The InterRidge Office has just received news from a January 2008 cruise on board R/V Hakuho-maru to the Southwest Indian Ridge near the Marion hotspot. We thank Dr. Nobukazu Seama, InterRidge Steering Committee Member from Japan, for providing a summary of the cruise objectives and sampling. Please visit the InterRidge-Japan webpage for ship track and sampling locations:
http://ofgs.ori.u-tokyo.ac.jp/~intridgej/report_html/KH07-4-2/KH07-4-2.htm
Preliminary report on the Hakuho-maru Cruise KH-07-4 Leg 2, Southwest Indian Ridge
Nobukazu Seama[1]; Kyoko Okino[2]; Toshinori Sato[3]; Hiroshi Sato[4]; Taichi Sato[2]; Shiki Machida[2]; Kentaro Nakamura[5]; Ryoko Senda[5]; Natsuki Neo[6]; Takayuki Kitamura[3]; Mariko Mizuno[3]; Teito Takemoto[3]; Ryosuke Tsujino[1]; Keiko Mizuma[1]; Katsura Kameo[2]; Akira So[7]; Masato Sugano[8]; Yoshifumi Nogi[9]; Hidenori Kumagai[5]; Masanao Shinohara[10]; Kimihiro Mochizuki[10]
[1] Kobe Univ.; [2] ORI, Univ. of Tokyo; [3] Chiba Univ.; [4] Senshu Univ.; [5] IFREE, JAMSTEC; [6] Niigata Univ.; [7] MWJ; [8] NME; [9] NIPR; [10] ERI, Univ. of Tokyo
The mid-ocean ridge process depends on a balance between spreading rate and melt supply, and the melt supply should reflect the physical and chemical character of mantle beneath the ridge. At 34-40ºE on the Southwest Indian Ridge, the spreading rate (ultraslow, ~15mm/yr) is the same along this long portion of the ridge. However, the eastern part of the area appears to have anomalously large melt supply maybe due to the proximity to the Marion hotspot, while this effect does not seem to reach the western segments where the ridge axis is offset by a series of fracture zones. In January 2008 on board R/V Hakuho-maru Cruise KH-07-4 Leg 2, we conducted a surface geophysical survey, marine magnetotelluric transect for imaging upper mantle structure, seismic survey for imaging crust structure, and rock dredge sampling to better understand the contribution of mantle geochemistry to the structure and spreading style of mid-ocean ridges. The surface geophysical survey was focused on ridge segments at 35-39ºE, and one third of the segments was covered by 11 survey lines, each with length 60 nautical miles. Seven OBEMs were deployed to carry out a marine MT transect across the Southwest Indian Ridge at 37ºE. The MT transect across the spreading center will provide an image of the electrical conductivity structure beneath the spreading system after the recovery of the OBEMs during a subsequent Hakuho-maru cruise in February 2009. The seismic survey also targeted the Southwest Indian Ridge at 37ºE to investigate seismic velocity structure. Ten OBSs, a 48-channel hydrophone streamer, and two 20-liter air guns were used, and all the data were successfully obtained. Eight rock dredge hauls were performed along the Southwest Indian Ridge 34-40ºE to investigate a variation due to different ridge segments. Rock samples were successfully recovered at 6 sites. All of these data sets and rock samples will provide geophysical and geochemical constraints for understanding diversity in spreading style and dynamics beneath an ultraslow-spreading system interacting with a hotspot.
