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• 1. [Article] Geochronology of Galapagos seamounts

  Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of ...

  Citation × Citation Citation Abstract Copy Title: Geochronology of Galapagos seamounts Author: Christie, David M., Duncan, Robert A., Sinton, Christopher W. Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of these features. The areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 Ma. Age-distance relationships are consistent with a decrease in the velocity of the Nazca plate relative to the Galápagos hotspot during the past 9 m.y., most likely at ~5 Ma when there was a change in Pacific-hotspot motion. Alternatively, the plate velocity remained constant and volcanism at a single seamount endured up to 7 m.y. lavas from seamounts along the Wolf-Darwin lineament to the northwest of the Galápagos Platform are young (<1 Ma), with the youngest measured ages closest to the platform. Title: Geochronology of Galapagos seamounts Author: Christie, David M., Duncan, Robert A., Sinton, Christopher W. Lavas from seamounts of the central Galápagos Platform and the Carnegie Ridge increase in age with distance from the western edge of the platform, consistent with a hotspot model for the generation of these features. The areal distribution of seamount ages suggests that the pattern of dispersed volcanism seen on the present islands also prevailed between 5 and 6 Ma. Age-distance relationships are consistent with a decrease in the velocity of the Nazca plate relative to the Galápagos hotspot during the past 9 m.y., most likely at ~5 Ma when there was a change in Pacific-hotspot motion. Alternatively, the plate velocity remained constant and volcanism at a single seamount endured up to 7 m.y. lavas from seamounts along the Wolf-Darwin lineament to the northwest of the Galápagos Platform are young (<1 Ma), with the youngest measured ages closest to the platform. Close

• 2. [Article] Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor

  Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge ...

  Citation × Citation Citation Abstract Copy Title: Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor Author: Mahoney, J. J., Pyle, D. G., Duncan, Robert A., Christie, David M. Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87SR/86SR (0.7025-0.7029) and 206Pb/204Pb (18.80-19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030-0.7035), low 206Pb/204Pb (17.99-18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma. Title: Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor Author: Mahoney, J. J., Pyle, D. G., Duncan, Robert A., Christie, David M. Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87SR/86SR (0.7025-0.7029) and 206Pb/204Pb (18.80-19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030-0.7035), low 206Pb/204Pb (17.99-18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma. Close