Petit-spot magmatic-volcanic processes and sediment interaction: new insights from deep-sea drilling at the Japan Trench

International Ocean Drilling Programme (IODP3) Expedition 502, scheduled for October – November 2025, will drill the Japan Trench subduction zone in the NW Pacific, in the region of largest slip associated with the 2011 tsunami-generating magnitude 9.0 Tohoku Earthquake. In this region, previous seismic surveys have identified anomalously thin seafloor sediments, hypothesised to be due to disturbance by lava flows, or magmatic intrusions, related to a recently discovered type of volcanism, called petit-spot volcanism. Petit-spot volcanism occurs at subduction zones, where fractures form due to the bending of the down going plate. It is expected that petit-spot activity disrupted and potentially thermally metamorphosed the sediments, thereby changing the nature and topography of the subducting plate, which are factors known to affect earthquake dynamics. First discovered in the Japan Trench and subsequently found in the Java and Mariana Trenches, petit-spot volcanism could be widespread in oceanic trenches worldwide, but is challenging to detect given their deep-marine location e.g. the drilling site for Expedition 502 is in a region with a water depth of nearly 5.5 km. To date, globally, petit-spot rocks have only been sampled via dredging and submersible dives, which can only provide incomplete snapshots of these volcanic systems and their influence on the surrounding sediment. The challenging nature of observing and sampling these systems means that many aspects of this type of volcanism remain poorly understood, including, its global distribution, eruption and intrusion dynamics, and its influence on seafloor sediment properties. IODP3 Expedition 502 will drill and retrieve a 225 m core, providing the first, unprecedented, opportunity to examine an entire stratigraphic sequence of petit-spot rocks, and their in-situ relationship and interactions with the surrounding seafloor sediments. The aim of this post-expedition research is to characterise the thus-far understudied petit-spot volcanism and magmatism, and its interaction with seafloor sediments, thereby advancing our understanding of submarine volcanic processes, petit-spot magmatism, and the physical and chemical properties of the subducting slab. The research objectives are to: 1) resolve the structural architecture of petit-spot basalt and intercalated sediments within the drill core; 2) characterise the interactions between hot basalt magma/lava and water-logged sediments and the effect on the mineralogical and textural properties of the sediments; and 3) reconstruct the eruptive and intrusive processes and dynamics of submarine petit-spot igneous activity, in order to contribute to broader models of deep-sea volcanism and the lithological and geochemical complexities of the subducting plate. The project aim and objectives will be achieved via the first ever macroscopic and microscopic core descriptions, and textural, geochemical and petrological analysis of the igneous rocks and their associated sediments. This post-expedition research aligns with all three scientific objectives for IODP3 Expedition 502, including understanding the nature and composition of the seafloor sediment, assessing the impact of petit-spot activity on the composition of this sediment, as well as improving estimation of geochemical cycles at subduction zones, crucial for understanding Earth processes. The outcomes of this proposed post-expedition research will significantly enhance our understanding of submarine volcanism, particularly the role of petit-spot activity and its impact on Earth systems, through characterising the lithological and geochemical characteristics of the down-going plate at subduction zones. This includes the influence of petit-spot volcanism on subduction processes and earthquake dynamics, with direct implications for understanding hazards and risk connected to volcanoes, earthquakes and tsunamis.