Magnetic Susceptibility as a Tool for Investigating Igneous Rocks—Experience from IODP Expedition 304

Continuous measurements of magnetic susceptibility have been commonly used on Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP)expeditions to study minor lithological variations (forexample, those related to climatic cycles) in sedimentary rocks, but they have been less frequently used on igneous rocks, although important post-cruise studies have utilized them (e.g., Ildefonse and Pezard, 2001). Here I report its use (and that of the closely related electrical conductivity) on IODP Expedition 304 to examine igneous crustal rocks. Expedition 304/305 targeted the Atlantis Massif, an oceanic core complex on the Mid-Atlantic Ridge, and recovered a suite of igneous rocks comprising mainly gabbros, troctolites, and some diabases (Blackman et al., 2006; Ildefonse et al., 2006, 2007; IODP Expeditions 304 and 305 Scientists, 2005). Shipboard measurements (on D/V JOIDES Resolution) of physical properties were made to characterize lithological units and alteration products, to correlate cored material with down-hole logging data, and to interpret broader-scale geophysical data.