Rock Magnetic Signal of Fluid Systems at IODP Site U1437 in the Izu Rear Arc

Abstract:

IODP Expedition 350 cored Izu rear-arc site U1437, and recovered a 1320 m thick distal sequence dominated by tuffaceous mud and subordinate fine tephra, overlying a proximal volcaniclastic sequence extending to the base of coring at 1800 mbsf. A continuous magnetostratigraphy was recovered from the muds of the distal sequence, despite challenges presented by downhole alteration. Rock magnetic parameters, which track diagenesis and alteration of the magnetic iron oxides and sulfides, exhibit a background downhole trend towards increasingly mature pyritization that is surprisingly similar to patterns seen in ODP sites drilled in hemipelagic sediments with much lower volcanogenic input. Superimposed on this trend are excursions which parallel features in pore-water geochemistry and physical properties, apparently marking fluid migration and diffusion at depth. Low values of saturation isothermal remanence (SIRM) after 300 mT back-field demagnetization, S_-0.3T, which typically indicate production of greigite, accompany a deep sulfate diffusion zone from a fluid source at about 275 mbsf. SIRM normalized for susceptibility (SIRM/k) reaches its maximum value at about 460 mbsf, matching an offset in foliation development in AMS (anisotropy of magnetic susceptibility), and corresponding to the position of the highest value in the sulfate profile (reaching seawater concentration), inflections in Li, Mg and other species, and an increase in lithification marked by a break in the porosity profile. A second fluid source at this depth, perhaps along the lithification front, seems indicated. Interruption to otherwise continuous magnetostratigraphy across the break between coring in Holes U437D and U1437E suggests the presence of a normal fault. A sudden change to multidomain-dominated magnetic mineralogy below the break between holes, and the equally sudden appearance of ethane in headspace gas, suggest a contrasting fluid circulation environment in the foot and hanging walls.