HR: 0830h
AN: GP31B-0754    [PDF]
TI: Rock Magnetic Investigation of Felsic Hydrothermal Vent System: Results from ODP Leg 193 to Eastern Manus Basin, Papua New Guinea
AU: * Lee, S
EM: smlee@kordi.re.kr
AF: Korea Ocean Research and Development Institute, Ansan P.O. Box 29, Seoul, 425-600 Korea, Republic of
AB: In November-December, 2000, an active hydrothermal vent field in the Eastern Manus back-arc basin, Papua New Guinea, known as the PACMANUS vent field, was drilled during ODP Leg 193. This vent field has been considered as a modern-day analog of massive volcanogenic sulfide deposits within felsic volcanic sequence. The recovery was generally low due to fragility of rocks. Detailed paleomagnetic and rock magnetic analyses were performed on rock samples recovered from three major sites (Sites 1188, 1189 and 1191). Site 1188, a low-temperature diffused venting region, was drilled to 370 mbsf utilizing a combination of RCB, Hammer Drill, ADCB and casing, and Site 1189, a black smoker region, was drilled to a depth of 200 mbsf using RCB. Paleomagnetic analysis shows that recovered rock samples have inclination close to the present-day Earth field. The top 35 m of PACMANUS vent field consists of fresh to moderately altered dacite-rhyodacite and exhibits moderately high natural remanent magnetization ($<$ 6 A/m). Although there are small intervals of markedly less intensive alteration, the region below this extrusive layer is largely comprised of pervasively altered rocks with little evidence of sulfide deposit and exhibits as a whole a low magnetization intensity. However, two intervals with high remanent magnetization ($>$ 6 A/m) were recognized 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 systems. Two possibilities of magnetite formation and its apparent alignment with the Earth field 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). Understanding the origin and behavior of these magnetic mineral assemblages may in turn provide a valuable constraint on the physical and chemical conditions of subseafloor hydrothermal systems, which are very poorly known at the moment.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
DE: 1550 Spatial variations attributed to seafloor spreading (3005)
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting