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 [1]. 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 [2]. Silicic crust formations by the intra-oceanic crustal subduction and the following arc-arc collision are probably significant processes of continental crust formation and growth in the early Earth’s history. We will report comprehensive oxygen and silicon isotope data of zircons from the IBM arc (~49Ma to <1Ma) and the ICZ (15Ma to 4Ma). Oxygen and silicon isotope measurements of zircons were performed with an ion microprobe, Cameca IMS-1280HR of the Kochi Institute, JAMSTEC. The instrumental bias of analyses was corrected based on results of bracketing analyses of the in-house kimberlite zircon standard, KC-KLV-Zrc1 (δ18O=5.43±0.13‰ & δ30Si=-0.38±0.19‰ [3,4]). Typical spot-to-spot reproducibility (2SD) of the bracketing standard analyses was ±0.25‰ for δ18O and ±0.3‰ for δ30Si, respectively. The average δ18O values of zircons from individual IBM arc samples (4.9 to 5.3‰) are generally consistent with the mantle zircon value [5]. This is probably because incorporation of sediments into the silicic parent magmas does not occur efficiently. One exception is zircons from diorite of the Omachi Seamount (δ18O=7.0‰) (~38Ma). Since serpentinite bodies are abundant around this seamount, elevated δ18O values of zircons could be a result of assimilation of serpentine during emplacement of the parent magma. On the other hand, zircons from ICZ granitic rocks show wider range δ18O values (4.8‰ to 6.4‰), and three samples out of six ICZ samples show δ18O values higher than 6.0‰, indicating incorporation of mature sediments from the Honshu arc during the emplacement of the granitic magmas at shallow levels by the arc collision. Contrary to the δ18O values, the average δ30Si values of zircons from all the studied samples (-0.20±0.37‰ to -0.57±0.34‰) are consistent with the kimberlite zircon data (-0.38±0.19‰) within analytical uncertainty. These results suggest that (1) silicic crusts can be formed by intra-oceanic crustal subduction, however, the d18O values of zircons commonly stay within the range of the mantle value, (2) silicic magmas with elevated d18O values by incorporation of sediments can be effectively formed by arc-arc collisions, (3) assimilation of serpentine is another possibility to form silicic crusts with elevated δ18O values [e.g., 6], (4) δ30Si values is much less sensitive to incorporation of mature sediments than δ18O values because of smaller isotopic contrast between mantle and mature sediments (larger than 10‰ for δ18O vs. a few ‰ for δ30Si).

Reference(s)

[1] Ishizuka, O., et al. 2011, EPSL, v.306, p.229-240. 

[2] Tani, K., et al. 2010, Geology, v.38, 215-218. 

[3] Ushikubo, T., et al. 2015, Goldschmidt abstract #3214. 

[4] Ushikubo, T., et al. 2017, GSA abstract #139-8. 

[5] Valley, J. W., et al. 2005, Contrib. Mineral. Petrol., v.150, p.561-580. 

[6] Trail, D., et al. 2018, PNAS, v.115, p.10287-10292.