Characteristics and transitions of titanomagnetite in the sheeted-dike basalts from the ODP drilled hole 504B---with implication for the magnetization of oceanic crusts

• Main Authors: Shu-Fang Ou, 歐淑芳
• Other Authors: Yen-Hong Shau
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/f65zb2

碩士 === 國立中山大學 === 海洋資源學系研究所 === 90 === Abstract The pattern of seafloor magnetic anomalies is a record for the self-reversals of the Earth magnetic field from the long past to the present. It has preserved crucial data for the formation and evolution of oceanic crusts and is one of the most important evidences for the theory of plate tectonics. However, the features and origins of magnetic carriers in the sheeted dikes of oceanic crusts have not been completely understood and are still in debate. In the present study, magnetic minerals in the core samples, which were drilled from the sheeted dikes at the DSDP/ODP 504B drill hole during Legs 83, 111, 137, 140, and 148, have been studied by using methods of rock magnetism and mineralogy with high-resolution petrographic tools (transmission electron microscopy, TEM). Our results indicate that the sheeted dike basalts have been subjected to different degrees of hydrothermal alterations, which are equivalent to greenschist facies to amphibolite facies metamorphism on the basis of the secondary mineral assemblages. The primary titanomagnetite in all the sheeted dike basalts has suffered high-temperature oxidation, exsolution, and hydrothermal alteration, and transformed into magnetite, which becomes the main magnetic mineral in the sheeted dikes. The lamellar widths of the secondary magnetite, as observed with electron microscopy, are consistent with the grain sizes inferred form the rock magnetic properties. The grain sizes of the magnetite are within the pseudo-single-domain field and increase with depths of the sheeted dikes. The consistent results of the whole-rock magnetic properties and the TEM observations have proved that the secondary magnetite and its textural features are representative of the features of magnetic mineral in the sheeted dikes. Therefore, on the basis of the formation model of the magnetite, it is inferred that the sheeted dike basalts obtained thermal chemical remanent magnetization (TCRM) at ~500°C (high-temperature oxidation, or exsolution), and then obtained chemical remanent magnetization (CRM) at ~350°C (hydrothermal alteration). The timing for the magnetization of the sheeted dike basalts thus lags slightly behind their formation. The primary titanomagnetite in the sheeted dikes has been completely transformed into pseudomorphs that consist of approximately half magnetite and half ilmenite or other phases. Thus, the natural remanent magnetization (NRM) of the sheeted dikes is only about half of that for the extrusive pillow basalts. However, the total thickness of the sheeted dikes is about three times of that for the pillow basalts. The sheeted dikes should have contributed to the seafloor magnetic anomalies to some extents.