11:00 〜 13:00
[SEM15-P03] Understanding of the role of silicate-hosted magnetic-mineral inclusions in paleomagnetic recording in sediments subjected to reductive diagenesis
キーワード：堆積物古地磁気、還元性続成作用、古地磁気記録、ケイ酸塩包有磁性鉱物、オントン･ジャワ海台
Diagenesis is ubiquitous in marine sediments, causing sedimentary iron-bearing minerals to undergo a series of redox reactions until they reach equilibrium with the reactive chemical components in sediments. Paleomagnetic records in sediments subjected to severe diagenesis may be distorted or lost due to iron mineral dissolution. Special mineralogic structures of the silicate-hosted magnetic-mineral inclusions, where fine-grained magnetic particles embedded in non-magnetic silicate minerals, could protect the magnetic particles from reductive diagenesis. Widespread occurrence of silicate-hosted magnetic-mineral inclusions in sediments could thus provide the possibility that sediments subjected to severe diagenesis still preserve some paleomagnetic records. To better understand this issue, we studied on a sediment core from the Ontong-Java Plateau, western equatorial Pacific Ocean. Magnetic susceptibility (k) has a sudden decrease at the depth about 6 m. The ratio between the anhysteretic remanent magnetization (ARM) susceptibility and saturation IRM (kARM/SIRM) shows a corresponding drop at the same depth as the k. These observations indicate that a decrease in magnetic mineral concentration occurs simultaneously with an increase in average grain size of magnetic minerals, which is likely due to diagenetic dissolution of magnetic minerals. Meaningful information on paleomagnetic declination and paleointensity, on the other hand, could still be successfully extracted even from deeper part of the sediment core. The silicate-hosted magnetic inclusions may play an important role in this sediment core. A considerable amount of the inclusions that preserved paleomagnetic records could survive from diagenesis in the studied sediments. To further investigate the contribution of the inclusions to paleomagnetic recording in the studied sediment core, we are conducting chemical extraction experiments on several samples throughout the sediment core. Any magnetic minerals that remain in the residues after the experiments are assumed to be magnetic-mineral inclusions in quartz and feldspar. We will present magnetic properties of the residues including first-order reversal curve (FORC) measurements and transmission electron microscope (TEM) observations, which would help better understand the role of the silicate-hosted magnetic inclusions in paleomagnetism.