Reflected Light Microscopy, SEM and Rock Magnetic Characterization of Magnetic Minerals Through an Intact Sequence of Oceanic Crust, IODP Hole 1256D

The identification of magnetic minerals throughout a complete oceanic crustal section yields important information about the carriers of the marine magnetic anomalies, one of the longest continuous archives of the behavior of the geomagnetic field. A study involving the determination of the magnetic hysteresis parameters, Curie temperatures, IRM acquisition curves, magnetic susceptibility, and the microscopic identification of magnetic minerals of samples from sections recovered during drilling of IODP Leg 206, Expedition 309 and 312. Identification of magnetic minerals was performed by reflected light microscopy and scanning electron microscopy and by classifying the oxidation state of titanomagnetites according to the oxidation stages of Haggerty (1976, 1991). The composition of Fe-Ti oxides has been determined semi-quantitatively by energy dispersive X-ray analysis (EDX). Analyses on these sections confirm that a number of downhole variations exist in the magnetic properties which can be attributed to variations in the composition and alteration state of the magnetic mineral phases. From the top of 1256D, the sheeted flows and massive basalts (Leg 206 and Exp. 309) have cruciform/dendritic titanomagnetite grain shapes typical for MORBs. There is huge variability in grain sizes from <1μm up to 100μm in this part of the section. The degree of alteration roughly increases with depth. This finding is mainly evidenced by microscopically visible shrinkage cracks that indicate a transition from titanomagnetite to titanomaghemite and by the continuously increasing Curie temperatures. Primary sulphides are present and in some cases are in contact with titanomagnetites. The magnetic minerals in these sections are likely to carry primary magnetization. In the sheeted dike section (Exp. 309), grain shapes are more ideomorphic, reflecting earlier stage crystallization and possibly a change of the primary composition of Fe-Ti- oxides. In some cases oxy-exsolution is seen with occurrences increasing with depth. An increase in the degree of alteration is evident by the kind of mineral replacement seen in these rocks (titanomagnetite - titanomaghemite-rutile/titanite/magnetite). Secondary sulphides are present in veins in addition to the primary sulphides found. In the plutonic section (Exp. 312), the gabbros contain massive titanomagnetite grains up to 2-3 mm in size. These grains are frequently oxy-exsolved to magnetite and ilmenite with the ilmenite lamellae being partially to totally replaced by secondary minerals like sphene and/or rutile. Hemoilmenite grains are also present as discrete crystals. In some cases, much smaller magnetite grains around 1μm in size are exsolved from silicate minerals. Sulphides are ubiquitous in the plutonic rocks. The present level of oxidation exhibited by the titanomagnetite grains in this section ranges from stage C2 to C3. The Curie temperature throughout the sheeted dikes and gabbroic units lies between 550 and 580°C confirming the presence of Ti-poor titanomagnetite either as the primary product of Fe-Ti-oxide crystallization or as a result of oxyexsolution. In both cases, the present composition would have been established during initial emplacement or shortly thereafter.