23. ORIGIN AND APPLICATIONS OF WHOLE-CORE MAGNETIC SUSCEPTIBILITY OF SEDIMENTS AND VOLCANIC ROCKS FROM LEG 1521Koji Fukuma2 |
ABSTRACTNearly continuous records of whole-core magnetic susceptibility obtained during Leg 152 reveal their potential as a lithostratigraphic tool for both sediments and volcanic rocks. For sediments, thermomagnetic and hysteresis measurements show that the whole-core magnetic susceptibility is primarily controlled by the concentration of magnetite associated with detrital minerals. The continuous susceptibility record for cores from Site 918 documents that the detrital influx gradually increased from about 11 Ma and that already at about 9.5 Ma the susceptibility reaches a high level about 50 m below the bottom of the oldest verified glaciomarine sediment (about 7 Ma). The two overlapping cores from Site 919 provide an opportunity for correlation and recovery of a complete composite sequence. In addition, the presence and type of dropstones can be detected by whole-core susceptibility if combined with the gamma-ray attenuation porosity evaluator (GRAPE) density data. For volcanic rocks, the whole-core susceptibility depends on the rock chemistry and the degree of oxidation. The susceptibility of the massive inner part of lava flows is lowest in the least evolved picrites and highest in the evolved olivine-free basalts in the Middle Series. Scoriaceous breccia and fractured rocks are characterized by highs of magnetic susceptibility. Although some of the lava flow tops are marked by susceptibility enhancement, very intense oxidation at other tops reduced the susceptibility by oxidizing magnetite into hematite. In situ susceptibility logging of subaerial volcanic sequences would be useful in taking a continuous profile of successions of lava flows. When combined with shore-based rock magnetic measurements, new applications of whole-core magnetic susceptibility data can be exploited to give new insights to lithostratigraphic variations. |
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