14:00 〜 14:15
[SCG55-02] Prolonged mantle history recorded in zircons from the Horoman peridotite, Hokkaido, Japan
キーワード：ジルコン、幌満、かんらん岩、マントル交代作用、リサイクル
The Horoman peridotite complex, exposed at the southwestern end of the Hidaka metamorphic belt, is a well-preserved peridotite suites containing highly depleted spinel-harzburgite to fertile plagioclase-lherzolite with a large scale layered-structure coexisting with types of gabbro, pyroxenite and dunite (Niida, 2007). Geochemical studies have shown that the Horoman peridotite have complex and prolonged history from mid-ocean ridge (MOR) stage ca. 1.0 Ga (Yoshikawa and Nakamura, 2000; Ranaweera et al., 2018) to final stage of up-lifting from the mantle ca. 20 Ma (Yoshikawa et al., 1993; Kaneoka et al., 2001). We recovered sixteen zircon grains from lherzolite rock sample, and examined U-Pb age, trace elements and mineral inclusions in zircon to constraint their formation and age relationship previously reported. Rock samples of harzburgite and lherzolite were collected directly from quarry site of TOHO Olivine Industrial Company and cut them into plats on site, then transferred to the laboratory. To avoid any contaminations, all procedures mineral separation were conducted at newly built facilities in Yokohana National University.
Zircons are transparent or slightly pink in color and show subhedral to rounded shape. Analysis using Raman spectroscopy on zircon grains show relatively narrow and intense Raman peak indicating well-crystallinity, probably due to long annealing under the mantle conditions. Spot analyses with LA-IC-PMS, assisted by cathodoluminescence (CL) images gave a wide age range, from the early Miocene to the Mesoproterozoic (ca. 18–1055 Ma). Zircon CL images show unclear oscillatory zoning and relatively uniform and/or complex internal structure, and some grains have inherited old core (911 Ma) with younger rim (620 Ma). Zircons recovered often have mineral and or fluid inclusions within them. A young zircon with age of 18 Ma has Mg-hydrous silicate inclusion (phase not determined), which suggest that such zircons were formed by hydrous fluid/melt metasomatism at the mantle environment. It has been suggested that zircon U-Pb age retain under the mantle conditions (> 1000 °C) due to low Pb diffusion rate of host minerals such as orthopyroxene and olivine (Bea et al., 2020). The minimum ages of ca 18 Ma, plotted on the concordia curve, were consistent with the age of metasomatic event previously reported by Ar-Ar plateau age (20.6 ±0.5 Ma), and the maximum age of 1055 Ma in the study would correspond to that of MOR setting ca. 1.0 Ga.
Zircons are transparent or slightly pink in color and show subhedral to rounded shape. Analysis using Raman spectroscopy on zircon grains show relatively narrow and intense Raman peak indicating well-crystallinity, probably due to long annealing under the mantle conditions. Spot analyses with LA-IC-PMS, assisted by cathodoluminescence (CL) images gave a wide age range, from the early Miocene to the Mesoproterozoic (ca. 18–1055 Ma). Zircon CL images show unclear oscillatory zoning and relatively uniform and/or complex internal structure, and some grains have inherited old core (911 Ma) with younger rim (620 Ma). Zircons recovered often have mineral and or fluid inclusions within them. A young zircon with age of 18 Ma has Mg-hydrous silicate inclusion (phase not determined), which suggest that such zircons were formed by hydrous fluid/melt metasomatism at the mantle environment. It has been suggested that zircon U-Pb age retain under the mantle conditions (> 1000 °C) due to low Pb diffusion rate of host minerals such as orthopyroxene and olivine (Bea et al., 2020). The minimum ages of ca 18 Ma, plotted on the concordia curve, were consistent with the age of metasomatic event previously reported by Ar-Ar plateau age (20.6 ±0.5 Ma), and the maximum age of 1055 Ma in the study would correspond to that of MOR setting ca. 1.0 Ga.