[M-IS23] New frontiers in geology

[MIS23-P02] Characterizing slab-derived fluids in Phanerozoic forearc serpentinites through boron isotopes

*Tatsuki Tsujimori¹, Qing Chang² (1.Tohoku University, 2.JAMSTEC)

Keywords:mantle wedge serpentinite, boron isotopic ratio, slab-derived fluids, global reference

Investigating serpentinites originating from the subforearc mantle wedge, which are affected by fluids emanating from the subducting slab, offers crucial insights into the nature of these fluids derived from the slab. Boron (B) has two isotopes, mass numbers 10 and 11, with ¹¹B selectively partitioning into the liquid phase, implying that the B isotopic ratio (δ¹¹B) of fluids released from the subducting slab would gradually become lighter (decrease in value). Thus, it is expected that the B isotopic ratio in serpentinites of the hydrated mantle wedge, directly affected by slab-derived fluids, would be lower in deeper parts. As shown by Martin et al. (2016, 2020) and Yamada, Tsujimori et al. (2019a,b), the B isotopic ratio in serpentinites exposed in orogenic belts allows for a rough distinction between serpentinites from deeper parts of the hydrated mantle wedge (or subducted serpentinites) and others, regardless of B concentration. However, B isotopic fractionation can significantly vary with temperature and possibly change with pH differences. Trace element concentration evaluations are effective but can also depend on the pre-serpentinization mantle peridotite. To assess the B isotopic ratios in forearc serpentinites, we obtained 420 data from Phanerozoic forearc serpentinites, including those from the Franciscan Complex (western USA), SW Japan, and the Southern Kitakami Mountains (NE Japan). Despite the wide range of δ¹¹B values from –12‰ to +30‰ (with boron concentrations ranging from 8 to 930 µg/g), it is possible to identify specific boron isotopic ratios and concentrations that are characteristic of forearc serpentinites influenced by fluids derived from the subducting slab. In this paper, we outline our efforts to establish a global reference value or range for the boron isotopic ratio in forearc serpentinites.