11:00 〜 13:00
[SMP25-P02] Geochemistry of zircon and monazite for geochronological interpretation of a felsic gneiss of Harvey Nunatak in the Napier Complex, East Antarctica
Ultra-high temperature (UHT) metamorphism plays an essential role in developing and stabilizing continents through accretionary and collisional orogenesis. The Napier Complex in East Antarctica is where the regional UHT metamorphism was first recognized (Dallwitz,1968). This complex experienced extremely high temperatures (>1100 °C) based on the mineral assemblage of sapphirine + quartz (Harley, 2016 and reference therein). The thermal history of the Napier Complex is essential for unraveling the earth’s crustal evolution, including deep crust; however, geochronological constraints, such as the timing and duration of the metamorphic events, are still debated.
Zircon is a valuable accessory-mineral for some geochronometers such as U-Th-Pb geochronology. The concentrations and isotopic ratios of major and trace elements other than U, Th, and Pb have indicated the magmatic and metamorphic environment. The concentrations and the isotopic ratios of Li in zircon analyzed using secondary ion mass spectrometer (SIMS) provide helpful information for the existence of liquid water in the early earth’s surface and the contribution to felsic magma (e.g., Ushikubo et al., 2008). There are Li-enriched zircons ([Li]: ~300-600 ppm) in orthopyroxene-felsic-gneiss collected from the Harvey Nunatak within the ultrahigh-temperature metamorphism region in the Napier Complex, East Antarctica by the 58th Japanese Antarctic Research Expedition (JARE-58) Geological Field Survey Team. The zircons are characterized based on the concentration of the other trace element (U, Th, Pb, K, Ca, Mn, Fe, Al, P, rare earth elements (REE), Nb) analyzed by a single-collector type sensitive high-resolution ion microprobe (SHRIMP-IIe) in National Institute of Polar Research (NIPR). The Li and oxygen isotope ratios of zircons are also analyzed by a multicollector type sensitive high-resolution ion microprobe (SHRIMP-Iie/AMC) in NIPR.
Several zircon grains collected from the Opx-felsic gneiss are hydrothermally altered since the zircons indicate high concentrations of non-formula elements such as Ca, Mn, Fe, Al, K, and light REE. The unaltered zircon grains, which show the concordant data on the Concordia diagram, indicate the weighted mean of 2499.5 ± 8.2 Ma (95% conf., MSWD: 16) Ma. The altered zircon grains indicate lower Li concentrations ([Li]: 12-612 ppm, average: 286 ppm) than those of unaltered zircon grains ([Li]: 311-668 ppm, average: 525 ppm). The Li isotope ratios (δ7Li) of the unaltered zircons indicate the wide range from -2.8‰ to 12.7‰ (average is 3.5‰), which suggest the sources of the zircons were affected by contamination of sediment. The concentrations of U, Nb, and Yb of the unaltered zircons fall in the magmatic arc array in the discrimination diagram (Nb/Yb vs. U/Yb) proposed by Grimes et al. (2015). It suggests that the zircons crystallized from the sources incorporating crustal material and water. The oxygen isotope ratios (δ18O) in the unaltered zircons indicate a mean of 4.93 ± 0.09 ‰ and range from 4.31 to 5.34 ‰. The values are close to oxygen isotope ratios of mantle-derived zircons (~5.3‰; Valley, 2003).
Monazites in the Opx-felsic gneiss were analyzed using the electron microprobe (EMP) chemical dating method to verify the U-Pb zircon ages. We will add monazite geochronological data and discuss the interpretation in the presentation.
Reference
Dallwitz (1968) Nature, 219, 476–477.
Harley (2016) J. Mineral. Petrol. Sci., 111, 50–72.
Ushikubo et al. (2008) Earth Planet. Sci. Lett., 272, 666-676.
Grimes et al. (2015) Contrib. Mineral Petrol. 170, 46.
Valley (2003) Rev. Mineral. Geochem. 53, 343–385.
Zircon is a valuable accessory-mineral for some geochronometers such as U-Th-Pb geochronology. The concentrations and isotopic ratios of major and trace elements other than U, Th, and Pb have indicated the magmatic and metamorphic environment. The concentrations and the isotopic ratios of Li in zircon analyzed using secondary ion mass spectrometer (SIMS) provide helpful information for the existence of liquid water in the early earth’s surface and the contribution to felsic magma (e.g., Ushikubo et al., 2008). There are Li-enriched zircons ([Li]: ~300-600 ppm) in orthopyroxene-felsic-gneiss collected from the Harvey Nunatak within the ultrahigh-temperature metamorphism region in the Napier Complex, East Antarctica by the 58th Japanese Antarctic Research Expedition (JARE-58) Geological Field Survey Team. The zircons are characterized based on the concentration of the other trace element (U, Th, Pb, K, Ca, Mn, Fe, Al, P, rare earth elements (REE), Nb) analyzed by a single-collector type sensitive high-resolution ion microprobe (SHRIMP-IIe) in National Institute of Polar Research (NIPR). The Li and oxygen isotope ratios of zircons are also analyzed by a multicollector type sensitive high-resolution ion microprobe (SHRIMP-Iie/AMC) in NIPR.
Several zircon grains collected from the Opx-felsic gneiss are hydrothermally altered since the zircons indicate high concentrations of non-formula elements such as Ca, Mn, Fe, Al, K, and light REE. The unaltered zircon grains, which show the concordant data on the Concordia diagram, indicate the weighted mean of 2499.5 ± 8.2 Ma (95% conf., MSWD: 16) Ma. The altered zircon grains indicate lower Li concentrations ([Li]: 12-612 ppm, average: 286 ppm) than those of unaltered zircon grains ([Li]: 311-668 ppm, average: 525 ppm). The Li isotope ratios (δ7Li) of the unaltered zircons indicate the wide range from -2.8‰ to 12.7‰ (average is 3.5‰), which suggest the sources of the zircons were affected by contamination of sediment. The concentrations of U, Nb, and Yb of the unaltered zircons fall in the magmatic arc array in the discrimination diagram (Nb/Yb vs. U/Yb) proposed by Grimes et al. (2015). It suggests that the zircons crystallized from the sources incorporating crustal material and water. The oxygen isotope ratios (δ18O) in the unaltered zircons indicate a mean of 4.93 ± 0.09 ‰ and range from 4.31 to 5.34 ‰. The values are close to oxygen isotope ratios of mantle-derived zircons (~5.3‰; Valley, 2003).
Monazites in the Opx-felsic gneiss were analyzed using the electron microprobe (EMP) chemical dating method to verify the U-Pb zircon ages. We will add monazite geochronological data and discuss the interpretation in the presentation.
Reference
Dallwitz (1968) Nature, 219, 476–477.
Harley (2016) J. Mineral. Petrol. Sci., 111, 50–72.
Ushikubo et al. (2008) Earth Planet. Sci. Lett., 272, 666-676.
Grimes et al. (2015) Contrib. Mineral Petrol. 170, 46.
Valley (2003) Rev. Mineral. Geochem. 53, 343–385.