HR: 1330h
AN: V42D-0383    [PDF]
TI: In-situ Hf isotope analysis of early Archean zircons in the Acasta Gneisses from the Slave province, Northwestern Canada
AU: * Iizuka, T
EM: tiiduka@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Oookayama 2-12-1, Meguroku, 152-0081 Japan
AU: Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Oookayama 2-12-1, Meguroku, 152-0081 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Oookayama 2-12-1, Meguroku, 152-0081 Japan
AU: Hirata, T
EM: hrt1@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Oookayama 2-12-1, Meguroku, 152-0081 Japan
AB: Lu-Hf and Sm-Nd isotopic systems of early Archean rocks provide insights into the early crustal evolution and early mantle differentiation of the Earth. The Acasta Gneisses have been established as the oldest known intact terrestrial rocks (Bowring et al., 1999). The Acasta Gneiss Complex comprises mainly of Gray Gneiss (granodioritic gneiss), White Gneiss (tonalitic to granitic gneiss), and Foliated Granite, with many aplite and basaltic intrusions, and the relation between these rocks is very complex. Bowring et al. (1989) carried out the whole-rock Sm-Nd isotopic system measurement of the Acasta gneisses, and demonstrated that the gneisses exhibit a wide range of initial $\epsilon$(Nd) (+3.5 to -4 at 4.0 Ga and +4 to -7 at 3.6 Ga). However, because most of the Acasta gneisses have experienced amphibolite facies metamorphism, it is difficult that the whole-rock isotopic system remains closed. Zircon, which is extremely resistant against erosion and/or metamorphic events, and it can be also dated precisely by U-Pb chronometer. Because of high Hf content (ca. 1 wt%) and low Lu/Hf ratio, zircon has been widely used for the isotopic study using Lu-Hf system, too. Recent Lu-Hf isotopic studies were carried out using a multiplecollector inductively coupled plasma mass spectrometer (MC-ICPMS). Amelin et al. (2000) carried out the Hf isotope analyses of some zircon grains from the Acasta Gneisses using MC-ICPMS. The zircon grains exhibit enriched initial $\epsilon$(Hf) (+0.16 to -4.1 at ca. 3.6 Ga), while other early Archean zircon grains from the Amitsoq gneisses and the Barberton gneisses indicate depleted signature (Amelin et al., 2000). One possible reason is that the zircon grains from the Acasta Gneisses are grown at partial melting of the gneisses and/or underwent isotopic disturbance caused by intrusion of younger granites. Therefore, it is very important to reveal the growth features of zircon, such as oscillatory zoning, in order to derive inherent information of the early Archean rocks, because zircon grains in the rocks usually have complex internal structures. Moreover, precise in-situ Hf isotope microanalysis is required in order to avoid the disturbance of inclusions and cracks in the zircon grain. In this study, we separated many zircon grains from rock samples, and carefully observed the internal texture using cathodluminescence. In addition, we carried out in-situ U-Pb analysis and in-situ Hf isotope analysis from ablation crater size of $<$20 $\mu$m and 30 $\sim$ 60 $\mu$m diameter, respectively, using laser ablation-MC-ICPMS. We established to estimate the original isotopic signature of protoliths of the Acasta Gneisses from comprehensive analyses of zircon.
UR: http://www.agu.org/rb33ti
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting