16:15 〜 16:30
[BCG09-04] 伊豆・小笠原・マリアナ（IBM）島弧と伊豆衝突帯（ICZ）の珪質岩ジルコンの酸素と珪素同位体比の特徴
キーワード：ジルコン、IBM島弧、二次イオン質量分析計、酸素同位体、珪素同位体
The Izu-Bonin-Mariana (IBM) Arc is an active intra-oceanic arc for ~52 million years, and silicic crusts have been generated by subduction zone magmatism [e.g., 1,2]. The north end of the IBM arc has been colliding with the Honshu Arc at the Izu Collision Zone (ICZ) for ~15 million years and voluminous syn-collision granitic plutons are exposed in ICZ [e.g., 3]. Silicic crust formations by intra-oceanic crustal subduction and the following arc-arc collision are probably significant processes of continental crust formation during the early Earth history.
Elevated δ18O values (6.0 – 7.5‰, VSMOW) in Hadean and Archean zircons are interpreted as evidence for incorporation of low temperature aqueous alteration products to the zircon-forming magmas as early as 4.3 Ga [4,5]. Similarly, low δ30Si magmas can be caused by incorporation of severely weathered materials [6,7]. Oxygen and Si isotope ratios of zircons can be useful to investigate incorporation of low-temperature weathered material to their parent magmas. We established zircon standards, a kimberlite xenocryst KC-KLV-Zrc1 from Kaalvallei, South Africa ( δ18OVSMOW=5.43±0.14‰, δ30SiNBS-28=−0.38±0.19‰, 2SD) and a detrital zircon megacryst KC-MGK-Zrc2 from Mogok, Myanmar ( δ18OVSMOW=19.14±0.14‰, δ30SiNBS-28=−0.18±0.09‰, 2SD), for in situ O and Si isotope measurements by SIMS. And we performed O and Si isotope analyses of zircons from IBM Arc (6 granitic rocks and 1 gabbro, ~52 to 0.15 Ma) and ICZ (6 granitic rocks, 15.7 to 4.3 Ma) by SIMS. Typical spot-to-spot reproducibility (2SD) of SIMS measurements was ±0.25‰ for δ18O and ±0.3‰ for δ30Si, respectively. Part of isotope data have already reported in JpGU 2016 [8].
The zircon δ18O values of most IBM granitic rocks (4.9 to 5.2‰) are within the range of the mantle-like zircon value (5.3±0.6‰, [4,9]) with one exception of the Omachi Seamount sample (7.0‰). Zircons from IBM fore-arc gabbro have slightly low values (4.6‰). In contrast, zircons from ICZ tend to have elevated δ18O values (up to 6.4‰), indicating incorporation of accreted sediments in the Honshu Arc. The δ30Si values of all measured zircons (−0.4±0.3‰) are consistent with the kimberlite zircon within analytical uncertainty (~±0.3‰). The present results suggest that incorporation of sediments to produce elevated d18O magmas effectively occurs by the arc-arc collision process but production of low d30Si magma require further accumulation and incorporation of matured sediments.
References:
[1] Ishizuka et al. (2011) EPSL, 306, 229-240.
[2] Tani et al. (2015) EPSL, 424, 84-94.
[3] Tani et al. (2010) Geology, 38, 215-218.
[4] Valley et al. (2005) Contrib. Mineral. Petrol., 150, 561-580.
[5] Valley et al. (2015) Am. Mineral., 100, 1355-1377.
[6] Savage et al. (2014) Lithos, 190-191, 500-519.
[7] Savage et al. (2016) Goldschmidt Conference abstract #2749.
[8] Tani et al. (2016) JpGU abstract #SGL38-08.
[9] Grimes et al. (2011) Contrib. Mineral. Petrol., 161, 13-33.
Elevated δ18O values (6.0 – 7.5‰, VSMOW) in Hadean and Archean zircons are interpreted as evidence for incorporation of low temperature aqueous alteration products to the zircon-forming magmas as early as 4.3 Ga [4,5]. Similarly, low δ30Si magmas can be caused by incorporation of severely weathered materials [6,7]. Oxygen and Si isotope ratios of zircons can be useful to investigate incorporation of low-temperature weathered material to their parent magmas. We established zircon standards, a kimberlite xenocryst KC-KLV-Zrc1 from Kaalvallei, South Africa ( δ18OVSMOW=5.43±0.14‰, δ30SiNBS-28=−0.38±0.19‰, 2SD) and a detrital zircon megacryst KC-MGK-Zrc2 from Mogok, Myanmar ( δ18OVSMOW=19.14±0.14‰, δ30SiNBS-28=−0.18±0.09‰, 2SD), for in situ O and Si isotope measurements by SIMS. And we performed O and Si isotope analyses of zircons from IBM Arc (6 granitic rocks and 1 gabbro, ~52 to 0.15 Ma) and ICZ (6 granitic rocks, 15.7 to 4.3 Ma) by SIMS. Typical spot-to-spot reproducibility (2SD) of SIMS measurements was ±0.25‰ for δ18O and ±0.3‰ for δ30Si, respectively. Part of isotope data have already reported in JpGU 2016 [8].
The zircon δ18O values of most IBM granitic rocks (4.9 to 5.2‰) are within the range of the mantle-like zircon value (5.3±0.6‰, [4,9]) with one exception of the Omachi Seamount sample (7.0‰). Zircons from IBM fore-arc gabbro have slightly low values (4.6‰). In contrast, zircons from ICZ tend to have elevated δ18O values (up to 6.4‰), indicating incorporation of accreted sediments in the Honshu Arc. The δ30Si values of all measured zircons (−0.4±0.3‰) are consistent with the kimberlite zircon within analytical uncertainty (~±0.3‰). The present results suggest that incorporation of sediments to produce elevated d18O magmas effectively occurs by the arc-arc collision process but production of low d30Si magma require further accumulation and incorporation of matured sediments.
References:
[1] Ishizuka et al. (2011) EPSL, 306, 229-240.
[2] Tani et al. (2015) EPSL, 424, 84-94.
[3] Tani et al. (2010) Geology, 38, 215-218.
[4] Valley et al. (2005) Contrib. Mineral. Petrol., 150, 561-580.
[5] Valley et al. (2015) Am. Mineral., 100, 1355-1377.
[6] Savage et al. (2014) Lithos, 190-191, 500-519.
[7] Savage et al. (2016) Goldschmidt Conference abstract #2749.
[8] Tani et al. (2016) JpGU abstract #SGL38-08.
[9] Grimes et al. (2011) Contrib. Mineral. Petrol., 161, 13-33.