Comparative Boron Budgets of Subduction Zones and Implications for Volatile Cycling

Boron is a key volatile tracer in subduction systems. It is concentrated in the pore waters of subducting sediments prior to diagenesis, partitions between aqueous and solid phases, is highly fluid-mobile, and is progressively released during devolitization. Exchangeable (aqueous + adsorbed) boron is primarily released by desorption at low temperatures (≤150 °C) and lattice bound boron is released by breakdown of hydrous phases at higher temperatures (<350 °C). During compaction, diagenesis, and dehydration of sediment in the forearc, released boron migrates with fluids and volatiles to seafloor seeps and mud volcanoes, or is retained and entrained within the subducting slab, where it can appear in arc lavas. Its widespread distribution makes boron an effective tracer throughout subduction. However, previous studies have not examined which subduction zone characteristics control the concentrations of exchangeable boron in subducting sediments, nor how these characteristics - and the resulting boron fluxes - vary among margins, limiting its use as a quantitative tracer of volatile recycling.

To address this, we investigated the mechanisms governing boron adsorption on compositionally representative Circum-Pacific trench sediments and quantified adsorbed boron input to subduction zones. We conducted boron adsorption experiments on sediments from Costa Rica, Barbados, Cascadia, Nankai, and the Hikurangi trench obtained from IODP drill cores. Sediment surface areas (SA) and aluminum-oxide (Al-O) contents were characterized using multi-point Brunauer–Emmett–Teller (BET) analysis and X-ray photoelectron spectroscopy (XPS), respectively.

Results show that boron adsorption onto marine sediments is controlled by SA and lithology, specifically the abundance of surface aluminum-oxide sites. SA varies by up to a factor of 4.6 among margins (e.g., 130 m²/g at Hikurangi versus 28 m²/g at Nankai), while aluminum-oxide surface concentration varies by a factor of 1.3 (e.g., 16 Al-O At% at Nankai versus 22 Al-O At% at Japan). Adsorbed boron flux varies by up to a factor of 9 between sampled subduction zones, being greatest at Hikurangi (5 kg yr⁻¹ m⁻¹) and similar at the Japan and Nankai trenches (0.6 kg yr⁻¹ m⁻¹) - higher SA and Al–O content at the Japan Trench is offset by reduced sediment input compared to Nankai - producing similar adsorbed boron fluxes. Adsorbed boron dominates the total exchangeable boron entering the trench, accounting for 97%, 84%, and 82% of the flux at Hikurangi, Nankai, and Japan, respectively, and contributes the largest variability among aqueous (up to 1.6×), adsorbed, and lattice-bound (~3.5×) boron. Total boron flux is highest at Hikurangi (18 kg yr⁻¹ m⁻¹), lowest at northern Japan (5 kg yr⁻¹ m⁻¹), and intermediate at Nankai (8 kg yr⁻¹ m⁻¹).

These findings indicate that boron fluxes into subduction zones vary substantially among margins, primarily due to differences in adsorbed boron at the trench, which largely reflect variations in sediment SA. Adsorbed boron dominates early boron release through desorption during initial burial and devolatilization, making it the most important boron reservoir for volatile tracing in the shallow forearc. The zone with the highest adsorbed boron flux, Hikurangi, corresponds to the most elevated boron concentrations observed at seeps. This work highlights that adsorbed boron - a previously overlooked component- is critical to volatile transport and geochemical cycling throughout the subduction complex.