Magnetic Characterization of Borehole Samples from IODP Expedition 399: Atlantis Massif, Mid-Atlantic Ridge

Serpentinization is a hydrothermal process that often forms magnetite, significantly altering the magnetic properties of ultramafic rocks at mid-ocean ridges. However, the evolution of these magnetic properties during serpentinization and their stability over geological timescales are not completely understood. The Atlantis Massif, one of the best-studied oceanic core complexes, is an ideal place to study serpentinization's effects on rock magnetism. IODP Expedition 399 drilled a deep (1268m) borehole (Hole U1601C) into uplifted lower crustal and upper mantle rocks on the Mid-Atlantic Ridge, providing an excellent opportunity to study the variation in rock magnetic properties with spatial context at mid-ocean ridges. In-depth magnetic properties were analyzed using facilities at Stanford and the Institute for Rock Magnetism at the University of Minnesota. We measured room temperature hysteresis loops, back field curves, magnetic properties measurement system, first-order reversal curves, low and high-temperature magnetic susceptibility, and anisotropy of magnetic susceptibility. We found that the magnetic carriers for serpentinized peridotites consisted predominantly of stoichiometric magnetite. Magnetic carriers for gabbros were dominated by magnetite and titanomagnetite, with noticeable contributions from monoclinic pyrrhotite in some samples. Most of the serpentinized samples exhibited vortex (pseudo-single domain-like) domain behavior. Ongoing scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy measurements are being used to contextualize the spatial distribution of magnetic minerals in relation to primary phases, secondary minerals (ex. lizardite, brucite), cracks, and void spaces. Tentative results indicate that iron sulfides in gabbros were predominantly located in cracked regions, while SEM-detectable magnetite grains in serpentinized peridotites were typically found along the rims of relict olivine grains.