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• 1. [Article] Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027.

  Citation × Citation Citation Abstract Copy Title: Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites Author: Ford, Mark T., Duncan, Robert A., Grunder, Anita L. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027. Title: Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites Author: Ford, Mark T., Duncan, Robert A., Grunder, Anita L. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1525-2027. Close

• 2. [Article] Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/.

  Citation × Citation Citation Abstract Copy Title: Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti Author: Krawl, Kyle, Kent, Adam J. R., Duncan, Robert A., Loewen, Matthew W. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Title: Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti Author: Krawl, Kyle, Kent, Adam J. R., Duncan, Robert A., Loewen, Matthew W. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gc/. Close

• 3. [Article] Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier ...

  Citation × Citation Citation Abstract Copy Title: Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/ Title: Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-volcanology-and-geothermal-research/ Close

• 4. [Article] DuncanTimingCompositionContinental.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinental.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinental.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 5. [Article] DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinentalAppendixAGeochemicalData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 6. [Article] DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf

  Citation × Citation Citation Abstract Copy Title: DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Title: DuncanTimingCompositionContinentalAppendixAGeochronolgyData.pdf Author: Schlieder, Tyler D., Kent, Adam J.R., Duncan, Robert A., Thornber, Carl R., Al-Amri, Abdullah M. Close

• 7. [Article] Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

  The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province ...

  Citation × Citation Citation Abstract Copy Title: Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197 Author: Keller, Randall A., Duncan, Robert A. The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar-39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2s) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N-R-N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n-22r-22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot. Title: Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197 Author: Keller, Randall A., Duncan, Robert A. The Hawaiian-Emperor seamount chain is the ‘‘type’’ example of an age-progressive, hot spotgenerated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar-39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2s) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N-R-N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n-22r-22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot. Close

• 8. [Article] Tahiti: Geochemical evolution of a French Polynesian volcano

  The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system ...

  Citation × Citation Citation Abstract Copy Title: Tahiti: Geochemical evolution of a French Polynesian volcano Author: Fisk, Martin R., White, William M., Duncan, Robert A., Nielsen, Roger L. The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (~1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and the late-stage, valley-filled phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg#60-64), while the later valley-filled lavas are basanitic (Mg#64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also changes systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this ~0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0 .9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield-building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25-50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognized a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radically zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted viscously coupled sheath. A correlation between Pa/Ce and isotope ratio suggest that the Society plume contains deeply recycled continental material. Title: Tahiti: Geochemical evolution of a French Polynesian volcano Author: Fisk, Martin R., White, William M., Duncan, Robert A., Nielsen, Roger L. The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (~1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and the late-stage, valley-filled phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg#60-64), while the later valley-filled lavas are basanitic (Mg#64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also changes systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this ~0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0 .9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield-building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25-50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognized a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radically zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted viscously coupled sheath. A correlation between Pa/Ce and isotope ratio suggest that the Society plume contains deeply recycled continental material. Close

• 9. [Article] Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas

  The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya ...

  Citation × Citation Citation Abstract Copy Title: Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas Author: Denyer, Percy, Duncan, Robert A., Sinton, Christopher W. The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jaco, and the Quepos Peninsula using Ar-40-Ar-39 radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88-90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83-84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid-ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau [Donnelly, 1994]. The Jacó lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ~64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks, Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jacó lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume. Title: Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas Author: Denyer, Percy, Duncan, Robert A., Sinton, Christopher W. The pre-Tertiary oceanic crust exposed on the west coast of Costa Pica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jaco, and the Quepos Peninsula using Ar-40-Ar-39 radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88-90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83-84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid-ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau [Donnelly, 1994]. The Jacó lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ~64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks, Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jacó lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume. Close

• 10. [Article] Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion

  copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion Author: Chauvin, Annick, Duncan, Robert A., Bonhommet, Norbert, Levi, Saul copyrighted by American Geophysical Union. Title: Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadiere volcanic flow (Central France) : new evidence for the Laschamp excursion Author: Chauvin, Annick, Duncan, Robert A., Bonhommet, Norbert, Levi, Saul copyrighted by American Geophysical Union. Close