels de Winter, Donald DePaolo, Philippe Claeys
The influence of foreland structures on hinterland cooling: evaluating the drivers of exhumation in the eastern Bhutan Himalaya The influence of foreland structures on hinterland cooling: evaluating the drivers of exhumation in the eastern Bhutan Himalaya
Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three...
Authors
Nadine McQuarrie, Paul Eizenhofer, Sean Long, Tobgay Tobgay, Todd Ehlers, Ann Blythe, Leah Morgan, Michelle Gilmore, Gregory Dering
Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada
Miocene calc-alkaline volcanic rocks, part of the southern segment of the ancestral Cascades magmatic arc, are spatially, temporally, and likely genetically associated with precious metal epithermal deposits in the Tonopah, Divide, and Goldfield Districts of west-central Nevada. In the Tonopah mining district, volcanic rocks include the Mizpah Trachyte, Fraction Tuff, and Oddie Rhyolite...
Authors
Edward du Bray, David John, Joseph Colgan, Peter Vikre, Michael Cosca, Leah Morgan
Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada
The purpose of this report is to summarize geochemical, petrographic, and geochronologic data for samples, principally those of unmineralized Tertiary volcanic rocks, from the Tonopah, Divide, and Goldfield mining districts of west-central Nevada (fig. 1). Much of the data presented here for the Tonopah and Divide districts are for samples collected by Bonham and Garside (1979) during...
Authors
Edward du Bray, David John, Peter Vikre, Joseph Colgan, Michael Cosca, Leah Morgan, Robert Fleck, Wayne Premo, Christopher Holm-Denoma
Revisiting Herto: New evidence of Homo sapiens from Ethiopia Revisiting Herto: New evidence of Homo sapiens from Ethiopia
Localities in the radiometrically dated Upper Herto Member of Ethiopia’s Bouri Formation continue to produce new data that complement and extend initial reports of fossils and artifacts published in 2003. Results of these revisits are reported here and include the in situ recovery of artifacts from the same sediments containing hominid fossils. We evaluate the absolute and relative...
Authors
Yonatan Sahle, Yonas Beyene, Alban Defleur, Berhane Asfaw, Giday WoldeGabriel, William Hart, Leah Morgan, Paul Renne, Joshua Carlson, Tim D White
A stratigraphic approach to inferring depositional ages from detrital geochronology data A stratigraphic approach to inferring depositional ages from detrital geochronology data
With the increasing use of detrital geochronology data for provenance analyses, we have also developed new constraints on the age of otherwise undateable sedimentary deposits. Because a deposit can be no older than its youngest mineral constituent, the youngest defensible detrital mineral age defines the maximum depositional age of the sampled bed. Defining the youngest `defensible' age...
Authors
Samuel Johnstone, Theresa Schwartz, Christopher Holm-Denoma
A supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system A supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system
Establishing temporal constrains on major volcanic eruptions is limited by the precision of existing geochronometers. Prior work on the La Garita caldera, created by the eruption of the Fish Canyon Tuff, failed to resolve temporal differences between pre-, syn-, and post-collapse eruptive units. Here, we report 40Ar/39Ar geochronologic data supporting a ca. 100 ka eruptive history of the...
Authors
Leah Morgan, Samuel Johnstone, Amy Gilmer, Michael Cosca, Ren A. Thompson
C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA
The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late...
Authors
Allen Andersen, Peter Larson, Michael Cosca
Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes
The ore-genesis model for world-class deposits of the Butte mining district, Montana, USA, is deep pre-Main Stage porphyry Cu-Mo and overlying Main Stage Ag-Zn-Cu zoned-lode deposits, both of which formed from hydrothermal fluids driven by minor volumes of rhyolitic magma. The lode-specific model is that hydrothermal processes diminished in intensity outward from district center along...
Authors
Karen Lund, Ryan J. McAleer, John Aleinikoff, Michael Cosca, Michael Kunk
Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA
On active alluvial fans, debris-flow deposits and frequent avulsions produce a rough topographic surface. As is the case in many initially rough landforms produced by catastrophic processes, the topography of alluvial fans is progressively smoothed, producing textural differences useful in establishing relative age criteria for fans. Here, we outline an approach for defining a...
