Magnetic structure of the upper kilometer of the marine crust at Deep Sea Drilling Project Hole 504B, eastern Pacific Ocean

Abstract

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low‐temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200‐m transition zone, consisting of a mixture of extrusives and dikes. High‐temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation‐exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe‐rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation‐exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magnetic properties.

Structures and Magnetic Fabrics from the Lower Sheeted Dike Complex of Hole 504B Reoriented Using Stable Magnetic Remanence

“…This may produce a declination error of about 17° estimated from McFadden et al (1988). This is similar to the standard deviation of the stable inclination of the sheeted dike complex (-23°, Smith and Banerjee, 1986;-17°, Dick, Erzinger, Stokking, et al, 1992). We consider that this controls the precision of our reorientations.…”

“…It is possible that some lithologic units are normally magnetized, but for the majority the assumption is probably valid. Smith and Banerjee (1986) have described a secondary magnetization in a few samples in the upper part of the dike sequence, which they used to identify the polarity of the stable remanence component. This yielded approximately equal numbers of reversed and normal polarities, although it was not clear whether this secondary component represented a component acquired in situ, in the Earth's field, or a later viscoremanent contamination.…”

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…At least three processes can affect the magnetic properties of crustal rocks; low temperature oxidation/alteration of the magnetic minerals, geochemical variability and variations in geomagnetic field intensity. In axial and near axis locations extrusive basalts are assumed to comprise the majority of the magnetic source layer Smith and Banerjee, 1986;Pariso and Johnson, 1991]. The primary carrier of magnetic moment in basalts is titanomagnetite, which is altered to lower magnetic moment titanomaghemite Marshall and Cox, 1973;Pariso and Johnson, 1991].…”

Effect of low-temperature oxidation on the remanence properties of titanomagnetites.

“…The deepest penetration of marine crust (at 1076 msub-basement) was achieved on DSDP hole 504B and low-temperature altered titanomagnetite was reported to be the dominant magnetic carrier (SMITH and BANERJEE, 1986). They showed that the NRM in the lower two sections was a chemical rather than a thermal remanence.…”