TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 3), pp. 277-291, 2017

Low ratio of sediment recycling at Northeast Japan Arc: Constraints from 10Be isotopes and B-Ba-K-Be systematics

Takashi Sano,1* Toshiaki Hasenaka,2 Akiko Shimaoka,3 Takaaki Fukuoka4 and Hisao Nagai5

1Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan
2Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto-shi, Kumamoto 860-8555, Japan
3Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunko-ku, Tokyo 113-0032, Japan
4Faculty of Geo-Environmental Sciences, Rissho University, 1700 Magechi, Kumagaya, Saitama 360-0914, Japan
5College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

(Received April 8, 2016; Accepted December 29, 2016)

Abstract: The contribution of subducted sediments to the compositions of arc magmas is now widely accepted, but ratios of the sediment recycling have not been examined in detail. Here we compare the Be isotopes and B-Ba-K-Be systematics of descending sediments and erupted lavas from a cold subduction zone, the Northeast (NE) Japan Arc, to examine the sediment recycling. The descending sediments were recovered from Japan Trench, and erupted lavas were collected from three active volcanoes on the volcanic front of the NE Japan Arc. Since the subducting slab is cold, the pressure-temperature (P-T) path of its upper surface does not cross the sediment solidus, which means that the sediments would not be melted and release hydrous fluids to the overlying mantle wedge. In addition to the sediment-derived fluid, contributions of fluids from subducted altered oceanic crust (AOC) is proposed to satisfy the Be isotopes and B-Ba-K-Be systematics. Comparisons of NE Japan to other cold subduction zones show that 10Be input from Japan Trench (10Be = 488 × 106 atms/g; 10Be/9Be = 471 × 10−11) is middle range, but 10Be output from NE Japan volcanoes is lower (10Be < 0.8 × 106 atms/g; 10Be/9Be <3.1 × 10−11) than the other arcs (Kurile and Tonga-Kermadec arcs). This fact suggests that the ratio of sediment recycling at the NE Japan Arc is distinctly lower than the other cold subduction zones. The low ratio of sediment recycling is probably due to the lower temperature of descending slab beneath the NE Japan Arc than the other arcs, because cold slab induces both low extraction rate of Be from descending slab and long travel time beneath the arcs. Also, significant scraping of sediments from descending slab is another candidate to explain the low extraction rate of 10Be.
Key words: arc magma, trench sediment, recycling, beryllium-10, Northeast Japan

*Corresponding author E-mail: sano@kahaku.go.jp

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