Heavy Si isotope compositions of subduction zone fluids controlled by fluid-rock interaction and fluid evolution

Abstract

Jadeitites are formed either through direct precipitation from Na-Al-Si rich fluids (P-type), or by replacement of magmatic protoliths (R-type) in subduction zones. They are valuable targets for investigating the mobility behavior and chemical composition of subduction zone fluids. The Rio San Juan Complex (RSJC) in the northern Dominican Republic hosts both P- and R-type jadeitites and jadeite-rich rocks, which provide ideal samples for addressing such issues. Here, we present trace element and Sr-Nd-O-Si isotope compositions of RSJC jadeitites and related rocks. Most samples show similar REE patterns, trace element distributions and δ¹⁸O values to those of plagiogranite protoliths, indicating the predominance of R-type origin in RSJC. The P-type samples exhibit slightly higher δ³⁰Si values (−0.15‰ to 0.25‰) than that of R-type samples (−0.20‰ to 0.08‰), which place above the igneous array. The low (⁸⁷Sr/⁸⁶Sr)_i (0.70346 to 0.70505) and high ε_Nd(t) values (4.6 to 6.8) of the P-type jadeitites and quartzites, along with relatively low δ¹⁸O values (4.7‰ to 6.4‰) of their forming fluids, indicate that the fluids are likely derived from the altered basaltic crust rather than from oceanic sediment. However, the estimated jadeitite- and quartzite-forming fluids exhibit distinct δ³⁰Si values (0.76‰ to 0.99‰ and –0.48‰ to –0.08‰, respectively), implying an evolution of the fluids that modified the Si isotopic compositions. Since fluid metasomatism and related desilication process could have lowered the whole-rock δ³⁰Si values, the heavy Si isotope compositions of the R-type samples are produced from the external fluids. Combing Rayleigh distillation and binary mixing simulations, we propose that fluids derived from altered oceanic crust obtained high δ³⁰Si values after crystallization of minerals enriched in light Si isotopes. The P-type jadeitites are formed through direct precipitation from this fluid. As the plagiogranite protoliths were continuously replaced by this fluid, the formed R-type samples (jadeitites and quartzites) also exhibit high δ³⁰Si values. Such rocks could significantly alter the Si isotope compositions of local mantle when they are deeply subducted at convergent plate margins.