Magnetic properties and opaque mineralogy of drilled submarine intrusive rocks

Summary Measurements of natural rémanent magnetization (nrm), Curie temperature, ferrimagnetic and paramagnetic susceptibility, saturation induced and remanent magnetizations, coercive forces, alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during Legs 30, 37 and 45 of DSDP. The collection includes doleritic sill rocks, fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites, and serpentinized lherzolites believed to have originated in Layers 2B, 3B and 4 respectively. Magnetite, with a Curie temperature between 520 and 580°C, is the principal magnetic mineral in all samples. There is no indication of maghemitization or of metamorphism to greenschist facies or above. The magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene; the stable nrm is a primary trm. The magnetite in the serpentinized rocks is a secondary product of serpentinization; the stable nrm is a crm. In most rock types, the magnetite is of single‐domain or pseudo‐single‐domain size and soft components of nrm are small. The magnetite grain size in some of the doleritic gabbros is much coarser; these rocks acquire large viscous magnetizations, which however are readily removed by alternating field cleaning. The cleaned nrms of Legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded Leg 37 units have been dispersed by tectonic rotations. The doleritic gabbros and the serpentinized peridotites have stable, directionally coherent nrms ≳ 10 ‐3 emu cm ‐3 in intensity. Their counterparts in Layers 2B and 3B/4 are likely contributors to oceanic magnetic anomalies. Layer 3B cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources.