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• 31. [Article] Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume Author: McBirney, Alexander R., White, William M., Duncan, Robert A. copyrighted by American Geophysical Union Title: Petrology and Geochemistry of the Galápagos Islands : portrait of a pathological mantle plume Author: McBirney, Alexander R., White, William M., Duncan, Robert A. copyrighted by American Geophysical Union Close

• 32. [Article] Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models Author: Roperch, Pierrick, Chauvin, Annick, Duncan, Robert A. copyrighted by American Geophysical Union Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models Author: Roperch, Pierrick, Chauvin, Annick, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 33. [Article] ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81

  ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project ...

  Citation × Citation Citation Abstract Copy Title: ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81 Author: Duncan, Robert A., Sinton, Christopher W. ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project Leg 81, southwest Rockall Plateau (56°N). Both of these legs drilled into thick sections of submerged lava flows, known as seaward-dipping reflector sequences, that are part of the Tertiary North Atlantic Volcanic Province (NAVP). Results show that subaerial volcanism began at the southeast Greenland Margin as early as 6162 Ma with the eruption of continentally contaminated lavas. After an apparent hiatus, volcanism continued with the subaerial eruption of noncontaminated, mid-ocean-ridge basalt (MORB) -like lavas. A sill or flow was emplaced at 52 Ma into the sediments overlying the basement lavas at the most seaward site (918). The MORB-like lavas of the Rockall Plateau gave poor results, although the data suggest that they erupted at 5758 Ma and may be contemporaneous with the compositionally similar oceanic lavas of Leg 152. Compiled ages from the NAVP indicate that most of the lavas were erupted in two distinct magmatic episodes. The first occurred prior to 60 Ma with volcanism at the southeast Greenland Margin, in West Greenland, and the northwest British Isles. This early stage of widespread volcanism most likely records the arrival of the Iceland mantle plume. The second phase of magmatism began about 57 Ma with voluminous basaltic eruptions centralized along the eventual line of continental separation. Magmatism at this stage was associated with continental breakup and asthenospheric upwelling with probable thermal input from the mantle plume. Title: ⁴⁰Ar-³⁹Ar ages of lavas from the Southeast Greenland margin, ODP leg 152, and the Rockall Plateau, DSDP leg 81 Author: Duncan, Robert A., Sinton, Christopher W. ⁴⁰Ar-³⁹Ar incremental heating experiments were performed on volcanic rocks recovered by drilling during Ocean Drilling Program Leg 152, southeast Greenland Margin (63°N), and Deep Sea Drilling Project Leg 81, southwest Rockall Plateau (56°N). Both of these legs drilled into thick sections of submerged lava flows, known as seaward-dipping reflector sequences, that are part of the Tertiary North Atlantic Volcanic Province (NAVP). Results show that subaerial volcanism began at the southeast Greenland Margin as early as 6162 Ma with the eruption of continentally contaminated lavas. After an apparent hiatus, volcanism continued with the subaerial eruption of noncontaminated, mid-ocean-ridge basalt (MORB) -like lavas. A sill or flow was emplaced at 52 Ma into the sediments overlying the basement lavas at the most seaward site (918). The MORB-like lavas of the Rockall Plateau gave poor results, although the data suggest that they erupted at 5758 Ma and may be contemporaneous with the compositionally similar oceanic lavas of Leg 152. Compiled ages from the NAVP indicate that most of the lavas were erupted in two distinct magmatic episodes. The first occurred prior to 60 Ma with volcanism at the southeast Greenland Margin, in West Greenland, and the northwest British Isles. This early stage of widespread volcanism most likely records the arrival of the Iceland mantle plume. The second phase of magmatism began about 57 Ma with voluminous basaltic eruptions centralized along the eventual line of continental separation. Magmatism at this stage was associated with continental breakup and asthenospheric upwelling with probable thermal input from the mantle plume. Close

• 34. [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

• 35. [Article] Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record

  This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/.

  Citation × Citation Citation Abstract Copy Title: Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record Author: Wessel, Paul, Duncan, Robert A., Mahoney, John J., Ray, Jyotisankar, Naar, David F., Ray, Jyotiranjan S. This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. Title: Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record Author: Wessel, Paul, Duncan, Robert A., Mahoney, John J., Ray, Jyotisankar, Naar, David F., Ray, Jyotiranjan S. This is the publisher’s final pdf. The published article is copyrighted by Oxford University Press and can be found at: http://petrology.oxfordjournals.org/. Close

• 36. [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

• 37. [Article] Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range Author: Cox, Allan, Magill, James, Duncan, Robert A. copyrighted by American Geophysical Union Title: Tillamook volcanic series : further evidence for tectonic rotation of the Oregon Coast Range Author: Cox, Allan, Magill, James, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 38. [Article] Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine Author: Roperch, Pierrick, Duncan, Robert A. copyrighted by American Geophysical Union Title: Records of geomagnetic reversals from volcanic islands of French Polynesia 1. Paleomagnetic study of a polarity transition in a lava sequence from the island of Huahine Author: Roperch, Pierrick, Duncan, Robert A. copyrighted by American Geophysical Union Close

• 39. [Article] Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System

  copyrighted by American Geophysical Union

  Citation × Citation Citation Abstract Copy Title: Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System Author: Watts, A. B., Weissel, J. K., Duncan, Robert A., Larson, R. L. copyrighted by American Geophysical Union Title: Origin of the Louisville ridge and its relationship to the Eltanin Fracture Zone System Author: Watts, A. B., Weissel, J. K., Duncan, Robert A., Larson, R. L. copyrighted by American Geophysical Union Close

• 40. [Article] Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau

  In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar ...

  Citation × Citation Citation Abstract Copy Title: Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau Author: Spadea, Piera, Tarney, John, Marriner, Giselle F., Nivia, Alvaro, Duncan, Robert A., Kerr, Andrew C, Kempton, Pamela D. In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar to 90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high-MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The Ar-40/Ar-39 ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)(n) > 1 and (ε(Nd))(i) < +4.1) to relatively depleted ((La/Nd)(n) < 1 and (ε(Nd))(i) > +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas. Title: Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean-Colombian oceanic plateau Author: Spadea, Piera, Tarney, John, Marriner, Giselle F., Nivia, Alvaro, Duncan, Robert A., Kerr, Andrew C, Kempton, Pamela D. In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the similar to 90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high-MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The Ar-40/Ar-39 ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)(n) > 1 and (ε(Nd))(i) < +4.1) to relatively depleted ((La/Nd)(n) < 1 and (ε(Nd))(i) > +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas. Close