Marine magnetic anomaly patterns record the history of global plate movements and variations in the Earth's magnetic field. Interpretation of those patterns 40 yr ago shed light on the theory of plate tectonics (Vine and Matthews, 1963) and is fundamental to understanding the evolution of oceanic crust. However, there have been questions and debates regarding the source layers of the marine magnetic anomalies and the magnetization of oceanic crust with respect to the change of time and space (e.g., Dunlop and Özdemir, 1997; Johnson and Tivey, 1998). For example, what is the thickness and nature of the magnetic source layer that contributes to the marine magnetic anomalies? Is there a significant time lag for the magnetization of oceanic crust after their formation at the spreading centers (e.g., Hall and Muzzatti, 1999; Shau et al., 2000)? How and why does the natural remanent magnetization (NRM) of the oceanic crust vary with respect to the age and depth of the crust (e.g., Johnson and Pariso, 1993)? What is the exact magnetization process of the oceanic crust? Is the contribution of NRM of the crust mainly from thermoremanent magnetization (TRM) acquired during subsolidus cooling or from chemical remanent magnetization (CRM) formed as a result of alteration? In order to understand these fundamental questions concerning the marine magnetic anomalies, we first need to know the nature and origin of main magnetic carriers in the oceanic crust, including mineral type, composition, grain size, texture, mineral paragenesis, and their magnetic properties. On the basis of our knowledge of the magnetic minerals in oceanic crust, it is then possible to solve the debates and to answer the above questions.
In the present study, we combined rock magnetism, mineralogy, and petrography methods to study magnetic minerals in the core samples from Ocean Drilling Program (ODP) Leg 187. The study aimed to (1) present magnetic properties of basaltic samples from the Southeast Indian Ridge (SEIR) area and correlate the whole-rock magnetic properties with petrographic features of magnetic minerals, (2) characterize the nature and formation mechanism of magnetic minerals in the oceanic-ridge basalts, and (3) study variations of magnetic properties and related magnetic minerals with crust age.