First Downhole and Core-Based Petrophysical and Hydrological Measurements in a Submarine Volcano: Brothers Volcano, Kermadec Arc

Downhole measurements at two sites at Brothers volcano, Kermadec arc, during IODP Expedition 376, provided the first continuous, in-situ petrophysical and temperature data below the surface of an active submarine volcano. Petrophysical properties obtained from core samples at four sites expand the spatial coverage of this unique dataset, and help with calibrating downhole measurements subject to variable borehole conditions. Density, porosity and P-wave velocity measurements show variations correlated with igneous units and, occasionally, alteration types in the recovered hydrothermally altered dacitic volcaniclastics and lava flows. High potassium content obtained from downhole spectral gamma-ray wireline logging correlates with illite-rich zones described from core. The resistivity image log (FMS) in Hole U1530A (NW caldera wall) shows textural variations which were not possible to interpret from cores because of partial (17%) recovery. Fluid temperature profiles acquired in Holes U1528D (upflow zone influenced by magmatic degassing; Upper Cone) and U1530A (seawater-dominated; NW caldera) show two different hydrological systems. Temperature profiles in U1528D show a convective system with some zones of permeability interpreted to be structurally controlled, reaching 247°C at ca. 310 meters below seafloor. The downhole temperatures had not yet reached equilibrium 7 hours after stopping circulation of cold seawater, as temperature was still increasing in-between successive measurement runs. On the contrary, temperature profiles in Hole U1530A show a gradual increase in temperature with depth, and concave-upward shape, indicative of a largely conductive-dominated and recharge regime. Fluid samples were successfully recovered from Hole U1528D in challenging borehole conditions. Given the partial core recovery, joint interpretation of core and downhole petrophysical properties will be crucial in the interpretation of the igneous stratigraphy and alteration zones. In turn, this will assist with determining the nature and location of past and contemporary fluid pathways.