HR: 1340h
AN: GP23A-0166    [Abstracts]
TI: Magnetic and Electron Microscopic Investigation on Rock Samples from the PACMANUS Hydrothermal Vent Field in Papua New Guinea
AU: * Lee, S
EM: smlee@snu.ac.kr
AF: Seoul National University, School of Earth and Environmental Sciences, Sillim-dong, Gwanak-gu, Seoul, 151-747 Korea, Republic of
AU: Doh, S
EM: sjdoh@korea.ac.kr
AF: Korea University, Dept of Earth and Environmental Sciences, Anam-dong, Sungbuk-gu, Seoul, 136-701 Korea, Republic of
AU: Kim, Y
EM: hesse67@freechal.com
AF: Seoul National University, School of Earth and Environmental Sciences, Sillim-dong, Gwanak-gu, Seoul, 151-747 Korea, Republic of
AB: The PACMANUS hydrothermal vent field in the Eastern Manus back-arc basin, Papua New Guinea, is considered as a modern-day analog of massive volcanogenic sulfide deposits within felsic volcanic sequence. This active vent field was drilling in November-December 2003 by Ocean Drilling Program Leg 193. The recovery was generally low with less than 15% due to fragility of rocks. Paleomagnetic measurements and scanning electron microscope observations were performed on samples from three major sites (Sites 1188, 1189 and 1191). Site 1188, a low-temperature diffused venting region, was drilled to 370 mbsf. Site 1189, a black smoker region, was drilled to a depth of 200 mbsf using RCB. The recovered rock samples have inclination close to the present-day Earth field ($-7\deg$), but those near the seafloor have much steeper inclination of up to $-25\deg$. The upper 35 m of the sites consists of fresh to moderately altered dacite-rhyodacite, which exhibits moderately high natural remanent magnetization ($<$ 6 A/m). The region below this extrusive layer largely comprises of pervasively altered rocks with little evidence of sulfide deposit and as a whole exhibits a low magnetization intensity. However, two intervals with extremely high remanent magnetization were discovered below the upper extrusive layer at Site 1188 (135-211 mbsf and 280-370 mbsf) and one interval at Site 1189 (137-190 mbsf). In particular, the samples between 135-211-mbsf interval at Site 1188 have extremely high remanence with intensities ranging up to 300-500 A/m. Although pockets of magnetite are not uncommon in the ancient hydrothermal ore bodies, they have seldom been documented in modern-day system, and little is known about the physical and chemical condition that allows the magnetite to form in hydrothermal vent systems. Two possibilities of magnetite formation are explored: one that these magnetites precipitated from magnetite-rich fluid as it cooled from above the Curie temperature (TRM) and the other that magnetization was acquired by the growth of magnetite grains below the Curie temperature (CRM).
DE: 3035 Midocean ridge processes
DE: 1519 Magnetic mineralogy and petrology
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting