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

In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400–16,000 mg/L), Br (6–70 mg/L), I (0.4–72 mg/L), Sr (0.1–200 mg/L), B (3–130 mg/L), Li (0.1–13 mg/L), and Na (33–6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ³⁷Cl, δ¹¹B, and δ⁷Li values range from −1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and −3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ³⁷Cl = 0‰; δ¹¹B = 40‰) and sedimentary pore fluids (δ³⁷Cl ≈ −8‰ to 0‰; δ¹¹B > ∼17‰). Br/Cl (0.0025–0.005) and I/Cl (0.00005–0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ⁷Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (<15 km) and therefore play a limited role in the transport of FMEs to great depths in subduction zones.