Second Call to Participate: Expedition 502: Impact of Petit-Spot Magmatism on Subduction Zone Seismicity and Global Geochemical Cycles

Second Call to Participate in IODP³ Expedition 502: Impact of Petit-Spot Magmatism on Subduction Zone
Seismicity and Global Geochemical Cycles now open!

Petit-spots are a recently discovered volcanic system characterised by highly alkalic and CO2-rich basalt generated by a low degree of partial melting of the asthenosphere. These deep mantle melts have been proposed to form and upwell due to the flexure of the oceanic Pacific Plate prior to subduction. IODP³ Expedition 502 aimed to explore the nature of the acoustic basement in the outer rise of the off-Tohoku Japan Trench and test the hypothesis that this acoustic basement is related to petit-spot magmatism. If this hypothesis is correct, petit-spot magmatism at the outer rise may be more widely distributed than previously thought. More extensive petit-spot volcanism could strongly impact our understanding of subduction systems, including rupture nucleation and slip propagation of plate boundary megathrust earthquakes.

IODP³ Expedition 502 successfully conducted logging and coring operations at Site C0027. Logging data were acquired while tripping the bit out of the drillhole from ~86 meters below sea floor (mbsf) to 23.5 mbsf, and cores were recovered from 0–106.5 mbsf. The boundary between the sediment layer and the acoustic basement is observed at ~60 mbsf, marked by a sharp increase in the electrical resistivity logs and a change in lithology. Recovered siliceous muds extend in age back to the late Pliocene. Core recovery in sediment units was nearly 100%, whereas core recovery from the underlying basaltic unit was limited, with an average of ~18%. The basement is composed mainly of fresh glassy and cryptocrystalline basalts. The basalt clearly differs from typical oceanic crust basalts of the Pacific plate in chemical composition, degree of alteration, and physical properties. Although no significant thermal alteration, decomposition, or deformation associated with the petit-spot magmatism was observed in the sediment, changes in sediment colour and pore fluid geochemistry in an interval of ~9 m above the sediment-basement boundary suggest that some fluid-rock reactions occurred between sediments and petit-spot basalt.

The original Expedition 502 Science Team has planned to conduct broad and multidisciplinary analytical approaches using the collected cores and logging/drilling data for petrochemical/geochemical, sedimentological and geochronological analysis, alongside palaeontological studies, palaeomagnetic and rock magnetic analyses, structural, geomechanical and petrophysical analysis, microbiological analysis, logging while drilling analysis, and numerical modelling.

With this work ongoing there are now opportunities to include new analytical directions that could complement the original research portfolio and further enhance the scientific value of Expedition 502 materials.

These include, but are not limited to:

• Non-destructive core analysis, including XRF core scanning
• Stratigraphic correlation of sediments with previous DSDP/ODP/IODP sites in the northwestern Pacific
• Geochemical/biological analysis of gas, interstitial water, sediment, and basalt.
• Experimental petrology (melting and quenching) with viscosity measurements
• Hydrological study of sediment and fractured basalt in relation to hydrothermal alteration

Applications are welcomed from scientists scientists based in IODP³ Core and Associate Member nations, proposing to either: (a) adopt these or any other approaches to address the original objectives of the expedition without duplicating the plans of the original Science Team; or (b) use core samples recovered during the expedition to address new objectives and deliver new science not envisaged by the original expedition proposal

For more information please read the full call on the PDF below.

Applications must be submitted by 23:59 BST on Monday 2 November 2026 using the IODP³ Gateway system, accessed via Apply to Participate on the IODP³ webpage.