Formation processes of magnetic spherules collected from deep-sea sediments -Observations and numerical simulations of the orbital evolution-

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Spherules collected from deep-sea sediments were analyzed for major chemical compositions and examined for textural relationships. Two main types of internal texture are observed in Ni-bearing I-type spherules : spherules with Ni-free Fe-oxide mantle and Fe-Ni metallic cores and Fe-Ni oxide spherules without metallic cores. Cores are often observed off-centered. S-type spherules show porphyritic, barred, and fine-grained types of texture. Relict olivine grains are found in some porphyritic S-type spherules. A cooling rate of 55℃/s was calculated for one of these grains based on chemical zonal patterns. For I-type spherules, numerical simulations were performed to constrain the relationship between temperatures, radii of spherules, and the apparent gravitational acceleration with the spherule as a stationary frame of reference. Results indicate that a) spherules lose most of their original mass by evaporation over 1900℃, and, b) the apparent gravitational acceleration is at a maximum during the last stage of evaporation. Based on the observations and numerical simulations of I-type spherules, we propose the following model for their formation : during ablation, the metallic core is off-centered, a surface of the core is then exposed to the air due to the strong apparent gravitational acceleration and, finally, oxidation reactions can proceed on the exposed core surface.
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