Boron desorption and fractionation in Subduction Zone Fore Arcs: Implications for the sources and transport of deep fluids

Abstract

At many subduction zones, pore water geochemical anomalies at seafloor seeps and in shallow boreholes indicate fluid flow and chemical transport from depths of several kilometers. Identifying the source regions for these fluids is essential toward quantifying flow pathways and volatile fluxes through fore arcs, and in understanding their connection to the loci of excess pore pressure at depth. Here we develop a model to track the coupled effects of boron desorption, smectite dehydration, and progressive consolidation within sediment at the top of the subducting slab, where such deep fluid signals likely originate. Our analysis demonstrates that the relative timing of heating and consolidation is a dominant control on pore water composition. For cold slabs, pore water freshening is maximized because dehydration releases bound water into low porosity sediment, whereas boron concentrations and isotopic signatures are modest because desorption is strongly sensitive to temperature and is only partially complete. For warmer slabs, freshening is smaller, because dehydration occurs earlier and into larger porosities, but the boron signatures are larger. The former scenario is typical of nonaccretionary margins where insulating sediment on the subducting plate is commonly thin. This result provides a quantitative explanation for the global observation that signatures of deeply sourced fluids are generally strongest at nonaccretionary margins. Application of our multitracer approach to the Costa Rica, N. Japan, N. Barbados, and Mediterranean Ridge subduction zones illustrates that desorption and dehydration are viable explanations for observed geochemical signals, and suggest updip fluid migration from these source regions over tens of km.

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

“…With increasing temperature during subduction, 10 B is preferentially desorbed from clays, leading to increasing boron concentration and decreasing δ 11 B values documented in sedimentary pore fluids as well as in mud volcanoes (You et al, 1996). Recent modeling work suggests that at non-accretionary margins, which receive less incoming insulating sediments, like the northern Hikurangi, boron desorption and fractionation lead to higher concentrations of boron and lower δ 11 B values in the pore fluids (Saffer and Kopf, 2016). If the Raukawa sample is ignored (off-scale point in Fig.…”