Characteristics and transitions of Ti-magnetite from sheeted-dike basalts at Hole 504B: Implications for magnetization processes of oceanic crusts

Shu-Fang Ou[1] ,*Yen-Hong Shau [1],Masayuki Torii [2]
Chorng-shern Horng [3]
National Sun Yat-Sen University[1]
Okayama University of Science[2]
Institute of Earth Science, Academia Sinica[3]

Magnetic properties and transmission electron microscopic analyses have shown that the magnetic mineral in the sheeted-dike basalts from the DSDP/ODP hole 504B is end-member magnetite. The magnetite has formed via high-temperature oxidation, exsolution, and greenschist-facies hydrothermal alteration of primary tita-nomagnetite. Therefore, the NRM of the sheeted-dike basalts is characteristic of CRMs, which were acquired and modified by magnetite during subsolidus cooling to the onset of hydrothermal alteration in the ridge-axis areas. The magnetization of the sheeted-dike layer occurs at temperatures much higher than that for magnetization of pillow layer of oceanic crust.

In order to understand magnetization processes for the lower oceanic crust, we have used methods of rock magnetism, conventional petrography, and transmission electron microscopy (TEM) to study characteristics and origin of magnetic minerals in the sheeted-dike basalts, which were drilled at the Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) Hole 504B. We distinguished four types of microtextures for the titanomagnetite from the sheeted-dike basalts. The first type shows apparently homogeneous titanomag-netite grains without opti-cally observable lamellar texture, the second type shows one generation of lamellar texture, the third is composed of thick ilmenite lamellae and host areas that consist of finer lamellar texture, and the fourth contains very fine titanomagnetite grains (< 5 micron m) without lamellar texture. The genesis for the four types of microtextures of titanomag-netite appears to be due to different cooling rate and oxygen fugacity during sub-solidus cooling of the sheeted dike basalts. In spite of the different microtextures of the titanomag-netite, whole-rock magnetic results and TEM analyses have consistently shown that the magnetic mineral in these samples is end-member magnetite. The magnetite has formed via high-temperature oxidation, exsolution, and greenschist-facies hydrothermal alteration of primary titao-nomagnetite on the basis of mineral parageneses and microtextures within the primary tita-nomagnetite. The effective grain size for the magnetite that contributes to the whole-rock magnetic properties is mainly reflected by the lamellar thickness of the microscopic to submicroscopic textures (trellis-type lamellae). Therefore, the primary titanomagnetite (TM~60) with Curie temperature of ~180 C could not acquire thermoremanent magnetization (TRM) for these sheeted-dike basalts. The natural remanent magnetization (NRM) of the basalts is characteristic of chemical remanent magnetizations (CRMs), which were acquired and modified by end-member magnetite during subsolidus cooling to the onset of hydrothermal alteration in the ridge-axis areas. The magnetization of the sheeted-dike layer of oceanic crust thus occurs at temperatures (~500 - 350 C) much higher than that for magnetization of pillow layer of oceanic crust (TRM obtained by primary titanomagnetite).