OXYGEN AND SILICON ISOTOPE RATIOS OF ZIRCONS OF SILICIC ROCKS FROM THE IZU-BONIN-MARIANA (IBM) INTRA-OCEANIC ARC AND ARC COLLISION ZONE

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., Ishizuka et al., 2011 EPSL; Tani et al., 2015 EPSL). 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., Tani et al., 2010 Geology). 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 (Valley et al., 2015 Am. Mineral.). Similarly, low δ³⁰Si magmas can be caused by incorporation of severely weathered materials (Savage et al., 2014 Lithos; Savage et al., 2016 Goldschmidt Conf.). Oxygen and Si isotope ratios of zircons can be useful to investigate incorporation of low-temperature weathered material to their parent magmas. 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. A kimberlite zircon megacryst, KC-KLV-Zrc1 (δ¹⁸O=5.43±0.13‰, δ³⁰Si measurement by ICP-MS is in progress), was used as a standard.

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‰, Valley et al., 2005 CMP; Grimes et al., 2011 CMP) 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 (0.2 to −0.2‰ relative to kimberlite zircon) are consistent with the kimberlite zircon within analytical uncertainty (~±0.3‰). The present results suggest that incorporation of sediments to produce elevated δ¹⁸O magmas effectively occurs by the arc-arc collision process but production of low δ³⁰Si magma require further accumulation and incorporation of matured sediments.