[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-01] Contribution of volcanic magnetite to the magnetism of pelagic sediment from magnetite morphology

Kaisei Hayakawa¹, *Yoichi Usui¹ (1.Kanazawa University)

Keywords:Mariana Basin, subduction zone, Cenozoic

So-called detrital magnetite in pelagic sediments has been regarded as a proxy record for continental eolian dust. At the same time, some microscopic observations show the presence of magnetite of volcanic origin. To understand the contribution of volcanic magnetite to sedimentary magnetism, we perform a comparative study of magnetite morphology and sedimentary rock magnetism. We use pelagic clay from ODP Site 777 in the Mariana Basin. Magnetically separated grains contain iron-rich particles with rounded morphology as well as angular, often octahedral morphology. We interpret the former as continental dust and the latter as volcanic grains derived from the Mariana Arc. Grain morphology shows no clear relationship to grain size or chemical composition. We count 40 grains from each sample and derive relative abundances. Angular (volcanic) grains account for up to 50% of the grains, indicating significant contributions to bulk magnetism. The proportion of angular grains was relatively high (tens of %) before 4 Ma, but lower ( < 10 %) thereafter. This timing is consistent with the increased continental dust flux reported elsewhere in the North Pacific. This age also corresponds to the change in the depth trend of the bulk isothermal remanent magnetization (IRM) acquisition curves. IRM decomposition analysis suggests an increased high coercivity ( > 100 mT) component from 4 Ma. Since volcanic antiferromagnetic minerals should be minor, the change in IRM can be explained as reflecting the relative abundance of continental and volcanic magnetic minerals. This interpretation supports the contribution of volcanic materials to sedimentary magnetism. Furthermore, rock magnetism does not require a change in continental dust properties, implying rather stable dust source during the last 25 Myr. IRM decomposition does not clearly distinguish between volcanic and continental magnetite, indicating simple interpretation of detrital magnetite as continental dust can be misleading. If we can assume a constant magnetic mineralogy for continental dust, it may be possible to quantify volcanic component from average coercivity of detrital magnetic components.