A tracing study of sediment diagenesis in the Hikurangi subduction zone, New Zealand: Evidence from Sr isotope of pore fluid

The subduction zone, as one of the most active zones on Earth, has great significance in the evolution of Earth's surface and interior. Slow slip events have not been recognized as an important form of faulting by geophysicists until recent decades. Slow slip event occurring in shallow sediments can rupture the seafloor and trigger large-scale tsunamis. Understanding the source of pore fluids and the diagenesis of subduction zone sediments is helpful to understand the formation mechanism of slow slip events. This paper presents the results of SO₄^2-, Ca²⁺, Mg²⁺ and Sr²⁺ and the radiogenic Sr isotope (⁸⁷Sr/⁸⁶Sr) on the pore fluid collected from the drilling sites on the subducting plate (Site U1520) and the deformation front (Site U1518) of the Hikurangi subduction margin, offshore New Zealand, drilled during the International Ocean Discovery Program IODP Expedition 375. The results show that Ca²⁺ and Mg²⁺ concentrations, Sr²⁺ concentration and ⁸⁷Sr/⁸⁶Sr ratio are negatively correlated, indicating the widespread occurrence of volcanic ash alteration at both sites. In addition, the simultaneous decrease of Ca²⁺ and Mg²⁺ concentrations in the pore waters of the shallow 0～14.3 and 0～37.3 mbsf sediments is due to authigenic carbonate precipitation. In addition, the significant decrease in Mg²⁺ concentration with depth and increase in both Ca²⁺ and Sr²⁺ concentrations with depth accompanied by relatively constant ⁸⁷Sr/⁸⁶Sr values at the lithologic Unit IV (509.82～848.45 mbsf) of Site U1520 point to the ongoing carbonate recrystallization. In the lithological Unit V (848.45～1016.24 mbsf) which is dominated by pyroclastic rocks, the SO₄^2-, Ca²⁺, and Mg²⁺ concentrations in pore fluids are all close to seawater values. This observation likely implies the lateral flow of seawater-like fluid within the more permeable pyroclastic rocks compared to neighboring lithologic units. Therefore, we propose that the lithologic heterogeneity and various diagenetic processes of the subducting slab may lead to the formation of abnormal stress field and high fluid pressure after being subducted, which is likely related to the frequent occurrence of slow slip events in the Hikurangi subduction zone.