Alteration processes at Deep Sea Drilling Project/Ocean Drilling Program Hole 504B at the Costa Rica Rift: Implications for magnetization of oceanic crust

Abstract

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first-order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well-defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This posternplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to posternplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic propellies of rock saniples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth. INTRODUCTION The recognition of polarity reversals of the geomagnetic field [Cox et al., 1964] and the identification of the record of this behavior in the magnetic anomalies of oceanic crust [Vine and Matthews, 1963; Morley and Larochelle, 1964] were fundamental in placing the process of ocean crust formation within the context of plate tectonics. Over time, however, our concept of oceanic crust as a magnetic tape recorder has evolved from the original, uniformly magnetized block model of Vine and Matthews, into a much more complex picture. Although progress has been made on understanding ocean crustal magnetization, we still do not have a complete model of the processes associated w...

Behavior of oceanic crustal magnetization at high temperatures: Viscous magnetization and the marine magnetic anomaly source layer

“…For this reason, each sample was first annealed for approximately 12 hours at 600øC to remove crystal defects and internal strain imparted during the drilling/sampling process [Padso and Johnson, 1991Johnson, , 1993. The annealing process eliminates defect migration or strain relaxation over the course of multiple heating experiments.…”

“…Vine and Matthews [1963] recognized early that a major contribution comes from the intensely-magnetized extrusive basalt layer. Recent evidence suggests that dike [Smith, 1990;Pariso and Johnson, 1991;Johnson and Salem, 1994] and gabbroic [Pariso and Johnson, 1993 a and b] sections can also be strongly magnetized.…”

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].…”

Paleomagnetic and Rock Magnetic Properties of Hole 896A

“…There is no need to apply a correction such as those described for other studies of the magnetism of the ocean basement (e.g., Pariso and Johnson, 1991). Plotting the intensity against depth below basement (Fig.…”

“…Our understanding of the magnetization of the oceanic crust comes largely from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilling (e.g., Furuta and Levi, 1983;Smith and Banerjee, 1986;Pariso and Johnson, 1991) and from ophiolites (e.g., Vine and Moores, 1972;Banerjee, 1980). Within these studies, ODP Hole 504B holds a special place as the deepest hole in ocean basement, penetrating 1.8 km of oceanic basement, including more than 1 km of sheeted dikes.…”