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.