Rock magnetic properties in the sulfate reduction zone in IODP 350 Hole 1437B, Izu Bonin rear arc; preliminary results

Author(s): Musgrave, Robert J.; Kars, M. A. C.; Kodama, K.
Author Affiliation(s): Primary:
University of Sydney, School of Geosciences, Sydney, N.S.W., Australia
Geological Survey of New South Wales, Australia
Kochi University, Japan
Volume Title: AGU 2014 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2014; American Geophysical Union 2014 fall meeting, San Francisco, CA, Dec. 15-19, 2014. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: During the northern Spring 2014 (April-May), IODP Expedition 350 drilled a 1806.5 m deep hole at Site U1437 in the Izu-Bonin rear arc, in order to understand, among other objectives, the compositional evolution of the arc since the Miocene and track the missing half of the subduction factory. The good recovery of mostly fine grained sediments at this site enables a high resolution paleomagnetic and rock magnetic study. Particularly, variations in magnetic properties and mineralogy are well documented. The onboard magnetostratigraphy established from the study of the archive halves highlighted remagnetized intervals that produced "ghost" repetitions of geomagnetic reversals ∼10's meters below their actual stratigraphic position in specific intervals. Onboard paleo- and rock magnetic analyses showed that remagnetization is probably due to a chemical remanence carried by iron sulfides (putatively identified as greigite). The rock magnetic parameters, SIRM/k and the S-ratio are consistent with the presence of ferromagnetic iron sulfides in Site U1437. A mixture of iron oxides and iron sulfides was found within the sulfate reduction zone, which was identified by onboard pore water analyses at ∼50-60 meters below sea floor (mbsf) by a minimum in sulfate (∼5 mM) coupled with a maximum in alkalinity. Below 50 mbsf, the sulfate content increases up to ∼29 mM at ∼460 mbsf. The particular downhole profile of the sulfate content in Site U1437 is probably triggered by fluid circulation. Evolution of sulfate content, pyritization process and fluid circulation are closely linked. Onshore research is focusing on further downhole characterization of the iron sulfides including their abundance, grain size and composition. Routine magnetic properties (NRM, magnetic susceptibility) and rock magnetic analyses at high resolution (every ∼20-50 cm), including hysteresis properties and low temperature magnetic measurements, have been conducted on about 400 discrete samples in the first 200 m of Hole 1437B (Cores 1H to 30X). As suggested by the onboard data, hysteresis measurements showed the occurrence of both single domain-pseudo single domain iron oxides and iron sulfides in the sulfate reduction zone. Preliminary results will be compared with the geochemical data.
Year of Publication: 2014
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Expedition 350; IODP Site U1437; International Ocean Discovery Program; Izu-Bonin Arc; Magnetic properties; Marine sediments; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; West Pacific
Coordinates: N314723 N314724 E1390136 E1390134
Record ID: 2015065268
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