Authors
Samuel Johnstone, Adam Hudson, Sylvia Nicovich, Chester Ruleman, Robert Sare, Ren A. Thompson
In addition to the USGS National Cooperative Geologic Mapping Program and the Geosciences and Environmental Change Science Center, below are partners associated with this project.
This project supports the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This laboratory develops methodology for small and difficult sample analysis often at the limits of existing mass spectrometer technology.
Media
Sources/Usage: Public Domain. View Media Details
Science Issue and Relevance
The 40Ar/39Ar Method: 40Ar/39Ar geochronology is an experimentally robust and versatile method for constraining the age and thermal history of rocks. Such information is extremely valuable for understanding a variety of geological processes including the formation of ore deposits, mountain building and history of volcanic events, paleo-seismic events, and paleo-climate. The 40Ar/39Ar isotopic dating method has evolved into the most commonly applied geochronological method, and can be applied to many geological problems that require precise and accurate time and temperature control.
Methodology to Address Issue
This project provides partial support for the USGS argon geochronology laboratory in Denver. The USGS 40Ar/39Ar geochronology laboratory is a state-of-the-art research facility for determining absolute ages of minerals and rocks. The 40Ar/39Ar laboratory contributes critical geochronology to individual USGS research projects and to partners in academia and other Federal agencies. This facility houses necessary equipment for sample preparation and analysis, including high-sensitivity noble gas mass spectrometers and ultraviolet (UV) and infrared (IR) lasers. The versatility of the 40Ar/39Ar method permits determining the timing of processes and events such as igneous intrusions and extrusions, ore mineralization and hydrothermal fluid circulation, metamorphic cooling and exhumation, mineral formation and recrystallization, and shallow crustal faulting. Scientists are dependent on the geochronologist for data and interpretations to determine these parameters. This laboratory develops methodology for small and difficult sample analysis often at the limits of existing mass spectrometer technology.
Below are other science projects associated with this project.
Below are data releases associated with this project. Visit USGS Geochron - a database of geochronologic and thermochronologic dates and data.
Filter Total Items: 30
40Ar/39Ar geochronology of the Tonopah, Divide, and Goldfield districts, Nevada 40Ar/39Ar geochronology of the Tonopah, Divide, and Goldfield districts, Nevada
Ar/Ar data are reported from minerals and rocks that were collected as part of a mineral resource investigation of the Tonopah, Divide, and Goldfield districts in Nevada. Data are reported from 92 samples and associated standards from eight separate neutron irradiations in the Denver USGS TRIGA reactor, including separated mineral grains and whole rocks. Data were collected by infrared...
Argon geochronology data from the Austroalpine-Pennine boundary, Central Alps, Switzerland Argon geochronology data from the Austroalpine-Pennine boundary, Central Alps, Switzerland
Ar/Ar data are reported from eight samples of white mica separated from rock samples collected along the Austroalpine-Pennine boundary in the Central Alps of Switzerland. These samples were collected as part of a Caltech PhD study to examine the tectonic history of this boundary and these argon geochronology data represent one small part of the study. Data are reported from these eight...
Argon data for: Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province, New Mexico Argon data for: Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province, New Mexico
This dataset accompanies publication 'Elemental, isotopic, and geochronological variability in Mogollon-Datil Volcanic Province archaeological obsidian, Southwestern New Mexico: Solving issues of inter-source discrimination', published in Geoarchaeology (https://dx.doi.org/10.1002/gea.21672), which studied the ability of isotopic and argon geochronologic analyses to discriminate between
Data release of Geologic Map of the Upper Arkansas River Valley Region, North-Central, Colorado Data release of Geologic Map of the Upper Arkansas River Valley Region, North-Central, Colorado
This 1:50,000-scale geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley, between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This work also incorporates new detailed...
