Rock Magnetic Properties of Hydrothermally Formed Iron Sulfides from Middle Valley, Juan de Fuca Ridge

Abstract

Hydrothermally formed sulfide samples recovered from the Juan de Fuca Ridge during Ocean Drilling Program Leg 139 were investigated to determine the carrier of remanent magnetization and to study rock magnetic properties. The samples were examined by isothermal remanent magnetization acquisition, alternating field demagnetization, back field measurements, thermomagnetic and hysteresis measurements, X-ray diffraction, and ore microscopy. Grain sizes for magnetite and pyrrhotite particles were estimated from hysteresis parameters on samples containing only one magnetic phase. Magnetomineralogical analyses made on whole-rock samples show that massive sulfides recovered from Holes 856G and 856H are composed predominantly of pyrite, pyrrhotite, magnetite, and minor amounts of sphalerite and chalcopyrite. Magnetite together with two types of pyrrhotite determines the magnetic properties of these rock samples. The first type of pyrrhotite is ferrimagnetic up to 320°C. X-ray diffraction data are in good agreement with those for monoclinic pyrrhotite. The second type of pyrrhotite is antiferromagnetic at room temperature and ferrimagnetic between 200°C and 270°C. X-ray diffraction data show typical peaks for hexagonal pyrrhotite. Thermomagnetic analyses indicate that samples from Hole 856G contain mainly magnetite, whereas parameters from Hole 856H were also influenced by pyrrhotite. Pyrrhotite and magnetite were distinguished by the large J rs /J s and the small B cr /B c ratios for pyrrhotite compared with those for magnetite, and through direct methods such as ore microscopy and X-ray diffraction.

A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

“…A magnetization of 5 A/m implies an average volume susceptibility of 0.11 SI units assuming an entirely induced magnetization in the ambient field of 54,500 nT. This value compares well with the average susceptibility measured on sulfide samples from ODP drill cores (arithmetic mean = 0.17 ± 0.15 SI; geometric mean = 0.08 SI [ Körner , 1994]). All samples from the sulfide mound contain predominantly pyrite (90%), with 2–10% magnetite and smaller amounts of hematite, pyrrhotite and other sulfides [ Körner , 1994].…”

Hydrothermal Activity at the Ultraslow‐Spreading Mohns Ridge: New Insights From Near‐Seafloor Magnetics

“…We observe that these observations of two strong positive anomalies correlate with the presence of previously collected sulfide material at the same location (Pedersen, Rapp , et al, ) and suggest that these sulfide deposits are creating a magnetic signal. This type of magnetic signature was observed in several locations around the world and is explained by the contrast between nonmagnetic sediments and the massive sulfide deposit usually containing highly magnetized magnetite, pyrrhotite (Gee et al, ; Körner, ; Pedersen, Rapp , et al, ; Tivey , ). The interpretation for the smaller anomaly is confirmed by video material and sampling of hydrothermal material composed of pyrite and heterogeneous fine‐grained chimney material (Pedersen, Rapp , et al, ) and corresponds to the Mohn's Treasure extinct hydrothermal field (MT‐1 in Figure ), as no water column indications of venting are registered at the site.…”

Middle Valley: Bent Hill Area (Site 1035)

“…At 0.2 T, only a 80%−90% saturation is achieved (cf. Körner, 1994). Comparable to the intensity after the 15-mT demagnetization step, the intensities of IRM @1T are highest in Subunit VIA, and Units VII and VIII.…”