Presentation Information
[T6-O-8]Drilling Sub-Seafloor Petit-Spot Basalts: Latest Results from IODP3 Expedition 502
*Asuka Yamaguchi1, Hiroko Kitajima2, Natsumi Okutsu3, Shipboard Scientists Expedition 502 (1. Atmosphere and Ocean Research Institute, the University of Tokyo, 2. Texas A&M University, 3. JAMSTEC)
Keywords:
Petit spot,IODP3,Expedition 502,Japan Trench
The International Ocean Discovery Program (IODP3) Expedition 502 was conducted from October 30 to November 23, 2025. The primary objective of this expedition was to evaluate the impact of petit-spot magmatism on subduction zone seismicity and global geochemical cycles. To achieve this objective, drilling was conducted to determine whether the unusually shallow acoustic basement observed beneath oceanic sediments on the Pacific Plate within the outer rise off the northeast coast of Japan is related to petit-spot basalts. Expedition 502 aimed to: (1) characterize the nature of the acoustic basement in areas with thin sediment cover; (2) assess the impact of petit-spots on the composition of pelagic sediments; and (3) estimate the geochemical fluxes associated with petit-spot volcanism.
During Expedition 502, a total of six holes (C0027A–F) were drilled at Site C0027. Holes C0027A and C0027B were test holes; C0027C was drilled for geophysical logging (logging while tripping); C0027D targeted shallow core sampling (HCPS and ESCS); C0027E was drilled for deep core sampling (SD-RCB); and C0027F targeted the sediment–basement boundary (SD-RCB). All holes reached the basement at depths between 57 and 63.5 m below seafloor (mbsf).
The underlying basement was extremely hard, making drilling operations challenging. Nevertheless, fresh basalt was recovered between the sediment-basement boundary and 106.5 mbsf, total drilled depth at Hole C0027E. The basalt exhibits minimal alteration and contains acicular phenocrysts of olivine and orthopyroxene, as well as olivine xenocrysts, but lacks feldspar phenocrysts. Whole-rock chemical compositions of basalts are plotted within the field of previously reported petit-spot basalts on the TAS (Total Alkali–Silica) diagram. Based on these characteristics, the recovered basalt is interpreted to be of petit-spot origin.
At the sediment–basalt contact, a reaction zone characterized by browning of the sediments was observed on the sediment side; however, no clear evidence of contact metamorphism was identified. In addition, phenocrysts in the basalts are generally small (up to a few millimeters), and no systematic increase in phenocryst size with depth was observed from the boundary. These observations suggest that the basalt at Site C0027 did not intrude into unconsolidated sediments but extruded onto the seafloor as lava and was subsequently covered by sediments.
The eruption age of the basalt is estimated to be approximately 3.6 Ma, based on the age of the overlying sediments constrained by integrated magnetostratigraphic and biostratigraphic analyses. Reevaluation of the geochemical flux associated with petit-spot volcanism and further clarify the roles of petit-spots in subduction zone systems are expected by ongoing and future analyses of the collected samples and datasets.
During Expedition 502, a total of six holes (C0027A–F) were drilled at Site C0027. Holes C0027A and C0027B were test holes; C0027C was drilled for geophysical logging (logging while tripping); C0027D targeted shallow core sampling (HCPS and ESCS); C0027E was drilled for deep core sampling (SD-RCB); and C0027F targeted the sediment–basement boundary (SD-RCB). All holes reached the basement at depths between 57 and 63.5 m below seafloor (mbsf).
The underlying basement was extremely hard, making drilling operations challenging. Nevertheless, fresh basalt was recovered between the sediment-basement boundary and 106.5 mbsf, total drilled depth at Hole C0027E. The basalt exhibits minimal alteration and contains acicular phenocrysts of olivine and orthopyroxene, as well as olivine xenocrysts, but lacks feldspar phenocrysts. Whole-rock chemical compositions of basalts are plotted within the field of previously reported petit-spot basalts on the TAS (Total Alkali–Silica) diagram. Based on these characteristics, the recovered basalt is interpreted to be of petit-spot origin.
At the sediment–basalt contact, a reaction zone characterized by browning of the sediments was observed on the sediment side; however, no clear evidence of contact metamorphism was identified. In addition, phenocrysts in the basalts are generally small (up to a few millimeters), and no systematic increase in phenocryst size with depth was observed from the boundary. These observations suggest that the basalt at Site C0027 did not intrude into unconsolidated sediments but extruded onto the seafloor as lava and was subsequently covered by sediments.
The eruption age of the basalt is estimated to be approximately 3.6 Ma, based on the age of the overlying sediments constrained by integrated magnetostratigraphic and biostratigraphic analyses. Reevaluation of the geochemical flux associated with petit-spot volcanism and further clarify the roles of petit-spots in subduction zone systems are expected by ongoing and future analyses of the collected samples and datasets.
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