Argon data for Poncha Pass Geologic Map Argon data for Poncha Pass Geologic Map
This dataset was collected by Leah E. Morgan and Michael A. Cosca in the Argon Geochronology Laboratory of the USGS in Denver, Colorado in 2015. The dataset contains full raw argon isotopic data for samples presented in: Geologic Map of the Poncha Map Area, Chaffee, Fremont, and Saguache Counties, Colorado.
40Ar/39Ar data for: Characteristics and 40Ar/39Ar geochronology of the Erdenet Cu-Mo deposit, Mongolia 40Ar/39Ar data for: Characteristics and 40Ar/39Ar geochronology of the Erdenet Cu-Mo deposit, Mongolia
This dataset was collected by Leah E. Morgan in the Argon Geochronology Laboratory of the USGS in Denver, Colorado in 2015. The dataset contains full raw argon isotopic data for samples presented in the manuscript: Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia. Imants Kavalieris, Bat-Erdene Khashgerel, Leah E. Morgan, Alexander Undrakhtamir, Adiya...
Below are publications associated with this project.
Filter Total Items: 87
Detrital record of the late Oligocene – Early Miocene mafic volcanic arc in the southern Patagonian Andes (~51 °S) from single-clast geochronology and trace element geochemistry Detrital record of the late Oligocene – Early Miocene mafic volcanic arc in the southern Patagonian Andes (~51 °S) from single-clast geochronology and trace element geochemistry
Retroarc foreland basins are important archives of continental arc magmatism and upper plate deformational processes that control the evolution of continental lithosphere. However, resolving source areas in foreland basin infill dominated from mixed mafic and recycled sediment using conventional methods such as detrital zircon geochronology poses a challenge to thorough analysis due to...
Authors
Rebecca VanderLeest, Julie C Fosdick, Joel Leonard, Leah Morgan
Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.) Dilution and propagation of provenance trends in sand and mud: Geochemistry and detrital zircon geochronology of modern sediment from central California (U.S.A.)
Integrated, multi-method provenance studies of siliciclastic sedimentary deposits are increasingly used to reconstruct the history of source-to-sink transport, paleogeography, and tectonics. Invariably, analysis of large-scale depositional systems must confront issues regarding how to best sample the system and adequately cope with the details of sediment mixing. Potential biases...
Authors
Matthew A. Malkowski, Glenn Sharman, Samuel Johnstone, Marty Grove, Dave Kimbrough, Stephen Graham
Multiproxy Cretaceous-Paleogene boundary event stratigraphy: An Umbria-Marche basin-wide perspective Multiproxy Cretaceous-Paleogene boundary event stratigraphy: An Umbria-Marche basin-wide perspective
The complete and well-studied pelagic carbonate successions from the Umbria-Marche Basin (Italy) permit the study of the event-rich stratigraphical interval around the Cretaceous-Paleogene (K-Pg) boundary (e.g., Deccan volcanism, boundary impact, Paleocene recovery and climate). To test the robustness of various proxy records (bulk carbonate δ13C, δ18O, 87Sr/86Sr and Ca, Fe, Sr and Mn
Authors
Matthias Sinnesael, Alessandro Montanari, Fabrizio Frontalini, Rodolfo Coccioni, Jerome Gattacceca, Christophe Snoeck, Wencke Wegner, Christian Koeberl, Leah Morgan, Niels de Winter, Donald DePaolo, Philippe Claeys
The influence of foreland structures on hinterland cooling: evaluating the drivers of exhumation in the eastern Bhutan Himalaya The influence of foreland structures on hinterland cooling: evaluating the drivers of exhumation in the eastern Bhutan Himalaya
Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three...
Authors
Nadine McQuarrie, Paul Eizenhofer, Sean Long, Tobgay Tobgay, Todd Ehlers, Ann Blythe, Leah Morgan, Michelle Gilmore, Gregory Dering
Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada
Miocene calc-alkaline volcanic rocks, part of the southern segment of the ancestral Cascades magmatic arc, are spatially, temporally, and likely genetically associated with precious metal epithermal deposits in the Tonopah, Divide, and Goldfield Districts of west-central Nevada. In the Tonopah mining district, volcanic rocks include the Mizpah Trachyte, Fraction Tuff, and Oddie Rhyolite...
