Magnetic Properties of Gabbros from Hole 735B, Southwest Indian Ridge

Abstract

A total of 500.7 m of continuous, vertical, oceanic gabbroic section was recovered during Leg 118. The gabbros obtained exhibited various degrees of alteration and deformation, which gave us a good opportunity to study the magnetic properties of oceanic gabbros. Many of these gabbros, which are mainly Fe-Ti oxide gabbros, have strong and unstable secondary magnetic components that were acquired during drilling. Stable inclinations, which are probably in-situ magnetic directions, show a single polarity, with an average value of 66° (±5°), meaning that the studied 501-m oceanic gabbroic block may be a candidate for the source of the marine magnetic anomaly. This may also imply that the metamorphism of oceanic gabbros causing acquisition of magnetization probably occurred within one geomagnetic polarity chron (about 0.3 to 0.7 m.y.) after these gabbros formed at the ridge, leading us to conclude that oceanic gabbros record the so-called Vine-Matthews-Morley type of initial magnetization at the ridge. The average intensity value of stable magnetic components of individual samples, which may be a minimum estimate for remanent magnetizations, is 1.6 A/m. Assuming this magnetic intensity value and a uniform magnetization within an oceanic gabbroic layer having a thickness of 4.5 km (i.e., whole layer 3), it is possible to explain most of the marine magnetic anomaly. If magnetic properties of the samples obtained from Hole 735B are common to oceanic gabbros, layer 3 may contribute more significantly to seafloor spreading magnetic anomalies than previously thought.

Paleomagnetism of Gabbroic Rocks and Peridotites from Sites 894 and 895, Leg 147, Hess Deep: Results of Half-Core and Whole-Core Measurements

“…The ratio of these two parameters suggests that the in situ magnetization of this crustal section is carried by remanent magnetization. These magnetic properties are similar to those obtained for Leg 118 gabbros which were from the slow-spreading Southwest Indian Ridge (Kikawa and Pariso, 1991). This fact may suggest that magnetic properties for oceanic intrusive rocks are similar between fast-and slow-spreading ridges, thus for both ridge systems oceanic intrusive rocks contribute significantly to the marine magnetic anomalies.…”

“…2B). However, most of the values lie between 0.005 and 0.035, which is consistent with previous studies (e.g., Fox and Opdyke, 1973;Kikawa and Pariso, 1991). Results of minicore samples show a similar range and identical mean values (see Pariso et al, this volume).…”

“…As mentioned above, the calculated average of stable inclinations of 51.2°, which is significantly steeper than the expected value close to 0°, was probably caused by tectonic rotation after cooling of the crustal section. This rotation was also observed for Hole 735B gabbros (Kikawa and Pariso, 1991), suggesting that crustal rotation after cooling of the oceanic intrusive section commonly occurs during the process of its emplacement to the seafloor at a mid-ocean ridge system. …”

“…Because the axial geocentric geomagnetic field inclination at Site 894 (2°N) is very close to 0°, it is unlikely that the secondary component was acquired parallel to the ambient geomagnetic field at the site. Previous studies of DSDP/ODP drill cores have shown that soft and steep components of magnetization are probably remanent magnetizations acquired during drilling (e.g., Lowrie and Kent, 1978; Kikawa and Pariso, 1991). The observed secondary magnetization which generally had soft and steep components is probably drilling-induced remanent magnetization.…”

Magnetic polarity structure of the lower oceanic crust

“…Schulz et al, 1988;Tivey and Tucholke, 1998). Rock magnetic studies and paleomagnetic studies in unoriented dredge samples [Fox and Opdyke, 1973;Kent et al, 1978], partially oriented Ocean Drilling Program (ODP) cores [Pariso and Johnson, 1993a,b;Kikawa and Pariso, 1991] and ophiolites [Banerjee, 1980] all illustrate that oceanic gabbros are capable of retaining a stable magnetization. Inherent in the VineMatthews hypothesis is the idea that oceanic crustal magnetization is acquired while cooling from magma close to the ridge axis, either by cooling through the Curie temperature or by high-temperature alteration soon after eruption or intrusion.…”