BORON AS A HYDRAULIC TRACER WITHIN THE SW JAPAN SUBDUCTION ZONE-SEDIMENT DEHYDRATION FROM TRENCH TO SUB-ARC

The active SW Japan subduction zone is a site of extensive geologic, seismic, hydrologic, and volcanic research to date. Material input comprises both sediments and altered oceanic crust (AOC) as evidenced by cores obtained from seafloor drilling sites, ultimately resurfacing as arc lavas. However, mass balance of fluids in and out of the system between the time of subduction and arc magmatism remain poorly constrained. Boron can be a useful hydraulic tracer given its solubility and high concentrations in subducted sediments relative to the overlying mantle wedge. Under lower PT conditions, diagenetic processes drive fluid loss. At higher PT, the predominant driver transitions to metamorphic dehydration, or breakdown of hydrous mineral phases. This study uses B concentration and δ¹¹B values as proxies to quantify this fluid loss.

Preliminary [B] and δ¹¹B data have been collected from (meta)sediments along a SE-NW transect across Shikoku Island from the Nankai trough, through the Shimanto belt, to the Sanbagawa metamorphic belt. The data show decreasing [B] and δ¹¹B with increasing temperature, covering a range of ~25-500°C. This suggests progressive fluid loss with increasing metamorphic grade due to slab dehydration, with ¹¹B being preferentially incorporated into the fluid. To assess provenance, ⁸⁷Sr/⁸⁶Sr data have also been obtained for samples from the Shikoku transect including clays, shales, schists, and basalts. Five of the seven schist samples have an ⁸⁷Sr/⁸⁶Sr value range of 0.709-0.715, which largely fall within the range for Nankai Trough clays (0.710-0.715) and Shimanto shales (0.710-0.717). This overlap suggests shared input sources, thus a viable analog of a continuous sedimentary suite. This assessment of initial homogeneity establishes a strong base for interpretation of boron cycling in global subduction zones.

Recent fieldwork (May, 2023) has expanded the existing dataset to fifty samples in total, including clays, shales, and schists from the same Shikoku transect. ⁸⁷Sr/⁸⁶Sr, [B], and δ¹¹B data acquired for each sample will help constrain volatile behavior from when sediments first enter to the system, undergo metamorphism at depth, and resurface.