Authors
Edward du Bray, David John, Joseph Colgan, Peter Vikre, Michael Cosca, Leah Morgan
Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada Petrographic, geochemical, and geochronologic data for cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada
The purpose of this report is to summarize geochemical, petrographic, and geochronologic data for samples, principally those of unmineralized Tertiary volcanic rocks, from the Tonopah, Divide, and Goldfield mining districts of west-central Nevada (fig. 1). Much of the data presented here for the Tonopah and Divide districts are for samples collected by Bonham and Garside (1979) during...
Authors
Edward du Bray, David John, Peter Vikre, Joseph Colgan, Michael Cosca, Leah Morgan, Robert Fleck, Wayne Premo, Christopher Holm-Denoma
Revisiting Herto: New evidence of Homo sapiens from Ethiopia Revisiting Herto: New evidence of Homo sapiens from Ethiopia
Localities in the radiometrically dated Upper Herto Member of Ethiopia’s Bouri Formation continue to produce new data that complement and extend initial reports of fossils and artifacts published in 2003. Results of these revisits are reported here and include the in situ recovery of artifacts from the same sediments containing hominid fossils. We evaluate the absolute and relative...
Authors
Yonatan Sahle, Yonas Beyene, Alban Defleur, Berhane Asfaw, Giday WoldeGabriel, William Hart, Leah Morgan, Paul Renne, Joshua Carlson, Tim D White
A stratigraphic approach to inferring depositional ages from detrital geochronology data A stratigraphic approach to inferring depositional ages from detrital geochronology data
With the increasing use of detrital geochronology data for provenance analyses, we have also developed new constraints on the age of otherwise undateable sedimentary deposits. Because a deposit can be no older than its youngest mineral constituent, the youngest defensible detrital mineral age defines the maximum depositional age of the sampled bed. Defining the youngest `defensible' age...
Authors
Samuel Johnstone, Theresa Schwartz, Christopher Holm-Denoma
A supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system A supervolcano and its sidekicks: A 100 ka eruptive chronology of the Fish Canyon Tuff and associated units of the La Garita magmatic system
Establishing temporal constrains on major volcanic eruptions is limited by the precision of existing geochronometers. Prior work on the La Garita caldera, created by the eruption of the Fish Canyon Tuff, failed to resolve temporal differences between pre-, syn-, and post-collapse eruptive units. Here, we report 40Ar/39Ar geochronologic data supporting a ca. 100 ka eruptive history of the...
Authors
Leah Morgan, Samuel Johnstone, Amy Gilmer, Michael Cosca, Ren A. Thompson
C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA
The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late...
Authors
Allen Andersen, Peter Larson, Michael Cosca
Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes
The ore-genesis model for world-class deposits of the Butte mining district, Montana, USA, is deep pre-Main Stage porphyry Cu-Mo and overlying Main Stage Ag-Zn-Cu zoned-lode deposits, both of which formed from hydrothermal fluids driven by minor volumes of rhyolitic magma. The lode-specific model is that hydrothermal processes diminished in intensity outward from district center along...
Authors
Karen Lund, Ryan J. McAleer, John Aleinikoff, Michael Cosca, Michael Kunk
Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA
On active alluvial fans, debris-flow deposits and frequent avulsions produce a rough topographic surface. As is the case in many initially rough landforms produced by catastrophic processes, the topography of alluvial fans is progressively smoothed, producing textural differences useful in establishing relative age criteria for fans. Here, we outline an approach for defining a...
Authors
Samuel Johnstone, Adam Hudson, Sylvia Nicovich, Chester Ruleman, Robert Sare, Ren A. Thompson
In addition to the USGS National Cooperative Geologic Mapping Program and the Geosciences and Environmental Change Science Center, below are partners associated with this project.